Document 4vRM5jYmo1v31dj0pBawJVmDQ

tr.d Ciav \4;irah. Vol. 23. po LM-U3 Pcrgamon Press Printed in Greal Britain MINERALOGY AND ORIGIN OF THE COALINGA ASBESTOS DEPOSIT F. A. Mumpton* and C. S. Thompson-!- 'Department of The Earth Sciences, State University College, Brockport, New York 14420 and +R. T. Vanderbilt Corporation. Norwalk. Connecticut U.S.A. (Received 27 July 1973) Abstract--Since 1960 asbestos production of the United States has more than tripled, a phenomenon due in large part to the development of the Coalinga asbestos deposit in western California. Although most asbestos ores contain 5-10 per cent chrvsotile in the form of cross- or slip-fiber veins within massive serpentinite bodies, no such veins are to be found in the Coalinga deposit despite the fact that it contains more than 50 per cent recoverable chrysolite. The Coalinga ore consists of soft powdery, pellet-like agglomerates of finely matted chrvsotile surrounding blocks and fragments of serpentinite rock. The highly sheared and pulverized nature of the ore favors its exploitation by simple, open-pit mining, and high quality products can be prepared by either wet or dry milling processes. Four types of serpentine material are distinguishable: (1) hard, dense blocks of serpentinite rock, ranging from fractions of an inch to several tens of feet in diameter; (2) tough, leathery sheets, resembling mountain Ieatiier, up to seyera! square feet in size; (3) brittle blades and plates of green serpentine, a few square inches in size; and (4) friable agglomerates of soft, powdery chrysotile contain ing appreciable amounts of the other three. Chrysotile is the principal component of all the above materials, with the exception of the serpentinite rock which consists mainly of lizarditc and/or antigorite. wifh small amounts "of brucitc, magnetite, and very short fiber chrysotile. Although chrysotile fibers up to several microns in length are present in the leathery sheets, most Coalinga chrysotile is much shorter and arranged in a swirling mesh or disoriented, tangled fibers, much like cellulose fibers in paper. Fragments of serpentinite gangue are scattered throughout the ore and contain most of the lizardite. antigorite. and brucite. Chemical, electron probe, and X-ray aoalyses confirm the iron-rich nature of the brucite. a critical factor in the susceptibility of this phase to oxidation in the surface weathering zone. Here brucite either dissolves, leaving behind a residue of brown, amorphous iron oxides, or transforms in situ to pyroaurite or coalingite. Dissolved magnesium precipitates as hydromag nesite immediately above the water table throughout the deposit. ' The abnormally high chrysotile content of this deposit is probably a result of the intensive shearing that it underwent during or after emplacement. If the friable, cbrysctile-rich ore was produced during this pulverization episode, (I) lizardite and/or antigorite in the serpentinite must have been transformed into chrysotile and (2) brucite must have been removed. It is likely that earlv-formcd lizardite. antigorite dissolved in the ground waters which pervaded the highly sheared body and that chrysotile later precipitated from these waters, coating all available surfaces. INTRODUCTION Although the New Idria serpentinite has been known since 1853, there is little difficulty in realizing why it went unrecognized for more than 100 yr as one of the world's largest deposits of chrysotile asbestos. Despite its enormous size and high-grade nature, the physical appearance of the ore is markedly unlike that of almost any other asbestos ore in the world. Most asbestos ores consist of chrysotile in the form of cross- or slip-fiber veins within massive serpentinite rock. Such veining is almost absent from the New Idria body and the ore consists of friable masses of matted chrysotile surrounding fragments of serpen tinite rock. The ore is highly sheared and pulverized allowing mining to be carried out by simple, open-pit methods and the production of high quality asbestos by straightforward wet or dry separation processes. The New Idria serpentinite outcrops over a 50 mile2 area in Fresno and San Benito Counties. 35 airline miles northwest of Coalinga. California. F.ariy reports described the serpentine material as "moun tain leather" or "antigorite... the platy, noncommer cial form of serpentine". A minor amount of cross fiber ore was produced from thin veins and seam fillings near the northwestern edge of the body (Laizure. 1926), but this ore played out in just a few years. The California Division of Mines (Anonymous. 1957) made brief mention of "chrysotile" in the cen tral part of the massif but no evidence was given for this identification. In the late 1950's, j. H. Bright, a Union Carbide Corporation geologist, examined the area and noted that although cross-fiber veins were not to be found, the entire southeastern third of the serpentinite consists of leathery sheets and pel lets of friable, asbestos-like material. He reported that the friable ore is not merely a surface weathering phenomenon, but that it extends several tens of feet deep and is relatively uniform over a 15 square-mile area. I.'2 F. A. VfvMPTOv and C. S. Thompson Subsequent X-ray diffraction and eiectron micro scope examination indicated that the major consti tuent of the ore is indeed, chrysotile. the fibrous member of the serpentine group. This identification samples ground to 525 and 400 mesh were slightly contaminated by the tungsten carbide balls and mill. Mineralogical examinations were made also on pressed filter cakes which had been prepared by mix led to considerable speculation on commercial uses ing the sample for several minutes in a blender and for this unusual form of asbestos, and by late 1959. filtering immediately. Highly purified, gangue-free the town of Coaiinga had become the center of samples were prepared by dispersion in an aqueous asbestos exploration activities of several industrial solution of aluminum acetate at about pH 4-5. The organizations, and the deposit become known as the dispersed fiber was then decanted from coarser mag Coaiinga asbestos deposit. (See also Munro and netite and platy serpentine, flocculated with sodium Rcim, 1962: Merritt. 1962; Rice. 1963.) carbonate, filtered, anti dried. This technique is a The Coaiinga ore contains more than 50 per cent modification of procedures reported by Barbaras chrysolite, in contrast to the <10 per cent recoverable (1953). By changing settling times and centrifugation fiber in typical cross- or slip-fiber ores. The derived conditions, various degrees of purification could be asbestos product was found to be comparable with achieved. Grade-7 Canadian fiber and thus in the 1960's appeared potentially useful in standard applications Apparatus of this grade. Mining of the Coaiinga ore in recent X-ray powder diffraction studies were made with years by Union Carbide Corporation, Coaiinga commercial high-angle diffractometers and 114-6 mm Asbestos Company (a joint venture between Johns dia. powder cameras, using Mi-filtered CuK* radia Manviile Corporation and Kern County Land Com tion. Onc-inch circles cut from dried filter cakes of pany) and Atlas Minerals Corporation has caused asbestos were examined directly by pasting them to the asbestos production of the United States to triple glass sample holders. Samples thus prepared have since 1960. These companies currently produce nearly a strong tendency to orient, and basal spacings of 75.000 tons of powdered asbestos each year by both the serpentine mineral are greatly enhanced on the dry and wet processing techniques. In addition to X-ray diffraction patterns. Orientation effects can tie conventional uses for Grade-7 asbestos, wet-processed reduced by spinning the sample in the X-ray beam fiber has found many new applications in the paper by means of a rotating specimen holder or by using and plastic industries, in water purification, and in packed sample holders of 450 mesh pow-der. several other specialized businesses (Woolery. 