Crystal structure and investigation of Bi₂TeO₆·nH₂O (0 ≤ n ≤ 2/3): natural and synthetic montanite

Abstract

The crystal structure of montanite has been determined using single-crystal X-ray diffraction on a synthetic sample, supported by powder X-ray diffraction (PXRD), electron microprobe analysis (EPMA) and thermogravimetric analyses (TGA). Montanite was first described in 1868 as Bi₂TeO₆·nH₂O (n = 1 or 2). The determination of the crystal structure of synthetic montanite (refined composition Bi₂TeO₆·0.22H₂O) has led to the reassignment of the formula to Bi₂TeO₆·nH₂O where 0 ≤ n ≤ 2/3 rather than the commonly reported Bi₂TeO₆·2H₂O. This change has been accepted by the IMA–CNMNC, Proposal 22-A. The PXRD pattern simulated from the crystal structure of synthetic montanite is a satisfactory match for PXRD scans collected on both historical and recent natural samples, showing their equivalence. Two specimens attributed to the original discoverer of montanite (Frederick A. Genth) from the cotype localities (Highland Mining District, Montana and David Beck’s mine, North Carolina, USA) have been designated as neotypes. Montanite crystallises in space group P6 ¯ , with the unit-cell parameters a = 9.1195(14) Å, c = 5.5694(8) Å, V = 401.13(14) Å3, and three formula units in the unit cell. The crystal structure of montanite is formed from a framework of BiOn and TeO₆ polyhedra. Half of the Bi3+ and all of the Te6+ cations are coordinated by six oxygen atoms in trigonal-prismatic arrangements (the first example of this configuration reported for Te6+), while the remaining Bi3+ cations are coordinated by seven O sites. The H₂O groups in montanite are structurally incorporated into the network of cavities formed by the three-dimensional framework, with other cavity space occupied by the stereoactive 6s² lone pair of Bi3+ cations. While evidence for a supercell was observed in synthetic montanite, the subcell refinement of montanite adequately indexes all reflections in the PXRD patterns observed in all natural montanite samples analysed in this study, verifying the identity of montanite as a mineral.