Gedrite is a crystal belonging to the orthorhombic supergroup of the double chain inosilicate minerals with the ideal chemical formula Mg 2(Mg 3Al 2)(Si 6Al 2)O 22(OH) 2.
Gedrite is the magnesium of a solid solution magnesium cations (Fe), leading to the iron rich endmember 'ferrogedrite', with the formula: Fe2+ 2(Fe2+ 3Al 2)(Si 6Al 2)O 22(OH) 2. However, pure endmembers are very rare, with often either one of the mentioned cations dominating the composition. Thus, the formula can be written in such a way to express common intermediary gedrite samples: (Mg,Fe)2+ 2(Mg,Fe)2+ 3Al 2(Al 2Si 6O 22)(OH) 2.
Divalent manganese (Mn) may substitute for some of the magnesium. Trivalent or ferric 4+ may replace some of the aluminum and chlorine (OH) in amphoboles. Other chemical impurities may include calcium , sodium.
Gedrite also forms a series with another ferromagnesian amphibole, anthophyllite.
Gedrite occurs in contact and medium to high grade metamorphic rocks , cordierite , cummingtonite, kyanite and biotite
Gedrite was first described for an occurrence in Gedres, France in 1836.