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Fluor-tsilaisite, NaMn3Al6(Si6O18)(BO3)3(OH)3F, a new tourmaline from San Piero in Campo (Elba, Italy) and new data on tsilaisitic tourmaline from the holotype specimen locality

Published online by Cambridge University Press:  02 January 2018

Ferdinando Bosi*
Affiliation:
Dipartimento di Scienze della Terra, Sapienza Universita` di Roma, P. le Aldo Moro, 5, I-00185 Rome, Italy CNR- Istituto di Geoscienze e Georisorse, VOS Roma, P. le Aldo Moro, 5, I-00185 Rome, Italy
Giovanni B. Andreozzi
Affiliation:
Dipartimento di Scienze della Terra, Sapienza Universita` di Roma, P. le Aldo Moro, 5, I-00185 Rome, Italy CNR- Istituto di Geoscienze e Georisorse, VOS Roma, P. le Aldo Moro, 5, I-00185 Rome, Italy
Giovanna Agrosi
Affiliation:
Dipartimento di Scienze della Terra e Geoambientali, Universita` di Bari - Campus, via E. Orabona 4, I-70125 Bari, Italy
Eugenio Scandale
Affiliation:
Dipartimento di Scienze della Terra e Geoambientali, Universita` di Bari - Campus, via E. Orabona 4, I-70125 Bari, Italy

Abstract

Fluor-tsilaisite, NaMn3Al6(Si6O18)(BO3)3(OH)3F, is a new mineral of the tourmaline supergroup. It occurs in an aplitic dyke of a LCT-type pegmatite body from Grotta d'Oggi, San Piero in Campo, Elba Island,Italy, in association with quartz, K-feldspar, plagioclase, elbaite, schorl, fluor-elbaite and tsilaisite. Crystals are greenish yellow with a vitreous lustre, sub-conchoidal fracture and white streak. Fluor-tsilaisite has a Mohs hardness of ∼7 and a calculated density of 3.134 g/cm3.In plane-polarized light, fluor-tsilaisite is pleochroic (O = pale greenish yellow and E = very pale greenish yellow), uniaxial negative. Fluor-tsilaisite is rhombohedral, space group R3m, a = 15.9398(6), c = 7.1363(3) Å, V = 1570.25(11) Å3,Z = 3. The crystal structure of fluor-tsilaisite was refined to R1 = 3.36% using 3496 unique reflections collected with MoKα X-ray intensity data. Crystal-chemical analysis resulted in the empirical formula: X(Na0.69〈0.29Ca0.02)Σ1.00 Y(Mn2+ 1.29Al1.21Li0.56Ti0.03)Σ6.00 ZAl6 T(Si5.98Al0.03)Σ6.00B2.92O27 V(OH)3 W[F0.39(OH)0.25O0.36]Σ1.00.

Comparisons were performed between fluor-tsilaisite and a tsilaisitic tourmaline from the same locality as the holotype specimen. This latter tourmaline sample was selected for this study due to its remarkable composition (MnO = 11.63 wt.%), the largest Mn content found in tourmaline so far.

Fluor-tsilaisite is related to tsilaisite through the substitution WF ↔ W(OH) and with fluor-elbaite through the substitution Y(Al + Li) ↔ 2YMn2+ , and appears to be a stepwise intermediate during tourmaline evolution from tsilaisite to fluor-elbaite.

Information

Type
Research Article
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 2015

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