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Why are clay minerals small?

HYDRASA, Bât. Sciences Naturelles, Universitè de Poitiers, 40 avenue Recteur Pineau, 86022 Poitiers cedex, France

^* E-mail: alain.meunier{at}hydrasa.univ-poitiers.fr

(Received 11 February 2005; revised 5 December 2005)

The most abundant silicates formed under the Earth’s ‘normal’ surface conditions, i.e. clay minerals, are always of small grain size. Under the same conditions, other mineral species such as carbonates, sulphates and oxides may form much bigger crystals. The reason why phyllosilicates formed in soils or in weathered rocks are always of small grain size is not related to the low-temperature-pressure conditions but rather to particular aspects of their crystal structure. Many recently published works describe the order-disorder cation distribution in the tetrahedral and octahedral sheets and the crystal defects in the layer stacks. Related to the Periodic Bond Chains (PBCs) theory, these data suggest that the size and the shape of clay crystallites could depend on the amount of crystal defects in the three axes of symmetry [100], [10] and [0]. The accumulation of crystal defects poisons the crystal growth along one, two or three PBCs. Then, nucleation becomes less energy-consuming than crystal growth and favours the formation of numerous smaller crystals rather than fewer bigger ones.

KEYWORDS: crystal growth, crystal defects, order-disorder, polymerization, illite, smectite

This article has been cited by other articles:

				A. MEUNIER, P. SARDINI, J. C. ROBINET, and D. PRET The petrography of weathering processes: facts and outlooks Clay Minerals, December 1, 2007; 42(4): 415 - 435. [Abstract] [Full Text] [PDF]