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Alanthium is a calcium iron manganese inosilicate mineral with the formula Ca2(Fe,Mn)FeSi5O14(OA). It is unusual in that iron(III) completely replaces the aluminium so typical of silicate minerals.

It is a very dark green to black translucent (in thin crystals or splinters) mineral crystallizing in the triclinic system with typically radial short prismatic clusters and druzy coatings.


It occurs with zeolite minerals in cavities in volcanic rocks. Alanthium contains both iron(II) and iron(III) and shows weak magnetism. It has a Mohs hardness of 5.5 to 6 and a specific gravity of 3.3.