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Formula: ☐Na2(Mg3Fe3+2)Si8O22(OH)2
Crocidolite is a fibrous form of riebeckite or
magnesio-riebeckite.
Inosilicate (chain silicate), riebeckite group
Crystal System: Monoclinic
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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