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 cm⁻³. The mineral is inert in concentrated HCl at room temperature. Optically, alpeite crystals are biaxial (−), with α = 1.747(3), β = 1.785(3) and γ = 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 (Ca_3.84Mn²⁺_0.16)_Σ4.00(Mn³⁺_1.33Al_0.67)_Σ2.00(Al_1.29Mn³⁺_0.60V⁵⁺_0.10)_Σ1.99(Mn³⁺_0.70Mg_0.65Al_0.42Co²⁺_0.20)_Σ1.97(SiO₄)₂(Si₃O₁₀)[(V⁵⁺_0.75Si_0.25)O₄](OH)₆, based on 28O atoms 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) ų and Z = 2. The four strongest lines in the X-ray powder diffraction pattern are [d_obs/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 [F_o > 4σF]. 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.