1966). A Powertron 300 W Ultrasonic Gcaner was used In support of raining, milling and product develop to disaggregate and to assist in dispersing serpentine ment activities of Union Carbide Corporation, a specimens. Electron microscope investigations were detailed mineralogical study was carried out on the made by W. Roettgers with a Siemens Research Coaiinga ore. The results of this study have suggested Model electron microscope, using both water-suspen a formation mechanism which may be applicable to sion and preshadowed. carbon-replica preparation chrysotile-rich zones and deposits in other serpen- techniques. Critical samples were also examined by tinites of the world. selected-area electron diffraction. Differential thermal analysis' patterns were recorded on an X-Y recorder EXPERIMENTAJ. as a function of e.m.f., using calcined kaolinite as Sample collection and preparation a reference material. More than 500 samples were collected from sum mer 1962-1964 from the surface weathering zone, the resistant serpentinite knobs and outcrops, and the pits and stockpiles of the companies then active on the deposit. Nearly 2100 interval samples were also examined from 160 drill holes located on a grid pat tern across Union Carbide's property. These samples were collected bv W. T. Cohan and M. Genes and represent half of the material recovered from 10 ft intervals of 6 in. rotary drill holes. They were split at the drill site to about 25 lb and further reduced in size to about 51b of -jin material by alternate grinding, screening and splitting. To obtain finer grained material for analysis. 10-20g splits were then ground dry in a Waring Blendor to about 10 mesh. Additional cominuntion could be achieved in a Pitchford Selective Uniform Particle Size grinder; however. * Sec Mumpton. F. A. (1974) Oharacteri/ation of chryso lite asbestos and other members of the serpentine group of minerals. Siemens Renew XL(. 7th Spec. its.--X-ray and Electron Microscopy Sews. 75-H4. CHARACTERIZATION OF SERPENT1NEGROI.P MINERALS The unequivocal identification of members of the serpentine group of minerals is a difficult task which is made even more complex by the presence of more than one polymorph in the same specimen. Most of the samples examined in this study contain at least two of the three principal serpentine poly morphs. A critical review of the probiem of the char acterization of the serpentine minerals is being pre pared for publication elsewhere: however, to support the identifications made in this study, a brief discus sion of this problem is warranted.* .. Chrysotile itself is readily distinguished in Tfirtlec- tron microscope by its tubular-fibrous habit but the morphological similarity of lizardite and antigorite precludes their differentiation by this technique alone. X-ray diffraction provides a reliable means of dis tinguishing the serpentine polymorphs, provided that samples are relatively mor.oininerallic. Mixtures are ill >n v id I cu us ,;r he Sm 0 * a as >r, ">e th rn of 10 of m *g 5 d e e h n v d $ I h t: fe i::: Fig. i. Electron micrograph and diffraction pattern of chrysoiile from the Coaiinga asbestos deposit (Green-bladed material). Magnification - 21,000 X. Fig. 2. Electron, micrograph and diffraction pattern of lizardite from the Coaiinga asbestos deposit, in anligorite from Santa Rita Peak (487-52-8). Magnification = 21,000 x. Fig. 3. E:ectron micrograph and diffraction pattern of antigorite from Santa Rita Peak, Coaiinga asbestos deposit (487-52-8). Magnification -- 21,000 " . CCM--f.p. 132 & v~ j ' : Fig. 5. White, asbestos-strewn slope in the central part of the New Idriaserpentinite. Fig. 6. Barren slopes covered with green-bladed chrysotile, Coaltnga asbestos deposit. Fig. 7. Electron micrograph of chrysotile from the Coalinga asbestos deposit. Magnification 15,500 x ax's* Fig. 8. Electron micrograph of chrysotile from fragments of massive serpentinite in the Coalinga asbestos deposit (248-43-4). Magnification = 19,500 Fig. 9. Electron of a replica of chrysotile from green-bladed fragments from the Coalinga asbestos deposit. Magnification - 47,000 >.. ) Fig. 10. Electron micrograph of a replica of chrysotile from the leathery sheets from the Coalinga asbestos deposit. Magnification -= 5000 x. Fig. II. Photomicrograph of a thin section of sample 487-44-6, showing serpentine, bruciteand magnetite. Crossed nicols. Magnification -- 50 ............ uit- * I Fig. 12. Photomicrograph of a thin section of sample 487-44-9 showing serpentine (white-gray) with characteristic "mesh" structure and magnetite (black). Polari7cd light. Magnification -- 50 x. Fig. 13. Photomicrograph of a thin section of sample 487-44-6 showing serpentine, brucite and magnetite. Crossed nicols. Magnification - 50 . I Fig. 14. Photomicrograph of a thin section of sample 487-44-9, showing serpentine and magnetite, and a cross-cutting vein in which magnetite is concentrated near the walls and brucite at the outermost edge and near the center. Crossed nicols. Magnification -- 85 . Fig. 15. Photomicrograph of a thin section of sample 487-44-6, showing fibrolamellar serpentine surrounding an opaque inclusion. The center of the inclusion is chromite (black) which is surrounded by carbonate (gray-white) which is in turn surrounded by an outer rim of magnetite (black). Crossed nicols. Magnification -- 150 ? . Fig. 16. Photomicrograph ofa thin section ofsample 487^14-28, showing relict olivine (Mg..8iFe0.ls)SiOi in serpentine. Polarized Sight. Magnification - 130 .'<. Fig. 17. Photomicrograph of a thin section of sample 487-44-28, showing a large reiict grain of olivine in serpentine. Polarized light. Magnification -- 130 ". Pig. 19. Electron micrograph of antigoritc from sample 487-52-8, collected at Santa Rita Peak. Coalinga asbestos deposit. Magnification = 18,000 i 4 Fig. 20. Electron, micrograph of an unusual form ofchrysotile in sample 487-44-6. Note the extremely wide tubes of varying diameter. Magnification = 41,000 x. Fig. 21. Electron micrograph and diffraction pattern of an unusual form of chrysotile in sample 487-44-6. Note the extremely wide tubes of varying diameters. Magnification -- 20.200 x. J Mineralogy and origin of the Coalinga asbestos deposit 133 more difficult to decipher by X-ray diffraction because ail three polymorphs yield very similar and overlap ping patterns. Although Brindley and Zussman (1959) noted that infrared (i.r.) absorption methods could distinguish antigorite from the other serpentines, results obtained in the present study suggest that the technique is of little use in characterizing mixtures of serpentinegroup minerals, due to the close similarities of their i.r. patterns. (See also Luce. 1972.) Faust and Fahey (1962) found that antigorite could be distinguished from the pair chrysotilc/lizardite by differential ther mal analysis because the former gives rise to an endothermic peak at a considerably higher tempera ture than does either of the other two. Unfortunately Table !. Standard X-ray diffracdon data for serpentinegroup minerals Antigorite New Zealand 487-39-5 d I,.I,, Lizardite Cornwall 568-22-2 d I/l,, Chrysotile Thetford 568-30-1-1 d f/I,, Lizardite/ chrysotile indices fhfcl) 7-25 5-19 4-59 4-24 -- 3-62 -- -- -- -- 2-52 -- 2-44 2-38 2-16 -- 1-829 -- 1-776 1-740 -- -- -- 1-562 1-536 1-522 1-506 I ---63 1-414 1-384 1-342 1-315 1-298 1-263 1-207 1-152 l-07> 1-055 1-001 0-972 095! 