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As-bearing new mineral species from Valletta mine, Maira Valley, Piedmont, Italy: II. Braccoite, NaMn2+5 [Si5AsO17(OH)](OH), description and crystal structure

Published online by Cambridge University Press:  02 January 2018

Fernando Cámara ,
Erica Bittarello ,
Marco E. Ciriotti ,
Fabrizio Nestola ,
Francesco Radica  and
Marco Marchesini
Fernando Cámara*
Affiliation:
Dipartimento di Scienze della Terra, Universita` degli Studi di Torino, via Tommaso Valperga Caluso 35, I-10125 Turin, Italy CrisDi, Interdepartmental Center for Crystallography, via Pietro Giuria 7, I-10125, Turin, Italy
Erica Bittarello
Affiliation:
Dipartimento di Scienze della Terra, Universita` degli Studi di Torino, via Tommaso Valperga Caluso 35, I-10125 Turin, Italy CrisDi, Interdepartmental Center for Crystallography, via Pietro Giuria 7, I-10125, Turin, Italy
Marco E. Ciriotti
Affiliation:
Associazione Micromineralogica Italiana, via San Pietro 55, I-10073 Devesi-Cirie´, Turin, Italy
Fabrizio Nestola
Affiliation:
Dipartimento di Geoscienze, Universita` degli Studi di Padova, via Giovanni Gradenigo 6, I-35131 Padova, Italy
Francesco Radica
Affiliation:
Dipartimento di Scienze Geologiche, Universita` degli Studi Roma Tre, largo San Leonardo Murialdo 1, I-00146 Rome, Italy
Marco Marchesini
Affiliation:
EEEP house, UNIR, Basing View, Basingstoke, Hampshire RG21 4YY, UK
*
* E-mail: [email protected]
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Abstract

The new mineral species braccoite, ideally NaMn2+5[Si5AsO17(OH)](OH), has been discovered in the Valletta mine dumps, in Maira Valley, Cuneo province, Piedmont, Italy. Its origin is probably related to the reaction between ore minerals and hydrothermal fluids. It occurs as subhedral crystals in brown-red coloured thin masses, with a pale-yellow streak and vitreous to resinous lustre. Braccoite is associated with tiragalloite, for which new data are provided, as well as gamagarite, hematite, manganberzeliite, palenzonaite, quartz, saneroite, tokyoite, unidentified Mn oxides, organic compounds, and Mn arsenates and silicates under study.

Braccoite is biaxial positive with refractive indices α = 1.749(1), β = 1.750(1), γ = 1.760(1). It is triclinic, space group P1̄, with a = 9.7354(4), b = 9.9572(3), c = 9.0657(3) Å, α = 92.691(2), β = 117.057(4), γ = 105.323(3)°, V = 740.37(4) Å3 and Z = 2. Its calculated density is 3.56 g/cm3. The ten strongest diffraction lines of the observed powder X-ray diffraction (XRD) pattern are [d in Å, (I), (hkl)]: 3.055 (69)(22̄1), 3.042 (43)(102), 3.012 (65)(32̄1̄), 2.985 (55)(23̄1̄), 2.825 (100)(213̄), 2.708 (92)(220), 2.627 (43)(23̄2̄), 2.381 (58)(41̄1̄), 2.226 (25)(214̄) and 1.680 (433̄)(36). Chemical analyses by wavelength-dispersive spectroscopy electron microprobe gave (wt.%): Na2O 4.06, CaO 0.05, MnO 41.76, MgO 0.96, Al2O3 0.04, CuO 0.02, SiO239.73, As2O5 6.87, V2O5 1.43, SO3 0.01 and F 0.04. H2O 2.20 was calculated on the basis of 2OH groups p.f.u. Raman spectroscopy confirmed the presence of (SiO4)4–, (AsO4)3– and OH groups. The empirical formula, calculated on the basis of Σ cations-(Na,K) = 11 p.f.u., in agreement with the results of the crystal structure, is Na1.06(Mn2+4.46Mn3+0.32Mg0.19V3+0.01Al0.01Ca0.01)[Si5(As0.48Si0.37V5+0.15)O17(OH)](OH0.98F0.02); the simplified formula is Na(Mn,Mg,Al,Ca)5[Si5(As,V,Si)O17(OH)](OH,F).

Single-crystal XRD allowed the structure to be solved by direct methods and revealed that braccoite is the As-dominant analogue of saneroite. The structure model was refined on the basis of 4389 observed reflections to R1 = 3.47%. Braccoite is named in honour of Dr Roberto Bracco (b. 1959), a systematic minerals collector with a special interest in manganese minerals. The new mineral was approved by the International Mineralogical Association Commission on New Minerals, Nomenclature and Classification (IMA 2013-093).

Keywords

braccoitesaneroitearseno-silicatestiragalloitenew mineral speciescrystal structureRamanVallettaPiedmontItaly
Type
Research Article
Information
Mineralogical Magazine , Volume 79 , Issue 1 , February 2015 , pp. 171 - 189
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 2015

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