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ANTIGORITE from the VICINITY of CARACAS, VENEZUELA* H. H. Hbss,R.J.Surrn, Princeton(Jni.Uers,Ity, Princeton, N

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ANTIGORITE from the VICINITY of CARACAS, VENEZUELA* H. H. Hbss,R.J.Surrn, Princeton(Jni.Uers,Ity, Princeton, N ANTIGORITE FROM THE VICINITY OF CARACAS, VENEZUELA* H. H. HBss,R.J.Surrn, princeton(Jni.uers,ity, princeton, N. J., AND G. Dnxco, Direcc|on d,eMi,neria y Geologia,Caracas, Venezuela. Ansrnncr fNrnooucrrox CnBlrrcar Couposrrrom Though much has been written on antigorite there are no superior chemical analysesof the mineral availabre in the literature so far as the writers are aware. An analysis by peck is given in Table 1. The materiar analyzed is a rock consisting armost entirely of antigorite, the onry impurity noted being a fraction of a per cent of unidentified dusty in- clusions of fibrous character and having much lower indices of refraction than antigorite. An attempt to recalculate the analysis into the usual serpentine for_ mula, MgoSirOro(OH)sshows a poor fit (columns 7 and g, Table 1). rt is evident that the Mg:Si ratio is closeto 7:5. A number of trial cal- culations to determine the probable relationship of (OH) and HzO in the mineral assumingMg: Si: 7:5 resulted in arriving at columns5 and 6 of Table 1 as the most likely solution. The formula deduced indicates 21 O(OH) of which 8 are (OH) and leaves a remainder of HzO. What in_ formation is available in the literature indicates a rather wide range of variation in Hzo content-from slightly less than 12 weight per cent to somewhat over 14 per cent. This suggeststhat the number of molecules * Princeton Investigations of Rock Forming Minerals, number 6. 68 ANTIGORITE FROM CARACAS, VENEZAELA 69 Tasrn 1. CnBurcer-CouposrrroN Cations Cations Ion Theo- Theo- to 2l to 18 retical retical Ratios o(oH) o(oH) sio, 43.60 Jt_ 497 +t6 AI,OA AI+3 20 Li CrzOr .02 Cr+3 FerOe .90 Fe*3 11 FeO .81 Fe+'z l1 MnO .04 Mn*2 Mgo 41.00 Mg{ 1017 Nio .16 Ni+2 2 CaO .05 La- I NarO .01 Na*r K,O .03 K+l 1 H,O+ 12.18 Ti+4 HrO- .08 H 1201. 8.05 8 TiOr .01 o 3131 99.92 HzO 1to3 Analyst Lee C. Peck, Rock Analysis Laboratory, University of Minnesota. Density 24" C/4' C :2.607. Approximate general Jormula: MgzSiron(OH)e 'zH,O, Ni+z, Fei2, Mn+2, etc', may substitute for Mg+ in part, Al+3Al+3, Fe+3AI+8,Cr+3AI+3, etc., may substitute for MgflSi+a to a limited extetrt. of H2O in the formula (representedby "n" in Table 1) might vary from n:! to n:3.In the present analysis la is near the lower valte, n:l'4' X-nev DnlnacrroN Darn The powdered antigorite was examined with a low powered Philips f-ray spectrometer.Three recordswere made using an iron Larget with a manganesefilter and three with a copper target and nickel filter' The averagedvalues for three runs on eachsample are given inTable 2,columns 2 and 3. In the conversionof 2 0 to d spacingsthe tables made by Switzer, Axelrod, Lindberg and Larsen (1948)were used.In theseK" Cu is 1.5418 A. and K" Fe is 1.9373 A. ttte estimated relative intensities are given in columns adjacent to the d spacings of columns 2 and 3 but are not directly comparable to the intensities of column 1. Dr. H. S. Yoder of the Carnegie GeophysicalLaboratory very kindly offered to run the same powder on a Philips f-Iay spectrometer having greater accuracy with respect to 2 0 and higher resolving power. His results are siven in Table 2 column 1. The following pertinent data with 70 H. H. EESS, R. T. SMITH, AND G. DENGO Tl^r;l:n2, X-nly Drrlnll.crrow pnrrenr CopperTarget Iron Target Copper Target (Yoder) (aver. 3) (aver. 3) dur A dorrA du.rA 8.05 10 7.97 1 7.30 <400 7.23 8 7.34+ 8 6.95 nn 6.51 lo 6.10 6 s.78 I 4.67 6 4.62 a 4.62 1 4.& 1 4.27 4 4.27 1 4.01 6 3.99 3.63 <300 3.612 10 3.625 10 3.51 24 2.88 n 171) I 2.686? 