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January-February 2011

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Word to the Wise: Serpentine, California's State Rock

Serpentine is a mineral group of about twenty 1:1 layer-silicate minerals with the general composition M3T2O5(OH)4, where M can be occupied by Mg, Fe2+, Fe3+, Al, Ni, Mn2+, Zn, and systematic or random vacancies, and T can be occupied by Si, Al, Fe3+. What we usually think of as serpentine, in the more restricted sense, are the common species antigorite, lizardite, and chrysotile, all polymorphs with the ideal composition Mg3Si2O5(OH)4. They most frequently form by the hydrous alteration of ferromagnesian silicates, such as olivine and pyroxene. The structures of each of these minerals has some modification of a two-dimensional sheet of corner-sharing silica tetrahedra that is joined to a two-dimensional sheet of edge-sharing magnesium octahedra (fig. 1). Because the lateral dimensions of the tetrahedral and octahedral sheets in these 1:1 layer structures can be slightly but significantly different (with the tetrahedral sheet typically being smaller), strain is created when the sheets are linked. The three serpentine polymorphs have long been thought to be the result of different structural mechanisms for reducing this strain (Wicks and O'Hanley 1988; Veblen and Wylie 1993; Evans 2004).

Dr. John Rakovan, an executive editor of Rocks and Minerals, is a professor of mineralogy and geochemistry at Miami University in Oxford, Ohio.

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