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CALORIMETRIC STUDY OF NATURAL POLLUCITE
Ogorodova L.P., Melchakova L.V., Kiseleva I.A. (Lomonosov Moscow State University, Department of Geology, Moscow, Russia)
logor@geol.msu.ru; (095) 939-13-49 Key-words: thermochemistry; pollucite; heat capacity; enthalpy of formation

Pollucite is found in rare-element-rich granitic pegmatites having lithium and niobium-tantalum mineralization. It is an aluminosilicate phase with a cubic lattice representing the Cs end-member of the analcime NaAlSi2O6.H2O - leucite KAlSi2O6 - pollucite CsAlSi2O6 series. The knowledge of the thermodynamic properties of pollucite, analcime, leucite and their solid solutions is of geological and technological interest. We have determined previously the thermodynamic properties of analcime [1] and leucite [2]. This work is concerned with an experimental investigation of the thermodynamic properties of pollucite. Experimental data on the heat capacities, enthalpies of formation from the oxides and elements of the natural pollucites: Cs0.77Na0.14Rb0.04Al0.91Si2.08O6·0.34H2O (Minas Gerais, Brazil) (I) and Cs0.84Na0.11Al0.88Si2.10O6·0.17H2O (East Siberia, Russia) (II) have been determined. The heat capacities of the pollucite (II) were measured by DSC method from T=250 K to 810 K. A value of C o m (298.15 K)=172.2 J K-1 mol-1 was obtained and the resultant C o m (T) equation in the interval p, p, T=298.15-610 K was calculated: C
o p, m

= 131.37+181.97·10-3T­11.84·105T-2, J K-1 mol-1 (±0.36%). The

standard enthalpies of formation were determined by a set of thermochemical cycles using decomposition and subsequent dissolution of the dehydration products in molten lead borate at T=973 K. Values of fH o (298.15 K) are found to be -3104±13 (pollucite I) and -3090±14 (pollucite II) kJ m mol-1. The standard entropies for pollucites were estimated from adiabatic calorimetric data [3] for natural pollucite, taking into account their composition variations. Using these values we calculated the molar Gibbs free energies of formation fG o (298.15 K) to be equal -2921 (I) and -2911(II) kJ m mol-1. References: 1. L.P. Ogorodova, I.A. Kiseleva, L.V. Melchackova, I.A. Belytsky, B.A. Fursenko. Geokhimiya. 1996. 11. P.1088-1093 (in Russian). 2. I.A. Kiseleva, L.P. Ogorodova, L.V. Melchakova, I.A. Belitsky, B.A. Fursenko. Vestn. Mosk. Un-ta. Geol. 1996. 1. P. 48-53 (in Russian). 3. K.O. Bennington, R.P. Beyer, G.K. Johnson. U.S. Bur.of Mines Rept. of Inv. 1983. N 8779. P. 1-18.

Electronic Scientific Information Journal "Herald of the Department of Earth Sciences RAS" 1(21)2003 Informational Bulletin of the Annual Seminar of Experimental Mineralogy, Petrology and Geochemistry ­ 2003 URL: http://www.scgis.ru/russian/cp1251/h_dgggms/1-2003/informbul-1_2003/mineral-8e.pdf Published on July 15, 2003 © Department of the Earth Sciences RAS, 1997-2003 All rights reserved