0906 0895 0887 10 <i 3 3 -- 6 - 9 -- 1 <1 -- 4 - 1 -- 1 1 -- -- 4 4 <1 <1 < IB <I <1 <1 3 <1 1 1 <1 <1 1 l <1 <1 <1 <1 <1 7-34 -- 4-59 -- 3-88 3-63 -- 2-86 2-64 2-59 -- 2-49 -- 2-15 -- 1-830 1-792 -- 1-735 1-691 -- -- 1-531 -- 1500 1-4611-414 -- 1326 1-306 1-278 1-248 1-165 1073 1-0.57 0994 0968 0954 -- 10 7-31 ---- 5 4-55 ---- 3 6 .. < IB 1 3-65 3-40 -- -- < 1 2-60 _ 2-54 to -- -- 2-45 ---- -- 2-2 i 6 -- -- --- - 2-09 <1 -- 2-- ---- 1-743 1-- IB -- -- 1-614 _-- 7 1-535 ---- 6-- <1 -- --1 ---- <1 -- 3 1-316 1-- <1B -- 1- 1-- <1 1-043 1-000 l l ---- -- 0-886 - 10+ 4 -- -- 6 <1 -- 3 2 -- 4 -- <1 -- 1 -- -- <1 -- <1 6 -- -- -- _1- - -- IB IB -- --- -- IB -- 002 020 022 004 024 130 201 202 202 202 204 204 008 206 028 060 208 00.10 064 400 402 20-10 404 Underlined d values me regarded as diagnostic. lizardite and chrysotile yield similar patterns making their distinction impossible by D.T.A. methods. Limited use. however, has been made of D.T.A. data in this study to confirm and support identifications made by other techniques. To estimate the polymorphic composition of criti cal serpentine samples an identification scheme was developed using a combination of film X-ray diffrac tion, electron microscopic and electron diffraction techniques. The relative complexity of the procedure precluded its routine use and only about two dozen specimens were completely characterized in this man ner. Based upon the data of Whittaker and Zussman (1956) and upon X-ray patterns ofstandard chrysotile, lizardite and antigorite, estimates were made of the three principal serpentine polymorphs. The lines which are critical for differentiating the three poly morphs are underlined in Table 1. Although it is not possible to identify the nonfibrous varieties of serpentine by electron microscopy alone, certain features appear to characterize the polymorphs. Lizardite particles commonly have hexa gonal outlines (Fig. 2). Antigorite is generally form less, but thin plates may show characteristic moire patterns (Fig. 3). The most reliable identifications are achieved by selected area electron diffraction, a tech nique used successfully by Zussman, Brindley and Comer (1957). As seen in Fig. 1 the oriented layer lines of the electron diffraction pattern of chrysotile are distinctive. The pattern can also yield information as the exact polytype of chrysotile present (ortho, clino or para). The electron diffraction pattern of lizardite (Fig. 2) is a relatively simple hexagonal array of spots. Overlapping plates of lizardite may give rise to a multiple arrays, wherein hexagonal patterns are slightly rotated from one another, but the six-fold symmetry is clearly visible. The pattern of antigorite (Fig. 3) is unique in that clusters of closely spaced spots occur along layer lines at regular intervals, cor responding to the superlattice periodicity of the enlarged antigorite unit coll (Zussman, Brindley and Comer, 1957). GEOLOGY OF THE DEPOSIT Regional geology The New Idria serpentinite has been described by several authors and has been the scene of past mining operations for chromite, magnesite, mercury and gem benitoite. A simplified geologic map of the serpen tinite area, based or. the original work of Eckel and Meyers (1946) and the modifications of Coleman (1957) is shown in Fig. 4. The serpentinite is at the southern end of the Diablo Range, midway between the Great Valley of California on the east and the San Andreas fault zone of the west. It is an elongate oval flanked by upturned sedimentary rocks of the Franciscan and Panoche Formations of Cretaceous age. It generally forms ridges surrounded by lower, rounded hills. Most of the contacts between the ser pentinite and the enclosing sedimentary rocks are covered by landslides; however, where a contact is 134 F. A. M cmpros and C. S. Thompson ffiie. h 12e ;i illjt) Fig. 4. Simplified geologic map of the New Idria serpentinite. (From F.ckel and Meyers, 1946; modified by Coleman, 1957.) MS visible, it is generally steeply dipping and marked diameter, are scattered throughout the body and I Mi by high angle faults and/or zones of extensive shear stand out as resistant krfobs and ridges. Many are ing. Little evidence has been found of contact meta rounded and take the shape of resistant "boulders" morphism and it is likely that the present position encased in softer, friable material. iil of the serpentinite is the result of tectonic forces, A most pronounced feature of the body are myriads rather than of magmatic intrusion (op. cit.. p. 10). of fractures and joints, which crisscross the serpen tinite. This phenomenon is most noticeable in the i Geology of the serpentinite southeastern tliird of the body, coinciding with the The New Idria serpentinite is about 4 miles wide areas of highest grade asbestos. In the northwestern III and 12 miles long and appears to be a low-tempera part of the body massive serpentinite is much more jii ture alteration product of a pre-existing dunite, common and the quantity of soft, friable asbestos although only meagre evidence of the original igneous is much less. Figure 5 shows a typical asbestos- rock can be found today. Coleman (1957) indicates strewn slope in one of the high-grade areas of the that the parent olivine-rich assemblage was altered deposit. In other parts of the body slopes are covered to serpentine far beneath the surface. The resultant by hard chips of green, bladed serpentine, as shown serpentinite mass was then injected "cold" into the in Fig. 6. overlying sedimentary rocks by tectonic forces. Typi The total thickness of the serpentinite and of the cal mesh-type structures, pseudomorphous after oli asbestos ore is not known; however, gravity surveys <!' vine. are commonly observed in thin section, but few indicate that the body extends to a depth of 15,000 ft grains of relict olivine fiave been detected. Narrow (Byeriy, 1954, p. 48). Drill holes in the NW 1/4, Sec bands of pvrexene have been encountered in the field, tion 25, T18S, R12E collared at an elevation of but the overall textural charactistics of the serpen 4215 ft, and near the New Idria Mercury Mine in tinite suggest that pyroxene was not a major consti Section 32. T17S. R12E and collared at 3780 ft, both tuent of the original ultramafic rock (op. cit.. p. 99). penetrated serpentinite at a depth of 500 ft. Lenses The abundance of brucitc. overlooked by Coleman, and bands of silica-carbonate rocks are abundant in also suggests that the parent rock was a dunite. rather the interior of the serpentinite as well as along the than a peridotite (vide inji-a). edge of the mass. Many are dike-like bodies which VI The great bulk of the New Idria serpentinite has appear to follow prominent fault zones. These have been highly sheared and pulverized and consists of probably formed during localized hydrothermal alter soft, friable sheets and clumps of ashestiform material ation of the serpentine. According to Bright (1959) r&f admixed with a fine serpentine powder. The friable these rocks are composed of "chalcedony, quartz, ore has little strength or coherence and crumbles with opal and possibly ankerite" and form tough, porous a touch. Veins of cross-fiber asbestos have been dis outcrops. covered in only three localities, at or near contacts, and may be the result of local hydrothermal pheno MINERALOGY OF TKF. FRKSH ASBESTOS ORE mena. Hard, dense, competent masses of serpentinite. The Coalinga asbestos ore is a mixture of several ranging from fractions of an inch to tens of feet in physically different types of serpentine materials, with Mineralogy and origin of the Coaiinga asbestos deposit 135 gradations between the types being the rule rather than the exception. The four main types of material are: 0) hard, dense, dark-blue and green fragments of blocky serpentinite. ranging from fractions of an inch to tens of feet in diameter; (2) large, tough, leathery sheets of malted chrysotile, resembling mountain leather, up to several square feet in size; (3) small, brittle, bladed and platy fragments of green serpentine, up to several square inches in size; and (4) soft, friable greenish-white agglomerates of flaky asbestos, varying from about {-1 in dia,, and contain ing small quantities of both the green-bladed material and the hard, dense, serpentinite rock fragments. Type 4 material makes up the bulk of the ore. Scattered through it are large, massive serpentinite "boulders" (type 1. above), up to several feet in diameter, which have apparently withstood the intense deformation which accompanied or followed the initial serpentini- zation process. The grade