Crystal chemistry of povondraite by SCXRD, EMPA, Mössbauer spectroscopy and FTIR

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Marco E. Ciriotti
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Crystal chemistry of povondraite by SCXRD, EMPA, Mössbauer spectroscopy and FTIR

Messaggio da Marco E. Ciriotti » lun 28 nov, 2022 13:08

Prossima pubblicazione.

Referenza:
▪ Bosi, F., Skogby, H., Hovis, G.L. (2022): Crystal chemistry of povondraite by SCXRD, EMPA, Mössbauer spectroscopy and FTIR. Mineralogical Magazine, 86, (in press).

Abstract:
Five povondraite crystals from San Francisco Mine, Villa Tunari, Bolivia, have been structurally and chemically characterized by single-crystal X-ray diffraction and electron microprobe analysis. For the first time, this characterization is accompanied by Mössbauer spectroscopic and single-crystal infrared spectroscopic data, which show the exclusive presence of Fe3+ 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 oxytourmalines show a complete substitution series described by the reaction YFe3+3 + ZMg +ZFe3+4 ↔ YAl2 + YMg + ZAl5 (i.e. Fe3+Al–1) with variation of the structural parameters dominated by Fe3+ (or Al). Povondraite is the tourmaline member having the largest unitcell parameters due to the larger size of Fe3+ 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 Fe3+ or decreasing Al. Endmember compositions related to the smallest and largest a- and c-parameters are, respectively, NaAl3Al6(Si3B3O18)(BO3)3(OH)3(OH) (synthetic tourmaline) and NaFe3+3(Fe3+4Mg2)(Si6O18)(BO3)3(OH)3O (povondraite).
Marco E. Ciriotti

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