Crystal structure and high-temperature transformation of RbAlGe₃O₈, a germanium analogue of rubicline, RbAlSi₃O₈

Abstract Rubicline, RbAlSi 3 O 8 , is one of three known minerals containing essential rubidium and a geochemically significant member of the feldspar family. In the course of current work, RbAlGe 3 O 8 (germanium analogue of rubicline) was grown hydrothermally via the formation of leucite-like RbAlGe 2 O 6 as an intermediate phase. The crystal structure of RbAlGe 3 O 8 was determined for the first time by direct methods from single crystal X-ray diffraction data and refined to R 1 = 0.0528. It has monoclinic symmetry (space group C 2/ m, a = 9.1237(9) , b = 13.5679(6) , c = 7.4677(4) Å, β = 116.687(6)° and V = 825.95(11) Å 3 ), with cell parameters typical for disordered feldspars. According to the high-temperature study, feldspar-like RbAlGe 3 O 8 irreversibly transforms into the leucite-like phase RbAlGe 2 O 6 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 (α V = 20.3(1) ×10 –6 °C –1 ), is slightly higher than that of other feldspar-related compounds with essential rubidium.