Thermal behaviour of filatovite, a rare aluminoarsenate mineral of the feldspar group

The high-temperature behaviour of a feldspar-group mineral, filatovite (with the simplified formula: K(Al,Zn) (sub 2) (As,Si) (sub 2) O (sub 8) ), in which the Al:As:Si ratio is close to 2:1:1), was studied by in situ high-temperature single-crystal X-ray diffraction and in situ high-temperature (hot stage) Raman spectroscopy up to 600 degrees C. In the temperature range studied (25-600 degrees C) filatovite does not undergo any phase transition, whereas at 800 degrees C it decomposes to X-ray amorphous phase(s). The evolution of 12 main Raman bands was traced during heating, which indicates a gradual change in the crystal structure. The thermal expansion coefficients of filatovite demonstrate a sharply anisotropic character of thermal expansion: the maximal expansion is close to the a axis (alpha (sub 11) = 17.7(1) X 10 (super -6) degrees C (super -1) ), whereas along the b and c axes the thermal expansion coefficients are close to zero. Such behaviour is typical for minerals with a similar crystal structure topology; it indicates the dominant role of structure geometry in the thermal behaviour of the mineral.