Rectorite (=allevardite) is a perfectly ordered mixed layer clay mineral consisting of double layer units of alternating 2:1 (or T-O-T) nonexpandable mica-like and expandable smectite-like 10Å component layers (1,2). Complete dispersion of Na-saturated rectorite results in flakes equal in thickness to the basic unexpanded (20Å) double layer unit. Mixed layered (or interstratified) clay minerals are ubiquitous in both sediments and epithermal deposits; however their routine identification is difficult and is based upon various assumptions concerning their X-ray powder diffraction characteristics. Therefore, rectorite was studied by high resolution transmission electron microscopy (HRTEM) to test existing mixed layering models. Oriented specimens of air-dried rectorite (Fort Sandeman district, Baluchistan, Pakistan) were prepared by three methods: (a) ultrasonic dispersion in acetone, (b) diamond knife ultrathin-sectioning of epoxy embedded dispersions and (c) ion thinning of epoxy embedded aggregates. Dispersed mounts were also prepared after treatment with n-dodecylammonium ions to expand the smectite-like interlayers, producing a double layer repeat of 28Å (3).
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McKee et al. (1978) studied this question.