File:Kaolinite after agate (Salta Jacui, Rio Grande do Sul, Brazil) (29806949113).jpg

Kaolinite pseudomorph after agate from Brazil

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 5100 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 & 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.

Famous localities for kaolinite (a.k.a. kaolin) include Cornwall & 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 remarkable specimen shown above is kaolinite that has replaced agate. "Agate" is a rockhound/collector term for cavities in rocks (usually sedimentary rocks such as limestone or igneous rocks such as basalt) that have been partially or completely filled with irregularly concentric layers of microcrystalline, fibrous quartz (chalcedony - SiO2). Agate is quartz.

Minerals that have replaced other minerals, while retaining the crystal shape of the original mineral are called "pseudomorphs".

Locality: Salta Jacui, north-central Rio Grande do Sul, far-southern Brazil

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