Document zzG5xDrqKbxKvr783Ky06bQ17

Special Publication Crystalline Silica Primer Staff, Branch of Industrial Minerals U.S. Department of the Interior Manuel Lujan, Jr., Secretary U.S. Bureau of Mines T S Ary, Director ir']R>HrQrrA F I FA1J7 SFF F/T' A U>1P>F7UTFT 1D> i^AX A Ls A J%jL/JL/AJ. V1U/ j mi 11^11 q. ^^-11 JTJL\JLLVJL1U*A\. Library of Congress Cataloging-in-Publication Data: Pcrny/et'sllina ^enilSiv/*a^ pnr (ci / wQiann/*ukii ui iImn/u^iuoui<ai iMvu!nii/dM<>ani5lM>. p. cm. - (Special publication) Includes bibliographic references. 1. Silicate industry-health aspects. 2. Siiica-Toxicoiogy. 3. Silica-Carcinoaenicitv. 1. United States. Branch of Industrial Minerals. II. Series: Special publication (United States. Bureau of Mines) RC965.SSC79 1992 615.9'25682-dc2Q 92-16938 CIP II CONTENTS PARTI Understanding Crystalline Siiica? What Is Silica? Silicon Is an Element Silica Is a Chemical Compound Silicates Are Compounds of Silicon and Oxygen plus other elements PART II What Is_ Me__ant bv/ C- ry/ stalline-? The Crystalline State The Noncrystalline State Focusing on Crystalline Silica Crystalline Silica's Forms mvvln__e__r_er*v_^_r_y__sju_aIIn:_m__e_tro:lm:__c_a_i_isr*r_u_u___n__ul The Natural Occurrence of Crystalline Silica In Igneous Rocks In Sedimentary Rocks In Metamorphic Rocks Crystalline Silica in the Industrial Setting The Many Uses of Crystalline Silica In Glass, Ceramic, and Fine China Manufacturing In Construction In Heavy Industry High-Tech Applications Synthetic Crystalline Silica The Regulation Of Crystalline Silica OSHA's Hazard Communication Standard The IARC Evaluation Process IARC Classification of Silica Sidebar: More About IARC'S Evaluation Process Regulatory Activities of Other Agencies The Complexities of Measurement Conclusions List of Resources and Selected Readings G--l-o--s-s--a/ry iii CM YSTAEEENE SRMCA MMRMEM ILLUSTRATIONS 1. Terms Related to Crystalline Silica 2 Si!icon-Oxygen Tetrahedron 3_._D_i_a_tomite (. amoro| hou- s- ). a- nd- O-vuartz (' cry/ stalline)' 4. Repeating Pattern of Crystalline Structure 5. Random Pattern of Amorphous Structure 6. Nonrepeating Patterns of Glassy Structures 7. Relationship Between Forms of Silica oo. oPji.a_uLi:iIu:i.y. rn:e_ilus ui mxle ume_r_e_nxi rro__r_m__spu:Ii:jinc1 _d iri Glass, Ceramic, and Fine China ,Manufacturing 9. The Geologic Cycle 10. The Glassmaking Process 11. Smithsonian Castle Built of Sandstone 12. Oil Rig 13. Quartz dock 14. Synthetic Quartz Crystal TABLES 1. Silica in Commodities and End-Product Applications 2. Common Products Containing 0.1% or More 3. Methods Used to Detect Quartz In a Sample IV a raxc?rm> a /r"rr UU ABSTRACT Crystalline silica is the scientific name for a group of minerals composed of silicon and oxygen. The term crystalline refers to the fact that the oxygen and silicon atoms are arranged in a three-dimensional reiDeatineu iDattern. This uerouDI of mine-ra-ls--ha-s-sha- D,- ed human history since the beginning of civilization. From the sand used for making glass to the piezoelec tric quartz crystals used in advanced communication systems, crystal line silica has been a part ofour techno logical development. Crystalline silica's pervasiveness iiini rvtui iir tivov.r1kini invilum^rrxj/ iUc 1m1 ia-aivt.rikts/ijr\ji nvin11l\j/ kvjir/ Iiutc? aikviuiiniurla'iinivm^ inni nature. It's found in samples from every geologic era and from every location around the globe. Scientists have known for decades that pro longed and excessive exposure to crystal line silica dust in mining environments can cause silicosis, a noncan- cerous lung disease. During the 1980's, studies were conducted that suggested that crystalline silica also was a carcinogen. As a result of these findings, crystal line silica has been regulated under the Occupational Safety and Health Administration's (OSHA) Hazard Communication Standard (HCS). Under HCS, OSHA- regulated businesses that use materials containingO.1 % or more crystalline silica must follow Federal guide lines concerning hazard communication and worker training. Although the HCS does not require that samples be analyzed for crystalline silica, mineral suppliers or OSHA-regulated businesses may choose to do so if they wish to show that they are exempt from the requirements of HCS. Because crystalline silica is an extremely com mon mineral and the HCS will affect many mineral rnmmnHitiAc itic Imnnrtantthpn thatthorp hp ac rlp^r .. IWiW .w.7 V. .UVU*WIW MW -- an understanding as possible of what is and what is not crystalline silica, and where it is found and used, and how it is qualitatively and quantitatively identified. This primer is an attempt to accomplish this in as nontechnical a manner as possible. This primer wiii 1 CMYSTAEMNIB SEEECA IPMMMEIR AA DD3CT1 InV/AW/-'T1 examine crystalline silica. Part i will describe, in nonscientific terms, what crystalline silica is and how wp rome in rontart with it. Part II will dismss thf regulatory decisions that have created new interest in this ancientand widespread substance and will present a nontechnical overview of the techniques used to measure crystalline silica. Because this primer is meant to be a starting point for anyone interested in 1I_C_d_lll_ll_lg__I_II_U_I_C_dLW__U4.lU__y_dkldilill:l_lC__a:illM:_-_d-, d\-.I-I31UI S_CII_C_LI.C_Ui rparlinoc anrl nth<r rpcmirrpc ic inr~lnrloH Tho rlo. tailed glossary, which defines many terms that are beyond the scope of this publication, is designed to help the reader move from this presentation to a more technical one, the inevitable next step. ACKNOWLEDGMENTS We wish to acknowledge the U.S. Bureau ofMines commodity specialistsfortheirinputon thisprimer, withspecialappreciation to Sands Q Ampian andRobertL Vina, whose technicalpaper formedthe foundation ofthis primer, andto Marilyn K. Nelson, Blue Pencil Group, Inc, Vienna, VA, forher valuable contribu tions to the content and editing ofthe primer. We also wish to acknowledge Or. Rustu Kalyoncu and Dr. Garrett Hyde, U.S. BDUi iIhCaC^Ui iJaU/iAnrUiJC9/1V VCLX III IgVUIt,TLVA*-, fUWl.MVV/TII/IIZCiUim19A/r|^iC//v3rD/wIiUfr>M* rUAVnG Carp, Denver, CO; Dr. RobertHamilton, Manville Corp., Den ver, CO; Dr. C. S. Thompson, R. T. VanderbittCo. Inc, Norwalk, CT; andMs. Elizabeth Festa-Watson, Chemical Manufacturers Association, Washington, DC, for their reviews ofthe primer. Editor, NancyRyan forherpatientediting ofthefinalmanuscript and Visual Information Specialist, S. Ann Lewis for thegraphics, design, photographs, and layout ofthe primer, U.S. Bureau of Mines, Branch ofPublications, Washington, DC IPAMT E UNDERSTANDING CRYSTALLINE Crystalline silica is an indispensable partot Doth the natural and the technological worlds. We all come ir> rnntTrt u/ith it rlailw kiwo all ru ir liuoc It Kac Kcuan wi ttiui iiwuiijr .................................................................... - -----------U----J----f-c--S--V--WI called one o- f t-h-e- buildinOg blocks o- f our |D---l-a--n--e--t-. AlthougCJh it is a mainstay of modern technology, it is neither modern nor manufactured. It was known to the ancients, and its uses are still being expanded today. tTiidi is jiiiui; coiner suosiances wnose names souna simiiar- with silica. The terms may sound alike, but each means someth i ng qu ite d isti net. Knowi ngthe differences among these four substances is crucial to understanding what crystalline silica is and, perhaps just as important, what it is not. All matter in the universe is formed from the 107 FlprriAnt or so chemical elements known to exist A chemical Silicon // // \\ \\ X /XX /// \ \\ \\ M etal j/ / // .Qilirnnps \ Silica \ \ \\ Qiliratoc viiivuivw FIGURE 1. Terms Relating to Crystalline Silica 1 CM YSTAEEHNE SEMCA MMEMEM element is the simplest form of mutter, 3 fundamental substance that consists of only one kind of atom. Silicon is the second most common element in the Earth's crust, second only to oxygen, and together silicon and oxygen make up approximately 75% ofthe Earth on which we iive and from which we get aii that we use in our daily lives. S---tric- t-lv/ s-D|..e...a..k..i.n..(ej.f-s--i-li-c--o--n- '(whosec--h--e--m----ic--a---l-s/vmbol is Si) is classified as a nonmetal, but it possesses some of the properties associated with metals. There are eight elements, in fact, that fall on the borderline between metais and nonmetais. Some scientists refer to these as metalbias. One property associated with ___ati Ic ^nr ov'amnli 1 ic.iaiiui uAampn a'akh/il I11if\.ty/ tiruv.rv//'M1 iiu/'lui ivc.itcnilcnvc,'utri1 ity. Silicon's electronic capabilities are unusual: At high temperatures, it acts like a metal and conducts electricity, but at low temperatures, it acts like an insulator and does not. It is said to be a semiconductor. This unusual property made silicon the perfecteiement to move technology first into the world of transistors, 4-1_______________ U :_____ _J r:__11.. UICII IIIUS UIC WVJI IU Ul IlllCglCtlCU I.IIV.UIU, dllU lllldliy into the world of today's computer chip. Silicon is the backbone ofthe computer chip. The pure silicon needed for this use, however, does not exist in nature; it is formed from silica sand. Thin slices of pure silicon are then etched with the intricate electronic circuits needed to run the computer. Silica Is a Mineral Compound The_ c_ o_ mDound silic-a (*"SiO- i.') is formed from silicon and oxygen atoms. A chemical compound is defined as a distinct and pure substance formed by the union of two or more elements. Because oxygen is the most abundant element in tka rn icf anrl c 11irnn ic tho cflrr\nri mnct hi u iw uai u 1 a vi ujv anu jiiiwii uiv 7vwi ivi 11 ivji abundant, the formation of silica is quite common in nature. The silica sand, just mentioned as the sub stance used to derive pure silicon, is made of quartz. 