Ramazzoite, [Mg_8Cu_(12)(PO_4)(CO_3)_4(OH)_(24)(H_2O)_(20)][(H_(0.33)SO_4)_3(H_2O)_(36)], the first mineral with a polyoxometalate cation

Abstract

Ramazzoite (IMA2017-090), [Mg_8Cu_(12)(PO_4)(CO_3)_4(OH)_(24)(H_2O)_(20)][(H_(0.33)SO_4)_3(H_2O)_(36)], is a new polyoxometalate (POM) mineral from the Monte Ramazzo mine, near Genova, Liguria, Italy. It occurs on magnetite-rich matrix in association with chlorartinite, chrysotile, dypingite, goethite, lepidocrocite and nesquehonite, and is a late-stage, secondary mineral crystallizing from low-temperature, aqueous solutions. Ramazzoite forms as simple cubes up to about 0.15 mm on edge. The mineral is blue to greenishblue with a vitreous to oily lustre and pale blue streak. Crystals are very brittle with conchoidal fracture, and a perfect cleavage on {100}. The Mohs 'hardness is 2½. The measured density is 1.98(1) g cm^(–3). The mineral is soluble with mild effervescence in dilute HCl at room temperature. Optically, ramazzoite is isotropic with n = 1.491(1) (white light). Electron microprobe analyses (with CO2 from structural data and H2 O from density) gave the empirical formula [(Mg_(8.00))(Cu_(8.00)Mg_(3.78))(PO_4)(CO_3)_4(OH)_(24)(H_2O)_(20)][(H_(0.65)S_(1.01)O_4)_3(H_2O)_(36)], based on 1 P atom per formula unit (apfu). Ramazzoite is cubic, P4 3m, with the unit-cell parameters: a = 13.3887(10) Å, V = 2400.0(5) Å^3 and Z = 1. The crystal structure, refined to R_1= 0.064 for 803 observed reflections [I> 2 σ I], contains a novel [Mg_8Cu_(12)(PO_4)(CO_3)_4(OH)_(24)(H_2O)_(20)]^(5+)POM cation.

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