Belmonteite, CaMn 2 (AsO 4 ) 2 (H 2 O) 5 ⋅ 2H 2 O, a new arsenate mineral from the Mn ore deposits of the Graveglia Valley, eastern Liguria, Italy

Belmonteite (IMA 2024-040), ideally CaMn2(AsO4)2(H2O)5⋅2H2O, is a new arsenate mineral discovered in the Gambatesa mine, Graveglia Valley, Ne, Genoa Province, Liguria, Italy. It occurs as aggregates up to 0.5 mm in length, formed by thin tabular crystals, white in color, with a silky luster and white streak. A perfect {010} cleavage was observed. Associated minerals are tennantite-(Cu), calcite, and a still unidentified (Ca,Cu)-arsenate secondary phase. Calculated density is 2.677 g cm−3. The empirical chemical formula of belmonteite is (Ca0.86Mn0.11K0.01)Σ0.98(Mn1.90Cu0.08Al0.02)Σ2.00As2.01O8⋅7H2O. Unit-cell parameters are a=8.8418(9), b=23.031(2), c=13.5270(14) Å, V=2754.6(5) Å3, space group Cmce, Z=8. The crystal structure of belmonteite was refined to R1=0.0385 for 1195 unique reflections with F>4σ(F) and 129 refined parameters. It can be described as being formed by {010} layers of six-fold-coordinated Mn atoms decorated on both sides by (AsO4) groups and Ca atoms. These heteropolyhedral layers are connected along b through H bonds. Belmonteite displays {010} layers topologically similar to those occurring in switzerite. The genesis of belmonteite is probably related to the circulation of As-rich oxidizing fluids within the Mn ore deposit exploited at the Gambatesa mine during its late-stage evolution. The name honors Donato Belmonte (born 1978) for his earlier contribution to the knowledge of the mineralogy of Liguria.