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The Thermodynamic Properties of Fluor'topaz

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The Thermodynamic Properties of Fluor'topaz American Mineralogist, Volume 67, pages 350-355, 1982 The thermodynamicproperties of fluor'topaz M. D. BentoNr Department of the GeophysicalSciences University of Chicago, Chicago, Illinois 60637 H. T. HnseLToN.Jn., B. S' HelarNcwav U. S. Geological SurveY MS 959, Reston, Virginia 22092 O. J. Kleppe, Department of the GeophysicalSciences University of Chicago, Chicago, Illinois 60637 eNo R. A. RouB U. S. Geological SurveY MS 959, Reston, Virginia 22092 Abstract The standardthermodynamic properties of fluor-topaz,AlzSiOnFz, have been calculated from low- and high-temperatureheat-capacity measurements and from high-temperature, oxide-meltcalorimetry. Fluor-topaz (from TopazMountain, Thomas Range, Utah) con- taining 0.04 wt. percentwater was used in all the experiments.Adiabatic calorimetry performedfrom 10.6to 379.2K givesSi9E - Si of 105.4* 0.2 J/mol'K. Combinedheat capacitiesdetermined by adiabaticcalorimetry (200-380 K) and differentialscanning calorimetry(340-800 K) werefit to the followingpolynomial (equation valid 200-1,000K): c; (ymol.K)= 471.41-0.08165r + l.2695xlO6T-2- 5485.510'5eO.1%). The enthalpyof the reactionCaF2 + Al2O3+ SiO2: CaO + Al2SiO4F2was measured at 970K by oxide-meltcalorimetry and gave L'rtels= 9l'E8-f3'56 kJ' Fromour heat-capacity measurementsand ancillary data, we calculateLrt2es = 96.12!3.95kJ for the reactionand LI4.2e8:-3084.45t4.70 kJ/mol, and AGi,zqe : -2910.66-+4.74kJlmolfor fluor-topaz. Introduction synthesized topaz containing more than 50% hy- Topaz occurs frequently as an accessorymineral droxyl-topaz component. (1972, 1978)reported synthesisresults in fluorine-rich granitic rocks and associatedhydro- Rosenberg solid solutions; his is the only work thermally altered rocks. As one of the principal involving topaz thermodynamic data might be derived. fluorine bearing minerals, it offers a key to under- from which was undertaken to measurethe thermo- standing the genesisof these rocks. Topaz is a solid This study properties of fluor-topaz as part of a proj- solution between fluor-topaz (Al2SiO4F and (hy- dynamic of topaz and other pothetical) hydroxyl-topaz (Al2SiOr(OH)z).Natural ect on the thermochemistry and their petrological application. topazes vary from nearly pure fluor-topaz to about fluoro-silicates (197 Al2SiO4F1.4(OH)0.0, althougtr Rosenberg 2) has Experimental methods and results Starting materials rPresent address: Geophysical Laboratory, Carnegie Institu- tion of Washington, 2801Upton Street, NW, Washington,D. C. Natural fluor-topaz from Topaz Mountain, 2flnE. Thomas Range, Juab County, Utah, was collected 0003-(M)vE2l0304-0350$02.00 350 BARTON ET AL,: FLUOR.TOPAZ 351 from the Pliocene topaz-rich alkali rhyolites (Lind- Adiabatic calorimetry sey, 1979).This topaz is known to be very closeto Heat-capacity measurements the fluorine end member (Penfieldand Minor, lg94; were made from 10.6to 379.2K by adiabatic Ribbe and Rosenberg,l97I) and occurs as crystals calorimetryat the U.S. Geological Survey, as large as 3 cm in the rhyolite lithophysae. Associ- Reston, Virginia. The cryostat and calorimeter ated minerals include quartz, sanidine, plagioclase, have been described in detail by biotite, Robie and Hemingway (1972) and Robie er a/. fluorite, beryl, hematite, pseudobrookite, (re76). spessartine,and bixbyite (Ream, 1979).About 60 g The topaz sample (50.5654g, of transparent, inclusion-free crystals and cleavage corrected for buoy- ancy) was loaded into fragments were handpicked from a few hundred the calorimeter. After evacu- ation of air from the calorimeter, grams of rough material. Approximately one-third it was backfilled with dry_helium gas at pascals of the crystals were light brown; this color is 6x103 pressure (4.0x 10-5 mole of He) to promote apparently caused by electronic defect cenrers thermal equilibra- tion, and sealed.The reported (Dickinson and Moore, 1967),not chemical impuri- temperaturesrefer to the International Practical ties. As previously demonstrated(e.g., Nassauand Temperature Scale of 1968(rPTS-68). Prescott, 1975),the color is removedby heatingto Table2 gives the results, 500"C for several hours. We did not heat treat the corrected for curvature, for the four series (1-4) material used for calorimetry. We did not attempt to of experiments in their order of collection. synthesize fluor-topaz by the reaction 2AlF, + The formula weight for pure fluor-topaz of 184.043g/mol used 2AlzO3 + 3SiO2: 3Al2SiOaF2nor did we attempt to in the calculations is based on the 1975values for use this reaction for the solution calorimetry be- the atomic weights (Commission on Atomic