Monteneroite, Cu2+Mn22+(AsO4)2•8H2O, a new vivianite-structure mineral with ordered cations from the Monte Nero mine, Liguria, Italy

Monteneroite (IMA2020-028), Cu2+Mn22+(AsO4)(2)center dot 8H(2)O, is a new vivianite-structure mineral from the Monte Nero mine, Rocchetta di Vara, La Spezia, Liguria, Italy. It is a secondary mineral that crystallised from As-, Cu- and Mn-rich fluids and it is associated with braunite, copper, cuprite, rhodochrosite and strashimirite. Monteneroite occurs as light green, thick blades up to similar to 2.5 mm long. The streak is white. Crystals are transparent with vitreous lustre. The mineral has Mohs hardness of 2, is somewhat sectile, exhibits two cleavages ({010} perfect and {001} fair) and has irregular stepped fracture. The measured density is 2.97(2) g cm(-3). Monteneroite is optically biaxial (+), with alpha = 1.604(2), beta = 1.637(2) and gamma = 1.688(2), determined in white light; 2V = 80(1)degrees; slight dispersion is r < v, orientation: X = b; Z <^> c = 52 degrees in obtuse beta. Electron microprobe analyses provided the empirical formula (Cu0.882+Mn0.112+)(Sigma 0.99)Mn-2.00(2+)(As1.00O4)(2).8H(2)O. Monteneroite is monoclinic, C2/m, a = 10.3673(14), b = 13.713(2), c = 4.8420(8) angstrom, beta = 105.992(8)degrees, V = 661.72(18) angstrom(3) and Z = 2. Monteneroite has a vivianite-type structure (R-1 = 0.0535 for 534 I > 2 sigma I reflections). It is the first mineral with this structure type to be defined with ordered octahedral cation sites.