Dumortierite - Encyclopedia

    Class : Silicates
    Subclass : Nesosilicates
    Crystal system : Orthorhombic
    Chemistry : Al7(BO3)(SiO4)3O3
    Rarity : Rare


Dumortierite is a rare mineral present in different rocks (granulites, pegmatites and quartzite from Madagascar, garnet gneiss from France), having in common the presence of boron which suggests high temperature magmatic emanations (pneumatolites). It owes its name to the French paleontologist Eugène Dumortier. It generally occurs in compact fibrous or fibroradiated aggregates, in columnar masses, rarely in distinct prismatic crystals, of a magnificent cobalt blue to lavender blue color. It is a locally important mineral for the manufacture of refractories and ceramics (Oreana in Nevada, Minas Gerais in Brazil). It is also known to play an ancillary role as an insulating material and sometimes as a fine or ornamental stone.

25.90 ct dumortierite from Italy
39.65 ct dumortierite of  from Africa
40.30 ct dumortierite from Nevada, USA
34.30 ct dumortierite from Peru

Dumortierite in the world

Exceptional crystals are known in Ambatalahinanahary and in other pegmatites of Madagascar : deep blue in color, embedded in quartz, they reach up to 10 cm by 0.3 cm. These are the best crystals in the world. In Brazil, remarkable crystals (nearly 10 cm), similar in quality to the Malagasy ones, are extracted around Boquira (Bahia). Fibrous masses radiated in a superb blue reminiscent of lapis lazuli are extracted near Los Angeles (California). Very abundant in Oreana (Nevada), it was exploited there as a refractory. This is currently the case in Minas Gerais (Brazil), where large fibrous masses have ruthlessly passed through the crusher... In Europe, the finest dumortierite comes from Tvedestrand (Norway).

Dumortierite in France

In France, we should point out the type locality of Beaunan (Rhône) where dumortierite appears in the form of microscopic blue-violet fibers in a pegmatite vein inserted in a micachist.

Twinning

Twinning is common on {110}, sometimes producing trills.

Fakes and scams

No fake inventories for this mineral species. Quartz crystals containing dumortierite fibers were able to see some of their faces repolished in order to improve the visibility of their inclusions.



Hardness : 7 to 8
Density : 3.21 to 3.41
Fracture : Undetermined
Trace : White to blue



TP : Translucent
RI : 1.659 to 1.692
Birefringence : 0.027
Optical character : Biaxial -
Pleochroism : Strong
Fluorescence : White to purple


Solubility : Insoluble

Magnetism : None
Radioactivity : None