GE-ANALOGS OF ALUMINUM SILICATES - HIGH-PRESSURE SYNTHESIS AND PROPERTIES OF ORTHORHOMBIC AL2GEO4(OH)(2)

Orthorhombic Al2GeO4(OH)(2) has been synthesized at 20 kbar and 650 de grees 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) Angstrom, b = 9.107(2 ) Angstrom, c = 8.551(2) Angstrom, V = 374.0(1) Angstrom(3), rho calc. = 3.987 g/cm(3). 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 Al2GeO4(OH)(2), the molar volume o f the orthorhombic polymorph is smaller by about 4.0 %. The mean index of refraction was determined to be (n) over bar = 1.746. In analogy t o the infrared spectrum of Al2SiO4(OH)(2), the OH-streching vibration is split into two bands at 3604 and 3502 cm(-1). Rietveld structure re finement confirmed that orthorhombic Al2GeO4(OH)(2) is isostructural w ith the silicate analogue topaz-OH. Most important, and in agreement w ith earlier experiments with Ge-analogues of silicates, high-pressure Al2GeO4(OH)(2) crystallizes some 30 kbar below the limiting pressure f or silicate topaz-OH.