RECTORITE AND THE RECTORITE-LIKE LAYER STRUCTURES*

Recurrent reports of analyses of X-ray diffraction data from regularly alternating and nearly regularly alternating structures like rectorite provide opportunity for a descriptive discussion of their nature. MacEwan-transform-based analyses have uniformly shown higher incidence of unlike adjacent layers than would be expected from random distribution. The continuous Fourier transforms for pairs of 2 : 1 layer silicate structures evaluated normal to the layers, provide a family of similar curves that are differently stretched out in reciprocal space for different assumed intervals between the two members of a pair. For each exactly alternating example the relative amplitudes of the ordered reciprocal nodes dis- tinguish the instances for which fixed interlayer populations are Na § from those for which they are K § For sequences not exactly alternating scattering maxima are displaced from exact incremental positions in conformity with the Hendricks and Teller mixing functions, but relative intensities of adjacent observations still distinguish the alkali identities. THE ORIGINAL description of rectorite (Brackett and Williams, 1891) antedated X-ray diffraction analyses. The schematic interpretations for alterna- tion of complex layers for rectorite and related examples, as first presented, have since been amenable to distinct improvement as the quality of data from X-ray diffractometry has improved on that available from photographic observations. It seems to be especially true that some latitudes both of composition and of regularity of alternation exist. Rather comprehensive collections of rectorite (or allevardite) from extensive occurrences in some Paleozoic shales in north-western Utah, (Ehlmann. 1958; Henderson, 1968) have provided a ground for extended study. Details of exact distribution of the framework cations in the silicate skeleton and consequent allocations of layer charges are dependent on the purity of chemically analyzed specimens and on arbitrary reductions of the chemical analyses to structural formulae. They establish with certainty no more than the di-octahedral character. The distribution of fixed or exchangeable interlayer *Acknowledgement is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for partial support of research related to this analysis. cations has been more reliable. A first diffraction analysis (Bradley, 1950) did not disclose evidence for the alkali cations, and they were presumed to be associated with vermiculite-like layers. Brindley (1956) established that in allevardite, their presence between the fixed mica-like layers was more important than association in swellable interlayer spaces. This disposition is confirmed by Brown and Weir (1963) and by Kodama (1966). These deter- mined relative abundances of fixed and of ex- changeable base ions establish that these fixed- layer pairs have the character of paragonite. Brown and Weir (1963) conclude that rectorite and alle- vardite are synonomous, and Kodama's observed abundance of fixed sodium ions indicates a related nature. Tomita and Sudo (1968) have more recently utilized a procedure of heating to de- hydroxylation followed by boiling in 6NHC1 to prepare a nearly regular alternating sequence from muscovite, in which the fixed alkali ions are potassium.