Ge-analogues of aluminium silicates: High-pressure synthesis and properties of orthorhombic Al2GeO4(OH)2
Wunder, Bernd; Marler, Bernd
European Journal of Mineralogy Volume 9 Number 6 (1997), p. 1147 - 1158
published: Dec 2, 1997
manuscript accepted: Jun 11, 1997
manuscript received: Nov 4, 1996
ArtNo. ESP147050906002, Price: 29.00 €
Abstract Orthorhombic Al2GeO4(OH)2 has been synthesized at 20 kbar and 650°C. This is the Ge-analogue of the high-pressure phase topaz-OH, Al2SiO4(OH)2, which occurs in the system Al2O3-SiO2-H2O (ASH). X-ray powder diffraction analysis yields a = 4.804(1) Å, b = 9.107(2) Å, c = 8.551(2) Å, V = 374.0(1) Å3, pcalc. = 3.987 g/cm3. The molar volume is about 5.1 % larger than in topaz-OH, because of the substitution of silicon by germanium. Compared to the monoclinic low-pressure form of AhGeO4(OH)2, the molar volume of the orthorhombic polymorph is smaller by about 4.0 %. The mean index of refraction was determined to be n = 1.746. In analogy to the infrared spectrum of AhSiO4(OH)2, the OH-streching vibration is split into two bands at 3604 and 3502 cm-1. Rietveld structure refinement confirmed that orthorhombic AhGeO4(OH)2 is isostructural with the silicate analogue topaz-OH. Most important, and in agreement with earlier experiments with Ge-analogues of silicates, high-pressure Al2GeO4(OH)2 crystallizes some 30 kbar below the limiting pressure for silicate topaz-OH.