4 IPAMT E which is the most common form of silica found in nature. ran alcn ho hinl/viiral in rtrioin nrnrli irorl WI IVU V.WI I u 1 ov vv* v/iwaw^ivui 11 i WI ii wiUWVl bv tinv organisms. The most significant of these are t /u ij ' " diatoms (plants) and radiolarians (animals), both of which extract silica from the water around them to form their structures or shells. For both organisms, silica is a nutrient they must have to survive. In nature, they use the dissolved silica that originates from sedimentary rocks at the bottom of a lake, river, or ocean. When diatoms grown in the laboratory extract all the available silica from the aquarium water, they attach themselves to the walls of the aquarium and use the small amounts of dissolved silica etched from the glass itself. In nature, when diatoms and radiolarians die, they sink to the radiolaritC; Diatomite is a commercially useful rock. It's highly porous and, thus, is effective for filtering as well as for use as filler and as a mild abrasive. 5 CM YSTALEENE SEEECA IPMEMEM Thus, silica can be found in more than one state-amorphous as in the remains from a diatom and crystalline as in a quartz crystal, as we shall explain later. Both are Si02 but they are quite different physically. What's more, silica in its crystalline state is found in more than one form. This phenomenon is called polymorphism (literally "many forms"). Silicates Are Compounds of Silicon and Oxygen Plus Other Elements When silicon and oxygen bond with other elements, they do so in a paired formation. Scientists call this pairing the silicon-oxygen (Si04)tetrahedron because it is made of four oxygen atoms and one silicon atom. Tetrahedron means "four surfaces" and refers to the shape ofthe Si04 compound. The siliconoxygen tetrahedron bonds most frequently with so dium; potassium, calcium, magnesium, iron, and aluminum to form silicates. Silicates constitute the most abundant class of minerals. Geologists regard silicate minerals as the basic materials out of which most rocks are created. Silicones Are Synthetic Compounds Silicones are polymers, a type of synthetic compound. Developed commercially during World War II, silicones are formed from two or more silicon atoms linked with carbon compounds (referred to as organic compounds). Most silicones contain oxygen as well. Unlike what happens when silica and sili cates form, in silicone, the silicon and oxygen do not take the tetrahedral shape but instead form chain like structures called silicon polymers. Polymerization is a chemical reaction in which small organic molecules combine to form larger molecules that contain repeat ing structural units of the original molecules. Silicones can range from liquids (used as water repel I ants and defoamers) to greases and waxes (used as water- and heat-resistant lubricants) to resins and solids (used to make special heat- and chemical- 6 PART M rntuf*u' iKi nCr j. D latum ite (amorphous) and Quartz (crystalline) resistant products including paints, rubbers, and plastic DI--a--r-t-s'). Pr-o-b--a--b-l/v-s--il-i-c--o-n--e--'-s--m---o--s-t--h- iOehl/vI-o-u--b--l-i-c-i-z--e--d---u-s--e--isin the manufacture of breast implants. What Is Meant by We mentioned that the compound silica, which Crystalline? is formed by thechemical reaction ofsilicon and oxygen, 7 CMYSTALMNE SIMCA IPMEMEIR FIGURE 4. Dan3tino Pattom n"& C'rvcta.l.li.n..o. Qtnirfiirp can be either crystal 1 i ne or noncrystal 1 ine. Depending upon the extremes of temperature and pressure it has been subjected to or, in some cases, the speed at which it cooled, a solid can take on different forms. Diatomite, described earlier, and quartz are identical c-tn_e__m:i_c_aInI.y. /[!_o_a.*ml_ oik^2j\a__n_a1 1a_u_u_i_a_r_e_m_jli:i-ul-^ at mum temperature, but their physical forms-and their inter n* 'al---t-f-mct-ur-ps--ar- p v* prv/ di.f.f.e..r..e..n..t.. The Crystalline State In a crystal 1 ine substance (such as quartz), the atoms and molecules make up a three-dimensional repeating pattern. The pattern unit is repeated indefi nitely in three directions, forming the crystal line struc ture. This is similar to floor tiles, in which a vo= dimensional pattern unit, say one made of two black tiles and fourwhite tiles, is repeated indefinitely in two directions. 8 JPAMT E This repeating pattern can be altered. It would be possible to change the positions of the two black tiles and four white tiles in relationship to one another and still have a pattern that could be repeated indefinitely in two directions, but the resulting design would be different. Likewise, the internal structure of the crystal can be changed and the resulting crystalline substance would be changed. The Noncrystalline State Now, picture the black tiles and white tiles, still in the same relative proportions of two to four, randomly placed on the floor, forming no pattern whatsoever. Such is the structure of a noncrystalline, or amorphous, substance. A diatom is an example of silica in a noncrystalline state. Some amorphous materials exhibit shortrange ordering oftheir atoms. Using the analogy of the floor tiles one last time, suppose the two black tiles and FIGURE 5. Random Pattern of Amorphous Structure 9 CMYSTALMNE SEMCA PMEMEM four white tiles formed a pattern, and it was a pattern governed by some sort of rule, but it was not a repeating pattern. The distinguishing feature of a crystalline substance is that you can take any portion of it and see the whole. With a nonrepeating pattern, you can't do that. Some short-range orderliness may exist, but no predictable order extends over a long distance. FIGURE 6. Nonrepeating patterns of Giassy Structure Scientists call this state glassy. Not surpris ingly, window glass, which forms when molten glass is quenched, is an example of silica in a glassy state. It is not crystalline because it cooled too rapidly for the atoms to arrange themselves into a long-range periodic structure, but it contains short-range ordering that many amorphous materials do not possess. Glassy and amorphous materials are considered to be synony mous by many scientists because both are noncrystalline. 10 IPAMT 1 C Wrt^Ui i9cIini iag on Crystalline Silica confused-silicon, silica, silicates, and silicone, and we have narrowed our discussion to silica, the compound formed from the elements of silicon and oxygen. We have seen how siiica can be crystalline or noncrystalline This primer's focus is on silica in its u.--puti1u1:iic. auuc --umI.y. . FKiUkE 7. Relationship Between Forms of Siiica Crystalline / Silica's Forms Crystalline silica exists in seven different forms / or polymorphs, four of which are extremely rare. The three major forms, quartz, cristobalite, and tridymite, are stable at different temperatures. Within the three major forms, there are subdivisions. Geologists distin guish, for example, between alpha and beta quartz, nntinir iv/u u law at ai R 7t *3 n11art*r rhannoc frnm nno fnrm tr\ W/ vjuoi uu \*i iui ii v 11 vi iv ivi tvs the other, Each of these subdivisions is stable under different thermal conditions. Foundry processes, the burning of waste materials, and other manufacturing procedures can create the kinds of conditions neces sary for quartz to change form. In nature, quartz in its alpha, or low, form is most common, although both lightning strikes and meteorite impacts can change alpha quartz into keatite or coesite. Alpha quartz is 11 CMTSTAEEENE SEEECA JPMIMEM abu- nd- a- ntv, fo--und o- n- - e' ve- rv/ c' o"ntinent "in largOe---o1u--a-n--tities. In fact, alpha quartz is so abundant and the other polymorphs of crystalline silica are so rare, some writers use the specific term quartz in place of the more general term crystalline silica. 4000 2000 Low Quartz 1000 High Quartz Cristobaltte Liquid UUU 500 1000 1500 20001 FIGURE 8. Stability Fields of the Different Forms of Silica In Glass, Ceramic, and Fine China Manufacturing \A/horo W V IIVI V. Crystalline Silica Is Found Crystalline silica, usually in the form of alpha quartz, is everywhere. It is in every part of every continent It occurs plentifully in nature and is used commonly in industry. The Natural Occurrence of cZ.'_ry__s1ia--u11m*__e Calls*'* ZIIIV.O All soils contain at least trace amounts ot crystalline silica in the form of quartz. It may have koAn n*i+ k/Vsti i pai v v/i u rA/*l/ i vsv,iv tkof u lai wcau iti caj trt lu //M*m tv/i 111 fka uic pcaiI jvi i it may have been transported, or It may have crystal lized from an amorphous (that is, a noncrvstalline) IPAMT E silica thatformedduringthe weathering process. Quartz is also the major component of sand and of dust in the air. O^uartz is IDresent in ieneous rocks-but onlv/ those that contain excess silica. As magma cools, olivine, pyroxenes, amphiboles, feldspars, and micas form first. These minerals (all silicates) need silica to form, because silicates are made from silicon, oxygen, and a metal, usually one of the six most common metals. Quartz forms only if sufficient silicon and oxygen are left over after these silicates have formed. Nature's odds are stacked in quartz's favor, however. The fact that quartz is the second most common mineral in the world (feldspar is most com mon) indicates that plenty of silicon and oxygen were left over during the cooling process to allow ample quantities of quartz to form. In fact, the average quartz content of igneous rocks is 12%. In geologic history, igneous rocks originated from magma, the material carried to the surface from the Earth's molten core. The othertwo types of rocks are sedimentary and metamorphic. Quartz is abundant in all three types of rock. It is one of Earth's primary u..;M;nrr kl A/'l/C UUIIUIIIg The rock cycle describes the relationship between the three types of rock. Igneous rocks reflect activity (heat andpressure) beneath Earth's crust; meta morphic rocks reflect activity both beneath the crust and within and at the surface; and sedimentary rocks reflect conditions (wind, water, and ice) at the Earth's surface. Over geologic time, sedimentary rocks may be altered by heat and/or pressure to create metamorphic or igneous rocks. All rocks may be eroded to make sediments that, in turn, harden (lithify) into sedimentary rocks. Thus, the history of the Earth's crust, the lithos phere, is one of continuous change. During these changes, quartz endures. It is one of Earth's harder minerals, so it resists erosion, and it is soluble in very few chemicals, so it is seldom dissolved. 