Muséum national d'histoire naturelle

GENERAL DESCRIPTION

Diaspore is an aluminium hydroxide. It can be found in the laterites (red soils found in tropical countries) and their older equivalents, the bauxites (the main ore of aluminium).

Laterites are formed thanks to the weathering of aluminosilicates contained in igneous rocks (feldspars, certain pyroxenes and amphiboles).

Bauxite is formed by diagenesis of the laterites. The diagenesis is a physicochemical highly complex and variable process which affects any fresh deposit of sediments (often wet, porous and not very compact) by gradually transforming them into a rock, i.e., a material much more compact and endured (also called “diagenesis”). This transformation is often initiated by a progressive burying of the fresh deposits, exposing gradually them to increasing pressures and temperatures. In these rocks, the diaspore is frequent in volume.

But, on the other hand, it is very rare to observe individual crystals of diaspore other than under the microscope. In an extreme case of diagenesis and if the burying is very important (hundreds of meters), a more important mineralogical transformation takes place that is called metamorphism. At this stage, one can hardly recognize the starting sediment because all the minerals have recristallized as other minerals.

However, this is rather in these metamorphic rocks that one can find large crystals of diaspore. With even higher pressures and temperature, the diaspore dehydrates into corundum, as observed in the charnockites of Ceylon.

The crystals of diaspore can flat and be lengthened, colourless to grayish, usually transparent with a vitreous luster. They can also form twins resembling the letter V. Crystals up to 40 cm in length were discovered. It is associated to other species, such as the corundum (Al2O3), magnetite (Fe3O4) or the spinel (MgAl2O).

Concerning the deposits, the diaspore is present in many localities (what includes the numerous mines of bauxite). But very few of these mines produced remarkable samples of diaspore. It is present in the province of Mugla in Turkey, where large gemmeous crystals were discovered. The metamorphic deposits producing remarkable diaspore are Bisbee, Red Mountain in Arizona; Champion Mines in California or the district of Rosita in Colorado, to quote only some of the many US localities.

In France, it is present in the historical mines of bauxite of Les Baux of Provence in the Rhône delta (from where bauxite tooks its name) but also in many mines where weathering in relation to hydrothermalism are important such as in Chizeul (Saone-et-Loire), Costabonne (Pyrénées-Orientales) but also in many crystalline massifs under weathering as in the Vanoise (Savoie, French Alps).

The type of this species is preserved in the collection of R-J Haüy, and is kept at the Muséum national d’Histoire naturelle in Paris. It is sample H 1556, extracted from the Mramorsk (Mramorskoye) Zavod mine near the village of Kosoi-Brod, located 35 km at the south of Yekatrinburg in the Ural Mountains of Russia. This new species was published by Rene-Just Haüy in 1801.

Diaspore

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Mineralogy