As-bearing new mineral species from Valletta mine, Maira Valley, Piedmont, Italy: II. Braccoite, NaMn52+[Si5AsO17(OH)](OH), description and crystal structure

作者：Camara Fernando^*; Bittarello Erica; Ciriotti Marco E; Nestola Fabrizio; Radica Francesco; Marchesini Marco

来源：Mineralogical Magazine, 2015, 79(1): 171-189.

DOI：10.1180/minmag.2015.079.1.14

• braccoite
• saneroite
• arseno-silicates
• tiragalloite
• new mineral species
• crystal structure
• Raman
• Valletta
• Piedmont
• Italy

The new mineral species braccoite, ideally NaMn52+[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 alpha = 1.749(1), beta = 1.750(1), gamma = 1.760(1). It is triclinic, space group P (1) over bar, with a = 9.7354(4), b = 9.9572(3), c = 9.0657(3) angstrom, alpha = 92.691(2), beta = 117.057(4), gamma = 105.323(3)degrees, V = 740.37(4) angstrom(3) and Z = 2. Its calculated density is 3.56 g/cm(3). The ten strongest diffraction lines of the observed powder X-ray diffraction (XRD) pattern are [d in A, (I), (hkl)]: 3.055 (69)(2 (2) over bar1), 3.042 (43)(102), 3.012 (65)(3 (2) over bar(1) over bar), 2.985 (55)(2 (3) over bar(1) over bar), 2.825 (100)(21 (3) over bar), 2.708 (92)(220), 2.627 (43)(2 (3) over bar(2) over bar), 2.381 (58)(4 (1) over bar(1) over bar), 2.226 (25)(21 (4) over bar) and 1.680 (43 (3) over bar)(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, SiO2 39.73, 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 Sigma cations-(Na,K) = 11 p.f.u., in agreement with the results of the crystal structure, is Na-1.06(Mn4.462+Mn0.323+Mg0.19V0.013+Al0.01Ca0.01)[Si-5(As0.48Si0.37V0.155+)O-17(OH)](OH0.98F0.02); the simplified formula is Na(Mn,Mg,Al,Ca)(5)[Si-5(As,V,Si)O-17(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 R-1 = 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).

• 出版日期2015-2