Gedrite
Gedrite on a feldspar matrix
General
Category	Inosilicate, ferromagnesian amphibole group
Formula (repeating unit)	Mg2(Mg3Al2)(Si6Al2)O22(OH)2
IMA symbol	Ged[1]
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
Identification
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	[2][3][4][5]

Gedrite is a crystal belonging to the orthorhombic ferromagnesian subgroup of the amphibole supergroup of the double chain inosilicate minerals with the ideal chemical 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 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 impurities 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.^[2]

Gedrite was first described for an occurrence in Gèdre, Hautes-Pyrenees, France in 1836.^[3]

References