THE MINERAL SUGILITE

Chemistry: KNa₂Li₃(Fe, Mn, Al)₂Si₁₂O₃₀, Potassium Sodium Lithium Iron Manganese Aluminum Silicate.
Class: Silicates
Subclass: Cyclosilicates
Group: Milarite - Osumilite
Uses: As an ornamental and semi-precious stone used in jewelry and as a mineral specimen.
Specimens

Sugilite is a somewhat obscure mineral named for the Japanese geologist who discovered the first specimens in 1944, Ken-ichi Sugi. It is becoming very popular in the jewelry trade. It does not form well shaped crystals but is usually massive. This is OK, as it is used mostly for ornamental and semiprecious stone purposes. It has a very distinctive opaque purple color when found in its most desirable color shades. The polished stones are mostly opaque with an almost waxy luster and a deep reddish purple color. It has been described as a purple turquoise although there is no relation between the two minerals. Its jewelry uses are becoming widespread and sugilite is being used in jewelry styles with turquoise, malachite and coral. Some sugilite has been cut into gemstones, but these are very rare.

The primary structural unit of sugilite is a most unusual double ring, with a formula of Si₁₂O₃₀. Normal rings of cyclosilicates are composed of six silicate tetrahedrons; 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 and is shared by other members of the Milarite - Osumilite Group.

PHYSICAL CHARACTERISTICS:

Color is purple, brown to yellow, pale pink and even black.
Luster is vitreous to dull or waxy.
Transparency: Crystals are translucent to opaque.
Crystal System is hexagonal; 6/m 2/m 2/m.
Crystal Habits include rare striated prismatic crystals, but specimens are usually massive.
Cleavage is poor in one direction.
Fracture is subconchoidal.
Hardness is 6 - 6.5
Specific Gravity is approximately 2.75 - 2.80 (average)
Streak is brown.
Associated Minerals include pectolite, poudretteite and polylithionite.
Notable Occurrences include the type locality of Iwagi Island, Shikoku, Japan as well as Mont Saint-Hilaire, Quebec, Canada and most importantly, South Africa.
Best Field Indicators are color, hardness, luster, streak and locality.

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