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:
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).
Back to Minerals