Thermal expansion of minerals in the tourmaline supergroup

The thermal behavior of 15 natural tourmaline samples has been measured by X-ray powder diffraction from room temperature to similar to 930 degrees C. Axial thermal expansion is generally greater along the c crystallographic axis (alpha(c) 0.90-1.05 x 10(-5)/K) than along the a crystallographic axis and the symmetrically equivalent b axis (alpha(c) 0.47-0.60 x 10(-5)/K). Ferro-bearing samples show lower expansion along a than in other tourmalines. In povondraite the thermal expansion along the c axis is higher than in other tourmalines, whereas along a it is lower [alpha(a) = 0.31(2) and alpha(c) = 1.49(3) x 10(-5)/K]. Volume expansion in the tourmaline-supergroup minerals is relatively low compared with other silicates such as pyroxenes and amphiboles. Volume also exhibits a relatively narrow range of thermal expansion coefficients (1.90-2.05 x 10(-5)/K) among the supergroup members. An interpretation for the small changes in thermal expansion in a compositionally heterogeneous group like tourmaline is that all members, except povondraite, share a framework of dominantly ZAlO(6) polyhedra that limit thermal expansion. Povondraite, with a framework dominated by ZFe(3+)O(6) polyhedra, displays thermal expansion that is different from other members of the group. Unit-cell dimensions of tourmalines having significant Fe2+ deviate from linearity above 400 degrees C on plots against temperature (T); along with the resulting substantial reduction in unit-cell volume, these effects are likely the result of deprotonation/oxidation processes. Lithium-rich and Fe2+-free tourmalines deviate similarly at T > 600 degrees C. In Li- and Fe2+-free tourmalines, no such deviation is observed up to the highest temperatures of our experiments. It is not clear whether this is due to cation order-disorder over Y and Z sites that occurs during the highest temperature measurements, a phenomenon that is apparently inhibited (at least in the short term) in Li-free/Mg-rich samples. If so, this must occur at a relatively rapid rate, as no difference in unit-cell values was detected at 800 degrees C after heating in both one- and 12-h experiments on Na-rich rossmanite.