Best viewed using Adobe Flash Player (8 or higher).
Download hereGENERAL DESCRIPTION
Spinel is an magnesium and aluminium oxide. But it is also the name of a group of minerals of composition AB2O4 (or A=Mg, Fe2+, Ni, Mn2+ or Zn whereas B=Al, Fe3+, Cr3+ and Mn3+).
The spinel "sensu-stricto" is magnesio-aluminous. It is formed in igneous rocks and in rocks having experienced contact metamorphism. The crystals are presented in the form of octahedra or of dodecahedra, generally transparent and with a red, red-pink to purple color but also blue, black, brown or green. The spinel often shows twins.
The crystals can be large-sized. One distinguishes pleonaste/hercynite which forms crystals with many facets and with a dark green to black color; picotite which is a chromiferous spinel (brown dark to black); the gahnite which is zinciferous (often black). The principal minerals of the group of the spinel gather chromite (Fe and Cr), magnetite (Fe2+ and Fe3+) and minium (Pb2+ and Pb4+).
Spinel deposits are numerous, but the large crystals (up to 120 mm of edge) coming from Amity and Franklin in the state from New York (USA) are remarkable such as the greenish spinels of Ambatomainly and the blue ones from Tiliana (Madagascar). Also in Québec, Brazil, Italy (Monte Somma, Vesuvius), Tajikistan (Kukh-i-Lal), Afghanistan (red-rosy; Jagdalak), Pakistan (often blue; from the valley of Hunza), Kenya (Kaiidado), Tanzania (Uluguru Mounts and Morogoro) and finally for the red gemmeous crystals with purple: alluvia and skarns of Mogok in Burma and Ratnapura in Sri Lanka.
In France, spinel is known in a number of localities related to the volcanism of the Massif Central, such as in Cantal, the Hate-Loire and Puy-de-Döme (and in the alluvia which result from it): Beaulieu in Cantal, Lavoute-Chilhac, around Puy-in-Velay in Haute-Loire, Arnats in Puy-de-Dôme, the Mont Sémiol in the Loire. But also in (or coming from) various basic rocks as in the island of Groix and various beaches of the Gulf of Morbihan (Brittany) and of Loire-Atlantique (Vendée), in various basic rocks as in Lherz (Ariège) or of the skarns (Costabonne, Pyrénées-Orientales) and in the alluvia of the Loire with Giens (Allier).
Did you know? The coloured and gemmy spinels are used in jewellery: superb red spinels were recently (2007) found in Sri Lanka and they reach amazing prices.
HISTORY : Name probably derived from Latin "spinella" meaning "thorn", ine reference to its crystals lines
Species would have been described when discovered in 1546 by Georgius Agricola (1494-1555). In 1779, Demestre made a complete description
Type-locality : unknown
ANCIENT NAME : Rubis balai, rubis spinelle, rubicelle, spinel
CHEMICAL FORMULA : Mg Al2O4
CRYSTAL SYSTEM : Cubic
COLOR : Colorless, brown, black, pink, red
DIAPHANIETY : Transparent to almost opaque
LUSTER : Vitreous, bright, dull
STREAK : White
MORPHOLOGIE : Octahedra crystals, seldomly cubic
HARDNESS : 7,5-8,0
CHEMICAL CLASS: 3,578
DENSITY : IV - Oxides and hydroxides
GROUP : Spinel
STRUNZ CLASS BEFORE 2001 : 4/B.01-10
STRUNZ CLASS AFTER 2001 : 4.BB.05
