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

Abstract

Piccoliite, ideally NaCaMn 3+ 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. Measured Mohs hardness is ∼5-5.5. Calculated density is 4.08 g⋅cm -3 . Piccoliite is non-fluorescent. Chemical spot analyses by WDS-EPMA resulted in the empirical formula (based on 10 anions pfu) (Na0.64Ca0.35)Σ0.99(Ca0.75Na0.24)Σ0.99(Mn 3+ 1.08Fe 3+ 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(Mn 3+ 0.63Fe 3+ 0.49Mg0.48Mn 4+ 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) Å 3 (Montaldo di Mondovì sample) and a = 8.8889(2), b = 7.5269(1), c = 11.6795(2) Å, V = 781.43(2) Å 3 (Valletta sample) with Z = 4. The seven strongest X-ray powder diffraction lines for the sample from

Thermodynamics and crystal structures of krautite, Mn[AsO3(OH)]⋅H2O, koritnigite, Zn[AsO3(OH)]⋅H2O and cobaltkoritnigite, Co[AsO3(OH)]⋅H2O

“…Zinc and cobalt for such minerals are released by weathering of primary ore minerals. Manganese, on the other hand, could be released from minerals of ore deposits (Holtstam et al, 2022;Cámara et al, 2022) but could have other sources in the environment (e.g., Gatuingt et al, 2021). This work focuses on thermodynamic properties and crystal structures on selected synthetic equivalents of the minerals in the krautite group.…”

“…Zinc and cobalt for such minerals are released by weathering of primary ore minerals. Manganese, on the other hand, could be released from minerals of ore deposits (Holtstam et al , 2022; Cámara et al , 2023) but could have other sources in the environment (e.g. Gatuingt et al , 2021).…”