Riebeckite

inosilicate

aegirine

Formula: ☐Na2(Fe2+3Fe3+2) Si8O22(OH)2 inosilicate (chain silicate) amphibole
Specific gravity: 3.4
Hardness: 5 to 5½
Streak: Greenish brown
Colour: Black, blue, grey, brown
Solubility: Insoluble in hydrochloric, sulphuric and nitric acid
Environments:

Plutonic igneous environments
Pegmatites
Carbonatites
Metamorphic environments

Riebeckite occurs most commonly in igneous rocks such as granite, syenite, nepheline syenite and related pegmatites, and in carbonatites. It is present in some schist of regional metamorphic origin.
Riebeckite also may be found in granite.
In contact metamorphic environments it may be associated with aegirine.

Alteration

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

Common impurities: Fe,Ti,Mg,Al,Mn

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