of the ore is directly related to the quantity of residual serpentinite rock present. Both the leathery sheets (type 2) and green-bladed material (type 3) occur on slickensided surfaces of the "boulders" and apparently coat joint and fracture surfaces. Chrysotile is the major serpentine constituent of the friable agglomerates and is yftually the only phase present in the leathery sheet! and the green- bladed serpentine fragments. The serpentinite "boul ders" are essentially lizardite and antigorite. with lesser amounts of very short-fiber chrysotile, brucite. and magnetite. Minor quantities of calcite and traces of chromite and uvarovite garnet are also present. Locally, concentrations of magnesite, chlorite, diopside, and a 14 A, expandable clay mineral have been identified. _ Chemically the Coaiinga ore is remarkably homo geneous. Analyses of 10 ft interval composites from a drill hole near Union Carbide's pit in Section 25, TI8S, R12E (see Fig. 4) are listed in Table 2. Aside from a surface weathering zone between 0 and 20 feet, few differences in composition are apparent although the recoverable fiber content of the samples ranges from 35- 74 per cent. The magnetite content as measured by a permeameter technique, is also fairly consistent and varies from about 4-6 per cent The magncsia/silica ratios of these samples leave little doubt that brucite is present a conclusion verified by X-ray diffraction analyses. Friable asbestos ore The soft, friable, greenish-white aggregates gener ally contain more than 80 per cent short-fiber chryso tile. When dried, this material breaks up readily into a fine powder, containing flakes of matted chrysotile and small particles of hard, gritty serpentinite. Although talc has been reported in the Coaiinga ore (Anonymous. 1957; Matthews, 1961; Miller. 1960), these identifications have not been confirmed and talc has not been found in any of the several thousand samples examined in this study. Strieb (personal com munication), however, reports traces of talc in sam ples from Johns Manville pit near the northeast con tact of the serpentinite. These may be due to local hydrothermal solutions and are not typical of the main body. The fibrous nature of the chrysotile from the Coai inga .deposit is readily apparent under the micro scope; however, individual fibers are much too small to be seen by this technique and optical microscopy provides only an equivocal means of identification. The mean indices of refraction of the fiber-bundles are close to the values reported by Deer. Howie and Zussman (1962) for cross-fiber chrysotile. X-ray dif fraction patterns of several chrysotiles, purified from various types of Coaiinga ore, are listed in Table 3 and agree well with those of standard chrysotile as listed by Deer, Howie and Zussman (1962). Although most of the samples appear to be the clinochrysotile variety, no rigorous attempt has been made to differentiate among the several chrysotile poly morphs in this study. Chemical analytes of chrysotile separated from several different types of ore are listed in Table 4. Although these data yield the approximate composi tion of Coaiinga chrysotile, the actual composition of a particular type of chrysotile in a given sample cannot be pinpointed. The processes used in purifying a sample for analysis tend to fractionate the sample and thus yield products which consist mainly of that material which is easily liberated and separated. The Table 2. Chemical analyses of drill hole samples Coaiinga asbestos deposit* 0-10' 10-20 20-30 30-40 50 60 80-90 110-120 SiO, MgO FeO FCjOj CaO co2 HjO Total Fiber Magnetite . 39-59 38-20 2-16 6-10 0-30 0-43 12-58 99-36 _ -- 33-88 39-82 1-55 5-44 1-02 4-16 14,3 95^ 74 4-4 36-73 39-00 1-94 5-19 0-54 1-29 13-88 98-57 53 4-8 36-75 40-76 1-98 5-80 0-67 0-82 13-50 100-28 63 7-5 37-34 40-57 2-66 5-06 0-56 0-67 13-68 100-54 43 4-8 36-91 40-31 239 5-22 0-82 0-79 13-47 99 91 52 4-4 35-68 41-15 2-46 4-91 0-69 0-70 14-20 99-79 35 3-3 * Located in the SW 1/4, NO 1/4, Section 25 (see Fig. 4/ t Dispersible fiber by Barbaras (19531 technique. 1 Permeameter measurement. 160-170 36-67 39-92 2-53 5-14 1-00 0-68 13-69 99-63 54 4-9 136 F. Mumvton and C. S. Thompson Table 3. X-ray diffraction dam* for chrysotiles Chrj#otile Zarcnatt Switzerland* Oiry*ot3c Thetftjrrf Quebecj: Chrysotile Tbetford Quebec Chrysolite Cassiar Brst. Col 4 Chrysotile Coulinga California!} ChrysotUc Coaling* California^ Chrysolite Coaltnga California!} Cbrytotifc Slragari Yugoslavia OoyS-liber Ptatv ('rott-fibw Cross-fiber Cross-fiber Grccn-bladcd bibcr Fiber <hkl> ASTM 10-381 d ri,, 367-2-1 d l/I,j 568-30-M d 1/J., 36',-92-1-5 d IX 487-52-1 $-1 d I'f 487-34-J J i.i, 367*49-1-1 d 1/Jn 487-66-1 ii I/Ir "" 002 020 7-36 4-N8 10 7-28 5 4-56 to 7-31 4 455 10+ 7-25 4 4-54 10+ 7-31 4 4-56 10 7-31 3 4-59 10 28 4 4-56 104 7-34 4 +55 004 366 10 3-65 - -- 3-40 6 MS 6 3-65 1 5-40 <1 -- 6 3 65 -_ 7 3-66 _ 6 J-fiJ __ . 8 3-65 __ 'laic -- - 3-13 <1 --. - __ _ _ 130 201 :o2 2*66 4 2-64 sl -- -- 2594 4 -- -- 2-60 3 2-GO 2-549 5 2-54 <1 2-54 2 - -- > 260 -- 2-65 <1 2 261 <1 2-60 2-59 3 HO l 2-55 <1 2-53 202 2-456 8 2-445 4 2-45 4 2-15 4 2-45 2 246 5 ?-45 4 245 203 204 204 2282 2-215 2-096 3 2-28 <1 -- X 2-21 5 2-09 1 2 2-21 209 <1 -- 2-21 <! 209 -- _ 2-0S -- 2-28 <1 2-27 <1 *i ?> <1 :-:i * 2-10 2 209 ._ 20.1 2-09 008 l S29 3 1-819 <1 -- -- - - 1-526 1 1-829 <1 I-523 206 1-748 5 l-'MO 2 1-743 <1 1-735 -- -- -- 1-614 <1 _ I 1-749 --- __ 2 1-742 - __ i-73i . IB 060 2(rt 1-536 s 1 su 5 1-535 6 1-534 6 1-536 6 1-540 S 1-533 6 1-535 6 - -- 1-506 -rl _-- - -- 1-509 <1 __ (Wi'jO 1-465 3 1-460 <1 -- -- - - -- _ 1-467 l 1-463 402 HP 4 1-316 3B 1-316 I --- -- --- 1-316 -- - 1-315 23 1-320 ---- 2D 1-315 3B 5-320 -- 1-274 <1 _:b 531 ---- -- 1043 IB - -- -- 1-047 CUB 1196 <1 0-997 <i - - 0-995 <1 -- 0-889 <1 K00 IB 0-992 <iB 0-995 <1 0-995 < IB 1006 IB 0-997 <\ IB - - 0-886 0**88 IB 0-88S 18 0887 \ * Film patterns, Cu Ka radiation, 114-6 mm camera. t Data listed in Whittaker and Zussman (1956) and represent a composite of two chrysotiles: one from Zermatt, Switzerland, and one from Rcichenstein. Silesia. Sample supplied by G. T. Faust, similar to the Coalmsfa grecn-bladed material. 5 Samples purified by dispersion techniques. -* * * Table 4. Chemical analyses of purified chrvsotiie samples from the Coalinga asbestos deposit Si02 ai,o3 Fe,()3 FeO (Fe) NiO MgO CaO HjO Total Leathery sheets 367-34-1* - 41-75 005 1-40 Oa27 (1-19) 0-03 4304 -- 13-46 i 00-00+ Greenbladed ' 367-34-2* 41-42 0-04 2-05 1-61 (2-65) 0-07 43-76 0-54 13-66 lOO-OOt Crossfiber 487-52-18* 41-02 0-31 2-03 2-44 -- -- 40-44 0-25 13-30 99-79 Leathery shects No. 4 42-2 0-66 1-2 0-09 - 0-04 41-4 0-04 14-25 Mill products^ No. 7 41-0 0-7 1-3 0-95 0-22 40-7 013 14-3 585-90-2 Random driil-hoie samples 58S-90-: 487-27-2-6 487-27-3-6 487-27-4-6 487-27-9-6 SiO, AI2O3 FcO (Fe) NiO MgO CaO H.O Total 41-92 0-36 0-99 0-64 -- 0-09 41-76 007 13-26 98-25 4i-88 - 1-09 1-08 0-07 42-11 0-17 13-60 100-00; 40-59 1-51 1-25 -- 0-29 41-94 0-14 14-21 tOO-Sqt 41---16 1-32 --0-41 0-26 42-18 0-22 14-44 100-00* 4.1_-59 1-17 0_-75 0-08 42-05 0-16 14-20 100-00i 41-52 _ 1-27 0-63 0 0-14 41-45 0-15 14-83 lOO-OOt * Untreated samples; all others were purified by chemical dispersion techniques and are therefore anomalously enriched in alumina (4-5"nl. Sample 585-90-2 was chemically dispersed without aluminum compounds, and was washed thoroughly afterwards. + Recalculated to I00.Q after correcting for calcite. 