Abstract

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 °C. The crystal was synthesized from the melt and thermally treated at T = 1450 °C for 146 h (a = 8.379, b = 12.963, c = 14.245 Å, β = 115.46 °, V = 1397.0 Å3; Qod = 0.82). At room temperature 1517 reflections of a-type and 988 reflections of b-type with Fo ≥ 2σ (Fo) were observed with R = 4.0% for refinement in space group I2/c. The dimensions of the tetrahedra do not change significantly with increasing temperature implying that the Al-Si configuration remains unchanged throughout the experimentally investigated temperature range. The Sr-coordination polyhedron expands regularly with temperature. The linear coefficient of volume expansion (αv = 1.69 × 10−5/°C) is close to that observed for the other feldspars. The thermal expansion ellipsoid shows a remarkable anisotropy and the main expansion occurs close to a*, as observed in the other monoclinic K-, Ba-, and Pb-feldspars. The variation along a* is related to the flexing of the double-crankshaft chains in response to the expansion of the Sr-polyhedron. As in Pb-feldspar, a progressive displacement of the non-tetrahedral cation towards the c-glide plane with increasing temperature is observed. However, in Sr-feldspar, the temperature 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 assume a configuration that may significantly favor Al-Si disorder.

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