13 CMYSTALMNIE SELHCA 1PMKMEM. Melting FIGURE 9. The Geologic Cycle In Igneous Rocks. Crystalline silica is present in igne ous rocks that contain excess silica. It is a common component of granite, rhyolite, quartz diorite, quartz monzonite, and andesite, to name a few. Crystalline silica as quartz also may be present in deposits of hardened, or consolidatedvolcanic ash, known as volcanic tuffs. When magma spews from a volcano, itdrops in temperature so rapidly thatthe ash is usually glassy, a noncrystalline state. The 1980 eruption of Mount St. Helens is a perfect example of this process. If the silica crystallizes before the molten rock leaves the volcano then the quartz is imbedded in a glassy matrix. Volcanic glasses do crystallize over time, so a complex mixture of finely crystalline quartz and silicates eventually replaces the volcanic glass. Cristobaliteandtridymite,the rarer forms ofcystal line silica, may also be present in volcanic tuffs. 14 JPAMT E In Sedimentary Rocks. Crystalline silica in the form of quartz is an extremely common component of sedimentary rocks. Sedimentary rocks form when minerals released during weathering or by chemical precipitation accumulate in a basin and are consoli dated. Quartz, which is extremely resistant to physical and chemical breakdown by the weathering process, stays intact chemically even when fragmented and d- is-D1~ e- rsed bv/ e- ro- s- io- n,' wind- , or ot'he- r we--a--the~ ringorDro' cesses. Quartz is present in a variety of sedimentary rock types, ranging from sandstones to conglomerates, in trace to major amounts. In Metamorphic Rocks. Metamorphic rocks, which form through heat or pressure, also contain crystalline silica as quartz. New textures may be created in the rock (for example, lineations or increased crystal sizes), and new minerals may be formed during metamorphism. Quartz may be present in the original rock, it may crystallize from silica-bearing fluids that entered the rock during metamorphism, or it may form as part of the metamorphic transformation. Because of its abundance in the Earth, silica, in both its crystalline and noncrystalline states, is present in nearly all mining operations. It is in the host rock, in the ore being mined, as well as in what geologists call the overburden, the soil and surface material above the ucuiuv^N* muaiuicDaicmuicu uuinucpuMidLuilutnmig crystalline silica. The mineralogy of the deposit and, to some extent, the processing of the ore determine the quartz content of the final product Sand and gravel consist mostly ofquartz, whereas the quartz content of crushed stone will vary from region to region. Table 1 listssome common commodities, the form of silica they contain, and their commercial uses. 15 CMYSTAEEENE SEEECA EMEMEM TABLE 1. - Silica In commodities and end-product applications Commodity Antimony Bauxite Beryllium Cadmium Cement Clay Copper Cashed stone Diatomite Dimension stone Feldspar Fluorspar Garnet Germanium Gold Gypsum Industrial sand Iron ore Iron oxide pigment (natural) Lithium Magnesite Mercury Mica Perlite Phosphate rock Form of Silica in the Deposit Quartz Quartz Quartz Quartz, jasper, opal,agate, chalcedony None Quartz, Cristobalite (or opal?) Quartz Quartz Quartz, amorphous silica Quartz Quartz Quartz Quartz Quartz, jasper. opal, agate, chalcedony Quartz, chert Quartz Quartz Chert, quartz Chert, quartz, amorphous silica Quartz Quartz Quartz Quartz Amorphous silica, quartz Quartz, chert Major Commercial Applications Flame retardants, batteries, ceramics, glass, alloys. Aluminum production, refractories, abrasives. Electronic applications. Batteries, coatings and platings, pigments, plastics, alloys. Concrete (quartz in concrete mix). Paper, ceramics, paint, refectories. Electrical conduction, plumbing, machinery. Construction. nitration aids. Building facings. Glass, ceramics, filler material. Acids, steelmaking flux, glass, enamel, weld rod coatings. Abrasives, filtration, gem stone. Infrared optics, fiber optics. semiconductors. jewelry, dental, industrial, monetary. Gypsum board (prefabricated building product), industrial and building plaster. Glass, foundry sand. Iron and steel industry. Construction materials, paint, coatings. Ceramics, glass, aluminum production. Refractories. Chlorine and caustic soda manufacture, batteries. joint cement, paint, roofing. Building construction products. Fertilizers. 16 IPAMT E I ABLE 1. - Siiica in commodities and end-product applications (Continued) Commodity D1 UnImIIIClr.aC Pyrophyllite Sand and gravel Selenium Silicon Form of Silica in ihe Deposit \/nlr^nir nl^cc v vivainx. giajj; Quartz Quartz Quartz Quartz c\U,*+ JIIVCI Talc Tellerium Thallium Titanium Tungsten Vanadium Zinc Zircon - 4^ yuoi \J-, uicil Quartz Quartz Quartz, jasper. opal, agate, 1U laiCCUUI J Quartz Quartz Quartz, amor phous silica QuHitzTTHSper, nna1 aoato chalcedony Quartz Major Commercial Applications r*vwnniivr.rio^tuo^ aonrormta uhnuiulHuinnir^r vhivlnv*rrlv^* Ceramics, refractories. Construction. Photocopiers, glass manufacturing, pigments. Silicon and ferrosilicon for ferrous foundry and steel industry; computers; photoelectric cells. Dky%fr\/fr<tr\ki/,< m<afAei^l AlArfri>sn/l i iivivgi apmv. iiiawnai/ cicv.u iv.cn ai iu electronic products. Ceramics, paint, plastics, paper. Steel and copper alloys, rubber compounding, electronics. Electronics, superconductors, glass alloys. Pigments for paint, paper, plastics, metal for aircraft, chemical processing equipment Cemented carbides for metal machining and wear-resistant components. Alloying element in iron, steel, and titanium. r" A ~ll_______________U__________________ ______ vjcu vai iiz.il ig, Z.IUV.-UCHCU anup, rh*miraU aorimltiirp Ceramics, refractories, zirconia production. The Many Uses Crystalline silica, again primarily in the form of of Crystalline quartz, has been mined for thousands ofyears. Inthefirst Siiica century A.D., the Roman scholar Pliny described in some detail, although his understanding was limited by the technology of his time. Tne ancients believed quartz to be very deep-frozen ice, which could no longer be rI ^a.1m1 1o^.1ltorl IIInI ar-t tIrV\ |n^rl nVtT/Vo tUfiiiicJ ihi\J/n^/nvtuhociC P l>Sa ns j\/ jn^vnraiani%tpvrwl outthatquartz seemed to be found most frequently in the vicinity of glaciers. Although they may not have under- 17 CMYSTAEEENE SEEECA JPMEMEJR TABLE 2.-Common products containing 0.1 % or more crystalline silica_________________________________________ At Work At Home Everywhere (in the process of manufactur- (as a consumer of the (exposure could be on V* M >4* lit ig ui using ui^iuuuwviug iicno/ n -- ----------1 lunuYving ncinyj m#Iiea yi/vtuk ni/ei ai ikiv/mmteA/) Asphalt filler-is usually com posed of quartz and stone ag gregate. Art clays and glazes-contain Caulk and putty-contain clay, clay, and sometimes crystal which can haivea lowto moderate line silica. crystalline silica content as a filler. Bricks-have a high concentra tion of sand. Contains quartz and possibly cristobal ite. Ceramic fixtures and tile-are made from clay, feldspar, talc, and sand. Present in raw ma terials, not in final product Concrefe-like asphalt filler, contains stone aggregate. Dishes and china-are made c_______:l:___C\_____C:____1.. ______ iruin Allied iiuui, imcty giuunu quartz in which 98% of the particles are below 5 microns in diameter. Present in raw materials, not in final product Jeweler's rouge contains cryp tocrystalline silica. Jewelry and crystals; amethyst and quartz arecrystalline silica. /Vfortar-contains sand. Municipal water filterbeds-are constructed from both sand (crystalline silica) and diatomite (amorphous silica). Sread-contains an amorphous Dusf-(whether household or in silica material, a biogenic subt is a natural part ofstance tha some grains. When heated, dustrial) silica. contains crystalline this amorphous silica can be come crystalline. Fill dirt and fopso/V-contain sand. Because the crystalline Cleansers-contain pumice and silica content ofcommon soil is feldspar as abrasives. so high, agricultural workers represent the occupational Ccsmedcs-contain talc and clay. group most at risk for exposure to respirable crystalline silica. Pet Ihter-is composed primarilyof uay* Foam in furniture and on rug Pottery-is made of clay. backings-contains talc and silica. Talcum powdercontains talc. Paint-contains clay, talc, sand, and diatomite. Unwashed root vegetablestsudh aspotatoesandcarrots) arecoated withsoil,which nasa nigh crystal contain.P--aPn--er a----n----d-| oa, oer---d---u----s---t--------------------- line silica content kaolin and clay. Pharmaceuticals<orttain clays and talc as filler. Often the dosage ofactive ingredient in a ii> f><<\ mini i+a Un* filler (listed as an inert ingredi ent) must be added to makethe substance manageable to take. Plaster-is made from gypsum Ui ell UUi 9UIIICIIIUC9 v.uiiiaiii9 amva. Plastic in appliances-can con tain clay, talc, crushed lime stone, and silica as fillers. *Qnnfmi"n0o Om' mt/pMK made from sand and aggregate. 