Document Rj9ZGzxorMqeQYy2Z4M4bL28X

206 FRANK. A. PATTY mixture contains, or is suspected of containing, feldspars (silicates) or- otlf minerals readily confused with quartz, it is necessary to purify it further. complish this, about 10 ml. of hydrofluosilicic or fluoboric acid is poured pf!| sample in a small platinum crucible and it is set aside for 3 to 6 days, being at least twice each day with a platinum loop'. At the end of the 3 to 6 days)ft pending upon the gross appearance of the residue, the remaining dust is -fil^ off, washed, dried, ignited, and weighed. It is again inspected under the petr graphic microscope in order to determine the nature and purity of the res0 which should now be quartz or possibly other forms of free silica, such as chalcv ony, tripoli, cristobalite, and tridymite, which dissolve in the above acids slopff or one of a few other minerals such as silicon carbide, which is not removed! these combined acid treatments but is readily distinguishable. The petrogralni identification of minerals is far beyond the scope of this chapter- but a sim* procedure for the identification of the quartz content of dusts, utilizing pel graphic technique, may be stated as follows. Place a minute amount (few particles) of the dust residue on a microscope slide, ai drop of immersion oil of refractive index 1.545, and cover with a cover slip. Observe wi petrographic microscope using crossed Nicols under about 450x magnification. If quart)? present the small particles will appear as milky white irregular shapes. To determine whe*' an individual particle is quartz, use the crossed Nicols, rotate the stage, and observ'd!" quartz particles (except when oriented with optic axis vertical) disappear four times pefif plete revolution of the stage. This characteristic establishes that quartz particles are anisj^L as distinguished from isotropic crystals, which are invisible when viewed with crossed,Pi' Bring the Btage to rest with a particle extinguished, then Blide the analyzer Nion' -:V of the system, focus on the edges of the crystal, and raise the microscope tube from focu light halo surrounding the particle will move into or away from the crystal dependihjll' its orientation, because this halo (Becke line) always moves toward the medium ofiiirefractive index when the microscope tube is raised from focus. Now turn the stage e'p 90 degrees in either direction and repeat this procedure. If the particle is quartz the.iu^ light will-move in the opposite direction. This is true because quartz has two indices of n tion, 1.544 and 1.553, both close to, and one on each side of, 1.545, the refractive index"' immersion oil. The refractive index of a submerged crystal may be judged to be very? that of the immersion oil if there is a lack of clear distinction of boundary lines; tjusijL when the difference in refractive indices is less than 0.01. When the difference is less thahj| a submerged crystal can be seen only with difficulty. If the particle under examinatio' these characteristics, that is, irregular shaped, milky white between crossed Nicols, two 3 tive indices one just above and one barely below 1.545, it may be suspected of bein#|f! -TriS`^(rsd pthcfi5e"f5'.'cl5eclFa sample;Bow"wf{EroirdrnnT5? to maK"certain that'thjsij have' both ihdices less- than! d'.57.. The identity of thb crystals may be verified by further scrutiny. Center a particle lira under the cross hairs and align the microscope system so that the particle does not mo'fijt position during a complete revolution of the stage. Now, by means of a Wright's diaphfi * 0r!ess-s4:tisfSdtdrilywitlr-th`d"Bdftrffad'l'en*s;'if the microscope is so equipped, and no^W, diaphragm is available--isolate this crystal and observe its inference figure." This test?! be applied successfully to particles appreciably smaller than 10 y in diameter. It is donfe crossed Nicols and upper lens condenser (converger) swung into place. Upon rotation^ 01E. E. Wahlstrom, Optical Crystallography. Wiley, New York, 1943. SAMPLING AND ANALYSIS OF CONTAMINANTS 207 .hon a light field, or portions of the arms of a cross, can be seen (interference ^crystal). If only portions of the arms making up the cross are visible, -the |me the complete figure in order to choose the section he wishes to investi- rV i", S.?as "l#- rarely fountl oriented so that the cross is in the center of the field, as in 1 ^infrequently, with the axis of the cross visible at or near the circumference ijVrbut- usually, as in 3, where only the arms of the cross, appearing as straight v-Hhgi i'" ir^yed bars extending perpendicular to the observer, move straight across the (right, or vice versa, or extending horizontal, move from top to bottom, or ** 1 k rystal may now be further classified as positive or negative by the use of a USX MgSA - He igure 10. Interference figures of uniaxial crystals (quartz). faica plate, or selenite plate. First choose the lower right quadrant, if only the Srarefyisible, by rotating the stage until a horizontal bar has passed across the field ^^{lieMiserv.er and a perpendicular bar is just out of view tq the left., b(pw, as a quartz sffinsSffdfSqiits slot as a bisector' of thi/field, color cmiVes mil move: butwqrd if the ^^q^o^^Msitive). With a mica platej.'sb'Inseiied, a black dot wiU appear in this and while with a seiehiS. plate these'same areas (3 to: 6, aiidjl1 tq 12 Ssf^^clfvfi^iwill have a yellow tinge and the other 2 quadrants wUFapp&f. blue. With results are obtained1.' The particle that has met:^these': requirements ^b|jm!'eiyridentified-as~quartZ'because-it-haS'been-'found"to-be-anisbtrop_ic,-uriiaxial, Itflicesbf 'refraction of barely iess than 1.545 (1.644) and between .1.545 and 1.57 ^Jf^pcrystalline material fits alloftHese characteristics. ' IS^^ffelqn'first observation is found to contain a large* percentage of particles other and. other silicates), it i? then nebessar/tqstreat witb^H^drdfluqsilicic g-Vcntni-int the residue is1 again examined and' if found/(olbe quartzA'-dorrection ^fenicf-'nuqeriH'ay should be added to the quartz residue tO'Compfensate' fqi'iB',siiglit solu- ^^^bflu^licic acid. .;. ( ^^fq^|fdf;:ffee silica as well'as all other mineraUimay)b^siWilarlyjf&ntmed';{byrthe fficj^^Sqicbpe. Anyone' interested in this method ''df:^ud|^h`t'ificati6nbsliouId',';dbtaiii o|?oa;qffimineral he wishes to be able to identify/pulvenze^s^&'of'each''id^dia'biond nd|prac'tice with known pure materials until all of these!',characteristics are"readily '^wit^&e'aid of samples and a table for the determination of minerals," along with e^^tB'Afield-may-be-broadened..to inSludArhanymineifilsiL... ....:. ` .... yji^S^mples ordinarily do not have a sufficient number of particles 10 y netefSrlareer upon which an interference figure can be obtained, and KnJd.'sjNatural Science Establishment Inc., Rochester, N. Y. SlgM=gn-d-'HyBerman. V.-S.Geol. Survey BulL Tto.-MS (1934),' ^^Snort, V. S. Oeol. Survey Bull. No. 914, 1940. A. N. Winchel), Elements of Wiley, New York, 1931. A. N. Winched, Artificial Inorganic Solid SubMinerals. Wiley, New York, 1931. E. S. Dana, A Textbook oj Mineralogy. " 1932. E. M. Chamot and C. W. Mason, Handbook of Chemical Microscopy. York, 1938. >