SEQUENCES OF CHARGED SHEETS IN RECTORITE

Two rectorite samples, a Na- and Ca-rich rectorite (sample 1) from Bea trix Mine, South Africa, and a Na-rich rectorite (sample 2) from Garla nd County, Arkansas, have been investigated quantitatively by solid-st ate Na-23, Al-27, and Si-29 magic-angle spinning (MAS) NMR spectroscop y, total chemical dissolution, cation exchange, and X-ray diffraction (XRD). Comparison of modeled and experimental diffractograms shows tha t both rectorite samples have a mica to smectite ratio of 1:1 and an i deal ordering in units of one mica plus one smectite layer. The averag e thicknesses of the coherent scattering domains are ten and eight 2:1 layers for samples 1 and 2, respectively. Quantification of the Na-23 ,Al-27, and Si-29 MAS NMR spectra has allowed determination of the com positions for the octahedral sheets and for the mica (paragonite and m argarite) and smectite tetrahedral sheets and determination of the dis tribution of interlayer Na+ ions between paragonite and margarite inte rlayers. Each 2:1 layer has an octahedral sheet of weak positive charg e sandwiched between two tetrahedral sheets of weak and strong negativ e charge. The top and bottom tetrahedral sheets of the coherent scatte ring domains have strong negative charges and very high cation-exchang e capacities for sample 1 and low negative charges for sample 2. Sampl e 1 is a three-component mixed layer (margarite, paragonite, and smect ite), and sample 2 a two-component mixed layer (paragonite and smectit e).