5anc/-iscrystallinesilica. Beach sand, piay sand for sandboxes, and the sand used on golf courses are no different than industrial sand used for con struction, in sandblasting, oron icy roads. All are largely crys talline silica. Wallboard-is made from gyp sum. 18 IPAJR7T H stoou its true nature, eariy civilizations aid understand its value as a gemstone. Today, quartz is used for a whole spectrum of products (Table 2) from high-technology applications in the electronics and optica! fields to i everyday usgs in building and construction. In Glass, Ceramic, and c:--,, /~u:__ mie vmiiid Manufacturing One of the major uses of crystalline siiica is as a raw material for glass manufacture. The first glass was probably made in Egypt more than 5,000 years ago. Today, the process has become highly refined. To ensure a very pure product, the specifications for glass are exceptionally stringent. A pure crystalline silica is used; the iron content must be less than 0.03%, and there are strict limits on the amounts of other impurities. Even the grain size of the crystals is specified. In the finished glass, the silica content must be at least 98.5%. Ceramics, porcelain, and fine china are made from finely ground crystalline silica, called silica flour. FIGURE 10. | The Glassmaking Process (Photo courtesy of ______________ FMC Corp.)_________________________________ CM YS TEA E L ENIE SEEECA IPMEMEM In Construction Building materials, such as concrete and di mension stone (sandstone, granite, and limestone are PV^mnlocl ^nntain r-rv/ct^ 11 ino i r\ rif wmuiu j .....................^iiiv.u ill wi 1*^ iv^i I I V-/I quartz. Dimension stone is commonly used to build churches, government buildings, and monuments. In the Nation's capital, for example, the White House is built of sandstone, the Smithsonian Institution's origi nal building of sandstone, the exterior of the Museum of Natural History of granite, and the Treasury build ing of granite and sandstone. Quartz is a component of cement, another technological development dating from ancient times. In the past, sandpaper and grinding wheels were made from quartz, and it was the primary abrasive used in sandblasting operations. FIGURE 11. Smithsonian Castle Built of Sandstone (Photo courtesy of l he Smithsonian) 20 IPAMT E Quartz is also used as functional filler in plas tics, rubber, and paint. In George Washington's time, Uu .w...... 4m-Ue id_sLm.:_u_ii.JluJd_u_u___>_d_ii_u1 iu p__d:mu -ril_n_u_s, .u1_ie w/fwipn pvtprior rvf KAni int \/omnn Wachino+nn'c Knmo in Virginia, was painted with a sand-paint mixture to give it the look of stone. In Heavy industry Foundry molds and cores for the production of metai castings are made from quartz sand. The manu facture of high-temperature silica brick for use in the linings of glass- and steel-melting furnaces represents another common use of crystalline silica in industry. The oil and gas industry uses crystalline silica to break up rock in wells. The operator pumps a watersand mixture, under pressure, into the rock formations to fracture them so that oil and gas may be easily brought to the surface i^tore than 1 million tons of a~*uartz sand- we- re- used a---nnuallv/ for t-h--i-si-o--u--r1D---o-s--e-d--u--r-i-nOg the 1970's and early 1980's when oil-well drilling was at its peak. Quartz sand is aiso used for filtering sediment and bacteria from water supplies and in sewage treat ment. Although this use of crystalline silica has in creased in recent years, it still represents a small propor tion of the total use. High-Tech Applications Historically, crystalline silica, as quartz, has been a material of strategic importance. During World War ii, communications components in telephones and mobi 1g military radios wrc made from cjuartz* With today's emphasis on military command, control, and communications surveillance and with modern ad vances in sophisticated electronic systems, quartz-crys tal devices are in even greater demand. in the fieid of optics, quartz meets many needs. It has certain optical properties that permit its use in polarized laser beams. The field of laser optics uses 21 /r,m>w<s*rr a tt tt ttrjk? it <as jj. j. v u^i tt tt tt ^ a m> m> tt tut n7 m> u 1ST JJ^U JimLblfi\ FIGURE 12. Oil Rig (Photo courtesy of the U.S. Department of Energy) quartz as windows, prisms, optical filters, and timing devices. Smaller portions of high-quality quartz crys tals are used for prisms and lenses in optical instru ments. Scientists are experimenting with quartz bars to focus sunlight in solar-power applications. 22 IPAMT E \ Quartz crystals possess a unique property called piezoelectricity. A piezoelectric crystal converts me chanical pressure into electricity and vice versa. When a quartz crystal is cut at an exact angle to its axis, pressure on it generates a minute electrical charge, and likewise, an electrical charge applied to quartz causes it to vioraie^more man. i_____x*__________j_________ia aaa mn! Ir^tinnc ap|^nv*auvi ij< _u__m____e__$____p___e__r____s__e_.<______u__nju in suuie Piezoelectric quartz crystals are used to make electronic oscillators, which provide accurate frequency control for radio transmitters and radio-frequency tele phone circuits. Incoming signals of interfering frequen cies can be filtered out by piezoelectric crystals. Piezo electric crystals are also used for quartz watches and other time-keeping devices. FIGURE 13. Quartz Clock (Bureau of Mines photo) CR YS FALL ENE SEEECA FREMER Synthetic Crystalline Silica Today, industry does not depend entirely on natural quartz for strategic applications. Since the 1940's, well-established techniques for synthetically growingquartz have been used and refined. Synthetic quartz crystals are grown in heavy-duty pressure cookers called autoclaves under pressures ranging from 1,500 to 20,000 pounds per square inch and at temperatures of 250 to 450C. Natural crystalline silica may contain impu rities or be flawed insome way, but synthetic crystals can be flawless. They can also be made to grow in a particular shape and size for specific needs. FIGURE 14. Synthetic Quartz Crystal (photo Courtesy of Motorola, Inc.) 24 nrmrjaa mttt.i l*fiT> nrritlt PART II: THE REGULATION OF CRYSTALLINE SILICA It has been estimated that over 3 million workers in the United States are exposed to silica dust in their occupations. Because ofthis contact, there has been much interest in how exposure to crystalline silica affects one's health. Scientists have known for decades that pro longed and excessive exposure to crystalli ne sil ica dust causes silicosis, a noncancerous lung disease. This was most dramatical ly demonstrated by the significant increase in reported si!icosis casesfollowingthe inven tion nfthn nnpumatir hammnrHrill in 1 AQ7 th* intro . w. ,,r...------------------.............................- -..........................--' / * ~--------------- duction of sand blasting in 1904, and the undertaking of a major tunneling project in the mid 1930s through a ridge of nearly pure quartz. These events helped spawn dust-control standards which have continued to evolve through the years. During the 1980's, studies were conducted that suggested that crystal I i ne si I ica also was a carci no gen. In 1987, the International Agency for Research on Cancer (IARC), an agency ofthe World Health Organi zation, evaluated the available medical literature on silica. Based on this evidence, IARC concluded that crystal line si! ica (but not noncrystal li ne, or amorphous, silica) was a 2A substance, a probable carcinogen for humans. These findings have attracted attention for several reasons. With the publication of the first study suggesting that crystalline silica was a carcinogen, crystalline silica became regulated under HCS as a carcinogen. Under HCS, OSHA-regulated businesses that use materials containing 0.1 % or more crystalline silica must follow Federal guidelines concerning haz ard communication and worker training under HCS. While the IARC finding did not trigger any Federal regulations, the work of that organization is important because it has captured the attention of the mining industry.__________________________________________ 25 CMYSTAEMNE SMLECA IFIREMEM l he foiiowing sections discuss OSHA's HcS and the IARC evaluation process. OSHA's Hazard Communication Standard In the development of the HCS, OSHA real ized that the task before it was herculean: to evaluate all the substances to which workers are exposed, as many as 650,000 ofwhich were potentially hazardous. In addition, OSHA's ruie-making process, once a sub stance had been determined to be hazardous, was I m/\ /'nr* r i im in/r Li A rln/'irlnrl I 14+1 /\ I <*! n n ui i ic-v-wi i?ui i in ig. \-/ji in ucuucvi u uu i iluc i iiiui i i iauui i about hazards and protective measures would be made available to employees if the substance-by-substance approach to analysis and regulation were the only one pursued. The Agency decided to adopt a generic approach, and promulgated the HCS, which requires container labeling, material safety data sheets, and u an m--ig* c------ .,1iv1.^.aiii_yir/-iciwL.,-i idi_d__u__i_c . irv^siiiiu__w__u__i_g__i_c_v__|u_ i:i_c_" merits: Chemical manufacturers and importers-cri\jst cJetermine the hazards of the product The regula tion states that "If a mixture has been tested as a whole to determine its hazards, the results of such testing shall be used to determine whether the mixture is hazardous. If a mixture has not been tested as a whole to determine whether the assumed to present the same health hazards as do the components that comprise 1 % (by weight or volume) or greater of the mixture, except that the mixture shall be assumed to present a carci nogenic hazard if it contains a component in concentrations of 0.1 % or greater which is con ___ I l________ :___ diucicu iu uc d Ldiunuj uiiuci pdidgidpn (d)(4) of this section." Chemical manufacturers, importers, and distributors-must communicate the hazard infor mation and associated protective measures to customers through the use of labels and "materiai safety data sheets."