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The thermal behavior of 15 natural tourmaline samples has been measured by X-ray powder diffraction from room temperature to ~930 °C. Axial thermal expansion is generally greater along the  c  crystallographic axis (α c  0.90–1.05 × 10 –5 /K) than along the  a  crystallographic axis and the symmetrically equivalent  b  axis (α a  0.47–0.60 × 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 [α a  = 0.31(2) and α c  = 1.49(3) × 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 × 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  Z AlO 6  polyhedra that limit thermal expansion. Povondraite, with a framework dominated by  Z Fe 3+ O 6  polyhedra, displays thermal expansion that is different from other members of the group.

Thermal expansion of minerals in the tourmaline supergroup