1 Recalculated to lOO0; after correcting for alumina contamination. From Page and Coleman (1968). Mineralogy and origin of the Coalinga asbestos deposit 137 composition of the separated chrysotile may therefore be significantly different from that of the very short- fiber chrysotile. which tenaciously adhers to the gan- gue materials in the original sample. From these analyses, however, the green-bladed chrysotile appears to contain more than twice as much iron as the lcathery-shcet chrysotile. The cross-fiber sample from Coalinga is even more iron-rich and m contains significantly more aluminum, likely due to the presence of minute particles of corundum locked between the fibers, as suggested by Faust and Nagy (1967) and Page and Coleman (1968). Naumann and Dresher (1966) reported that the surface area of Coalinga chrysotile is about 80 m2/g, compared with values of 20-30 m2/g for chrysotiles Si:i from most cross-fiber deposits. They attributed the higher values for the Coalinga material to the absence of "solid matter filling the voids between fibrils". From electron microscope studies the fiber leugths of chrysotile from the Coalinga deposit vary widely from sample to sample. The longest fibers are several microns in length and are generally found in the leathery sheets and in the flexible platelets of the friable type of ore (sec Fig. 7). Chrysotile from the serpentinite rock fraction is considerably shorter and thicker than normal and length/width ratios as small as 10 or 20 are not uncommon (see Fig. 8). The short, stubby nature of this material may be the result of inhibited growth within compact rock masses, rather than in the relatively "open" environments of fractures and cracks. A replica electron micrograph of grccn-bladed chrysotile. is shown in Fig. 9. The highly oriented nature of the fibers is readily appar ent and contrasts vividly with the swirling mesh of fibers in the leathery sheets, as shown in Fig. 10. Chrysotile from the green-bladed fragments is also significantly shorter than that from the leathery sheets. Within a single green-bladed lath, the fibers appear to be tightly held in a "whiskbroom" maimer with an occasional fiber peeling off an edge. The random, disoriented nature of chrysotile in the leath ery sheets, not unlike the arrangement of cellulose fibers in paper, may be the result of the squeezing and drying of a gelatinous chrysotile mass, which filled voids and coated surfaces during sftear-deforma- tiou of the serpentinite body. In addition to chrysotile. snail quantities of platy serpentines (antigorite and lizardite). brucite. and magnetite are also present in the friable portion of the fresh asbestos ore; however, these phases occur primarily in the tiny fragments of hard serpentinite rock which are caught up in the soft, flaky material. Of special importance, however, is the manner in which small flakes of leathery chrysotile completely enclose, in a sandwich fashion, harder plates of green- bladed material and fragments of serpentinite. The green-bladed material inside the soft packets is itself significantly softer and more pliable than the larger plates and laths found elsewhere. The soft flakes also enclose particles of white calcitc. the age of which relative to the asbestos is not precisely known. Large flakes and pellets of what appears to be secondary brucite have also been noted associated with the soft chrysotile agglomerates. Magnetite also is scattered throughout the sheets and embedded in flakes of chrysotile, and many asbestos aggregates can be picked up easily with a hand magnet. Serpentinite rock fragments The serpentinite rock fragments contain most of the gangue materials in the ore and must be separated during milling in order to produce clean, grit-free asbestos products. Nine hand specimens collected from different parts of the deposit, have been studied in detail by petrographic. X-ray diffraction, electron microscope, electron diffraction and thermal tech niques. Petrographically, these samples are similar to most other serpentinites. They consist of dense aggregates of serpentine-group minerals arranged in a brcccialikc manner as illustrated in Fig. 11. Randomly oriented aggregates of serpentine with shadowy extinction and fibro-lamellar structure vary in size from 0-05 to 100 mm and commonly form "mesh" structures, pseudomorphous after olivine (see Fig. 12). The serpentine has mean indices of refraction of 1-52 and 1-56. The laths or "fibers" of serpentine are length slow, with maximum birefringence of first order yel low. Veinlets up to 0-1. mm wide having parallel orientation and extinction cut across massive serpen tine and may be monominerallic chrysotile. This material is not cross-fiber chrysotile in the usual sense of the term. Brucite is abundant in most specimens examined and forms scaly aggregates from 0T to several milli meters in size. Intergrowths of brucite and serpentine are common with thin lamella of each mineral alter nating with the other, as shown in Fig. 13. Brucite is colorless in thin section and has a characteristic pinkish-yellow interference color. It has a mean index of refraction between t-57 and 1-58; it is optically positive and interference figures occasionally show anomalous colors. Brucite may be found in magnetite veinlets which are undoubtedly younger than the bulk of the serpentinite specimen (Figs. 11. 13) and which were probably formed during post-serpentinization deformation. Subhedral grains of magnetite occur in all specimens and are commonly associated with calcite and veinlets of brucite. Figure 14 shows such an association, with the outermost edge and center of the vein consisting of brucite and chrysotilef?). and with fine-grained magnetite making up the two inner zones. Dispersed magnetite varies from 0-01 to I 0 mm in size. The two size ranges of magnetite shown in Figs. 12 and 14 may represent two stages of crystallization. Some serpentinite fragments con tain as much as 15 or 20 per cent magnetite, although they average between 4 and 6 per cent. Calcite is commonly found in veinlets with magne tite and brucite. It is usually fine grained, about 0 05 mm in size, and may represent calcium originally present in olivine or pyroxene, and/or may have !?h F. A. Mumpton and C. S. Thompson Tabic 5. Mincraiogicai descriptions and identifications of serpentinite specimens from the Coalinga asbestos deposit Sample No. Location and description X-ray diffraction identification* 4S7-4-6 .187.444 187-52-7 437-52-1.1 487*52*1 Hard, deuae. dark green serpentinite from floor nf pit m NW 1/4 of Section 25, T18S. RUE Unweathered inner portion of large xerpentmile "boulder* in pit located in Section 29. T18S, R13E Hard, dense, dark blue serpentinite "boulder** from center of pit in NW 1/4 of Section 32, T18S. RUE Blue-brack vein cutting Sample 48 7-52-7 Serpentinite "boulder" from same locality as Sample 487-52*7 48744-14 48744-19 48"-52-8 Large, reddish colored "boulder" of eefpentinite. near nit in NW 1 4 of Section 35. T18S, R12E Large serpentinite mass near pit in NW 14 of Section 25. T18S, R12E Green serpentinite "boulder- near contact. Section 56, T18S, RUE Main serpentinite mass at Sants Rita Peak. Section 24. T1SS. RUE Antigorite Brucite Some magnetite Chrysolite Lizardite Some brucite Some magnetite Chrysotile Brucite Some magnetite Antigorite Some magnetite Some uvarovite Chrysotile Antigorite Brucite Some magnetite Trace lizardite Antigorite Trace chrysotile Some lizardite Trace magnetite Antigorite Some magnetite Antigorite Brudte Uvaravite Antigorite Some magnetite Trace garnet * Film methods used; d spacings listed in Table 6. t Sec Fig. 19. _ Electron microscope and diffraction identification Chrysotile Antigorite Lizordite Brucite Chrvsotile Some lizardite Soro* antigorite Antigorite Trace chrysotile Chrysolite Lizardite Trace antigorite Antigorite Some chrvsotile Some lizardite Antigorite Chrysotile Antigorite Some Lizardite Some chrvsotile Antigorite Differential thermal analysis idcntdicaticnt Chrysotile,'lizardite Antigorite Brucite Chrvwnle.il/arditf Brucite Chrysolite lizardite Brucite Chrysotile/lbrarditc Brucite Antigorite Clirysottle,lizardite Antigorite ChrysotUelizardite Antigorite Brucite Antigorite Trace chrysotile lizardite Table 6. Film X-ray diffraction data of serpentinite samples from the Coalinga asbestos deposit 4R7-44-6 487-44-13 d I/I., . d 1% 487-44-14 d l/I,, 487-44-19 d I/Tft 25 10 + 7*28 104 7*27 10+ 7*27 10 4*76 i -- -- -- 4*81 2 4-60 4-29 2 461 1 4 30 4 4*60 2 4 24 <1 4*60 4-26 2 3 362 9 3-63 9 361 9 3*63 8 *G4 <. S __ .... 