High-Temperature Crystal Chemistry of Acmite, Diopside, Hedenbergite Jadeite, Spodumene and Ureyite Available to Purchase

Crystal structure parameters have been determined for acmite, NaFe³⁺Si₂O₆ (at ∼400°C, 600°C, 800°C), diopside, CaMgSi₂O₆ (at ∼400°C, 700°C, 850°C, l00O°C), hedenbergite, caFe²⁺Si₂O₆ (at ∼24°C,400°C, 600°C, 800°C,900°C, 1000°c), jadeite,NaAlsi₂O₆ (at∼ 24°c, 400°C, 500°C, 800°C), spodumene,LiAlSi₂O₆ (at at∼300°C, 460°C, 76O°C) and ureyite, NaCrSi₂O₆ (at ∼400°C, 600°C). Refinements in space group C2/c using anisotropic temperature factors and over 640 reffections for each set of data resulted in unweightedR-factors ranging from 0.022 to 0.078.

The increase in mean Si–O bond lengths with increasing temperature is not statistically significant.but the mean M–O distances for both the 6- and 8-coordinated sites in the six pyroxenes exhibit relatively large increases. The mean thermal expansion coefficients for the various bonds increase in the following order: Si⁴⁺–O < Cr³⁺–O < Fe₃₊–O < Al³⁺–O < Fe²⁺–O < Na⁺–O < Mg²⁺–O < Ca²⁺–O < Li⁺–O. This series is essentially a function of decreasing bond strength, although the position of Mg²⁺–O may indicate an exception to this trend. The pyroxene structures accommodate the differential mean M-O expansions through (1) extension of the silicate tetrahedral chains, (2) distortion of the silicate tetrahedra, and (3) an increase in out-of-plane tilting of silicate tetrahedra. In all six minerals thermal expansion is accomplished principally by expansion of the non-tetrahedral polyhedra with concomitant expansion of the voids between the cation polyhedra, and rotation of the fairlyrigid tetrahedra about the bridging oxygen atoms. The rate of increase of the equivalent isotropic temperaturefactor for a cation is generally proportional to the coordination number of that cation.