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Title:
Thermodynamic Mixing Properties of Rb-K-Na Feldspars and Relevance to Rb-, NH4-, K-, Na-, and Li-Feldspar Thermal Expansion
Authors:
Hovis, G. L.; Morabito, J.; Mott, A.
Affiliation:
AA(Department of Geology and Environmental Geosciences, Lafayette College, Van Wickle Hall, Easton, PA 18042 United States ; ), AB(Department of Geology and Environmental Geosciences, Lafayette College, Van Wickle Hall, Easton, PA 18042 United States ; ), AC(Department of Geology and Environmental Geosciences, Lafayette College, Van Wickle Hall, Easton, PA 18042 United States ; )
Publication:
American Geophysical Union, Fall Meeting 2006, abstract #MR43B-1073
Publication Date:
12/2006
Origin:
AGU
AGU Keywords:
3611 Thermodynamics (0766, 1011, 8411), 3620 Mineral and crystal chemistry (1042), 3949 Thermal expansivity
Abstract Copyright:
(c) 2006: American Geophysical Union
Bibliographic Code:
2006AGUFMMR43B1073H

Abstract

We have investigated a ten-member Rb-K feldspar solid solution series having an ordered Al-Si distribution. Rb-microcline, or rubicline, was synthesized via repeated ion exchange experiments of microcline in molten RbCl. This resulted in Rb-feldspar containing ~92 mol percent Rb, which appears to be the maximum amount of Rb that ordered feldspars can take up using ion-exchange techniques at 1 kbar. [Similarly synthesized Al-Si disordered Rb-feldspar is even more limited in Rb content (Kovalskii and Kotelnikov, EMPG IX, 2002).] Compositions between 0 and 92 mol percent Rb were synthesized by combining rubicline and microcline in the desired molar proportions, then chemically homogenizing the samples at elevated temperature. Enthalpies of solution (20.1 wt percent HF, 50 °C, isoperibolic conditions) for the series are nearly linear with composition, with the possibility of low-magnitude positive enthalpies of Rb-K mixing at Rb-rich compositions. This behavior contrasts sharply with the considerably greater enthalpies of mixing in K-Na feldspars (Hovis, 1986, Journal of Petrology). Feldspars, therefore, which have but a single alkali site, exhibit larger enthalpies of mixing when there is a clear contrast in the sizes of the ions occupying that site. This is not surprising in light of the tendency of chemically homogeneous K-Na feldspars to undergo exsolution with cooling. In the case of the Rb end member, the feldspar structure appears to be stretched nearly to its limit. This is reflected by the coefficients of thermal expansion for Rb, NH4, K, Na and Li feldspar end members (all of which we have measured) that show a linear relationship with room-temperature unit-cell volume: The larger the unit cell at room temperature, the less potential there is for thermal expansion. [Similar relations are found as a function of K:Na in nepheline - kalsilite framework silicates (Hovis and others, Mineralogical Magazine, 2003).] Thus, even at room temperature the Rb-feldspar structure is severely stretched. The failure for K-sanidine to take up high amounts of Rb probably is related to this. This may account also for that fact that the volume of disordered Rb feldspar (synthesized via hydrothermal crystallization using the gel technique of Hamilton and Henderson, Mineralogical Magazine, 1968) is actually less than that of rubicline, even though disordering normally increases volume and despite the fact that the enthalpy of Al-Si disorder in Rb feldspar is what one would expect by comparison with those for K- and Na- feldspar. This also accounts for the stability of pollucite plus silica over Cs feldspar in laboratory ion-exchange experiments. We sincerely thank the Earth Sciences Department of Cambridge University, UK, where our high-T X-ray measurements on feldspars and feldspathoids were conducted. We are grateful to Andrey Kovalskii for helpful advice on the ion-exchange synthesis of Rb-K feldspars.
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