Alpeite, Ca4Mn3+2Al2(Mn3+Mg)(SiO4)2(Si3O10)(V5+O4)(OH)6, a new ardennite-group mineral from Italy

Alpeite (IMA2016-072), Ca4Mn3+2Al2(Mn3+Mg)(SiO4)2(Si3O10)(V5+O4)(OH)6, is a new member of the ardennite group from the Monte Alpe mine, Liguria, Italy. It crystallized from V- and Mn-rich hydrothermal fluids in an oxidizing environment. Alpeite occurs as intergrowths of plates, flattened on {1 0 0} and exhibiting the forms {1 0 0}, {0 0 1}, {0 1 2} and {1 0 2}. The mineral is brownish red and transparent with a vitreous to silky lustre and beige streak. Crystals are brittle with a curved and stepped fracture, and a perfect cleavage on {1 0 0} and good cleavages on {0 1 0 and {0 0 1}. The Mohs' hardness is 5½ to 6. The calculated density is 3.374 g cm3. The mineral is inert in concentrated HCl at room temperature. Optically, alpeite crystals are biaxial (-), with a = 1.747(3), b = 1.785(3) and g = 1.808(3) (white light); 2V = 73(2); dispersion r>v, strong; the optical orientation X = a, Y = b, Z = c. Pleochroism is Y>Z>X in shades of reddish brown. Electron microprobe analyses gave the empirical formula (Ca3.84Mn2+0.16)S4.00(Mn3+1.33Al0.67)S2.00(Al1.29Mn3+0.60V5+0.10)S1.99(Mn3+0.70Mg0.65Al0.42Co2+0.20)S1.97(SiO4)2(Si3O10)[(V5+0.75Si0.25)O4](OH)6, based on 28Oatoms per formula unit. Alpeite is orthorhombic, Pnmm,with the unit-cell parameters: a = 8.9421(11), b = 6.0534(6), c = 18.9781(18)Å, V = 1027.29(19)Å3 and Z = 2. The four strongest lines in theX-ray powder diffraction pattern are [dobs/Å (I) (hkl)]: 3.022 (93) (1 1 5), 2.673 (100) (1 1 6), 2.572 (69) (2 0 6) and 1.5112 (83) (0 4 0,4 2 7). The crystal structure was refined to R1=0.044 for 834 observed reflections [Fo>4sF]. The structure is the same as that of other members of the ardennite group. It contains chains of edge-sharing Mn3+ and Al octahedra extending along the b axis, which are linked into a framework by sharing corners with SiO4 tetrahedra and with Si3O10 tetrahedral groups. Within the cavities in this framework are 7-coordinated Ca sites. In the structure of alpeite, Mn3+ is the dominant cation in the M1 site and is the dominant trivalent cation in the M3 site, while in all other members of the ardennite group [ardennite-(As), ardennite-(V) and kannanite], Al is the dominant cation in the M1 site and is the dominant trivalent cation in the M3 site.