Paseroite, PbMn2+(Mn2+,Fe2+)2(V5+,Ti,Fe3+,〈)18O38, a new member of the crichtonite group

Abstract

Paseroite, ideally PbMn 2þ (Mn 2þ ,Fe 2þ )2(V 5þ ,Ti,Fe 3þ ,&)18O38, is a new mineral (IMA2011-069) from fossil wood in the upper part of the Molinello mine, Val Graveglia, Italy. Paseroite occurs in direct association with quartz, chalcocite, volborthite, metatyuyamunite and pyrophanite, and was also found as zones within V-rich senaite crystals with which it forms a solid-solution series. Paseroite forms as isolated submetallic, dark grey to black, elongated scalenohedral crystals between 50 and 100mm in length, with the forms {001} and {102} present. The tenacity is brittle, the fracture conchoidal, and the streak is black. The Vickers hardness is 847 kg mm � 2 (load 500g), which is equivalent to 6-6.5 on the Mohs scale. The calculated density is 4.315 g/cm 3 (on the basis of the empirical formula). In plane-polarised incident light, paseroite is greyish in colour, weakly bireflectant and non-pleochroic. Internal reflections are absent. Between crossed polars, paseroite is anisotropic, without characteristic rotation tints. Reflectance percentages (Rmin and Rmax) are: 18.4 %, 18.2 % (471.1 nm); 17.9 %, 17.7 % (548.3 nm); 17.6 %, 17.3 % (586.6 nm); and 17.0 %, 16.8 % (652.3 nm), respectively. The empirical formula, calculated on the basis of 38 O atoms pfu is: (Pb0.61Sr0.39)1.00 (V 5þ 7.78Ti 4þ 7.03Mn 2þ 1.86Fe 2þ 0.67Fe 3þ 0.37Zn0.24Na0.19U0.02Mg0.02&2.82)21.00O38. According to the structural results, the simplified formula is: PbMn 2þ (Mn 2þ ,Fe 2þ )2(V 5þ ,Ti,Fe 3þ ,&)18O38. Structurally, paseroite crystallises in the space group R3, with the unit-cell parameters a ¼ 10.3894(5), c ¼ 20.8709(8) A u , V ¼ 1950.98(15) A u 3 and Z ¼ 3. The crystal structure was refined to R ¼ 0.0234 for 632 reflections with Io . 2s(Io) and is isostructural with senaite and all other members of the crichtonite group. The eight strongest X-ray powder-diffraction lines (d in A u (I/I0 )( hkl)) are: 3.417 (100) (024), 3.012 (21) (300), 2.896 (61) (216), 2.858 (36) (214), 2.765 (27) (303), 2.260 (85) (144), 2.149 (65) (415) and 1.809 (57) (418). The name is after Marco Pasero (b. 1958), Professor of Mineralogy at the University of Pisa.