296 i 296 I 2*96 1 3 01 4 -_ __ -- - 2*69 1 _ -- 2 69 4 -- - -- 258 <1 2-58 <1 > o SO 2*52 10 1*52 ;0 2 52 10 2*46 1 2*45 <1 245 <1 2*46 4 -- 241 <1 244 2 -- -- 2 <7 2 -- - 2*37 5 221 <1 2 10 1 -- -- - _ 1 7*8 1 748 <1 1-711 <1 -- i 614 2 '575 1 1-556 4 1*5*8 * - 1-43*4 2 HI* -- 1 i/4 *U -- -- 1*513 -- 1*280 <1 i*:r>3 <1 221 209 1 :co <1 1-9<7 <1 1819 f PSI <1 1746 <1 1712 1 166X 1611 <\*v ---- 5 555 4 1 537 : S 615 i 1 481 2 UP <s 1 392 < i -- -- -- MI6 -- l 280 <! 1*459 <1 _ 2.79 1 --2 00 <1 -- i-823 <1 778 <1 1-742 <1 709 <1 -- 1*612 --- 1-558 *> 1-537 3 1 504 1 1*481 2 1 416 <1 - --- ---- 1-317 1 --1*278 <1 -- 2*20 <1 .NO 1 -- _-- 1955 2 1*831 | 1789 IB 1739 i -- 1671 2 1613 4 1*578 <1 1*558 2 1*537 X 1*504 2 1*485 1 1-416 <1 --- 1371 1 1347 1 -- 1-314 3B 1*299 <1 1-282 IB 1-262 <1B 487-44-26 d I/T0 457-52-1 d I/J0 487-44-9 i IT* 7-27 10 + 7-32 10+ 7*31 ! > -- -- 4*77 1 -- -- 4*59 4 4-60 2 4*55 4 - -- ----- -- 3*69 (0 3*64 10 3-65 to 303 7. __ _ 2*99 5 ? 97 2 - 2*88 2 . .-- -- 2-66 IB -- -- -- 2*61 <1 260 <1 2 50 9 2-53 8 250 4 -- ... -- -- 3*45 <1 -- 2*42 <1 -- " 7-37 4 -- -- 201 1 2*10 <.1 204 I 201 1 1*959 <1 _ 2 10 -- -- ~' _ l --- 2*22 2*09 -- - <1 <1 -- 1 628 1 J-78S 1 1-751 I --- 1672 1*673 2 _ 1-554 5B -- 1*504 j --- 1*423 1 1*407 1 -- ---- 1*328 2 1-310 2B -- 1082 2 1 262 < l _ 1-792 1 - 1-713 <! -- l-6!5 t 1-575 <1 1*543 SB -- 1*454 i -- .... --- -- 5-315 < IB ---- 1-281 <1 - _ 1*735 -- -- -- 1*541 -- 1*467 _ -- 1-310 -- -- -- <1 .. 4B <1 _ -- -- IB -- -- 487-52-7 d I/Io 487-52-7-1 d IIo 7-28 10+ 7*25 4*76 5 4 55 1 4*64 4*24 V62 7 3*62 _--_ 2-96 2 3-00 -- -- - 2*69 ------ 2*53 10 2*52 244 <1 2*45 _---- 2-37 9 2-36 2-15 <1 216 2-10 l M0 204 <1 -- - 1-854 " --_ 1-792 4 1-787 ---- !' 13 <\ -- 1614 -> 1 7H if/M 1610 1-577 _ -- 1 559 1*538 2B I 53? -- I-5I0 1-434 2 1-484 ---- _ 1369 l -- -- -- 1-314 IB 1315 --- --- - 10 3 4 10 _ j . 2 -- 4 <1 ) 2 <1 <1 <1 _ <1 _ <1 <: 2 -- 4 4 <1 <1 - _ - 3 - - V Mineralogy and origin of the Coalinga asbestos deposit 179 grains displaying the typical high relief and interfer ence colors of forsteritc (see Figs. 16, 17). In thin section olivine grains are in optical continuity and are undoubtedly relicts of larger, pre-existing olivine grains which were originally between 0-25 and 1-50 mm in size. The euhedral nature of the original crystals is apparent in Fig. 17. The optical properties of the olivine, as determined by H. W. Jaffe. are listed below: Indices of Refraction Mole Per Cent Fe,SiO+* a = 1-652 all 00005 ' 0 = 1-688 y = 1-685 Biaxial (+) 2V = 88'-' (Calculated refraction) 8-5 7-5 from indices of * Estimated from indices of refraction. - Using the optical curves of Poldevaart, as presented by Wahlstrom (1955) an empirical formula of (Mg1.82FeH.18) can be derived in which the olivine has an iron content of 8-8 weight per cent FeO. This composition is close to that of the dunitic olivines described by Hostetler el al. (1966) in their study of the formation of brurite during pervasive serpen- tinization of Alpine ultramafics. Mineralogical identifications of nine serpentinite hand specimens from the Coalinga deposit are listed in Table 5. X-ray data for the samples are listed in Table 6. Although the results of X-ray diffraction and electron microscopy are in good agreement for s'; formed from calcareous ground waters which per- most samples, the failure to recognize other poly -'f; meate-the mass. Chromite is usually present and is morphs in certain of the samples re-einphasizes the T commonly rimmed by either calcite or magnetite. It insufficiency of any one technique alone for unequivo ^ is much less abundant than magnetite and forms sub- cal identification of serpentine-group minerals. Differ '~t hedrai grains about 05- 2-0 Him in size. It is character- ential thermal analysis patterns are shown in Fig. tzed by a translucent, dark brown color at the edges 18. and generally confirm the presence of the major ,; of otherwise opaque grains. A chromite grain rimmed constituents of each sample. 'i by magnetite is shown in Fig. 15. Uvarovite garnet Chemical analyses of many of these specimens are ~ has been noted in many samples, usually associated listed in Table 7. The main differences are the wide with chromite or in veins cutting the serpentinite. variations in magnesia/silica ratios which correspond " Hostetler et al. (1966) reported trace amounts of to the presence or absence of brucite in the samples. .... awaruite (FeNi3) and heazelwoodite (N^Sj) in one These data also illustrate the low degree of confidence .. sample from this deposit by electron probe examina- which should be placed on discussions of serpentinite ; don. genesis which are based on chemical analyses of only Only two specimens examined in this study were one or two selected specimens from a large serpen found to contain relict olivine in 005-010 mm size tinite body. Tabic 7. Chemical analjses of serpentinite samples from the Coalinga asbestos deposit* 487-44-6 487-44-9 487-44-14 487-44-19 487-52-8 487-52-1 SiO, MgO FeO CaO CO, H>0 ai2o3 Cr2Oj( Total .'6-37 41-28 2-60 3-46 0-64 0-68 12-87 0-21 0-38 98-49 30-93 43-35 2-51 5-5}j2 o-is-!| 0-77 16-96 -- -- 100-20 40-26 38-16 2-57 5-08 0-17 0-18 10-22 1-45 0-32 98-41 35-69 36-74 2-77 6-26 3-26 0-18 11-97 0-34 1-71 98-92 40-67 38-87 2-35 4-10 0-23 007 1102 0-55 0-38 98-24 3645 40-67 2-14 5-78 0-48 0-60 13-15 024 -- 99-SI See Table 5 for locations and descriptions. 487-52-7 26-68 45-85 6-28 300 037 029 17-86 0-26 100-29 140 F. A. Mumpton and C S. Thompson Sample 487-52-8 was collected from an acre-size ore, and is most abundant in serpentinite fragments seipeatinste block at the top of Santa Rita Peak (see where it makes up between 10 and 25 per cent of Ftg. 4) and consists primarily of antigoritc, with a the rock. It is intimately associated with serpentine, small amount of magnetite arid andradite garnet. the coexistence of the two phases suggesting strongly Chrysolite, li/ardite and brucite are virtually absent that the original ultramafic rock was composed pri from this body which apparently has withstood the marily of olivine with very little pyroxene. Mumpton effects of weathering and stands topographically well and Thompson (1966) found that the brucite contains above the rest of the deposit. Chemical analyses of about 15 wt per cent iron and has an approximate two splits of antigoritc purified by heavy liquid and formula of (Mg^Fe^OH)!*. The iron content of magnetic techniques are listed in Table 8. The purified the brucite is a critical factor in the susceptibility product contains less than 05 per cent garnet, by of the ore to weathering, and exposure of brucite-rich visual estimate, and less than 01% magnetite, by per- ore to surface oxidation conditions invariably leads meair.cter measurements. An electron micrograph of this material is shown in Fig. 39. Relating these data to the composition diagrams of Page (1968) this sample falls at the edge of the antigorite field. It is relatively rich in silicon in the tetrahedral position and contains more octahedral cations than do most antigorites. An unusual form of chrvsotile was noted in sample to marked discoloration. Many large outcrops of massive serpentinite (e.g. Santa Rita Peak, the red dish outcrop south of the asbestos pit in Section 25. listed as samples 487-52-8 and 487-44-13. respect ively. in Table 4) are devoid of brucite suggesting that their "longevity" is due to the absence of this phase, which upon weathering would have led to a rapid disintegration. 