spinelle

OBS: Comme [...] les spinelles au sens large, ce sont des corps à faciès octaédriques quand ils sont bien cristallisés; [...] Les alumineux, ceux de cette famille, sont généralement transparents, souvent colorés [...], durs, d = 7,5 à 8, et de densité de 3,55 [...], indice variant autour de 1,8 donnant un bel éclat vitreux.
OBS: Formule chimique : MgAl[indice 2]O[indice 4]

OBS: The magnesium may be replaced in part by ferrous iron, and the aluminum by ferric iron. Spinel has great hardness, usually forms octahedral crystals (isometric system), varies widely in color (from colorless ... to black), and is used as a gemstone. It occurs typically as products of contact metamorphism of impure dolomitic limestones, and less commonly as an accessory mineral of basic igneous rocks; it also occurs in alluvial deposits.