Gedrite

gedrite

cordierite

cummingtonite

sapphirine

Formula: ☐Mg2(Mg3Al2)(Si6Al2)O22 (OH)2
Inosilicate (chain silicate), gedrite root name group, magnesium-iron-manganese amphibole subgroup, amphibole supergroup, forms series with anthophyllite
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:

Metamorphic 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.

Localities

At Yellowknife, North Slave Region, Northwest Territories, Canada, gedrite occurs in the meta-sedimentary rocks to the east of the Sparrow Lake granitic pluton, which lies about 40 km northeast of the town of Yellowknife. The gedrite-bearing rocks may be termed metagreywacke and argillite. Primary features, such as graded bedding and layering, are commonly preserved in these rocks and gedrite is restricted to certain layers. The gedrite has a dark green to brownish needle-like habit and commonly forms sheaf-like aggregates. The associated minerals are cordierite, biotite, almandine and oligoclase. Other phases, that may or may not be present, are ilmenite, magnetite and pyrrhotite. Gedrite-cummingtonite associations are observed most frequently in contact zones between gedrite-bearing rocks and siliceous concretions in which cummingtonite is abundantly developed. The concretions occur as thin layers and ellipsoidal lenses within the metasedimentary rocks. (CM 11.1012-1015).

At the type locality, Gavarnie-Gèdre, Argelès-Gazost, Hautes-Pyrénées, Occitanie, France, the low pressure high temperature metamorphic dome of Gavarnie-Gèdre harbours a small but special cordierite and orthoamphibole-bearing enclave embedded within an anatectic (re-melting) granite. Compared to other cordierite - orthoamphibole occurrences described in the literature, the enclave shows singular characteristics:
(1) its mineral composition is unique with cordierite + gedrite + biotite + staurolite + garnet + spinel + corundum + ilmenite and sulphides, but lacking quartz, plagioclase and aluminosilicate minerals.
(2) its chemical composition, with low SiO2, high Al2O3, K2O and TiO2 and high Fe2O3T/MgO.
The cordierite - orthoamphibole bearing rocks also occur as layers in stromatic migmatites that show a mineral composition (biotite + cordierite + gedrite + plagioclase + quartz + ilmenite + garnet +/- staurolite) and a chemical composition more akin to other described occurrences.
The cordierite - orthoamphibole enclave records a succession of metamorphic reactions:
(1) local cordierite - orthoamphibole growth at the expense of biotite with release of K2O
(2) development of coronas of cordierite +/- ilmenite +/- garnet around staurolite
(3) isochemical breakdown of staurolite to cordierite + spinel + corundum
In the gedrite-bearing layers in interlayered migmatites, the formation of the cordierite - orthoamphibole assemblage involves the consumption of biotite, plagioclase and quartz (CM 55.245-281).

At Simiuttat Island, Nuuk, Sermersooq, Greenland, the golden iridescence observed in orthoamphiboles from the Nuuk district has been shown to be the result of the interference of light reflected from alternate, coherent gedrite and anthophyllite exsolution lamellae (MM 64.5.885-889).

At the Ammonoosuc Volcanics, Grafton, Oxford County, Maine, USA, gedrite occurs in the metamorphosed volcanics at several localities, forming dark green needles or platy prisms about one centimetrer long, with some tendency to sheaf-like aggregation. It is partially altered to a brownish-yellow mineral of low birefringence that may be a serpentine or a chlorite (AM 46.734-740).

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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