Single-crystal in situ high-temperature structural investigation on strontium feldspar

A single crystal of ordered strontium feldspar (SrAl2Si2O8) was used for in situ X-ray intensity data collection at T = 20. 160, 330, 510, and 670 deg reesC. The crystal was synthesized from the melt and thermally treated at T = 1450 degreesC for 146 h (a = 8.379, b = 12.963, c = 14.245 Angstrom. bet a = 115.46 degrees, V = 1397.0 A(3); Q(od) = 0.82). At room temperature 151 7 reflections of a-type and 988 reflections of b-type with F-o greater than or equal to t 2 sigma (F-o) were observed with R = 4.0% for refinement in space group I2/c. The dimensions of the tetrahedra do not change significan tly with increasing temperature implying that the AI-Si configuration remai ns unchanged throughout the experimentally investigated temperature range. The Sr-coordination polyhedron expands regularly with temperature. The line ar coefficient of volume expansion (alpha, = 1.69 x 10(-5)/degreesC) is clo se to that observed for the other feldspars. The thermal expansion ellipsoi d shows a remarkable anisotropy and the main expansion occurs close to a*, as observed in the other monoclinic K-, Ba-, and Pb-feldspars. The variatio n along a* is related to the flexing of the double-crankshaft chains in res ponse to the expansion of the Sr-polyhedron. As in Pb-feldspar, a progressi ve displacement of the non-tetrahedral cation towards the c-glide plane wit h increasing temperature is observed. However, in Sr-feldspar, the temperat ure increase does not cause the atoms of the M polyhedron to approach C2/m symmetry. These results suggest that the atoms of the Sr-polyhedron retain I2/c symmetry at elevated temperatures and the Sr-polyhedron does not assum e a configuration that may significantly favor Al-Si disorder.