Deweylites, Mixtures of Poorly Crystalline Hydrous Serpentine and Talc-Like Minerals

David L. Bish and G. W. Brindley
Department of Geosciences, The Pennsylvania State University, University Park, Pa 16802, U.S.A.

Summary: An X-ray and chemical examination of deweylites reveals that they are intimate mixtures of very poorly ordered trioctahedral 2:1 and 1:1 layer silicates. The 2:1 mineral exhibits no swelling in the presence of ethylene glycol or water and is best described as an extremely fine-grained and highly disordered form of talc, for which the term kerolite is often used. Stevensite is not a component of deweylites. The 1:1 component most closely resembles a disordered chrysotile.

Evaluation of the chemical data, including values for H2O +, shows that both end-members are hydrated to various degrees. This hydration is most probably attributable to the presence of water molecules and hydroxyls associated with the large surface areas and unbalanced surface bonds. Formulae for the end-members are approximately R3Si4O10(OH)0·3 0·7 H2O and R3Si2O3(OH)4·0·3–0·7 H2O where R is principally Mg, but samples with more or less H2O are entirely possible due to variations in crystallinity. No specific formula can be given to deweylites as they are mixtures with variable proportions of the components. The name is useful, however, as a field or ‘box’ term, similar to the use of garnierite and limonite.

Mineralogical Magazine; March 1978 v. 42; no. 321; p. 75-79; DOI: 10.1180/minmag.1978.042.321.09
© 1978, The Mineralogical Society
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