_____________________ IPAMT EE Employers-must (1) identify and list hazardous chemicals in their workplaces; (2) obtain material _l__ _____ I l_l_l_ ______U I_____J___ SdlCiy Udld MICCIS dl IU IdLTCIS IUI CdCI I ll(UdIUUU3 chemical; {3} develop and implement a written hazard communication program, including labels, material safety data sheets, and employee training; and (4) communicate hazard information to their employees through labels, material safety data sheets, and formal training programs. Coverage of mixtures is based on the amount of the hazardous or carcinogenic material present. A mixture is considered to represent the same health hazard as any hazardous component present in con centrations of 1 % or greater (by weight or volume) or any carcinogenic component present in concentratlons 0 1 % qp groator Qijppjlore c ro hajrj rocpQri^ sible for determining whether a substance is covered.------ - ------ 0 ------------------ - ------------------------------------ - - - - - r including whetherthequantityofthe hazardous chemi cal in a mixture exceeds these cut-offs. Testing is not required; the employer may assume that ifthe hazard ous chemical is present, the mixture is covered. The employer must label all hazardous chemicals, m accordance with the HCS, provide material safety data sheets, and train exposed workers. I To help ensure that hazard evaluations by different suppliers would be consistent, OSHA refers to a number ofexisting documents as providing defini tive findings of hazard. These include iARC and the KI I Hrt i>v* npvranipiK/ POt/IO\i/C i^duuiidi i uXiv-uiugY i iugi ai 11 v^ai ui ifijr !n addition, hazardous and carcinogenic chemicals listed in the Code of Federal Regulations (29 CFR 1910, Subpart Z, Toxic and Hazardous Substances, OSHA) and in Threshold Limit Values for Chemical Substances and PhysicalAgents in Work Environment (American Conference of Governmental industrial 27 (CMYSTALLENIE SELECA IPMEMlEm The IARC Evaluation Process nygienisis/ are covered under Hc,:>. nowever, one properly conducted scientific study in the literature establishes a hazard for numnses of the The International Agency for Research on Can cer, headquartered in Lyon, France, began its program to evaluate chemicals and cancer risks in 1967. It holds working group meetings in Lyon, usuaiiy 1 week iong caui, auuu+i mKicA/eN +uI iiicAd/* aA myimcaami. caul giuup iucuimsaom uAMn a rhpmira 1 nr orni innf rhpmiralt Rorontlu rtthorsroac of study, such as radiation and viruses, have been included in addition to chemicals. Topics for study are selected on the basis of two criteria: there is both evidence of human exposure and some evidence or suspicion of carcinogenicity. IARC assigns an expert on the chemical to survev the scientific literature, review toxicoloev stud/ ' u/ ies, and summarize the results. Assignments to subject- matter experts are usually made 1 year in advance ofthe working group meetings. During the week-long meet ing, the working group breaks into two subgroups: chemistry and toxicology. These subgroups examine the ualiriitu ---------------v nf the ,v- matorialc --- --ci irtrtlior) hiz tho accianczi experts. The two subgroups also agree on terminology and any special definitions required. At the end of the week, the two subgroups come together for a plenary session; the monographs are issued from the plenary session, if significant new data become available after a monograph has been published, the chemical will be raawaliiatozi at a ciikca/inant mootina anrl a rovicorl 1 f Ul UUIVU UL U UIIU U 1 monoeoraioh mav/................ ............. be rDublished. IARC does not commission any health studies or replicate any studies it uses nor does it control the literature reviewed by the experts for its monograph assignments although research may be sponsored by 1 A Dr* ar\*> TUa pt i or tA ka in s'* It irlorl VJU ICI ir\l\V^ glUU|J9 1 1 IC U IUICC Ul 91UUIC9 LU 1II l\.IUUCU in the monograph is at the descretion of the expert panel. TTTN /) TTT Prri TT TT irj&MU IU1 Chemicals studied /'l'icci^io/1 in^A die uaddiucu iiuvr four categories mans. Group /.--The agent is carcinogenic to hu This classification is reserved for those sub stances for which sufficient evidence of carcinogenity in humans has been found. Sufficient evidence in iArulu<mt icmh ue iuc uc.i 11 (c>u ijkyi/i/ \xv-- tivn/ m11 ioc-aaini a rv'^aanuca^ali ri ovilauv/i i-- ship between exposure to the agent and the develop ment of human cancer. Group 2.--This group is divided into two subgroups: probably carcinogenic and possibly carci nogenic. Group 2/4.--The agent is probabiy carcino- - L_. _ _ -rl_:_ _l:f. __ l:1 .. .1 germ, iu uumdiis. mib cidbiiiicdiiuii is dppiitru wiien c.JItUiiVd4iIoWc.J rUloVMmInUnI ctrato tUhIaUtL t1h1 o1 r1o icJ limitod mmiiiwu m^ V/irlonl\r.ol. VrsSIf carcinoeenicitv in humans and sufficient evidence of Limited evicarcinogenicity in experimental animals. dence in humans is defined by IARC as evidence that shows some association between exposure to the agent and the development of cancer. The evidence is considered limited, however, because even though the .______ ____ ____ ,_________________ ... l,, ____ __._____________________________u:-.- ,,,,,, iwu pusmves nidy uc cicuiuic, ^iidiu_c, uids, ui <_un- founding factors cannot be ruled out. IARC defines sufficient " - - evidence -- _- in experimental / animals as docu- mentation of a causal relationship based on the results from studies involving two or more species of animals or from two or more independent studies in one species carried out at different times or in different laboratories. In its Supplement 7 to IARC Monographs on the Evalu- clukjukj! \~c'iur>u//*finiiy/rg/rc/rinliieV DiVLr rvijiv? iMu#Ului iiiaitj, 1 i/~\r\v^ r+^+nc siolcj; ^ln hi tho aheonro nf arlonnatP data rn humane it is hinlncn- .-tav. w. ------- ------------------ " ------------------------ , * --'----------------O' callv Dlausible and Drudent to reeard acents for which ----------/i---------------- ---------- r -- -- u vj sufficient evidencethere is of carcinogenicity in experi mental animals as if they presented a carcinogenic risk to humans" (p. 30). Group 25.--The agent is possibiy carcinogenic to humans. When there is limited evidence in humans, Ui + ^m k<*An/^a n itfiriant m/irlanro in ovnAfimontal UUl a\ 1 aU9CIIV.C UI 3UlllVICIltCVlUCiiV.C III CA|^imu-iiai animals thp rhpmiral is rlassifipd as 2B. 29 jj&ujmiaiK.//"'TZP W'C?<77t/l TT TT TTT\7TB TT TT TT /T* A U O' U jU1U1UUJ.V Jli OJlLlhU i/tTO 7733 TT TUT TE> 7733 IARC Classification of Siiica IARC Monograph 42 (1987) evaluates siiica as foiiows: Croup 3.-The agent is not classifiable as to its carcinogenicity to humans. When studies do not provide sufficient data to classify a chemical into any of the other categories, IARC assigns it to group 3. Croup 4.--The agent is probably not carcino genic to humans. IARC reserves this category for chemicals for which evidence from both human and experimental animal studies suggests a lack ofcarcino genicity. There is sufficient evidence for the carci nogenicity of crystalline silica to experimental animals. There is inadequate evidence for the carci nogenicity of amorphous siiica to experimental ani11 laid* There is limited evidence for the carcino genicity of crystalline silica to humans. There is inadequate evidence for the carcinogenicity of amorphous silica to humans (p.11). Thus, when the working group reviewed the medical and scientific literature submitted to it for study, it found that there was evidence of an increased incidence of malignant tumors in animals exposed to crystalline silica. The working group also found a causal relationship in humans, although other con founding factors could not be excluded. These are the criteria required by IARC to classifying crystalline silica as a 2A substance. It is important to note that not all the studies IARC examined show a link between exposure to cilira anrl ranrpr f)r imnnrtant aroi ir> cti irjifK cited in lARC's list of references, is compiled in Silica, Silicosis, and Cancer: Controversy in Occupational Medicine (1986), edited by David F. Goldsmith, Deborah M. Winn, and Carl M. Shy. Conclusions from several of the studies point to the controversy referred to in the volume's title: 30 IPAMT EE "Regulation of silica on the basis of potential carcinogenicity is premature" (p. 477). I.T.T. Higgins, "Is the Current Silica Standard Adequate?" "The present epidemiological and experimen tal data do not permit the conclusion that exposure to crystalline or amorphous Si02 is associated with in creased risk of lung cancer" (p. 491). E. Mastromatteo, "Silica, Silicosis, and Cancer: A Viewpoint from a Physician Employed in Industry" "In reviewing the information we have on the health hazards of silica exposure, we find we have received positive and negative findings on the issue of silica and cancer, and our staff have told us a lot about interactions. They have told us that they are not certain it is silica alone that may be causing cancer" (p. 529). M. Schneiderman and D.M. Winn, "Where Are We with the SiOz and Cancer Issue?" Regulatory Activities of Other Agencies MSHA and State legislators also are becoming involved in the silica controversy. MSHA has proposed an HCS similar to OSHA's. The proposal is still under review following a public comment period. MSHA's proposal also would use the results of IARC, NTP, and OSHA studies to determine which materials were car cinogenic. The enactment of such a regulation would affect nearly all mines because most ores are extracted from silica-bearing rock types. California passed the Safe Drinking Water and Toxic Enforcement Act of 1986 that includes crystall ine si I ica of respirable size on its list of carcinogens. California's Air Toxic Hot Spots Act and Air Quality Act have the potential to restrict the emissions of crystalline silica. There is concern that burning of rice hulls and straw after harvesting gener ates an ash that usually contains residual biogenic silica. These regulations would have an adverse effect of the economics of these crops. The Complexities Crystalline silica is a very common material. It of Measurement is present in a most mineral operations, many of which 31 CMYSTAEEENE MEECA PPEMEM -s-p--I-I--r-m---W---p---n--r, nrn--r-p----#---H -m---a--t--pri--ak' t--r> O^H...A....