|The Milarite - Osumilite
Group of Minerals
The Milarite - Osumilite Group is sometimes simply called the Osumilite Group, but it is also sometimes called the Milarite Group. It is a group of similarly structured cyclosilicates. The primary structural unit of this group is a most unusual double ring, with a formula of Si12O30. Normal rings of cyclosilicates are composed of six silicate tetrahedrons; with a formula of Si6O18. The double ring is made of two normal rings that are linked together by sharing six oxygens, one from each tetrahedron in each six membered ring (notice the loss of six oxygens in the double ring formula). The structure is analogous to the dual wheels of a tractor trailer.
The general formula for this group is X1-2Y2-3Z3(Si, Al)12O30 - 0-2H2O. The X ion can be either calcium, potassium, sodium and/or barium. The Y ion can be either iron, lithium, yttrium, magnesium, manganese, sodium, tin, titanium, zinc and/or zirconium. The Z ion can be either aluminum, beryllium, iron, lithium, magnesium and/or boron. Some members contain no aluminum ions or water molecules.
The Milarite - Osumilite Group is composed of rather rare minerals. Only sugilite, milarite and osumilite are somewhat well known minerals from this group. All others are very rare to extremely rare in occurrence.