Submicroscopic intergrowths of K biotite, Na biotite and intermediate Na-K biotite from a schist near Málaga (Betic Cordilleras, Spain) were discovered using high-resolution transmission electron microscopy and analytical electron microscopy. The sample was also studied with X-ray diffraction, electron microprobe analysis, and scanning electron microscopy. Scanning electron microscopy revealed that the Na-enriched biotite is concentrated in albite-rich microdomains, albite being partially replaced by biotite. These images also revealed that both K and Na-K biotite grains appear locally retrograded to kaolinite. Transmission electron microscopic data indicated that K biotite, Na biotite and Na-K biotite form parallel or subparallel packets with interfaces parallel to the basal planes of biotite. Potassium biotite forms thick packets, chemically homogeneous, with a basal spacing of 10.1 Å. Sodium biotite also occurs as chemically homogeneous stacks of layers with a 9.78 Å periodicity. Sodium-K biotite shows, on the contrary, variable composition and basal spacings intermediate between K and Na biotites. Analytical electron microscopic data revealed important chemical differences between Na and K biotites, which affect both the tetrahedral and the octahedral sheets. Both electron microprobe analysis and analytical electron microscopy indicated that the trioctahedral micas show relatively low interlayer occupancy, suggesting the presence of H3O+ replacing the interlayer cations. Partial hydration of biotite explains the presence of a weak 14 Å reflection in the X-ray patterns. Both chemical and textural data suggested that these trioctahedral micas grew during a common prograde metamorphic episode, the phases with intermediate composition probably being metastable.

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