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Category Inosilicate, ferromagnesian amphibole group
(repeating unit)
Strunz classification 9.DD.05
Crystal system Orthorhombic
Crystal class Dipyramidal (mmm)
H-M symbol: (2/m 2/m 2/m)
Space group Pnma
Unit cell a = 18.59, b = 17.89
c = 5.3 [Å]; Z = 4
Color White, gray, brown, green, black
Crystal habit Bladed and prismatic crystals; fibrous and sheath like aggregates.
Twinning contact twinning
Cleavage 56 and 126° - good; {210} perfect
Fracture Splintery
Mohs scale hardness 5.5 - 6
Luster Vitreous to silky
Diaphaneity Translucent, will transmit light on thin edges.
Specific gravity 3.18 - 3.33
Optical properties Biaxial (+)
Refractive index nα = 1.671 nβ = 1.681 nγ = 1.690
Birefringence δ = 0.019
Pleochroism Weak to moderate
2V angle Measured: 75°
References [1][2][3][4]

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 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 gerdite 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]