Abstract
Cassagnaite occurs at the Cassagna manganese mine (Eastern Liguria, Italy), filling fractures in braunite +quartz layered mineralizations together with piemontite. It occurs as very rare isolated prismatic to tabular {001} crystals, usually elongated along [100], and as entangled aggregates of a few crystals. Few aggregates consist of very small cassagnaite crystals (maximum size up to 0.05 mm) closely associated with much larger piemontite, quartz, and braunite crystals. The crystals are generally very small, with a maximum size up to 0.1 mm, golden brown in colour, transparent with vitreous lustre. The crystal structure, refined in the space group Cmcm with cell parameters a = 6.066(1) Å, b = 8.908(1) Å, c = 18.995(2) Å and Z = 2, may be described as a layer stacking along [001] of a fundamental building block of composition [M12(OH)2(SiO4)2]4− that alternates with intersheets, randomly occurring in a ratio ideally 1:1, of type 1 [(Ca, Mn2+)2SiO2]4+ and of type 2 (Ca, Mn2+)2M22(OH)2O2]4+, where Fe3+ and Mn3+ populate 3/4 of the M1 site and Al the remaining 1/4, while V3+, Mg and Al occupy in nearly equal proportions the M2 site. The simplified formula, inferred from chemical analyses, structure refinement and crystal-chemical considerations, may be written as (Ca, Mn2+)4(Fe3+, Mn3+, Al)4(OH)4(V3+, Mg, Al)2(O, OH)4(SiO4)2(Si3O10). From the composition of the “dominant” end-member of the complex solid-solution the end-member formula Ca4Fe43+(OH)4V23+O2(OH)2(SiO4)2(Si3O10) may be proposed for cassagnaite. So, cassagnaite may be classified as sorosilicate with insular and triple tetrahedral groups and belongs to the ardennite group in Dana’s classification.
