kaolinite - hard vitreous kaolin - hand specimen of the primary constituent of kaolin clay
Kaolinite is the primary constituent of kaolin clay and is one of the kaolin group of minerals. Its physical structure is like a sandwich, with aluminum hydroxide layers, called gibbsite layers, bonded to silicate sheets. These sheets are electrically neutral, so they are bonded by very weak van de Waals forces, easily sliding apart to give kaolinite its usual softness and greasy feel. It has a similar structure to minerals in the serpentine group. This kaolin is hard and breaks with a conchoidal fracture. It looks very much like the vitreous form of magnesite, but when powdered, it does not effervesce with hydrochloric acid as does magnesite, magnesium carbonate.
When the vitreous form of magnesite was first discovered in Muddy Valley, Nevada in the late 1800s, it was thought to be kaolin and plans were made to mine it as such. In 1915, before mining could begin, a specimen sent to the U.S. Geological Survey was identified as magnesite. X-ray diffraction confirms the identification of this material as kaolin.
Rocks with a silica-rich composition that contain aluminosilicate minerals weather to clays. This kaolinite was formed by the hydrothermal alteration of Pleistocene lacustrine sandstone and an underlying rhyolite. The sandstone was derived from granitic rocks, which contain abundant feldspar and after hydrothermal alteration or weathering are a common source for kaolinite.
Because kaolinite does not absorb water, it does not expand when it comes in contact with water and is the preferred type of clay for the ceramics industry. It is also used as a filler in rubber and in paper, paint, plastics and cosmetics.
This hard kaolin comes from the inactive Huntley Kaolinite Mine in Mono County, California.
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