Piccoliite, NaCaMn³⁺₂(AsO₄)₂O(OH), a new arsenate from the manganese deposits of Montaldo di Mondovì and Valletta, Piedmont, Italy

Autor(en)

Fernando Cámara, Cristian Biagioni, Marco E. Ciriotti, Ferdinando Bosi, Uwe Kolitsch, Hermann Werner Paar, Ulf Hålenius, Giovanni Lepore, Günter Blass, Erica Bittarello

Abstrakt

Piccoliite, ideally NaCaMn3+2(AsO4)2O(OH), is a new mineral discovered in the Fe-Mn ore hosted in metaquartzites of the Montaldo di Mondovì mine, Corsaglia Valley, Cuneo Province, Piedmont, Italy. It occurs as small and rare black crystals and aggregates hosted by a matrix of quartz, associated with calcite and berzeliite/manganberzeliite. It has been also found in the Valletta mine near Canosio, Maira Valley, Cuneo Province, Piedmont, Italy, where it occurs embedded in quartz associated with grandaite, hematite, tilasite/adelite and rarely thorianite. The mineral is opaque (thin splinters may be very dark red), with brown streak and has a resinous to vitreous lustre. It is brittle with irregular fracture. No cleavage has been observed. The measured Mohs hardness is ~5-5.5. Piccoliite is non fluorescent. The calculated density is 4.08 g·cm-3. Chemical spot analyses by electron microprobe analysis using wavelength dispersive spectroscopy resulted in the empirical formula (based on 10 anions per formula unit) (Na0.64Ca0.35)Σ0.99(Ca0.75Na0.24)Σ0.99(Mn3+1.08Fe3+0.59Mg0.20Ca0.10)Σ1.97(As2.03V0.03Si0.01)Σ2.07O9(OH) and (Na0.53Ca0.47)Σ1.00(Ca0.76Na0.23Sr0.01)Σ1.00(Mn3+0.63Fe3+0.49Mg0.48Mn4+0.34Ca0.06)Σ2.00(As1.97P0.01Si0.01)Σ1.99O9(OH) for the Montaldo di Mondovì and Valletta samples, respectively. The mineral is orthorhombic, Pbcm, with single-crystal unit-cell parameters a = 8.8761(9), b = 7.5190(8), c = 11.689(1) Å and V = 780.1(1) Å3 (Montaldo di Mondovì sample) and a = 8.8889(2), b = 7.5269(1), c = 11.6795(2) Å, V = 781.43(2) Å3 (Valletta sample) with Z = 4. The seven strongest powder X-ray diffraction lines for the sample from Montaldo di Mondovì are [d Å (Irel; hkl)]: 4.85 (57; 102), 3.470 (59; 120, 113), 3.167 (100; 022), 2.742 (30; 310, 213), 2.683 (53; 311, 023), 2.580 (50; 222, 114) and 2.325 (19; 320, 214, 223). The crystal structure (R1 = 0.0250 for 1554 unique reflections for the Montaldo di Mondovì sample and 0.0260 for 3242 unique reflections for the Valletta sample) has MnO5(OH) octahedra forming edge-shared dimers; these dimers are connected through corner-sharing, forming two-up-two-down [[6]M2([4]TO4)4φ2] chains [M = Mn; T = As; φ = O(OH)] running along [001]. These chains are bonded in the a and b directions by sharing corners with AsO4 tetrahedra, giving rise to a framework of tetrahedra and octahedra hosting seven-coordinated Ca2+ and Na+ cations. The crystal structure of piccoliite is closely related to that of pilawite-(Y) as well as to carminite-group minerals that also show the same type of chains but with different linkage. The mineral is named after the mineral collectors Gian Paolo Piccoli and Gian Carlo Piccoli (father and son) (1926-1996 and b. 1953, respectively), the latter having discovered the type material at the Montaldo di Mondovì mine.

Organisation(en)

Institut für Mineralogie und Kristallographie

Externe Organisation(en)

Associazione Micromineralogica Italiana, Naturhistorisches Museum Wien (NHM), Università degli Studi di Firenze, Università degli Studi di Milano-Bicocca, Università degli Studi di Pisa, Università degli Studi di Torino, Università degli Studi di Roma La Sapienza, Consiglio Nazionale delle Ricerche, Paris-Lodron Universität Salzburg, Swedish Museum of Natural History, Independent researcher

Journal

Mineralogical Magazine

Band

87

Seiten

204-217

Anzahl der Seiten

14

ISSN

0026-461X

DOI

https://doi.org/10.1180/mgm.2022.129

Publikationsdatum

04-2023

Peer-reviewed

Ja

ÖFOS 2012

104026 Spektroskopie, 105116 Mineralogie, 105113 Kristallographie

Schlagwörter

ASJC Scopus Sachgebiete

Geochemistry and Petrology

Link zum Portal

https://ucrisportal.univie.ac.at/de/publications/9f09878a-0d18-4a88-8ae6-b847562ba73d