RIEBECKITE

    Class : Silicates
    Subclass : Inosilicates
    Crystal system : Monoclinic
    Chemistry : Na2Fe5Si8O22(OH)2
    Rarity : Uncommon

Riebeckite belongs to the alkaline amphibole family. It forms a series with glaucophane, the intermediate terms being called "crossite". It is a classic mineral of alkaline granites and especially of nepheline syenites in association with aegyrine ; it is rarer in granitic pegmatites. The asbestiform facies (crocidolite variety) is mainly found in metamorphic environments, particularly in metamorphosed banded iron formations. It was named in honor of the German explorer Emil Riebeck who discovered the mineral. Riebeckite is black to bluish-black in color for the asbestiform varieties and has a good cleavage {110}. Its crystals are prismatic to acicular, and frequently asbestiform.

Main photo : Riebeckite from Zagi Mountain, Pakistan © Eugene & Sharon Cisneros

Riebeckite from Mount Malosa, Malawi © Martin Gruell
Riebeckite from Eureka Tunnel, Colorado, USA © Dan Polhemus
Asbestiform riebeckite from Transvaal, South Africa © Jolyon Ralph
South African hawk's eye cabochon (crocidolite)

Riebeckite in the World

The most beautiful Riebeckite crystals come from syenitic pegmatites, where they reach more than 20 cm in length, notably at Sultan Hamud (Kenya), with metric crystals even reported from Alter Pedroso (Portugal). The asbestiform variety is well represented in the BIF of South Africa in the Griquatown region, with its two variants, used as ornamental stones: hawk's eye (unaltered blue crocidolite) and tiger's eye (crocidolite turned golden brown-yellow by oxidation of iron).

Richelsdorfite in France

In France, the most beautiful riebeckite comes from Piana (Corsica) but it is also reported in Velay (Haute-Loire), in St-Véran (Hautes-Alpes) and in St-Jean-de-Maurienne (Savoie).

Twinning

No twinning known for this mineral species.

Fakes and treatments

No fakes listed for this mineral species.


Hardness : 5 to 5.5
Density : 3
Fracture : Conchoidal to irregular
Streak : Gray white

TP : Translucent to opaque
RI : 1.680 to 1.706
Birefringence : 0.008
Optical character : Biaxial -
Pleochroism : Strong
Fluorescence : None

Solubility : Insoluble

Magnetism : ParamagneticRadioactivity : None

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