Tissintite, (Ca, Na, □)AlSi2O6, a highly-defective, shock-induced, high-pressure clinopyroxene in the Tissint martian meteorite
Chi Ma, Oliver Tschauner, John R. Beckett, Yang Liu, George R. Rossman, Kirill Zuravlev, Vitali Prakapenka, Przemyslaw Dera, Lawrence A. Taylor
Earth and Planetary Science Letters
In Press, available online 24 April 2015
Tissintite, (Ca0.45Na0.31□0.24)(Al0.97Fe0.03Mg0.01)(Si1.80Al0.20)O6(Ca0.45Na0.31□0.24)(Al0.97Fe0.03Mg0.01)(Si1.80Al0.20)O6, is a C2/cC2/c clinopyroxene, containing 42–60 mol% of the Ca-Eskola component, by far the highest known. The cell parameters are a=9.21a=9.21 (17) Å, b=9.09b=9.09 (4) Å, c=5.20c=5.20 (2) Å, β=109.6β=109.6 (9)°, V=410V=410 (8) Å3, Z=4Z=4. The density is 3.32 g/cm3 and we estimate a cell volume for the Ca-Eskola end-member pyroxene of 411±13 Å3411±13 Å3, which is consistent with a previous estimate and, therefore, supports the importance of this component in clinopyroxenes from ultra-high pressure metamorphic rocks from the Earth’s upper mantle. At least in C2/cC2/c clinopyroxenes as sodic as tissintite, the a- and b-cell parameters as a function of vacancy concentration intersect at ∼0.3 vacancies pfu, much lower than the Ca-Eskola end-member (0.5), an inversion of anisotropy suggesting an elastic instability that drives clinopyroxene toward a disordered trigonal structure closely related to that of wadeite; it may mark the boundary beyond which the breakdown of vacancy-rich clinopyroxene to a wadeite-structured phase + stishovite becomes stable, although this was not observed in Tissint.