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.

Characteristics[edit | edit source]

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.

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