High-temperature crystal chemistry of phenakite (Be2SiO4) and chrysoberyl (BeAl2O4)
Thermal expansion and high-temperature crystal structures of phenakite and chrysoberyl have been determined by x-ray methods at several temperatures to 690° C. Phenakite (hexagonal, space groupR\(\bar 3\)) has slightly anisotropic thermal expansion; average expansions between 25 and 690° C perpendicular and parallel to thec axis are α⊥=5.2×10−6 °C−1 and α∥=6.4×10−6 °C−1, respectively. The unit cell volume of phenakite over this temperature range is given by the polynomial expression:V = 1102.9(2) + 0.010(2)T + 1.1(3) × 10-5T2.
Chrysoberyl (orthorhombic, space groupPbnm) has nearly isotropic thermal expansion, with maximum expansivity 8.5×10−6 °C−1 parallel to theb axis, and minimum expansivity 7.4×10−6 °C−1 parallel toa. Thec axis expansivity is 8.3×10−6 °C−1. Chrysoberyl volume between 25° and 690° C may be represented by:V = 227.1(2) + 0.003(1)T + 4(2) × 10-6T2.
The thermal expansion of beryllium, aluminum, and silicon cation coordination polyhedra in phenakite and chrysoberyl are similar to values found in previous studies of minerals in the BeO-Al2O3-SiO2 system. High-temperature structure studies of bromellite (BeO), beryl (Be3Al2Si6O18), phenakite and chrysoberyl all have beryllium tetrahedra that display the same near-zero expansion at room temperature, but increasing expansion at higher temperatures.
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