An intergrowth of mucovite and biotite from Mitchell Creek Mine, Upson County, Georgia, was studied with a high resolution transmission electron microscope. Atomic arrangements of the interface between muscovite and biotite have been derived, revealing far more complexity for the interface structure than that proposed previously based on optical and X-ray analysis (Gresens and Stensrud, 1971).

It was found that the muscovite has an almost perfectly ordered 2M1 structure, while the biotite is highly disordered. Despite such disorder in biotite, both octahedral layers and K ion interlayers of the two minerals are connected perfectly at the interface, while about 60% of the tetrahedral sheets are discontinued across the interface.

Occasional occurrences of coherent intergrowths of chlorite-like structure in biotite were recognized in the vicinity of the interface.

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