Portal de Periódicos da CAPES

Crystal chemistry of povondraite by single-crystal XRD, EMPA, Mössbauer spectroscopy and FTIR

2022; Cambridge University Press; Volume: 87; Issue: 2 Linguagem: Inglês

10.1180/mgm.2022.132

1471-8022

Ferdinando Bosi, Henrik Skogby, Guy L. Hovis,

Geological and Geochemical Analysis

Abstract Five povondraite crystals from San Francisco Mine, Villa Tunari, Bolivia, have been structurally and chemically characterised by single-crystal X-ray diffraction and electron microprobe analysis. For the first time, this characterisation is accompanied by Mössbauer spectroscopic and single-crystal infrared spectroscopic data, which show the exclusive presence of Fe 3+ at both the octahedrally-coordinated Y and Z sites as well as slight disorder of (OH) and O over the O(1) and O(3) sites. The data obtained along with those for earlier-studied bosiite and oxy-dravite oxy-tourmalines show a complete substitution series described by the reaction Y Fe 3+ 3 + Z Mg + Z Fe 3+ 4 ↔ Y Al 2 + Y Mg + Z Al 5 (i.e. Fe 3+ Al –1 ) with variation of the structural parameters dominated by Fe 3+ (or Al). Povondraite is the tourmaline member having the largest unit-cell parameters due to the larger size of Fe 3+ relative to other trivalent cations (V > Cr > Al). In the tourmaline-supergroup minerals, the a and c unit-cell parameters vary from ~15.60 Å to ~16.25 Å and ~7.00 Å to ~7.50 Å, respectively. Their values increase with increasing Fe 3+ or decreasing Al. End-member compositions related to the smallest and largest a and c parameters are, respectively, NaAl 3 Al 6 (Si 3 B 3 O 18 )(BO 3 ) 3 (OH) 3 (OH) (synthetic tourmaline) and NaFe 3+ 3 (Fe 3+ 4 Mg 2 )(Si 6 O 18 )(BO 3 ) 3 (OH) 3 O (povondraite).