Olmiite

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Formula: CaMn[SiO₃(OH)](OH)
Nesosilicate (insular SiO₄ groups), manganese-bearing mineral, forms a series with poldervaartite
Crystal System: Orthorhombic
Specific gravity: 3.05 measured, 3.102 calculated
Hardness: 5 to 5½
Streak: White
Colour: Pale to intense reddish pink
Luminescence: Many specimens Fluoresce deep red under short wave UV
Environments

Sedimentary environments
Hydrothermal environments

Poldervaartite amd olmiite are similar in appearance and physical properties, and they form a series together; they are difficult to impossible to tell apart without chemical analysis. If Mn is dominant at the second cation site the mineral is olmiite, but if Ca is dominant it is poldervaartite.
Poldervaartite was approved in 1992, and the type specimen, from the Wessels mine, was later determined to be, indeed, poldervaartite. Many specimens, however, subsequently found at the N'Chwaning II mine, also were assumed to be poldervaartite, but after olmiite was discovered (approved 2006) many poldervaartites from this locality were re-examined, and all of them proved to be olmiite, so olmiite turned out to be much more common than the now-known-to-be-rare poldervaartite (MinRec.39.5.373).

Localities

At the type locality, the N'Chwaning II Mine, N'Chwaning Mines, Kuruman, Kalahari manganese field, Northern Cape, South Africa, olmiite occurs as a product of hydrothermal alteration as wheat-sheaf aggregates consisting of pale to intense reddish pink minute crystals (MM 71.2.193-291).
The mineralised fissure zone where the olmiite occurred initially was found to contain hematite and gaudefroyite. Later, abundant fibrous celestine was identified at the base of the fissure, and, shortly thereafter, cream and then red olmiite was encountered. Associated minerals included oyelite, andradite, celestine, calcite, gageite, caryopilite, sturmanite, bultfonteinite and poldervaartite (R&M 87.2.150-155).

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