Sequences of charged sheets in rectorite

• Published in: The American mineralogist 1995-03, Vol.80 (3), p.247-252
• Main Authors: Jakobsen, Hans J., Nielsen, Niels C., Lindgreen, Holger
• Format: Article
• Language: English
• ISSN: 0003-004X; 1945-3027
• DOI: 10.2138/am-1995-3-407

Two rectorite samples, a Na- and Ca-rich rectorite (sample 1) from Beatrix Mine, South Africa, and a Na-rich rectorite (sample 2) from Garland County, Arkansas, have been investigated quantitatively by solid-state Na, Al, and Si magic-angle spinning (MAS) NMR spectroscopy, total chemical dissolution, cation exchange, and X-ray diffraction (XRD). Comparison of modeled and experimental diffractograms shows that both rectorite samples have a mica to smectite ratio of 1:1 and an ideal ordering in units of one mica plus one smectite layer. The average thicknesses of the coherent scattering domains are ten and eight 2:1 layers for samples 1 and 2, respectively. Quantification of the Na, Al, and Si MAS NMR spectra has allowed determination of the compositions for the octahedral sheets and for the mica (paragonite and margarite) and smectite tetrahedral sheets and determination of the distribution of interlayer Na ions between paragonite and margarite interlayers. Each 2:1 layer has an octahedral sheet of weak positive charge sandwiched between two tetrahedral sheets of weak and strong negative charge. The top and bottom tetrahedral sheets of the coherent scattering domains have strong negative charges and very high cation-exchange capacities for sample 1 and low negative charges for sample 2. Sample 1 is a three-component mixed layer (margarite, paragonite, and smectite), and sample 2 a two-component mixed layer (paragonite and smectite).