Abstract
Orthorhombic amphiboles with excess OH, which can be schematically deduced from anthophyllite by the combined substitutions Mg2+ + O2−→Li++OH− and Mg2+→2 Li+, were synthesized at 750–875° C/1 kbar \(P_{{\text{H}}_{\text{2}} {\text{O}}}\) in the system Li2O-MgO-SiO2-H2O. Their phase relations are presented for 800° C/1 kbar \(P_{{\text{H}}_{\text{2}} {\text{O}}}\). An amphibole with the analytical composition 2.70 wt% Li2O, 31.1 wt% MgO, 63.0 wt% SiO2, and 3.29 wt% H2O has lattice constants a 0 18.588 (11), b 0 17.966 (10), c 0 5.262 (3) Å, V 0 1,757.2 (1.5) Å3 (referred to Space Group Pnma). The OH-valence vibrational spectrum of this amphibole showed v OH bands at 3,667, 3,708, and 3,725 (shoulder) cm−1, which are ascribed to OH in the configurations (MgMgMg)-OH, (MgMgMg)-OH-Li (Li in the A-site) of the pseudotrigonal (M1M1M3)-OH arrangement in the amphibole structure, and to Si-OH, respectively. No explanation can at present be offered for an additional shoulder at 3,695 cm−1. The proposed structural formula is (Li0.27 □0.73)(Li1.11 Mg0.89)· (Mg5)(Si8.01O21.20(OH)0.80)(OH)2.00.
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Maresch, W.V., Langer, K. Synthesis, lattice constants and OH-valence vibrations of an orthorhombic amphibole with excess OH in the system Li2O-MgO-SiO2-H2O. Contr. Mineral. and Petrol. 56, 27–34 (1976). https://doi.org/10.1007/BF00375419
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DOI: https://doi.org/10.1007/BF00375419