THERMAL EXPANSION SYSTEMATICS FOR THE PYROXENE SYSTEM

Hovis, Medford, Conlon, Tether, & Romanoski (2010, Am. Mineral.) discovered straight-forward relations in the feldspar mineral system that allowed prediction of the thermal expansion coefficient for volume based simply on feldspar composition and room-temperature unit-cell volume. We undertook the present study to learn whether equally simple methodology might apply to the pyroxene system. Powdered samples of nine chemically analyzed pyroxenes (diopside, hedenbergite, johannsenite, acmite, jadeite, spodumene, enstatite, hypersthene, ferrosilite) were X-rayed from 22 to 928 (± 15) °C at 50° intervals using a PANalytical Empyrean X-ray system equipped with an Anton Parr furnace. Temperatures were checked via reversible phase transitions of several compounds. NIST SRM 640a Si was utilized as an internal standard. Unit-cell parameters were calculated using the software of Holland and Redfern (1997). Results for pyroxenes do not produce the same "magic bullet" for volume expansion determination that was the case for feldspars, but they do show systematic relationships. The length of the "a" unit-cell axis for all pyroxenes can be modeled simply from knowledge of the sum of the ionic radii for occupants of the M1 and M2 crystallographic sites; expansion of this axis can in turn be predicted from the room-temperature value for "a". For clinopyroxenes, including spodumene, the room-T value for "b" also can be predicted from the ionic-radii of M1 and M2 occupants. Expansion of "b" for C2/c pyroxenes is systematic as well, although the trend for these does not fit the data for spodumene. Both volume and volume expansion can be predicted from knowledge of the ionic radii of ions occupying M1 and M2. Generally, orthopyroxenes and Ca-clinopyroxenes have similar coefficients of thermal expansion, while such coefficients for aegirine, jadeite, and spodumene are significantly lower. By the time of the GSA meeting we shall have added the extensive data from the literature for comparison. This project was designed for undergraduate research participation; we thank the National Science Foundation and the Lafayette College EXCEL Program for their support of such efforts. We also thank Jeff Post and Mike White (NMNH), George Harlow (AMNH), and Don Lindsley (Stony Brook University) for generously contributing samples for this work.