Crystal chemistry of clinopyroxenes from the St. Marcel manganese deposit, Val d’Aosta, Italy Available to Purchase

The St. Marcel manganese deposit, Val d’Aosta, Italy, lies within the ophiolite sequence of the Zermatt-Saas unit of the Piemonte nappe, and formed as a hydrothermal sediment at an oceanic spreading center. Eoalpine metamorphism to eclogite facies was followed by retrogression to lawsonite–glaucophane facies and Lepontine greenschist facies assemblages. The Mn-rich rocks contain a wide variety of clinopyroxenes in the Di–Jd–Ac plane. Massive pyroxene-rich rocks (±albite ±quartz) contain aegirine-jadeite and impure jadeite, zoned via chloromelanite to omphacite. Violet pyroxenes (“violan”) in veins and in massive braunite ore are diopside, omphacite or chloromelanite. Di-Ac pyroxenes are breakdown products of omphacite/chloromelanite, or form late veins. Manganacmite substitution reaches 13% in some violans. X-ray data on 18 samples show that C2/c structures extend along the Di–Ac and Di–Jd joins to at least Ac₆₀; omphacites and an intermediate chloromelanite are of P type. Textural relations suggest that the various pyroxene compositions within each sample formed sequentially (zoning and/or replacement) through local changes in bulk composition, and should not be used to define solvi. The spread of all data points for 25 samples is consistent with a solvus between Jd₆₀ and Jd₈₅ on the Di–Jd join, but this gap closes at Ac > 20%.