In air, Tourmalines are stable up to 825-995 degree C, depending on their composition. The decomposition products are mullite and an X-ray amorphous phase, and also hypersthene in the case of magnesium-containing varieties and spodumene in the case of lithium-containing varieties. Up to 400 degree C, only a thermal expansion which is more intense along the c-axis occurs. In iron-containing tourmalines above 400 degree C oxidation of iron begins. The result is compression of the structure along the a-axis with compensation of the thermal expansion and a sharp increase in the c-parameter at high temperatures. The directions of the maximum thermal expansion, the maximum amplitudes of the thermal vibrations of atoms, and the minimum refractive index in the Tourmaline structure coincide with each other and are perpendicular to packets of the (001) structure.