487-44-6 by electron microscope techniques and is shown in Figs. 20 and 21. Concentric cylinders of varying diameters appear to be strung along a central MINERALOGY OF THE SERFAGE WEATHERING ZONE chrysotiie fiber. Electron diffraction patterns show it The mineralogy of the surface weathering zone of to be chrysotiie, or more precisely, orthochrysotile. the New Idria serpentinite lias been studied exten The thickness of the cylinders, from 700 to 3000A, sively by Mumpton. Jaffe and Thompson (1965) and is much larger than any values reported previously Mumpton and Thompson (1966) and need not be from chrysotiie. although similar materials have been discussed at length here. The reddish-brown weather observed in synthetic preparations. The origin of the ing zone extends to a depth of 20-30 ft and is present large-diameter chrysotiie in the natural sample is un over the entire deposit. Blocks and fragments of resi known. dual serpentinite have been transformed into soft, Brucite in the fresh asbestos ore brown, crumbly masses, while the surrounding friable asbestos ore has discolored only slightly. Surface Brucite plays an important role in the New Idria coatings of artinite and hydromagnesite are common, serpentinite, from the original emplacement of the the latter mineral also occuring as white, pea-size body to its final decomposition in the surface weath concentrations immediately above the water table. ering zone, and has been discussed in detail by Weathered serpentinite "boulders" are enriched in Mumpton and Thompson (1966) and Hostetler et ai. pyroaurite and coalingite [MgmFc-- (1966). It occurs in almost every type of material COj(OH)24.2HzO], a new mineral discovered in from the deposit averaging 7-8 per cent in typical the course of this investigation (Mumpton et al. Table ! Chemical analysis of antigorite from Santa Rita Peak 1965). Amorphous iron oxide is also abundant in highly weathered material, while brucite is virtually absent in the surface weathering zoue. Split A Split B Field observations and laboratory experiments have shown that in the weathering zone iron-rich SiO* AI.O, Fc.O* KeO MgO NiO (a O 11*0 rut iOOO'C Total 12 20 41-95 brucite either dissolves, leaving behind a brown, 107 105 0-40 0-46 amorphous residue or iron oxide, or transforms in W4 39-4K 3-33 39-44 situ into pyroaurite and/or coalingite by oxidation 022 0-3 \ and carbonation. Further weathering destroys these 033 12-68 0-36 12-39 phases, leaving behind more iron oxide. Dissolved 99-30 99-35 magnesium later precipitates as hydromagnesite Calculated formula by iccbnt'qje of Faust and Fahey (1961*1 from average throughout the deposit. composition. Weathered material has also been found at depths /Mgj ssFc,*iKc.J ;,,Ni0 0-Al,_ -w' sum = 5 96 iunt =4 0(1 well below the normal weathering zone; some drill holes have intercepted brown-colored, oxidized Spettio^opic Data o-o; >2 C? 0-1 -10` Cm o-oos ii-m Mn 60S 0 8 Pb 04X)? {1004-004 c-002 o-o: material as much as 200 ft beneath the surface. It is though that such materials oxidized as a result of surface conditions being brought beneath the sur face along faults or large fracture zones, etc. Fresh, massive serpentinite "boulders" were un covered at depths of 50-6011 in Union Carbide's Mineralogy and origin of the Coaiinga asbestos deposit 141 asbestos pit in Section 25 and found a few weeks Thus, `isocbemicar serpentinization is accompanied later to be coated with a thin layer of coalingite by a considerable increase in volume as follows: 18 and/or pyroaurite and brucite. It is likely that the 2Mg;SiO* f 3H,0 = Mg3Si,03(OHU + fcfg(OH), original iron-rich brucite in the ore dissolved in ground water and subsequently precipitated along fractures and joints, thus coating the serpenfinite Olivine Serpentine Brucite (2KIg,88cm3) (54g) (277g. 110cm3) (58g,23cm3) "boulders" at depth. When the "boulders" were These authors also argued that the ". . . nearly exposed to the atmosphere, the redeposited iron-rich universal occurrence (of Alpine ultraroafics and ser brucite oxidized and transformed into secondary pyr- pentinites) in and along structural discontinuities sug oaurite and coalingite, probably within a few days gests that much, or all, of the expansion is accomo or weeks of being exposed. Such a transformation dated during tectonism." It is not unlikely that the of iron-rich brucite to coalingite was observed to have volume increase attending serpentinization processes taken place in the laboratory within a few months actually assists in the diapiric ascent of the bodies. (see Murr.pton. Jaffc and Thompson, 1965). Coalingite The process may still be taking place today, account and pyroaurite have also been found as surface coat ing for the general occurrences of such bodies in II. ings or as fracture hliings in a small, highly sheared topographic highs. serpentinite at the southern end of Golden Gate It is likely that the original serpentinite at New Bridge, San Francisco, California; on ultraroafic rocks Idria formed in a manner similar to that described from the Thetford District, Quebec; and on surfaces above. The mineralogical composition of the serpen in a large serpentinized ultramafic near Radusa, tinite fragments suggests that the parent ultramafic Yugoslavia. Here also it is though that iron-rich bra- body was almost entirely olivine. The "serpentine" cite reprecipitated from ground waters onto fracture contains less iron, and the brucite, more iron, than surfaces and subsequently oxidized to coalingite or the parent olivine, a relationship brought out also pyroaurite. rather than that the secondary phases by Hostetler et al. (1966) for such bodies in general. were deposited directly from ground water solutions. According to Coleman (1957) the New Idria mass Artinite has been found in _the surface weathering was probably altered to serpentine far beneath the zone as white encrustations of acicular crystals and surface and injected "cold" into the overlying Francis as minute tufts of radiating needles on fracture and can rocks. During and after emplacement the body joints in the serpentinite. it is particularly abundant was extensively sheared and competent blocks of ser- in the Condor Pit area of Section 29. X-ray d values pentinite were broken and crushed against one 1 are essentially the same as reported by de Wolff another, much as a thick paste is smeared out and (1952). pulverized in a wet ball mill or in a mix-muller. The I Optical determinations by H. W. Jaffe are listed end product today is a highly friable, soft asbestos below; `` ore, containing variable amounts of uncrushed, resi Biaxial (-) x ft p y - 1-488 (all values +0001) = 1-535 = y = b -- elongation = 1-557 * 2V - 67" (Calculated y - v. = 0-069) UlSS GENESIS dual serpentinite rock. Several features of the Coaiinga asbestos deposit require additional discussion and explanation. Whereas most Alpine-type serpentinite consists mainly of lizarditc and/or antigorite with less than 10 per cent chrysotile in the form of slip- or cross fiber veinlcts. the New Idria body contains as much Classically, Alpine-type serpentinites are thougbt as 80 per cent chrysotile. none of which is in veins, to have formed by low temperature hydration of pre and only minor amounts of the plaly serpentine poly existing dunites or olivinc-rich peridotites. According morphs. Furthermore, lizarditc and/or antigorite to Bowen and Tuttle (1949) olivine reacts with water appear to be confined to the hard blocks and frag below 400"C to yield serpentine + brucite. Turner ments of massive serpentinite, with the soft, friable 1% and Verhoogcn (1960. p. 318) assumed, however, from part of the ore consisting almost entirely of chrysotile. petrographic evidence, tliat serpentinization takes Brucite is also much more abundant in the serpen place at constant volume, a concept which can only tinite rock portion of the ore (7-8 per cent) than be accounted for by the removal of copious amounts elsewhere. Thus, if the soft, friable asbestos agglomer of both magnesia and silica from the system. Hos ates are a "pulverization" product of the original ser tetler etui. (1966) found that brucite is a major consti pentinitc. (1) brucite must have been removed during tuent of many serpentinite bodies of the Circum-Paci- this process, and (2) lizardite and/or antigorite must fic qrogenic belt and stated that volume increases have been transformed into chrysotile. upljp 40 per cent are conceivable during the serpen The pervasive shearing which this body has experi tinization process. The general lack of field evidence enced no doubt permitted ground waters to move supporting the migration concept prompted them to freely through the rock. Much of the brucite produced conclude that pervasive serpentinization . . prob during the initial serpentinization probably dissolved ably results from incremental addition of water de in these waters and precipitated later as either hydro rived from country rocks during the tectonic ascent magnesite in the surface weathering zone, or as mas of the ultraroafic mass through the earth's crust." sive deposits of magnesite such as that found in the iM.wAvaa.w 142 F. A. Mumpton and C. S. Thompson nonhwest comer of the serpentinite. The absence of mmimmu-tit " *r "rp"- irrafehodk* has also been, noted by HosteJer et T^* transformation of lizardste/antigorite The only other asbestos deposit which has a physi cal appearance similar to that of the Coalinga deposit is the Stragari deposit in southern Serbia, Yugoslavia (Miller. 