Cassagnaite, a new, V-bearing silicate mineral from the Cassagna mine, northern Apennines, Italy

Authors: Basso, Riccardo; Carbone, Cristina; Palenzona, Andrea

Source: European Journal of Mineralogy, Volume 20, Number 1, February 2008 , pp. 95-100(6)

Publisher: E. Schweizerbart'sche Verlagsbuchhandlung

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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 Ca₄Fe³⁺₄(OH)₄V³⁺₂O₂(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.

Keywords: CASSAGNAITE; NEW MINERAL; CHEMICAL DATA; X-RAY POWDER PATTERN; CRYSTAL STRUCTURE; VANADIUM; ARDENNITE GROUP

Document Type: Research article

DOI: http://dx.doi.org/10.1127/0935-1221/2007/0019-1777

Publication date: 2008-02-01

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