Transmission electron microscopy, electron diffraction, and X-ray analytical electron microscopy indicate that the sodium mica wonesite from the Post Pond Volcanics, Vermont, has partially exsolved to a lamellar intergrowth of talc and a sodium mica having fewer interlayer site vacancies than the initial wonesite. This is the first known example of exsolution in a mica. The mica is enriched in Na, Al, K, Ti, Cr, and Fe relative to the talc. The lamellae, the wavelength of which varies from a few hundred ångströms up to about 0.5 μm, are inclined to the layers of the mica and talc structures at a variable angle that averages about 37°. This microstructure explains why wonesite is not expandable in water, unlike a number of other natural and synthetic sodium micas and smectites.

A new method of deriving the chemical compositions of the phases in fine lamellar intergrowths from electron microscope X-ray analyses is described. The method, which utilizes the bulk chemical analysis of the intergrowth as a standard, is used to derive the chemical compositions of the intergrown talc and mica. These analyses clarify the shape of the talc-wonesite miscibility gap, which is strongly asymmetric and skewed toward talc. In addition to clarifying the phase relations of the sodium micas, the exsolution of wonesite provides a structural analogy for amphibole exsolution phenomena that involve the amphibole A-site. The study clearly demonstrates that wonesite is a valid mineral species. However, the solid solution ranges exhibited by sodium micas suggest that the nomenclature used to describe them should be clarified.

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