Si8O22(OH)2 inosilicate (chain silicate)
Specific gravity: 3.4
Hardness: 5 to 5½
Streak: Greenish brown
Colour: Black, blue, grey, brown
Solubility: Insoluble in hydrochloric, sulphuric and nitric acid
Plutonic igneous 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.
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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