Gedrite - Encyclopedia

Class : Silicates
Subclass : Inosilicates
Crystal system : Orthorhombic
Chemistry : (Mg,Fe)₅Al₂(Si₆Al₂)O₂₂(OH)₂
Rarity : Common

Gedrite belongs to the group of orthorhombic amphiboles and forms two series with magnesiogedrite and ferrogedrite. Formerly classified as a variety of anthophyllite ; gedrite is now considered a mineral species in its own right which is distinguished from anthophyllite by its high aluminum contents. It is an amphibole of medium to high metamorphic grade, and rocks of contact metamorphism. It owes its name to its location of discovery : the Héas valley near Gèdre (Hautes-Pyrénées, France). Gedrite generally occurs in fibrous masses, sometimes in asbestiform aggregates (partly making up amosite), rarely in unfinished prismatic crystals. It is dark greenish brown to dark grayish green, sometimes brown. As with other asbestos, it was used in construction, insulation, the manufacture of fireproof materials, etc...

Main photo : Gédrite from Väster Silvberg, Smedjebacken, Dalarna County, Sweden © Michael Roarke

Gedrite in the World

Gedrite is known in beautiful crystals at Snarum (Norway) and in several sites in the United States, notably Grafton (Maine) and Chester (Connecticut). Penge (Northern Transvaal, South Africa) was home to an industrial exploitation of asbestiform gedrite where the fibers commonly reached 30 cm with a record at 76 cm. These are the largest known specimens.

Right photo : Gedrite from Chester, Middlesex County, Connecticut, USA © Harold Moritz

Gedrite in France

In France, the altered serpentinite of Cau, near Decazeville (Aveyron), gave crystals of asbestiform gedrite measuring 6 cm. It is also reported in the serpentinites and peridotites of Haute-Loire (Desges, Paulhac, Venteuges, Brioude, etc...) and at Gavarnie-Gèdre (Hautes-Pyrénées) its type locality.

Twinning

No twin known for this mineral species.

Fakes and treatments

No fakes recorded for this mineral species.

Hardness : 5.5 to 6
Density : 3.15 to 3.26
Fracture : Fibrous
Streak : White

TP : Translucent to transparent
RI : 1.671 to 1.690
Birefringence : 0.019
Optical character : Biaxial -
Pleochroism : Low
Fluorescence : None

Solubility : Insoluble

Magnetism : Paramagnetic
Radioactivity : None

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