High-temperature crystal chemistry of phenakite (Be₂SiO₄) and chrysoberyl (BeAl₂O₄)

Abstract

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^-5 T ².

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^-6 T ².

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.