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American Mineralogist; September 1999; v. 84; no. 9; p. 1360-1369
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Thermochemistry of stuffed quartz-derivative phases along the join LiAlSiO 4 -SiO 2

Hongwu Xu, Peter J. Heaney, Alexandra Navrotsky, Letitia Topor, and Jun Liu

University of California at Davis, Department of Chemical Engineering and Materials Science, Davis, CA, United States

Enthalpies of drop-solution (Delta H (sub drop-soln) ) of a suite of stuffed quartz-derivative phases with the composition Li (sub 1-x) Al (sub 1-x) Si (sub 1+x) O 4 (0< or =x< or =1) have been measured in molten 2PbO.B 2 O 3 at 974 K. Substitution of Si (super 4+) for Li (super +) +Al (super 3+) results in more exothermic enthalpies of drop-solution, which is consistent with behavior seen in other crystalline and glassy aluminosilicates. Al/Si ordering serves to stabilize these phases, and long-range ordering for compositions with x approximately <0.3 can be discerned in both calorimetric data and in structural data obtained by electron and synchrotron X-ray diffraction (XRD). In contrast, a structural but not an energetic discontinuity is apparent at x congruent to 0.65, which corresponds to a compositionally induced alpha -beta quartz transition with a small enthalpy of transformation. An enthalpy for the Al/Si order-disorder reaction in beta -eucryptite was measured as 25.9+ or -2.6 kJ/mol. Standard molar enthalpies of formation of the stuffed quartz-derivative phases from constituent oxides (Delta H 0 (sub f,ox) ) and elements (Delta H 0 (sub f,el) ) at 298 K also are presented. Delta H 0 (sub f,ox) = -69.78+ or -1.38 kJ/mol and Delta H 0 (sub f,el) = -2117.84+ or -2.50 kJ/mol for beta -eucryptite, which are in good agreement with results previously determined by HF solution calorimetry at 346.7 K (Barany and Adami 1966). The enthalpies of formation of other compositions are reported for the first time.

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This article has been cited by other articles:


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J. Zhang, A. Celestian, J. B. Parise, H. Xu, and P. J. Heaney
A new polymorph of eucryptite (LiAlSiO4), {varepsilon}-eucryptite, and thermal expansion of {alpha}- and {varepsilon}-eucryptite at high pressure
American Mineralogist, April 1, 2002; 87(4): 566 - 571.
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H. Xu, P. J. Heaney, and G. H. Beall
Phase transitions induced by solid solution in stuffed derivatives of quartz: A powder synchrotron XRD study of the LiAlSiO4-SiO2 join
American Mineralogist, July 1, 2000; 85(7-8): 971 - 979.
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B. L. Phillips, H. Xu, P. J. Heaney, and A. Navrotsky
29Si and 27Al MAS-NMR spectroscopy of {beta}-eucryptite (LiAlSiO4): The enthalpy of Si,Al ordering
American Mineralogist, January 1, 2000; 85(1): 181 - 188.
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