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Mineralogical ClassificationRevised formula of tinzenite

3rd May 2018 16:45 UTCMarco E. Ciriotti Manager

Reference:

▪ Grew, E.S. (2018): Tinzenite, a member of the axinite group with formula revised to Ca2Mn2+4Al4[B2Si8O30](OH)2. European Journal of Mineralogy, 30, 177-182.

Abstract:

The official International Mineralogical Association (IMA) list of minerals had given the formula for the tinzenite, an axinite-group mineral, as Ca6Al4[B2Si8O30](OH)2. However, this formula does not correspond to compositions reported for tinzenite, which contain less Ca than the other three members of the axinite group, and significantly more Mn2+ than the Mn end-member, axinite-(Mn), Ca4Mn2Al4[B2Si8O30](OH)2. Revision of the tinzenite formula to Ca2Mn2+4Al4[B2Si8O30](OH)2 was accepted by the IMA Commission on New Minerals, Nomenclature and Classification. A review of the compositions of 68 axinites containing over 10 wt% MnO reveals continuous solid solution between end-member tinzenite and end-member axinite-(Mn). Distinguishing tinzenite from axinite-(Mn) requires an analysis, either wet chemical or electron microprobe accompanied with an estimate of Fe2+/Fe3+ ratio and assuming stoichiometric H and B. The distinction should be based exclusively on Ca content with the cutoff being Ca = 3atoms per formula unit (apfu). Compositions closely approaching end-member tinzenite have been found at Klučov near Třebíč, Czech Republic and Jurvielle, Hautes-Pyrénées, France. The presence of tinzenite has been confirmed at ten localities worldwide, at nine of which tinzenite is found with quartz, rhodonite, Mn-bearing calcite or rhodochrosite in veinlets cutting stratiform manganese deposits or metachert. The tenth locality (Klučov) is entirely different: a pegmatite of the niobium-yttrium-fluorine family, where tinzenite occurs with quartz, K-feldspar and cassiterite.