Monteneroite, Cu2+Mn2+2(AsO4)2⋅8H2O, a new vivianite-structure mineral with ordered cations from the Monte Nero mine, Liguria, Italy

Abstract

AbstractMonteneroite (IMA2020-028), Cu2+Mn2+2(AsO4)2⋅8H2O, 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 ~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 α = 1.604(2), β = 1.637(2) and γ = 1.688(2), determined in white light; 2V = 80(1)°; slight dispersion is r < v, orientation: X = b; Z ^ c = 52° in obtuse β. Electron microprobe analyses provided the empirical formula (Cu2+0.88Mn2+0.11)Σ0.99Mn2+2.00(As1.00O4)2⋅8H2O. Monteneroite is monoclinic, C2/m, a = 10.3673(14), b = 13.713(2), c = 4.8420(8) Å, β = 105.992(8)°, V = 661.72(18) Å3 and Z = 2. Monteneroite has a vivianite-type structure (R1 = 0.0535 for 534 I > 2σI reflections). It is the first mineral with this structure type to be defined with ordered octahedral cation sites.