Coralloite, Mn2+Mn23+(AsO4)2(OH)2·4H2O, a new mixed valence Mn hydrate arsenate: Crystal structure and relationships with bermanite and whitmoreite mineral groups

Coralloite is a new mineral found at the Monte Nero Mine (Rocchetta Vara, La Spezia, Liguria, Italy) having the simplified formula Mn2+Mn23+(AsO4)2(OH)2·4H2O. It occurs as sub-millimetric lamellar cinnabar-red crystals elongated on [100] and flattened on (001), isolated or forming wisps up to 0.5–1 mm long. Associated phases are calcite, inesite, quartz, brandtite, sarkinite, and tilasite in a chert matrix. Crystals are pleochroic, yellow along [100] and orange-red in directions normal to it. Extinction is parallel to the cleavage traces and elongation is negative. The small crystal size does not allow accurate determination of refraction indices. Crossed polar observations of crystals placed in diiodomethane (n = 1.74) suggest that the mean refractive index is close to that value. Coralloite is triclinic, space group P1, a = 5.5828(7), b = 9.7660(13), c = 5.5455(7) Å, α = 94.467(3), β = 111.348(2), γ = 93.850(2)°, V = 279.26(6) Å3, Z = 1. The five strongest lines in the simulated powder diffraction pattern (dobs, I, hkl) are: 9.710 Å, 100.0, (010), 5.166 Å, 77.1, (100); 5.136 Å, 79.7, (001); 3.342 Å, 64.8, (1 2 1); 3.324 Å, 33.6, (12 1). The structure of coralloite (final Rall 0.044 for 3092 observed reflections) shows similarities with bermanite: Mn2+Mn23+(PO4)2(OH)2·4H2O and ercitite: Na2Mn23+(PO4)2(OH)2·4H2O. In particular, these three minerals exhibit the same structural slab formed by [[6]Mn23+([4]XO4)2(OH)2]; X = As or P. However, these structural slabs are connected by interposed layers of different polyhedra for each mineral species.