GRANDIDIERITE

    Class : Silicates
    Subclass : Nesosilicates
    Crystal system : Orthorhombic
    Chemistry : (Mg,Fe)Al3(BO3)(SiO4)O2
    Rarity : Rare

Grandidierite is a rare mineral, accessory to aplites and pegmatites rich in boron, and in certain rocks of regional metamorphism. It owes its name to the French geographer and naturalist Alfred Grandidier, who discovered it in Madagascar. It occurs in cleavable masses or in grains, sometimes in elongated orthorhombic prisms exceptionally terminate in a pyramid, and more rarely in tabular crystals. Translucent to transparent, grandidierite is dark bluish green to deep blue. It is a mineral occasionally cut into gemstone.

Main photo : Grandidierite from Tranomaro, Anosy, Madagascar © Rob Lavinsky

Grandidierite from Tranomaro, Anosy, Madagascar © Rémi Bornet
0.32 ct grandidierite from Tuléar, Madagascar © Rémi Bornet
0,72 ct grandidierite from Tuléar, Madagascar © Rémi Bornet
2,84 ct grandidierite from Tuléar, Madagascar © Rémi Bornet

Grandidierite in the World

It is a mineral that comes mainly from Madagascar, although it has been reported in several other places on the planet (especially in Malawi), all the beautiful specimens come from Madagascar. Superb crystals come from Marotrana which provided imperfect 15 cm prisms and perfect, finished 8 cm crystals, but the Ambaosara and Ampamatoa deposits also yielded large specimens.

Grandidierite in France

This mineral is not present in the French underground.

Twinning

No twin known for this mineral species.

Fakes and scams

No scam known for this mineral.


Hardness : 7.5
Density : 2.98 to 2.99
Fracture : Conchoidal
Trace : White

TP : Translucent to transparent
RI : 1.590 to 1.639
Birefringence : 0.033 to 0.037
Optical character : Biaxial -
Pleochroism : Visible
Fluorescence : None

Solubility : Insoluble


Magnetism : None
Radioactivity : None