Osumilite: Properties and Occurrences

Osumilite is a potassium-sodium-iron-magnesium-aluminium silicate mineral that is extremely rare. It was discovered in a volcanic rock in Sakkabira, Kyusyu, Japan, and was named after the Osumi province. It belongs to the cyclosilicate milarite group (also known as the milarite-osumilite group). Although osumilite is most likely a common mineral, it has always been misidentified as cordierite. It appears to be common in volcanic rocks and inclusions in them.

The chemical formula for osumilite is (K,Na)(Fe,Mg)2(Al,Fe)3(Si,Al)12O30. It is a rare mineral, but it is one of two minerals that give their names to a fairly large group of silicates known as the Milarite – Osumilite Group. This group is also known as the Milarite Group, the Osumilite Group, and, as it is done here, the Milarite – Osumilite Group.

General Information

  • Category: Cyclosilicate
  • Formula (repeating unit): (K,Na)(Fe,Mg)2(Al,Fe)3(Si,Al)12O30
  • Crystal system: Hexagonal
  • Crystal class: Dihexagonal dipyramidal (6mmm)
  • Color: Black, dark blue, dark brown, pink, gray

Properties

It is translucent and the typical coloring is either blue, black, brown, or gray. It displays no cleavage and has a vitreous luster. Osumilite has a hardness between 5-6 on the Mohs hardness scale.

  • Crystal habit: Crystals tabular to prismatic also anhedral and massive
  • Twinning: Rarely
  • Cleavage: None
  • Fracture: Subconchoidal
  • Mohs scale hardness: 5 – 6
  • Luster: Vitreous
  • Streak: Blue-gray
  • Diaphaneity: Translucent
  • Specific gravity: 2.62 – 2.64

Structure

Osumilite’s primary structural unit is an unusual double ring with the formula Si12O30. Normal cyclosilicate rings are made up of six silicate tetrahedrons; Si6O18. The double ring is composed of two normal rings joined together by the sharing of six oxygens, one from each tetrahedron in each six-membered ring (notice the loss of six oxygens in the double ring formula). The design is similar to the dual wheels of a tractor trailer.

Osumilite’s hexagonal crystal structure is an unusual molecular structure. The primary unit has the formula Si12O30 and is a double ring. Rings of normal cyclosilicate are composed of six silicate tetrahedrons; Si6O18. Two normal rings are linked in a double ring structure by sharing six oxygens, one from each tetrahedron in each six-membered ring.

Occurrence – In cavities and the groundmass of rhyolite and dacite, and in high-grade contactmetamorphic rocks and xenoliths.

Osumilite was discovered in volcanic rocks near Osumi, Japan, as grains. Because of their similar coloring, it was mistaken for the mineral cordierite. It can be found in high-grade metamorphic rocks, xenoliths, and rhyolite and dacite groundmasses.

Osumilite can be found in Oregon’s Obsidian Cliffs, Sardinia, Italy, Kagoshima and Yamanashi Prefecture, Japan, and Germany’s Eifel district. Osumilite pseudomorphs have been found in a variety of ultrahigh-temperature rocks, including those found in southern Madagascar.