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Published online by Cambridge University Press: 23 January 2025
Rubicline, RbAlSi3O8, is one of three known minerals containing essential rubidium and a geochemically significant member of the feldspar family. In the course of current work, RbAlGe3O8 (germanium analogue of rubicline) was grown hydrothermally via the formation of leucite-like RbAlGe2O6 as an intermediate phase. The crystal structure of RbAlGe3O8 was determined for the first time by direct methods from single crystal X-ray diffraction data and refined to R1 = 0.0528. It has monoclinic symmetry (space group C2/m, a = 9.1237(9), b = 13.5679(6), c = 7.4677(4) Å, β = 116.687(6)° and V = 825.95(11) Å3), with cell parameters typical for disordered feldspars. According to the high-temperature study, feldspar-like RbAlGe3O8 irreversibly transforms into the leucite-like phase RbAlGe2O6 at 1050°C. The thermal expansion of studied material displays a small negative change along the b axis. Its volume thermal expansion, fitted according to a linear model between 30 and 840°C (αV = 20.3(1) ×10–6 °C–1), is slightly higher than that of other feldspar-related compounds with essential rubidium.
Associate Editor: G. Diego Gatta