Arsenmedaite, Mn2+6 As5+ Si5 O18 (OH), the arsenic analogue of medaite, from the Molinello mine, Liguria, Italy: Occurrence and crystal structure

The new mineral species arsenmedaite, ideally Mn2+6 As5+ Si5 O18 (OH), has been discovered in quartz veinlets in the braunite ore deposit of the Molinello mine, Graveglia Valley, Ne, Genova, Liguria, Italy. It occurs as orange-reddish prismatic crystals, up to 200 μm in length, with a vitreous lustre, associated with braunite, quartz, calcite, As-rich medaite, calcian rhodocrosite, talc, and ganophyllite. Electron microprobe analysis of arsenmedaite gave (in wt%): V2 O5 = 1.84, As2 O5 = 6.81, SiO2 = 38.75, CaO = 0.70, MnO = 50.28, H2 Ocalc = 1.42, total 99.80. On the basis of ∑Me = 12 atoms per formula unit, the chemical formula of arsenmedaite is (Mn5.89Ca0.10)∑5.99[(As0.49V0.17)∑0.66Si5.35]∑6.01O17.69 (OH)1.31. The main diffraction lines are [d in Å (relative intensity) hkl]: 3.266 (100) 081, 3.159 (72) 052, 3.094 (82) 221, 2.963 (83) 091, 2.935 (79) 211, 2.788 (68) 072, and 2.612 (98) 082. The crystal-structure study gives a monoclinic unit cell, space group P21/n, with a = 6.7099(3), b = 29.001(1), c = 7.5668(3) Å, β = 95.469(3)°, V = 1465.7(1) Å3, Z = 4. The crystal structure has been solved and refined to R1 = 0.062 on the basis of 3526 reflections with Fo > 4σ(Fo ) and 287 parameters. Arsenmedaite is isotypic with medaite. In its crystal structure, six-to seven-fold coordinated Mn-centred polyhedra form wavy ribbons linked to TO4 tetrahedra centred by Si and (As,V). Its formation is likely related to the late-stage circulation of As-and Mn-rich hydrothermal fluids during the final stages of the Alpine tectono-metamorphic evolution of the manganese ore deposits of Eastern Liguria.