Gedrite
Mg2(Mg3Al2)(Si6Al2)O22(OH)2IMA 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]
H-M symbol: (2/m 2/m 2/m)
c = 5.3 [Å]; Z = 4
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
- ^ Warr, L.N. (2021). "IMA–CNMNC approved mineral symbols". Mineralogical Magazine. 85 (3): 291–320. Bibcode:2021MinM...85..291W. doi:10.1180/mgm.2021.43. S2CID 235729616.
- ^ a b http://www.handbookofmineralogy.org/pdfs/gedrite.pdf Handbook of Mineralogy
- ^ a b http://webmineral.com/data/Gedrite.shtml Webmineral
- ^ http://www.mindat.org/min-1665.html Mindat
- ^ IMA Master List
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