The Spinel Group of Minerals
The Spinel Group contains over twenty members, but only a few are considered common. They are a group of oxides that have very similar structures. Named after their sole gemstone representative, spinel, this is an important group of minerals.
It includes one of the most important ores of iron, magnetite; an important ore of chromium, chromite, an important ore of lead, minium; a once important ore of manganese, iron and zinc; franklinite and many other interesting members.
The general formula of the Spinel Group is AB2O4. The A represents a divalent metal ion such as Magnesium, Iron, Nickel, Manganese and/or Zinc. The quad valent lead ion can also occupy this site. The B represents trivalent metal ions such as Aluminum, Iron, Chromium and/or Manganese, Titanium may also occupy this site with a +4 charge and lead at +2 can occupy this site. Solid solutioning is common in this group of minerals, meaning that they may contain certain percentages of different ions in any particular specimen.
The structure of spinel is based on the structure of diamond, which has the same high symmetry, 4/m bar 3 2/m. The position of the A ions is nearly identical to the positions occupied by carbon atoms in the diamond structure. This could explain the relatively high hardness and high density typical of this group. The arrangement of the other ions in the structure conform to the symmetry of the diamond structure. But, they disrupt the cleavage as there is no cleavage directions in any member of this group. The arrangement of the ions also favors the octahedral crystal habit which is the predominant crystal form and is in fact the trademark of the spinels. All members of this group that share the spinel structure show the same type of twinning that is named after spinel, called the Spinel Law.