Gedrite is a crystal belonging to the orthorhombic ferromagnesian subgroup of the amphibole supergroup of the double chain inosilicate minerals with the ideal formula: Mg2(Mg3Al2)(Si6Al2)O22(OH)2

Gedrite on a feldspar matrix
CategoryInosilicate, ferromagnesian amphibole group
(repeating unit)
Strunz classification9.DD.05
Crystal systemOrthorhombic
Crystal classDipyramidal (mmm)
H-M symbol: (2/m 2/m 2/m)
Space groupPnma
Unit cella = 18.59, b = 17.89
c = 5.3 [Å]; Z = 4
ColorWhite, gray, brown, green, black
Crystal habitBladed and prismatic crystals; fibrous and sheath like aggregates.
Twinningcontact twinning
Cleavage56 and 126° - good; {210} perfect
Mohs scale hardness5.5 - 6
LusterVitreous to silky
DiaphaneityTranslucent, will transmit light on thin edges.
Specific gravity3.18 - 3.33
Optical propertiesBiaxial (+)
Refractive indexnα = 1.671 nβ = 1.681 nγ = 1.690
Birefringenceδ = 0.019
PleochroismWeak to moderate
2V angleMeasured: 75°

Gedrite is the magnesium (Mg) rich endmember of a solid solution series, with divalent magnesium cations readily replaced with ferrous iron (Fe), leading to the iron rich endmember 'ferrogedrite', with the formula: Fe2+2(Fe2+3Al2)(Si6Al2)O22(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+3Al2(Al2Si6O22)(OH)2.

Divalent manganese (Mn) may substitute for some of the magnesium. Trivalent or ferric iron, or titanium4+ may replace some of the aluminum (Al). Fluorine and chlorine are common substitutes for the hydroxyl (OH) in amphoboles. Other chemical impurites may include calcium, sodium, and potassium.

Gedrite also forms a series with another ferromagnesian amphibole, anthophyllite.

Gedrite occurs in contact and medium to high grade metamorphic rocks in association with garnet, cordierite, anthophyllite, cummingtonite, sapphirine, sillimanite, kyanite, quartz, staurolite and biotite.[1]

Gedrite was first described for an occurrence in Gedres, France in 1836.[2]


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