Mangano-mangani-ungarettiite

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Formula: NaNa₂(Mn²⁺₂Mn³⁺)Si₈O₂₂O₂
Inosilicate (chain silicate), double chains, w(O)-dominant amphibole group, amphibole supergroup
Crystal System: Monoclinic
Specific gravity: 3.52 measured, 3.45 calculated
Hardness: 6
Streak: Pale pinkish-white
Colour: Cherry-red to very dark red
Luminescence: Not fluorescent under long wave or short wave UV
Common impurities: Fe,Mg,Ca,K,Li
Environments

Metamorphic environments

Localities

At the type locality, the Hoskins mine, Grenfell, Forbes County, New South Wales, Australia, ungarettiite (mangano-mangani-ungarettiite) occurs in manganese silicate and oxide rocks from the abandoned mine. A steeply dipping stratiform suite of schistose metamorphosed manganese-rich rocks up to 8 m thick occurs with metamorphosed jasper, metabasalt, and metamorphosed siltstone. The manganese-rich rocks are submarine exhalative precipitates that were subsequently deformed and metamorphosed at upper middle to upper greenschist facies conditions. Amphiboles occur in rock-forming amounts in scattered samples obtained from the old mine dumps. In the former workings, the manganese-rich rocks were oxidised to manganese oxide assemblages to a depth of 45 m. The unweathered amphibole-bearing rocks are therefore likely to have come from below this depth.
Two major mineral associations are evident in the manganese-rich rocks:
(1) a reduced assemblage containing rhodonite, tephroite, hausmannite, garnet (spessartine - grossular - andradite - calderite), calcium manganese barium carbonates, quartz, manganese-bearing magnetite, manganese-rich chlorite, caryopilite, baryte and pyrite,
and
(2) an oxidised assemblage containing manganese-rich alkali amphiboles, clinopyroxenes ranging from namansilite to aegirine, manganoan pectolite - serandite, braunite, norrishite, calcium and barium carbonates, quartz, albite, K-feldspar, manganese-bearing sugilite and baryte.
Oxidised assemblage samples are moderately to strongly foliated and are commonly well laminated. Individual laminae are rich in one or more of the minerals amphibole, clinopyroxene, braunite, serandite, norrishite, carbonate, sugilite, quartz and alkali feldspar. Grain size ranges from <0.05 to 0.5 mm, with amphibole, serandite, norrishite and sugilite locally attaining 1.5 mm.
Amphiboles occur in two associations:
(1) a generally quartz-free assemblage containing amphibole, clinopyroxene, braunite, norrishite and manganoan pectolite - serandite, with minor alkali feldspars, carbonate and baryte
and
(2) a quartz-rich assemblage, with subordinate amphibole and clinopyroxene, and minor serandite, sugilite, norrishite and carbonate.
Amphiboles are disseminated in both assemblages but also occur as a major component of laminae from 0.1 to 4 mm thick in association (1). In hand specimens, they occur as lustrous dark red to brownish red acicular prismatic grains.
In amphibole-rich laminae, felted aggregates of elongate grains commonly display preferred orientation. Although amphibole is locally enclosed within porphyroblastic serandite, carbonate, braunite or sugilite and is intergrown with clinopyroxene, norrishite and braunite, it seems to coexist with all the primary metamorphic phases. Specifically, there is no textural evidence that could be interpreted as indicating replacement of clinopyroxene by amphibole.
Ungarettiite is a major component of two samples studied, numbered 820607 and 820240. In sample 820607, ungarettiite is locally enclosed in porphyroblastic (the texture of a recrystallised metamorphic rock having large mineral in a finer-grained crystalline matrix) serandite and carbonate, but it most commonly occurs in foliated aggregates with norrishite, accompanied by minor clinopyroxene and braunite. In sample 820240, ungarettiite occurs as isolated prismatic grains and aggregates in granular quartz, oriented in the plane of the foliation. Ungarettiite in aggregates is intergrown with clinopyroxene and norrishite. Leakeite, manganese-bearing leakeite, manganoan katophorite and manganoan arfvedsonite occur in both assemblages (AM 80.1.165-172 as ungarettiite).
Mangano-mangani-ungarettiite from the Hoskins Mine - Image

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