cause of the tendency of AlF3 to hydrolyze which Weights, 1976).Althoueh the sample topaz contains weight percent could result in considerable uncertainty as to the 0.04 OH, no correction compositions of the phasesunder study. was made to the measured heat capacities because Several crystals were analyzed on an automated the heat capacity of hydroxyl- topaz is not known. From electron microprobe at the University of Chicago comparisonsof the heat capacities of hydroxyl-apatite using a ZAF correction program written by L M. and fluor-apatite, and those of hydroxyl-phlogopite Steele of the University of Chicago. The beam and fluorphlogopite, we estimate that the difference current was 15 ma at an acceleratingvoltage of 15 in the heat capacity between fluor- kv. Andalusite served as the standardfor aluminum and hydroxyl-topaz would not ex- ceed approximately percent. and silicon. The water and fluroine were analvzed 7 The diference in SigrSo between hydroiyl-apatite at the U. S. Geological Survey with a perkin Eimer and fluor-apatite is 0.6 percent, and 2408 elemental analyzer (V2O5flux) and a specific between hydroxyl-phlogopite and fluor-phlogopite is -0.5 percent. ion electrode respectively. The water and fluorine On the basis analyses on this material are in good agreement penfield with the original analyses by and Minor Table l. Chemical and crystallographicdata (1894). Although we searchedfor other elements. none were detected. Crystallographic data were Chflistry" Cel I Paraneters collected using powder-diffraction methods (Ni-fiI- oxide wt g a = 0.4647s(3)nn tered CuKa radiation with a corundum internal !Sl-9!* Al 203 56.08 1.00 b - 0.87897(4)nn standarda : 0.47593(1)nm, c : 1.29917(5)nm).The Si02 32.74 0.99 c = 0.83920(4)n0 program of Burnham (1962)was used to refine the F 20.3 1.95 v = 0.34z8l(2)nn3 data. The chemical and crystallographic data are Hzo o.o4 o.orr given in Table 1 Other materials used in the high-temperature Total I 09. l6 oxide-melt calorimetry were (prepared CaO from Less F=0 roo.6i reagent CaCO3, sintered at 1,400.C for I week); optically pure natural CaF2 (southern Illinois, Uni- Al and Si analyses by microprobe, F by speclflc ion electrode, and versity of Chicago collection #1875); optically pure H20 by CHNelemental analJzer. natural quartz (locality unknown, University of Based on 5 oxygens. Chicago collection #2W9); and a-Al2O3 (prepared f As hydroxyl. from reagent AI(OH)3, fired at 1,300"Cfor 2 days). 152 BARTON ET AL.: FLUOR-TOPAZ of the above observations, we estimate that the Table 2. Experimental heat capacitiesfor fluor-topaz. Formula 184'M3 g/mol. correction to the measuredentropy of our sample, weight caused by the replacement of 10 percent (10 times Eee t what is actually present) of the fluorine by hydrox- TeuP' rcop. reDP. capeclty "";::i., ".li31., yl, would be less than 0.1 percent, which is some- R J/ (rol'K ) K J/ (ool'K ) K J/( ool'K) what less than our experimentaluncertainty. Figure 1 shows the measuredheat capacities. serles I Serl.eo 3 Serles 5 305.57 145.3 243.ar r2r-7 470.7 r87.3 Dffi rential scanningcalorimetry 3to,27 r48.0 249.36 124-O 480.7 r88.9 3 15 .93 l|s .7 254.E9 125.4 190.l r90 . 5 Heat capacity measurementswere madefrom 340 !2L.4a 151.4 260.41 r29.r 500.6 192.2 121.3r 153.5 255.9r r31.3 5r0.6 193.3 to 800 K with a differential scanningcalorimeter at 333.r 2 I 55.3 271 '40 133.4 520.6 194.5 338.92 r56.7 276.87 135.7 530.5 196.1 the U. S. Geological Survey. A cleavage flake of 344.72 158.3 282.34 137.8 540.6 196'1 350.49 160.2 2A7.79 139.7 550.6 198.0 topaz weighing 25 mg placed in an unsealed gold 356.26 162.0 293.23 142.O 560.5 r99 .2 pan and reduced 352.O2 163.0 298.65 144.0 570.5 200.8 was used. The data were collected 367.f7 164.4 304'(i7 145.8 580.5 200'9 using a computer program written by K. M. Krupka 373.50 156.3 309.47 147.6 590.5 20r.8 37g.21 167.7 314.87 L49.4 500.5 202.9 of Batelle Northwest Laboratories, which utilizes 320.25 I 50.9 serles 2 Serlca 7 scans made with a corundum disk over the same Serles I 54,20 6.547 770.2 213.3 temperature intervals as the unknown. The heat 50.48 E.687 lo .56 0.0189 780.2 213.1 given Ditmars and Douglas (1971)for 65.10 11.44 LL.77 o.026l 790.2 215.2 capacities by 70.54 14.92 t2.99 o.0356 800.2 215.6 corundum were used in the reduction. Additional 76.80 18.53 14.33 0.0538 82.86 22.23 r5.85 0.0768 s€rlea I details of the experimental method are given by 89.O2 26.11 17.s4 o.ll4r 95.18 30.r9 t9.42 0. t59r 470.7 87.1 Krupka et al. (1979).Because of dfficulties with lol.29 34.31 2L.52 o.2400 480.7 88.5 not be ob- 107.36 38.49 23.87 o.34rr 490.7 89.8 instrument stability, reliable data could 113.40 12,63 26.49 0.4993 500.7 91.1 tained at temperaturesgreater than 800K.
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