Ribbeite

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Formula: Mn²⁺₅(SiO₄)₂(OH)₂
Nesosilicate (insular SiO₄ groups), leucophoenicite subgroup, humite group, orthorhombic paramorph of monoclinic alleghanyite, manganese-bearing mineral
Crystal System: Orthorhombic
Specific gravity: 3.90 measured, 3.84 calculated
Hardness: 5
Streak: White
Colour: Pink
Luminescence: Not fluorescent under UV
Common impurities: Fe,Ca,H₂O
Environments

Sedimentary environments
Metamorphic environments

Localities

At the type locality, the Kombat Mine, Kombat, Otavi Constituency, Otjozondjupa Region, Namibia, the ore consists largely of copper and lead sulphides deposited in essentially un-metamorphosed dolostone. The dolostone contains lenses of iron and manganese oxides, the former dominated by magnetite and hematite and the latter by hausmannite, baryte, alleghanyite, manganese-bearing calcite and pyrochroite. The type specimen of ribbeite was found in one of the manganese-rich lenses, as pinkish lenses up to 5 cm thick and 20 cm long.
All of the minerals intergrown with ribbeite occur as approximately 0.5 mm diameter grains. There are four distinct bands in the type specimen that seem to have been inherited from a sedimentary protolith, each dominated by different minerals.
The first band is a 3 mm wide band composed primarily of ribbeite and also having subhedral pyrochroite and a mcgovernite-like mineral. This grades moderately sharply into the second band.
The second is a 2 cm wide band composed largely of ribbeite and chlorite, with minor spinel and calcite. The chlorite, which is not visible in the hand specimen, consists of subhedral colourless laths in thin section. Two spinels occur here; one is opaque, black, aluminium- and manganese- rich jacobsite and the other is transparent, orange, zinc- and magnesium- rich galaxite. Manganese-bearing calcite is intimately intergrown with ribbeite. The mcgovernite-like mineral occurs sparingly. The band containing ribbeite grades continuously into the third band.
The third band is a 2-mm-wide band of manganese-bearing calcite, nearly identical in composition to the calcite associated with ribbeite. These data imply that this band is of primary metamorphic origin rather than being a late-stage vein. The calcite unit in turn grades into the fourth band.
The fourth band consists almost entirely of equigranular alleghanyite, but with some galaxite, jacobsite and calcite.
The textural relations imply that large numbers of ribbeite grains separately formed in a prograde metamorphic environment only millimeters from the unit where alleghanyite of the same composition formed.
It is suggested that leucophoenicite subgroup minerals formed in the absence of fluorine. There is indeed no detectable fluorine in ribbeite and only a minor amount of fluorine in the associated alleghanyite. It is also pointed out that members of the leucophoenicite subgroup are limited in their manganese contents, especially in that they usually contain measurable amounts of calcium, zinc or magnesium substituting for manganese. For example jerrygibbsite from the Kombat mine.
Because ribbeite is polymorphic with alleghanyite, the manganese analogue of norbergite could be expected to occur with ribbeite and pyrochroite. The absence in this paragenesis of the manganese analogue of norbergite or of a leucophoenicite subgroup polymorph of such a phase implies that they may not be stable phases, at least at the Mn/(Mn + Fe + Mg) ratios observed (AM 72.213-216).

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