GRUNERITE

    Class : Silicates
    Subclass : Inosilicates
    Crystal system : Monoclinic
    Chemistry : Fe7Si8O22(OH)2
    Rarity : Quite common

Grunerite belongs to the group of monoclinic amphiboles and forms a series with magnesiocummingtonite (magnesian), cummingtonite being the intermediate term. It is an amphibole that is mainly found in banded iron formations (BIFs) as well as in iron-rich rocks having undergone contact metamorphism. It was named in honor of the French chemist Louis Emmanuel Gruner who made the first analysis. Grunerite is iron-bearing and occurs in fibrous to acicular crystals, often united in fibroradiated aggregates. Its color is dark green, gray to greenish gray, sometimes brown.

Main photo : Fibrous grunerite (amosite) from Penge Mine, Limpopo, South Africa © Bruce Cairncross

Grunerite in the World

The asbestiform variety called "amosite" was exploited in Penge (South Africa), where the fibers measured up to 70 cm. The exact nature of this amosite is however debated, with some authors considering it to be a variety of gedrite. Good specimens of grunerite come from the metamorphosed iron formations of Wabush (Canada), Rockport and Cummington (Massachussetts). Good samples are also known in Finland.

Right photo : Grunerite from Helen-MacLeod Mines, Ontario, Canada © Maggie Wilson

Grunerite in France

In France, grunerite is reported at Collobrières (Var), its type locality. It is also present on the island of Groix (Morbihan), in Cauro-Bastelica near Ajaccio (South Corsica), in Pierrefitte (Hautes-Pyrénées) as well as in Ouégoa in New Caledonia.

Twinning

Multiple twins over {100} are known.

Fakes and treatments

No fakes recorded for this mineral species.


Hardness : 5 to 6
Density : 3.4 to 3.6
Fracture : Fibrous
Streak : White

TP : Translucent
RI : 1.663 to 1.729
Birefringence : 0.033 to 0.043
Optical character : Biaxial -
Pleochroism : Visible
Fluorescence : None

Solubility : Insoluble

Magnetism : NoneRadioactivity : None