Formula: ☐Mg2(Mg3Al2)(Si6Al2)O22
(OH)2
Inosilicate (chain silicate),
magnesium-iron-manganese amphibole subgroup,
amphibole supergroup
Crystal System: Orthorhombic
Specific gravity: 3.15 to 3.26 measured, 3.25 calculated
Hardness: 5½: to 6
Streak: White
Colour: Pale greenish-grey to brown
Environments:
Gedrite occurs in contact and medium to high grade metamorphic rocks where it may be associated with
garnet, cordierite,
anthophyllite, cummingtonite,
sapphirine, sillimanite,
kyanite, quartz,
staurolite or biotite.
Alteration
gedrite-ferro-gedrite and quartz to
enstatite-ferrosilite,
Fe-rich cordierite and H2O
(Mg,Fe2+)5Al4Si6O22(OH)2 + 2Si2 →
3(Mg,Fe2+)SiO3 + (Mg,Fe2+)2Al4Si5O18 +
H2O
In the pyroxene-hornfels facies
enstatite-ferrosilite may develop from
gedrite-ferro-gedrite according to the
above reaction
(DHZ 2A p134).
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