MICROLITE

    Class : Oxides and hydroxides
    Subclass : Oxides
    Crystal system : Cubic
    Chemistry : (Ca,Na)2Ta2O6(O,OH,F)
    Rarity : Quite common

Microlite is a complex tantalum oxide that forms a continuous series with pyrochlore, its equivalent to niobium. These two minerals belong to the pyrochlore group of which it constitutes the two main representatives. Microlite is found mainly in sodolithic pegmatites associated with lepidolite and sometimes spodumene. Its name comes from the Greek mikros (small), in allusion to the dimensions of the defining crystals. Microlite is a frequently metamict mineral which crystallizes in octahedra sometimes modified by the shapes of the cube and the dodecahedron. Its color varies : pale yellow to brown, black-brown to red-brown, sometimes red. it is the principal ore of tantalum and an important rare earth ore.

Main photo : Microlite from Hollister prospects, Connecticut, USA © Harold Moritz

7.5 cm microlite from Amelia County, Virginia, USA © Kelly Nash
Microlite from Ipê mine, Golconda, Minas Gerais, Brazil © Jordy Fabre
Microlite from Ipê mine, Golconda, Minas Gerais, Brazil © Vincent Bourgoin
Microlite from Hewitt Gem Quarry, Connecticut, USA © David Busha

Microlite in the World

Large 7.5 cm octahedra on cleavelandite were extracted from the Rutherford mine near Amelia (Virginia): these are the best known crystals. The lithic pegmatites of the Appalachians are, as a whole, fairly constantly mineralized in microlite (Portland, Topsham, Chesterfield, etc...). The famous Brazilian pegmatites of Minas Gerais, so rich in fabulous crystals of many species, have also yielded large microlite crystals (Virgem da Lapa, Sao Joao del Rei...). The Canadian deposit of Mattawan (Ontario) for its part produced beautiful brown octahedrons measuring 1 cm.

Microlite in France

In France, microlite is relatively common in the lepidolite pegmatites of the Ambazac mountains near Limoges (Haute-Vienne) as well as in the leucogranite of Echassières (Allier).

Twinning

Twins are known on {111} (spinel twin law).

Fakes and treatments

No fakes recorded for this mineral species.


Hardness : 5 to 5.5
Density : 5.9 to 6.4
Fracture : Irregular to sub-conchoidal
Streak : Light yellow to brown

TP : Transparent to opaque
RI : 1.930 to 2.020
Birefringence : 0
Optical character : None
Pleochroism : None
Fluorescence : None

Solubility : Hydrochloric acid

Magnetism : NoneRadioactivity : Very low