The Thermal Expansion Of Feldspars

Hovis and others (1) investigated the thermal expansion of natural and synthetic AlSi3 feldspars and demonstrated that the coefficient of thermal expansion (α) decreases significantly, and linearly, with increasing room-temperature volume (VRT). In all such feldspars, therefore, chemical expansion limits thermal expansion. The scope of this work now has been broadened to include plagioclase and Ba-K feldspar crystalline solutions. X-ray powder diffraction data have been collected between room temperature and 925 °C on six plagioclase specimens ranging in composition from anorthite to oligoclase. When combined with thermal expansion data for albite (2,3,4) a steep linear trend of α as a function of VRT emerges, reflecting how small changes in composition dramatically affect expansion behavior. The thermal expansion data for five synthetic Ba-K feldspars ranging in composition from 20 to 100 mole percent celsian, combined with data for pure K-feldspar (3,4), show α-VRT relationships similar in nature to the plagioclase series, but with a slope and intercept different from the latter. Taken as a group all Al2Si2 feldspars, including anorthite and celsian from the present study along with Sr- (5) and Pb-feldspar (6) from other workers, show very limited thermal expansion that, unlike AlSi3 feldspars, has little dependence on the divalent-ion (or M-) site occupant. This apparently is due to the necessitated alternation of Al and Si in the tetrahedral sites of these minerals (7), which in turn locks the tetrahedral framework and makes the M-site occupant nearly irrelevant to expansion behavior. Indeed, in feldspar series with coupled chemical substitution it is the change away from a 1:1 Al:Si ratio that gives feldspars greater freedom to expand. Overall, the relationships among α, chemical composition, and room-temperature volume provide useful predictive tools for estimating feldspar thermal expansion and give insight into the controls of expansion behavior in this important mineral system. We thank the Earth Sciences Division of the National Science Foundation for support of this research via grant EAR-0408829, which has provided valuable learning experiences for the undergraduate coauthors of this abstract. We appreciate the cooperation of the Department of Mineral Sciences, Smithsonian Institution, which provided five of the plagioclase specimens. Thanks to Tony Abraham, Department of Earth Sciences, Cambridge University, who conducted a portion of the high-temperature X-ray experiments. The Ba-K feldspar crystalline solutions were synthesized and chemically characterized in the 1970's at Harvard University by our good friend, Dr. Jun Ito, now deceased. (1) Hovis, Morabito, Spooner, Mott, Person, Henderson, Roux and Harlov, 2008, American Mineralogist, (2) Stewart and von Limbach, 1967, American Journal of Science, (3) Hovis and Graeme-Barber, 1997, American Mineralogist, (4) Hovis, Brennan, Keohane, and Crelling, 1999, The Canadian Mineralogist, (5) Henderson, 1984, Progress in Experimental Petrology, NERC Report, Volume 6, (6) Benna, Tribaudino, and Bruno, 1999, American Mineralogist, (7) Lowenstein, 1954, American Mineralogist.