1 2.685 4 2.59 i 2.s7 8 2.554 2 2.52 70 2.522 5 2.53r 6 2.46 9 2.M3 I 2.462 2.42 38 2.4r9 6 2.429 1 a 2.39 9 2.392 2 2.399 2.35 5 2.286 ,t I 2.237 6 2.239 2 2.243 : 2.208 7 2.199 2.2@ 1 2.167 22 2.163 6 2.167 3 2.r50 20 2.126 A 2.1r9 I I 2.035 4 2.O32 6 1.886 J 1.884 I 1.830 12 1.833 1 I .833 ,I 1.815 ZJ 1.813 4 1.820 1.781 t4 1.779 a r.784 1 1.7s5 r.736 10 ., I r.733 1.738 4 1.688 a t.M0 2 1.584 3 1.560 T2 1.559 I I .559 I I .540 9 x:7.28 comparedto Muscovitestandard. ANTIGOMTE FROM CARACAS. VENEZUELA Tmr,r. 2-(conli,nued.) Copper Target Iron Target CopperTarget (Yoder) (aver. 3) d*r A d*r A dur.rA I ..)J.) 9 1.532 z 1 r.524 13 1.520 6 1.509 8 t.497 10 r.497 6 1.496 I I I t.479 7 L-+l I 2 1.480 2 r .4too 6 1.462 6 r.460 I 1.451 10 2 r.452 t.M8 9 t.M3 5 1.433 1.339 2 1.314 I r.279 2 t.261 I t.2w I regard to the measurements were supplied by Yoder: Dry powder, -150 +250 mesh *-rayed in shallow cell mount using Philips Geiger counter spectrometer.Copper radiation, nickel filter 25 Kv, 20 Ma, scan speed+o per minute, time constant 4 seconds,angular aperture 1", receiving slit 0.006 inch, goni- ometerradius 170 mm. Calibrated with silicon usins: Plane "24 (Cu K) rlrp :25.654 220ar :47 .302 3114r :56.r22 4004r :69. 130 331at : /o-J/o 422ar :88 .030 Error is approximately 0.01" 20 over region investigated, intensities subject to error as probable result of preferred orientations in sample. Scale intensity arbitrary. Spacings based on Cu Ko:1.5416 A in range 10o-40o20 and on Cu K"r:1.54650 lor 40'-70' 20. Tnnnuer ANer,ysrs Dare Thermal curves of the antigorite from room temperature to 1000' C. are given in Fig. 1. They show a strong endothermalpeak at782"+5" C. 72 E. H. HESS. R. J. SMITE. AND G. DENGO and a broad shallow endothermal peak starting a little above 100o C. The exothermal peak at 810oto 830oC. commonly reported for antigorite is absent. The broad shallow low temperature endothermal effect prob- ably representsthe lossof HzO while the larger peak at 782oC. represents dissociation of the antigorite structure probably with loss of its (OH). o o o Frc. 1. Thermal curves for antigorite DF180. Scale along top of diagram is in degrees C. Prrvsrcer, Pnopp'nrtps The antigorite rock is pale green in color, massive rather than foliated and has a dull luster rather than the resinousor the waxy luster common for serpentines.It is comparatively hard, approximately 3$ on Mohs scale. The density determined on seven 20 milligram fragments with a Ber- man balance using toluene averaged 2.603, 23" C/4" C. A seven gram cube weighed in air and toluene gave a density of 2.609, 24" C/4o C. Examination in thin section and in powders shows a well developed {001} cleavage.The presenceof a {100} parting is indicated in the p<.rw- ders but is rarely seenin the thin sections. The antigorite occurs in bladed aggregates.The crystals reach as much as a millimeter in length and are generally elongated parallel to o. There is considerablevariation in grain size, some portions of the rock being exceedingly fine-grained aggregates. Oprrcer- Pnoppnrrrs The indices of refraction were determined by immersion methods using a temperature control cell on the microscope stage and sodium light. Taking advantage of the {100} parting and {001} cleavagethe three principal indices of refraction were determined as follows with an accuracy of *0.0005: ANTIGORITE F ROII CARACAS, VENEZUELA 73 N":1.5670 lfu: L 5660 1r,: 1 5615 Ir,-rr,:0.0055 The optic angle was measured on the universal stage and found to be 47++l/2" with dispersionr)u moderate and sign negative. Z is in the plane of the {001} cleavageand X is perpendicular to ihis cleavage so far as it was possible to determine. A very slight departure from parallel or perpendicular might be overlooked inasmuch as the cleavagetraces are not quite straight as a rule and many of the antigorite blades have been slightly bent. The same difficulty arises in trying to measure the crystallographic angle B which is close to 90'. Y is parallel to the D crystallographic axis, assuming the cleavage represents {001} and the parting {100}. Under crossed nicols the antigorite shows anomalous interference colors. This is one of the most distinctive features of most antigorite as compared to chrysotile. Grains oriented so that they show moderate to low path difierence-in the grays-have a distinctly bluish tint' Pprnorocrclr. Dere aNo CoNcrusroNs Antigorite has not been found in laboratory investigations of the MgO-SiOrHzO system, hence its field of stability is not known (Bowen and Tuttle. 1949).
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