English: Kaolinite from the Quaternary of California, USA.

A mineral is a naturally-occurring, solid, inorganic, crystalline substance having a fairly definite chemical composition and having fairly definite physical properties. At its simplest, a mineral is a naturally-occurring solid chemical. Currently, there are over 5600 named and described minerals - about 200 of them are common and about 20 of them are very common. Mineral classification is based on anion chemistry. Major categories of minerals are: elements, sulfides, oxides, halides, carbonates, sulfates, phosphates, and silicates.

The silicates are the most abundant and chemically complex group of minerals. All silicates have silica as the basis for their chemistry. "Silica" refers to SiO2 chemistry. The fundamental molecular unit of silica is one small silicon atom surrounded by four large oxygen atoms in the shape of a triangular pyramid - this is the silica tetrahedron - SiO4. Each oxygen atom is shared by two silicon atoms, so only half of the four oxygens "belong" to each silicon. The resulting formula for silica is thus SiO2, not SiO4.

Kaolinite is a common & important clay mineral. “Clay” has more than one meaning in geology, which is unfortunate. Clay refers to a group of silicate minerals that result from chemical weathering of other silicate minerals. Clay also refers to very fine-grained sediment (each grain is less than 1/256 of a millimeter in size). Clay minerals include kaolinite, montmorillonite/smectite, illite, etc.

Earthy kaolinite masses consist of numerous microscopic crystals and appear deceivingly like chalk or diatomite. However, kaolinite won't bubble in acid as chalk does. Kaolinite is whitish, soft, and powdery. It has an earthy feel and an earthy smell. When wet, the earthy smell is stronger and kaolinite becomes noticeably sticky.

Kaolinite is an aluminum hydroxysilicate - Al2Si2O5(OH)4. Under a scanning electron microscope, kaolinite crystals are seen to be thin, hexagonally-shaped sheets. Kaolinite forms from weathering or significant hydrothermal alteration of aluminosilicate minerals, especially feldspar.

Famous localities for kaolinite (a.k.a. kaolin) include Cornwall and Devon in southwestern Britain, where hydrothermal metamorphism has completely altered the feldspars of granite batholiths. This material has been used to make English China. When heated over 500º C, the tiny crystal plates of kaolinite curl up as the hydroxyls (OH-) are driven away in the form of water. The tiny curled plates hook together to make ceramic. Ceramic remains in a hard state because the molecules won't take back the water to make kaolinite again.

Kaolinite is also moderately common in the Southern Appalachians of America (for example, in South Carolina and Georgia). In these areas, a kaolinite-rich clay occurs in Cretaceous-aged onlap deposits. The kaolinite comes from chemical decomposition of feldspars in sands produced by erosion of the ancient Appalachians during Triassic and Jurassic times.

The sample seen here is from a kaolinite mine in California. It formed by hydrothermal argillic alteration of lacustrine sandstones having sediments derived from granitic rocks. Alteration of an underlying rhyolite unit also produced kaolinite here. Kaolinitization in the area was caused by groundwater that was heated by subsurface magma - the mine is on the western edge of the Long Valley Caldera, a collapsed volcanic center. Active hot springs occur nearby.

Unlike other kaolinite samples I've observed, this material is hard and has a subvitreous luster. I haven't encountered an explanation for this, but I wonder if post-kaolinitization heating formed a natural ceramic?

Locality: Huntley Kaolinite Mine (a.k.a. Hot Creek pit), northeast of the town of Mammoth Lakes, southern Mono County, eastern California (37° 41' 31.13" North latitude, 118° 52' 00.02" West longitude)

---

Photo gallery of kaolinite:

www.mindat.org/gallery.php?min=2156