Hydroxylclinohumite

minerals

nesosilicate

metamorphic

skarn

Formula: Mg9(SiO4)4(OH)2 nesosilicate (insular SiO4 groups)
Specific gravity: 3.13
Hardness: 6½
Streak: White
Colour: Pale yellow to orange-yellow, almost colourless
Environments:

Metamorphic environments

Hydroxylclinohumite occurs in skarn rims around dolomitic marble xenoliths in gabbroic rocks.

Alteration

diopside, dolomite and H2O ⇌ hydroxylclinohumite, calcite and CO2
2CaMgSi2O6 + 7CaMg(CO3)2 + H2O ⇌ Mg9(SiO4)4(OH)2 + 9CaCO3 + 5CO2
In the nodular dolomites, hydroxylclinohumite associated with calcite occurs in a narrow zone in the central parts of the nodules due to the above reaction (DHZ 1A p264).

forsterite, dolomite and H2O to calcite, hydroxylclinohumite and CO2
4Mg2SiO4 + CaMg(CO3)2 + H2O → Mg9(SiO4)4(OH)2 +CaCO3 + CO2
A forsterite-clinohumite assemblage in the silica-rich dolomite in the aureole of the Alta granodiorite in Utah, USA, is probably due to the above reaction (DHZ 1A p264).

tremolite, dolomite and H2O ⇆ hydroxylclinohumite, calcite and CO2
Ca2Mg5Si8O22(OH)2 + 13CaMg(CO3)2 + H2O ⇆ 2Mg9(SiO4)4(OH)2 + 15CaCO3 + 11CO2
(DHZ 1A p264).

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