High pressure changes aluminum-rich minerals into Kyanite.
High pressure causes the chemical composition of aluminum silicate to change, forming kyanite. Classified as a polymorph (sharing the same chemistry but different crystal structure with another substance), kyanite forms translucent to transparent blue to blue-gray bladed quartz crystals (feldspar), appearing to have an almost pearly luster. Kyanite feldspar occurs in Brazil, the USA (North Carolina and Georgia), Switzerland, Russia, Serbia, India and Kenya.
Formation
Kyanite forms when elevated temperature and pressure exist, causing the mineral to undergo metamorphism. This pronounced change alters the characteristics of clay-rich sedimentary rocks, forming kyanite feldspar, which is often associated with other minerals such as garnet, staurolite and corundum.
Kyanite's typical habit includes embedded prismatic bladed crystals, although radiating masses of crystals commonly form.
Characteristics
The hardness of kyanite varies; the hardness of its long crystals measures about 4.5 if tested parallel to the length of the crystal, and 6.5 to 7 if tested across the crystal. Kyanite minerals were originally called "disthene," which means "two strengths." The crystals splinter when fractured.
Kyanite feldspar, although commonly blue, also displays white, gray or green colors. The color is not consistent throughout the mineral and often appears blotchy or with white streaks.
Heat Resistance
Unlike , kyanite expands significantly when heated--up to twice its original size, depending on particle size, temperature and heating conditions. The expansion of kyanite is predictable. During manufacturing of refractory products, the specified amounts of kyanite added to raw material shrink after heating, allowing the finished product to maintain its original volume.
Kyanite's heat resistance makes it exceptional for use as a binding agent in products such as high-refractory-strength porcelain; an example is the white porcelain insulators on spark plugs. Kyanite has a high-melting point, is a low thermal conductor and maintains its strength up to 1100°C.
Products
Kyanite is used in the manufacturing process for a variety of products, including bricks, mortars and high-temperature furnaces. In foundries, many of the molds used for high temperature metal castings are made of kyanite
Gemstone
Kyanite is classified as an exotic gemstone, although it is not often used in jewelry. Clear and colorful kyanite, although rare, is highly sought after by those who cut and facet gemstones for jewelry, beads or pendants.
Tags: forming kyanite, High pressure, Kyanite feldspar, other minerals