Magnesio-riebeckite

minerals

stilpnomelane

epidote

shortite

Formula: ☐Na2(Mg3Fe3+2)Si8O22(OH)2
Inosilicate (chain silicate), riebeckite group
Specific gravity: 3.19
Hardness: 5 to 5&$189;
Streak: Pale grey to bluish-grey
Colour: Light blue to blue-black
Common impurities:Ti,Al,Mn,Ca,K,H2O
Environments

Igneous environments
Carbonatites
Metamorphic environments

Magnesio-riebeckite occurs in sodium-rich granulite, ironstone, ferruginous metacherts, greenschist facies schist and carbonatites. Associated minerals include stilpnomelane, epidote, quartz, muscovite and winchite (HOM, Webmin).

Localities

At the type locality, the Alto Chapare mining district, Chapare Province, Cochabamba, Bolivia, magnesio-riebeckite occurs in veins in slightly metamorphosed sandstone and carbonates associated with evaporites (Mindat).

In Permian clays in Kazakhstan, magnesio-riebeckite occurs in a low temperature environment with opal (Dana).

In the Green River formation of the western USA, magnesio-riebeckite forms as an authigenic mineral around detrital grains of hornblende, and is associated with aegirine and shortite (Dana).

Alteration

serpentine, aegirine and quartz to magnesio-riebeckite and H2O
Mg3Si2O5(OH)4 + 2NaFe3+Si2O6 + 2SiO2 → Na2 (Mg3Fe3+2)Si8O22(OH)2 + H2O
(DHZ 2A p511)

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