The structural redetermination and crystal chemistry of sinhalite, MgAlBO4
Hayward, Chris L.; Angel, Ross J.; Ross, Nancy L.
European Journal of Mineralogy Volume 6 Number 3 (1994), p. 313 - 322
published: Jun 7, 1994
manuscript accepted: Jan 17, 1994
manuscript received: Jul 9, 1993
ArtNo. ESP147050603009, Price: 29.00 €
Abstract Electron microprobe analyses of a suite of four sinhalites (all of gem quality) indicate that sinhalites (ideal formula MgAlBO4) accommodate excess aluminium and small amounts of iron at the expense of magnesium. The results of structure determination and refinement, based upon single-crystal X-ray diffraction data, are in agreement with previous work. Sinhalite is isostructural with forsterite, having space group Pbnm. The cell parameters are a = 4.3320(4) Å, b = 9.8819(5) Å, c = 5.6813(4) Å. The structure was refined to Ru = 0.017, Rw = 0.019 with 427 observed reflections and 41 refinable parameters. The assumed ordering scheme of [Mg0.91Fe0.015Al0.0.05 □ 0.025]M2AlM1BO4 was deduced from the microprobe analyses. Average B-O, M1-O and M2-O bond lengths are 1.499, 1.903 and 2.092 Å respectively. Visible absorption spectra are indicative of extremely low Fe3+ contents in these sinhalites, suggesting that the main substitutional mechanisms are 3Mg2+ ↔ 2Al3+ + □ and Mg2+ ↔ Fe2+, where □ represents a vacancy.