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A hydrated aluminum silicate mineral which is the principal constituent of Kaolin clay. Kaolinite crystals have a lamellar or plate-like structure that gives the clay its slippery feel. It is a natural alteration product of aluminum silicate rocks, such as feldspar. Pure kaolinite is a primary clay. Most secondary clays contain only a small percentage of kaolinite (Fournier 1996). One exception is ball clay found in Wareham, England.

Synonyms and Related Terms

china clay, kaolin; gaoling (Chin.); Kaolinit (Deut.); caolinita (Esp.); kaolinite (Fr.); kaolien (Ned.); kaoliniet (Ned.); caulinite (Port.);


Kaolinite PMA.TIF


  • Noncombustible.
  • ThermoFisher: SDS

Physical and Chemical Properties

  • Insoluble in water, cold acids and alkalis.
  • Monoclinic system with with hexagonal, plate-like crystals 0.1-1.0 micrometers.
  • Perfect cleavage in one direction.
  • Plastic when wet. Brittle when dry.
  • Fracture = earthy (friable).
  • Luster =dull to pearly.
  • Streak = white.
Composition Al2Si2O5(OH4)
Mohs Hardness 2.0 - 2.5
Density 2.6-2.63 g/ml
Refractive Index 1.558; 1.565; 1.564

Resources and Citations

  • R. Fournier, Illustrated Dictionary of Practical Pottery, Chilton Book Co., Radnor, PA, 1996.
  • C.W.Chesterman, K.E.Lowe, Audubon Society Field Guide to North American Rocks and Minerals, Alfred A. Knopf, New York, 1979
  • R. J. Gettens, G.L. Stout, Painting Materials, A Short Encyclopaedia, Dover Publications, New York, 1966