1952; GrimSidar and Ofiepek. 1959). The fe-efiiysdtbe is still not explained, nor has it been deposit was examined by Mumpton in the summers cirried out experimentally. Page (1968) suggested the of 1967 and 1970. Chrysotile asbestos is being mined the compositional fields and stability ranges of the at Stragari from a 100-200 m wide shear zone at three members of the serpentine minerals may be the contact of a large serpentinite aud an enclosing different from one another, with partial pressure of limestone. The ore is almost identical to the Coalinga oxygen playing an important role in which of the ore in that blocks and fragments of hard, dense, mas three is formed in a given chemical environment It sive serpentinite are coated and surrounded by large may well be that Eh conditions during the initial leathery sheets and friable agglomerates of chrysotile. serpentinization favored the formation of a platy ser Fiber produced from this deposit is also of the high pentine. while those prevailing later give rise to the surface-area variety. Although only a trace of cross crystallization of chrysotile. fiber asbestos has been noted in this deposit, the From studies of spring waters issuing from serpen- ore contains 50-60 per cent recoverable chrysotile tinites and ultramafic bodies of California, Barnes fiber, most of which is also of the subgrade-7 type. et al. (1967) and Barnes and O'Neil (1969) suggested The geological location of the ore zone and the physi that chrysotile can precipitate directly from ground cal and mineralogical characteristics of the ore sug water. This hypothesis is noteworthy in the light of gest an origin similar to that postulated above for samples collected from a shallow ore-pit in Section the Coalinga deposit 34, TI8S, R13E near the northeastern edge of the A newly discovered "slip-fiber" asbestos deposit serpentinite. Here soft, spongy masses of leathery near Cuicatlan. Oaxaca, Mexico, was recently exam chrysotile occur saturated with water and admixed ined by Mumpton and found to be not-at-all like with a in pellets of hydromagnesite. The hydromagne a typical slip-fiber ore. Instead, the ore consists of site is undoubtedly a late-stage precipitate from fibrous asbestos coating all available surfaces of a ground water and there is reason to believe that the highly sheared and fractured serpentinite. The fractur chrysotile is also. If chrysotile can crystallize directly ing and shearing appear to be randomly oriented from ground water solutions, it is not uniikeiy that and individual "veins" can not be traced more than most of the chrysotile of the Coalinga asbestos depo an inch or two. The amount of chrysotile in this sit formed in a similar manner--by the dissolution of ore approches 25 per cent although the recoverable Jizardite and antigorite from the original serpentinite fiber (Grades 4, 5 and 6) by conventional dry-lift and the reprecipitation of chrysotile on all surfaces of methods is only about 8-10 per cent Here also the the sheared and pulverized mass. principal polymorph of the massive serpentinite is Although they have not-been examined in as much antigorite, while that of the fibrous material is chryso detail as the Coalinga deposit, several other asbestos tile. It is not unlikely that the asbestos fiber in this bodies and shear zones within serpentinite masses ore formed by the dissolution of antigorite from the exhibit mincralogicai features which lend credence to serpentinite rock and reprccipitation of chrysotile the dissoluiion-reprecipitation mechanism of asbestos from ground waters which saturated the sheared and formation discussed above. Although iizardite is the pulverized body. It appears that we must begin to dominant polymorph in the massive serpentinite, the consider such a mechanism of formation for all such powdery, friable material in zones of intense shearing highly sheared and fractured asbestos deposits, and in a small ultramafic body south of Clear Lake, Cali perhaps for cross-fiber ores aiso. fornia, is invariably chrysotile by X-ray diffraction It is indeed fool-hardy to offer a hypothesis on and electron microscopic techniques. Similar chryso- the. origin of chrysotile asbestos, especially when tiie-ricli shear zones have been noted in serpentinites detailed mineralogical and geological studies have at Red Mountain, California; Gasquct Mountain, been made on only one unusual type of deposit, and California; Rattlesnake Mountain, California; when necessary experimental data are not available. Ardino, Bulgaria; and Kraubath. Austria. Coleman However, the very fact that the Coalinga deposit and Leith (1971, p. 316) noted a "... general increase appeared to be unique, prompted the authors to de of clinochrysotile in highly sheared areas . . ." of velop ideas about its origin which were different from the Burro Mountain, California, massif. The chryso those established in the literature for "normal" cross tile content of the sheared serpentinite zone surround fiber deposits but which were not inconsistant with ing the Belvidere Mountain, Vermont, ultramafic is the mineralogical, chemical and field data. This apparently greater than that of the more massive mechanism involves (1) formation of serpentinite from parts of the body (Cady et al.. 1963). An apparent an original ultramafic body, (2) intense shearing and increase in chrysotile content of shear zones sur pulverization of the serpentinite and concomitant rounding other Vermont serpentinites was noted by saturation with ground waters from the enclosing Chidester (1962), although it is unfortunate that sedimentary column, (3) dissolution of lizardite-anti- detailed characterization of the serpentine poly gorite and brucitc from the hard, dense, massive ser- morphs was not undertaken in that study. pentinite during or after tectonic pulverization and Mineralogy and origin of the Coalinga asbestos deposit 143 . (4) subsequent precipitation of chrysotile from these solutions on all available surfaces in the sheared inass. The authors suggest that such a mechanism may also be responsible for the abundance of chrysotile in , shear zones of numerous serpentinite bodies through out the world, in the Stragari asbestos ore in Yugosla- V via, and perhaps in the intensely sheared serpentine zones of the several other deposits mentioned above. . Acknowledgments - In an industrial laboratory, no investiZ, gation is carried out by one or two individuals alone, but is the joint effort of many different people. The help x) and assistance of the following co-workers is therefore , gratefully acknowledged: W. B. Brown, R. W. Brown. W. ( T. Cohan, M. Genes. H. W. Jaffe, A. W. Naumann, J. S' L. Perry. P. E. Pezzella. K. 1.. Schmidt and W. K. Zwicker, J;' Special thanks are also due to R. G. Coleman. I. Barnes \ and G. T. Faust of the U.S. Geological Survey and H. W. Jaffe of the University of Massachusetts for numerous . discussions and advice during this study. 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