-.r"pOa-i-i-la--t-p- rl sites. Sampling at factories, construction sites, or even the mine site will be greatly increased if a mineral supplier or OSHA-regulated employer wishes to prove that the concentration of crystalline silica is less than 0.1% for carcinogens, thereby exempting the com pany from the requirements of HCS. Because crystalline silica is a common min eral.' it is assumed that it is easvI to measure. Unfortunately that isn't always the case. Determining whether a particular state of silica is present and how much of that state a sample contains, however, can be a much more complicated problem. Under certain condi tions, current techniques and equipment can't distin guish very well between its physical states at the low concentration level specified by HCS. Analysis can be difficult or even impossible for some samples. From an analyst's point of view, the problem is mainly one of equipment sensitivity. In short, these are the problems analysts face u/kan trwintT tr\ ninrvint cr\ r\rorlcol\/ tko m/ctsllinA silica content of a samole: v cu icxuuny. 11ic u pumiiniy ui dined iicmii different deposits^even from slightly different loca tions within the same deposits is not necessarily the same. This raises two problems. First, a single standard (that is, the reference material to which the silica in the sample is compared) may not be appro priate. Using a standard that matches the particle size _____l______j__II:_________4.1_____:l:___*1__________________i_ :___________ diiu ciyMdiimiiy ui uic Miicd 111 uic sdinpic is esseiiudi fr\r an az-miratp analv/cic Qornnrl nKtainino a rpnrp. .w. Ull UVV.U>Ul.b 7 uv w. IU, ............"CJ -- . v. s--ent--ative- s--amrole.' when the sam,ole size is so small and the deposit is so large, is nearly impossible. Presenceofotherminerals. Nearly all samples contain more than just silica. Some of these minerals interfere with the data interpretation, making the accurate de impossible. 32 PAMT EE Presence of other states. Some samples contain silica in more than one state. Because ofthe way it forms, silica may exhibit different degrees of crystallinity. Quartz present can be intermingled with noncrystalline silica since natural quartz may have a noncrystalline coating. Silica cementing quartz grains together in a sandstone may have a different crystallinity than the quartz grains. EvenXray diffraction, the most effective measurement technique, can distinguish between crystalline and noncrystalline silica only when rigid sampling pro tocol is followed and only when the sample has few other minerals. Effect of human interference. The very act of taking a sample can change the sample. The grinding and pulverizing required to get a small enough portion to analyze subjects the silica to heat and pressure, the very forces that, in nature, change crystalline surfaces to disordered amorphous sur faces. The analyst can't measure how much of it transforms from noncrystalline to crystalline during sample preparation. The seven techniques that scientists use to mea sure how much crystalline silica a given substance contains are summarized in table 3. 33 CMYSTAE.MNE SEEECA IPMEMSIE TABLE 3.~Methods used to Detect Quartz in a Sample Name Optical microscopy Description of Technique Accuracy Remarks Samples are visually examined Accurate to within Requires considerable skill by and the mineralogy is deter a few percent analyst to identify the miner mined. als present. Uses small samples. Electron microscopy Particle composition and mor Accuracy limited Cannotdifferentiate crystalline phology are determined. Crystal due to the nature of and amorphous silica except structure is determined with the analysis. when transmission electron transmission electron micros microscopy is used. Methods copy. Resolves very small par are slow, expensive, and ticles. samples are very small. Thermal analysis Measures a minerals response to Accurate only for Can be used only on very small temperature changes. quantities over 1 %. samples. Selective dissolution Minerals are dissolved selectively using acids. Quartz generally is less soluble than other minerals so it remains in the residue. The residue is analyzed to determine the content of crystalline silica. Not very accurate. Particle size and sample com position affectthe accuracy of this method. Fine-grained quartz, cristobalite, and tridymite may dissolve; other minerals may not dissolve. Separation based on density A finely ground sample is sus Not satisfactory for Particle size, shape, and sur pended in a heavy liquid. The routine analysis, facecharge affectsettling rates. denser minerals settle fasterthan routine analysis. Thetechnique is slow and dif less dense minerals. By varying ficult to perform. Many ofthe the density of the liquid, miner heavy liquids used are highly als with different densities can toxic. be separated from one another. Infrared spectros copy Minerals absorb infrared light at Accurate to about Requires very small samples, specific wavelengths. By exam 1%. the analyst must be sure that ining how the light is absorbed samples are representative of by the sample, the analyst can the deposit identify the minerals in the sample. X-ray diffraction X-rays are diffracted by the lat Mostaccurate;typi The degree of crystallinity tice planes ofthe minerals in the cally, the limit is (from amorphous to highly sample. By observing the inten about 1%. sity of the diffracted x-rays at crystalline) and the presence of silicates can affect the ac differentangles of incidence, the curacy of the quantitative analyst can determine the iden analysis. tity and concentration of miner als in sample. 34 JPAMT EE Conclusions Crystalline silica is the scientific name for a group of minerals composed of silicon and oxygen atoms that are arranged in a three-dimensional repeat : ---- -n___ ___ ____________i c____ _______ . i : mg fjdiicni. iiicic die scveicti luniii ui _i ysuii i me silica, the most common of which are cjuartz, cristobalite, andtridvmite. Crystalline silica is ubiqui tous, being in rocks from every geologic era and from every location around the globe. Crystalline silica's abundance in nature is matched by its pervasiveness in ourtechnoiogy. From ______i_____A C~-_____I.:____--I______________________________i____L_:_ UH5 SdMU U^CU IUI llldMllg gldbd LU U 1*5 piCZ.UCI tJCUl C quartz crystals used in advanced communication sys tems., crvt stalline silica has been a iDart of our te_ c- hno logical development. Scientists have known for decades that pro longed and excessive exposure to crystalline silica dust in mining environments can cause silicosis, a noncancerous lung disease. During the 1380's, stud- ioc ci inrroctorJ that rn/ctallino cilira u/ac a rarrin/vnan IVd II VI J III IV ll I IVU V V U7 U VUI VII IVgVI I* In 1987, IARC labeled crv/ stalline silica as a "2A substance," a probable human carcinogen following a review of the available medical literature on silica. With the publ ication ofthe fi rst study, crystal I i ne si I ica was regulated under OSHA's HCS as a carcinogen. To demonstrate that they are exempt from the require ments of HCS, suppliers must now analyze the crystal line silica content at the 0.1% level and must now more carefully consider whether the silica is crystal line or noncrystalline; whether it is a regulated form of crystalline silica; or whether it is a mixture of several silica types. Because this action has implications which gu ucjrui iu u ic m11 iiini iAc a'iai i/uJ mtniiniicaira^il nrAAorrlnrv iiniivrJjiuj- tries, it is important that there be as clear an under standing as possible about crystalline silica. This primer is meantto be a starting point for which to learn about its mineralogy, occurrences, uses in society, and regulations affecting its use. A detailed glossary and a list of selected readings and other resources are included to help the reader move from this presenta- tjon to a more technical one, the inevitable next step. 35 CRYSTAEEEN1E SEKECA PREMIER LIST OF RESOURCES AND SELECTED READINGS Recommended Readings Basic InformationBoegel, Hellmuth. The Studio Handbook of Minerals. John Sinkankas (ed.). New York: The Viking Press, 1968. Miles, W. J. The Mining Industry Responds to Crystal line Silica Regulations. Mining Engineering, 19,1990, rvr\ JTJ-JTU. Symes, R. F,, and R. R. Harding. Crystal and Gem. New York: Alfred A. Knopf, 1991. Zim, Herbert S., and Paul R. Shaffer. Rocks and Minerals. New York: Western, 1957. Technical Data Ampian, Sarkis G., and Robert L. Virta. Crystalline Silica Overview: Occurrence and Analysis. BuMines 1C 9317,1992. Craighead, John E., and the Silicosis and Silicate Disease Committee. Diseases associated with expo sure to silica and nonfibrous silicate minerals. Ar chives of Pathology and Laboratory Medicine, 112, moo------C71 77fl I 700, pp. U/ J"/ Hamilton, R. D., N. G. Peletis, and W. J. Miles. Detection and Measurement of Crystalline Silica in Minerals and Chemicals. In Regulation of Crystalline Silica. Littleton, Colorado: Manville Corporation, 1990. Murray, H. H. Occurrence and Uses of Silica and Siliceous Materials. Preprint from the Society for Mining, Metallurgy, and Exploration, Inc. Littleton, Colorado, 1990. World Health Organization, International Agency for Research on Cancer. IARC monographs on the evaluation of the carcinogenic risk of chemicals to humans, silica and some silicates. (Vol. 42), 1987. 