Spinels

The spinels are among mineral Oxyde S among most important and represent a notable source of metals for the mining. __TOC

Description

The structure of spinels is based on stacking 3 C and the two types of cavities, octahedral and tetrahedral, are occupied at the same time, but partially.

The general formula of spinels is X₂YO₄: one distinguishes spinels between “ normal ” (NR) and “ opposite ” (I) according to whether two cations X occupy two octahedral or octahedral and a tetrahedron - the cation Y going in the free cavity left by X.

The spinels are additional minerals in the volcanic rocks and metamorphic, and are also in clastic rocks.

the Magnétite is most abundant and forms economic iron lodgings of importance.
the Spinelle (mineral which bears the same name that the group) is in the metamorphic rocks of high degree and in the peridotites of the coat.
the Chromite is especially in the ultramafic volcanic rocks and can form lodgings of economic importance.
the Franklinite is almost only in the Calcaire S of Franklin (NJ, the United States), associated with Zincite and Willemite (Zn₂SiO₄).
the Ringwoodite is the phase of high pressure of Mg₂SiO₄, stable in the zone of transition from the coat; with more important depths it is the structure Pérovskite which becomes stable.

Note on classification

In several works, the spinels are classified into normal or opposite according to the distribution in the octahedral and tetrahedral sites of two cations trivalent and a bivalent cation. This criterion is not most general. In fact:

at least 30 different cations, with variable valence between +1 and +6, can be in the structure spinel
the Spinelle becomes a kind of hybrid of which the structure is often indicated like “seven the eighth I”, whereas ordonnément, even if the if partial one, of the cations shows that both octahedral site contain the same average cation: it is thus about a spinel normal.

Notes and references of the article

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