36 RART EE /^iL -- vainer Resources Chemical Manufacturers' Association, Chemstar Crys talline Silica Panel oireei,OPA1 k A Pi._____. kl\A/ \ A f____L!-___L____rv/^ AAnnT z.D\j\ m invv, vvasnmgion, ui~ zvuj/ Intornatinnal Hiatnnnito &ms4t irorc AccApntinn mw.iuuuvi ii i^iuiviiui^ iwiuk.gi a / u^wiauv/n 26 Wind lammer Court. Lone Beach. CA 90803 *' 'O ' 'f - - Refractories Institute 500 Wood Street, Suite 326, Pittsburgh, PA 15222 U. S. Bureau of Mines, Department of the Interior 810 7th St NW, Washington, DC 20241 GLOSSARY This glossary contains terms used inthis primer as well as terms the reader might encounter when consulting other sources for more information about crystalline silica. 2A SUBSTANCE--Ranked by IARC as prob ably carcinogenic to humans, that is, there is limited evidence of carcinogenicity in humans and sufficient evidence of carcinogenicity in experiments with ani mals. (See iARC Monographs on the Evaluations of Carcinogenic Risks to Humans.) Crystalline silica is currently ranked as a 2A substance. AGATE--Cryptocrystalline form of silica. Composed of extremely fine (submicroscopic) crys tals of silica. ArrocTATt nvjui\Lvjn l.--can mean ciuici a gi uup in matpria U nr a nv nf wpra 1 hard inprtih<tanrp< (vnrh --------------- -----------------------------------------------------------------------------' " ----------------------------------------------------------- as sand, gravel, or crushedstone) used for mixing with cement AMORPHOUS see NONCRYSTALLINE. ATOM--A minute particle of matter. The CMYSTAILJLENE SEJLECA PREMIER Smallest particle of an element that can enter into chemical reactions. BEDROCK--The rock underlying the soil or other surface material. CARCINOGEN--Causing cancer. In scien tific literature, the terms tumorigen, oncogen, and M ^ I t kntiA Uaam i iaaI ul< > urajiuiilugtii an navg dccii uscu 9|riiuiijmiuu9ijr wiui r--p--r-r-/-n---n- 0p-p--n--a--l-t-h~n--nOa' hnrr^^innfll/lv/ tumnr'iOa't-*-n ha*ht**n used specifically to connote a substance that induces benign tumors. The Federal Register (Vol. 52, No. 163, p. 31884) reports the following definition of carcinogen under Federal regulation 29CFR1201: "A chemical is considered to be a carcinogen if: (a) It has been evaluated by the International Agency for Re search on Cancer (!ARC), and found to be a carcinogen or DI Otential carcinogUen:' or ('b)' It is liste-d-a-s -a -ca-rc-in-o gen or potential carcinogen in the Annual Report on Carcinogens published by the National Toxicology Program (NTP) (latest edition); or (c) It is regulated by OSHA as a carcinogen." CHALCEDONY--Cryptocrysta!line silica. Composed of extremely fine (submicroscopic) silica crystals. CHEMICAL CARCINOGENESIS--An IARC term, although one that is widely used elsewhere, it refers to the "induction by chemicals (or complex mixtures of chemicals) of neoplasms that are usually observed, the earlier induction of neoplasms that are commonly observed, and/or the induction of more neoplasms than are usually found" (IARC Monograph 42). Neoplasms are tumors. pure substance formed by the union of two or more elements in definite proportion by weight. PART EE CHEMICAL ELEMENT--A fundamental sub stance that consists of only one kind of atom and is the simplest form of matter. CHERT-Cryptocrystalline silica. Composed of extremely fine (submicroscopic) silica crystals. COESITE--A rare form of crystalline silica. Believed at first to exist only as a synthetic form of crystalline silica, it was formed in the laboratory using two different methods. Subsequently, it has been found in nature in the Meteor Crater in Arizona. It was also found to occur as a result of shock-wave experi ments and nuclear explosions. COLLOIDAL SILICA--Extremely fine amor phous silica particles dispersed in water. Colloids do notsettleout ofsuspension overtime. Colloidal silica is used commercially as binders and stiffeners and as polishing agents. COMPOUND see CHEMICAL COM POUND. CONSOLIDATION--In geological terms, any process by which loose, soft, or liquid earth materials harden into rock. CRISTOBALITE-The form ofcrystalline silica that is stable at the highest temperature. It occurs naturally in volcanic rock. CRYPTOCRYSTALLINE SILICA--Silica with submicrometer crystals formed from amorphous, of ten biogenic, silica that undergoes compaction over geologic time. Examples are flint and chert Also called microcrystalline silica. CRYSTALLINE-Having a highly structured 39 ISm>*nAi m>7T 11\1 11 TTIT 11 11 ETHYL SILICATE, Si(OC2H^4-A colorless, flammable liquid with a faint odor. It is an OSHAregulated substance. FREE SILICA-Informal name for a pure crys talline silica,' which is chemically/ uncombined. FUSED QUARTZ-The material formed by the rapid melting of quartz crystals. A meteor strike or a lightning bolt striking sand can form fused quartz. The term quartz glass is often erroneously used to mean fused quartz, but quartz glass is a misnomer because quartz is crystal I ine and glass is noncrystal I i ne. FUSED SI LICA-The material formed by heat ing cristobalite to the melting point (1710C) and cooiing it rapidiy. GLASSY--Having a semistructural molecular arrangement. Atoms and molecules mav form a----------------O--------------------- - ----------- . . _ --------------- - - -/ - - pattern but it has only short-term or partial order and does not repeat predictably in three dimensions. GRAVIMETRIC SAMPLING-Quantitative chemical sampling in which the substances in a compound are measured by weight. HOST ROCK--A rock that contains ores or minerals of value. IARC -The International Agency for Research on Cancer, an agency of the United Nations' World Health Organization. IGNEOUS ROCK--A rock that has solidified from a molten state. JASPER--Cryptocrystal line silica. Composed of extremely fine (submicroscopic) silica crystals. 41 CM YSTAKE JIN13 SEMCA JPMEMEM moiecuiar arrangement. I he atoms and moiecuies form a three-dimensional, repeating pattern, or lattice. DERIVED--A substance formed from the Dr roducts that result when a more complex substance is destroyed. Pure silicon is said to be derived from quartz sand. nt/cciv/iiTiin\iicnv --~i ru_ c_ul__a__n___g__e__i_r__o__m___gi_l_a__s__s__y_ro___r_m__ iu the crystalline state. DIATOMACEOUS EARTH see DIATOMITE. DIATOMITE-A rock, high in amorphous silica content, formed from the structures of tiny fresh- and salt-water organisms called diatoms. Diatomite has DIMENSION STONE--Building stone quar ried and prepared in regularly shaped blocks to fit a particular design. i KAnoruK/vt/% i iurN--a nign- rn/ctil tnnc^nrm'jfinn f k-sf icriuw \_i y .31x11 u ai 11 lauui i n 101 n/*vfr im//\U/n tUa i ivjl ii i vui vc u ic breaking of molecular bonds. For example, the trans formation of low quartz to high quartz involves only the rotation ofthe si I icon-oxygen tetrahedra. Displacive transformations are usually rapid. ELEMENT see CHEMICAL ELEMENT. EPIDEMIOLOGICAL STUDIES -Studies of illness involving human subjects over the long term. They generally involve analyses of real-world inci dence of the illness with little or no attempt to control factors that potential Iy could contribute to the onset or severity of the illness. This contrasts with laboratory StijdiS, which 2FG gGflSFeuly performed Oil ailifTicuS, afe short term, and have variables that are controllable. 40 CMYSTAEEENIB SEEECA IPMEMEM KEATITE --A synthetic and rare form of crystal line silica, formed by the crystallization of amorphous silica. It is transformed to cristobalite at a temperature of 1620 C. LITHIFY--To harden into rock. MACROPHAGE--A large phagocyte, a type of cell in the body that engulfs foreign materials and consumes debris and foreign bodies. materials (primarily silicates) and steam. Magma is in motion. Geologists speculate that it originates just below the Earth's crust and is fed by new material from greater depths. /via i tKiAL i/virAiKmtrN i --c-iieo in me tea- fi /ia/qq) on rcD iom cuii r\cg/^ic \i/i J/U7/, icguiauuii \_i i\ 1710, as "...life threatening effects; disabling effects; various diseases; irritation to different organs or tissues; and changes in organ functions indicative of future health decrements" (p. 2361). METAL--A type of element. Metals are usually 1% aw4 la if+vaNi i<* 11 (fU/M # rtf/% i iaiu ai iu IU9U11 laucauic \u icy cai i uu pOui iucu miu sheets), ductile (they can be drawn into wires), and can conduct electricity and heat. METALLOID-A group of elements, eight in all, that form the boundary (on the periodic table) between the metai elements (such as copper, iron, tin, gold) and the nonmetal elements (such as carbon, nitrogen, hydrogen). Metalloids possess some of the nI--r-o- jn--e--r--t-ie--s- --o-f--t-h--e---m---e--t-a--l-s--a--n--d---s--o--m---e o~ f- t-he rDrolDerties of the nonmetals. METAMORPHIC ROCK-Rocks that have un dergone changes from the pressure and temperatures in the Earth's interior. 1m1 >JRA1 1T1MW1I1T 17 77 11 11 METASTAB LE-Possessi ng an energy state that is not stable, yet will not change spontaneously. An outside force is required to change. METHYL SILICATE-An OSHA-regulated sub stance, (CH30)4 Si; it exists in the form of colorless needles. MICROCRYSTALLINE SILICA see CRYP TOCRYSTALLINE SILICA. MIN ERAL--Natural ly occurri ng crystal I i ne sol ids, most of which are made from oxygen, silicon, sulfur, and any of six common metals or metal com pounds. MOLECULE--The smallest particle of a sub stance that retains the qualities of the substance and is composed of one or more atoms. NEOPLASM-A tumor. NONCRYSTALLINE-Havingan unstructured molecular arrangement. The atoms and molecules are randomly linked, forming no pattern. NONMETAL-Elementsthatdonotexhibitthe properties of metals. Usually poor conductors of electricity and heat. OPAL~An amorphous form of silica. ORGANIC COMPOUND--A chemical com pound containing carbon. OSHA--The Occupational Safety and Health Administration, an agency of the U.S. Department of Labor. 43 CMYSTAEEENE SEZECA IPMEMSM OVERBURDEN --Materia! overlying the ore in a deposit. PERMISSIBLE EXPOSURE LIMIT (PEL)--An OSHA term. It refers to the concentration of a sub stance to which a worker is allowed to be exposed as a time-weighted average. PHAGOCYTIZED--To be removed from the body by the action of phagocytes, cells in the body that engulf foreign materials and consume debris and foreign bodies. It is believed that upon exposure to airborne crystalline siiica particles, 80% of the par4m.:.,,ile_s__d_i_e__pIi_id_g_u_c_y-u--z-.-e--u1 diiu ,,cii:m_i_ii_id_i_e_ui .wiuim__a_. _sI_ii_u_i_i timo PI EZOELECTRICITY--The abi I ity ofsome crys tals to convert mechanical pressure to electricity and to convert electricity to vibration. A quartz crystal in a watch is an example of applied piezoelectricity. POLYMERIZATION--A chemical reaction in which small organic molecules combine to form larger molecules that contain repeating structural units ofthe original molecules. The product of polymerization is called a polymer. 111 < AMI # A ft uciany many luims* be polymorphic means to have or assume several forms. In reference to crystals, it is the characteristic of crystallizing in more than one form. For example, crystalline silica can be in the form of quartz, cristobalite, tridymite, or others. PRECIPITATED SI LICA--Amorphous si I ica that jc prgrjpjtgtgrj from either avapor or solution. QUARTZ--The most common type of crystal line silica. Some publications will use quartz and crystalline silica interchangeably, but the term crystal- i 44 IPAMT EE linesilica actually encompasses several forms: quartz, cristobalite, tridymite, and several rarer forms. RADIOLARIAN EARTH-Soil, high in amor phous silica content, composed predominantly from the remains of radiolaria. Radiolarian earth that has been consolidated (hardened) into rock is called radiolarite. RADIOLARITE--A rock, high in amorphous silica content, formed from the shells of tiny fresh- and salt-water organisms called radiolana. RECONSTRUCTIVE TRANSFORMATION--A crystal transformation that involves the breaking of molecular bonds. For example, the transformation of quartz to tridymite involves the restructuring of the molecules, it is generally a slow transformation. RESPIRABLE CRYSTALLINE SILICA (RESPI RABLE DUST)--May be defined as dust that contains particles small enough to enter the gas-exchange re gion of the human lung(about3.5 microns). Oneofthe studies to which IARC refers in its monograph (Vol. 42, 1987) found that particle size for crystalline silica (in IIIC IUIIII Ul dll dipild l|Udl \JL 3dllU3lUllC WIUI UIC UdUC" mark Min-U-Sil 15) was distributed as follows: par ticles larger than 5 microns constituted about 0.1 % of the sample, particles between 2 and 4.9 microns, about 7%; and particles less than 1.9 micron, 92.8%. The Silicosis and Sil icate Disease Committee (National Institute for Occupational Safety and Health) states that particles less than 1 micron in size are the most troublesome and that particles in the range of0.5 to 0.7 microns are re--t-aine- d- in t-he- lunc. (S ee Archives of Pathology and Laboratory Medicine, Vol. 112, July 1988) . As early as 1943, however, the Department of Labor established a limit of no more than 5 million particles of free silica under 10 microns in size per cubic foot of air. {See Silicosis, industrial Health Series 45 CMYSTAEEEN1B SEEECA IPMEMIEM No. 9, U.S. Department of Labor, Division of Labor Standards, 1943.) ROCK CYCLE--A cycle taking place over geo logic time in which the three types of rock are related. Sedimentary rocks are changed into metamorphic rocks or melted to create igneous rocks. Sedimentary, metamorphic, and igneous rock may be eroded to make sediments that then harden into sedimentary rock. SEDIMENTARY ROCK--A rock formed by the accumulation and consolidation of minerals that have been either transported to a particular site by wind, water, or ice or precipitated by a chemical reaction at the site. SEMICONDUCTOR--Materialsthatactas con ductors within certain temperature ranges; at other temperatures they act as insulators. The elements silicon and germanium are examples of semiconduc tors of electricity. This unusual electrical capability has led to silicon's use in transistors, integrated cir cuits, and computer chips. SILICA--A compound formed from silicon and oxygen. Silica is a polymorph, that is, it exists in more than one state. The states of silica are crystalline and noncrystalline (also called amorphous). Crystal* line silica can take several forms: quartz (most com mon), cristobalite, tridymite, and four more rare forms. SILICA BRICK--Brick composed of silica that is used as a iining in furnaces. SILICA GEL-Amorphous silica, prepared in formation with water. Removal of the liquid creates xerogels and further treatment with alcohol creates aerogels. Silica gels are used as drying agents and to alter viscosity of liquids. 46 1PAMT EE SILICA FLOUR--Finely ground quartz, typi cally 98% of the particles are below 5 microns in diameter. r3nil1i^4"'*ma coatik^iiun-aac__o___m____m___o___n___t_e__r__m__; _in:_m____uusiry. It generally is used to mean a sand that has a very high percentage of silica, usually in the form of quartz. Silica sand is used as a source of pure silicon and as a raw material for glass and other products. Also called quartz sand. 3i li i C3--c.ompounas rormea irom si i icon, ALS AND SILICON-OXYGEN TETRAHEDRON. SILICATE MINERALS--Minerals containing silicon, oxygen, and a metal or metal compound. Sii ica tetrahedra form the framework of si iicate miner als. Examples are olivine, pyroxene, amphibcle, feldspar, and mica. SILICA W--A synthetic form of crystalline silica. It reacts rapidly with water transforming into amorphous silica. ^lirrk lllgll specs; 1in11%j/ nuni ivo. rwninutaaiiniiuinirgr free silica rather than silicates. SILICIC ROCK~An igneous rock containing more than two-thirds Si04 by weight, usually as quartz or feldspar. Granite is an example of siiicic rock. SILICON--The second most common ele ment in the Earth's crust. (Oxygen is the most com mon.) Silicon's chemical symbol is Si. Silicon is a metal loid, possessing some ofthe properties ofa metal and some ofthe properties of a nonmetal. Pure silicon 47 CMYSTAILEENI3 SEEECA PMEMI3M does not exist in nature. Silicon derived in the laboratory exists as black to gray, lustrous, needlelike crystals, and is an OSHA-regulated substance. SILICON CARBIDE (SiC) -A green to blueblack iridescent crystal. It is an OSHA-regulated sub stance. SILICON DIOXIDE-Silica (Si02). SI LICON ES--Synthetic compounds formed from two or more silicon atoms linked with carbon com pounds. Most silicones contain oxygen as well. Sili cones are formed by a process called polymerization; the molecular structure is a chain, not the tetrahedral shape of the molecules of si I ica or the si I icates. See also POLYMERIZATION. SILICON-OXYGEN TETRAHEDRON-Silicon and oxygen bond in a paired formation with four oxygen atoms and one silicon atom. Its chemical symbol isSi04. Tetrahedron I iteral ly means "four surfaces" and refers to the way the molecule looks internally. Picture four spheres (the oxygen atoms) touching a smaller sphere (the silicon atom) held in the pocket in the middle of the spheres. Lines drawn between the centers ofthe spheres would form a regular, four-sided prism, a tetrahedron. Although many structures are possible in nature, geolo gists seldom encounter more than a relatively small number, primarily because most rocks are made up of silicate minerals, which combine in the silicon-oxygen tetrahedron. The silicon-oxygen tetrahedron bonds most frequently with sodium, potassium, calcium, mag nesium, iron, and aluminum. SILICON TETRAHYDRIDE (SiH4)-A colorless gas used in the manufacture of semiconductors. Also called silane. It is an OSHA-regulated substance. SILICOSIS--A pneumoconiosis characterized 48 TU) A m>*ff 77" 77" IT 11. lite. u. U.U by scarring of lung tissue, which is contracted by prolonged exposure to high levels of respirable silica dust or acute levels of respirable silica dust. *%TARI "F--P' npina an' #'w` n,'wp`rOcn// *'wv'' that k balanced and will not change spontaneously, resis tant to energy change. STATES OF MATTER-A substance can be in a solid, iiquid, or gas state. These three are called states of matter. ST!SHOV!TE~The most dense form of crys talline silica. It is rare and at first was believed to exist only as a synthetic. It was initially found to occur as a result of shock-wave experiments and of nuclear explosions. Subsequently, it was found in nature in the Meteor Crater in Arizona. TETRAHEDRON--A solid geometric shape with four surfaces. See SILICON-OXYGEN TETRA HEDRON. TRIDYMITE--A form of crystalline silica. It is found in nature in voicanic rocks and stony meteor ites. it is also found in fired silica bricks. VITREOUS SILICA-Glassy silica. The term is sometimes use to refer to any noncrystalline sub stance. VITRIFY--To form as a glass. V//UMLC/'/I\krNl!IC/' ti iuirrrr3r-ur-ei__|_j_u___s___i_i_i___u___i___v__u_Iic___c_m_:i,,c 49 I