Technique for determination of the content of smectite layers in the dispersed dioctahedral K-bearing micaceous minerals

Abstract

A technique was developed to determine the low contents of smectite layers in the dispersed dioctahedral K-bearing micaceous minerals of the illite-glauconite composition. To implement this technique, diffraction patterns were calculated for two-component mixed-layer mica-smectite structures, in which the content of smectite layers increased with a step of 2% from 2 to 20% at a short-range order factor R = 0 and from 2 to 14% at R = 1. On the basis of Mering’s principles (1949) and Q-rules (Moore and Reynolds, 1989), the half-width βcosθ of each basal refection and its distance Q to the nearest reflection corresponding to the periodical structure made up of 9.98 A micaceous layers were calculated for mixed-layer structures with the given content of smectite layers WSm. The Q dependence on βcosθ for the series of basal reflections observed in the calculated diffraction pattern was approximated by a straight line corresponding to the linear regression equation. Equations relating slope angles of straight line (α) and concentrations of smectite layers (WSm) were deduced for the analyzed mixed-layer structures with the short-range order factors R = 0 and R = 1, respectively. To determine the contents of smectite layers in natural micaceous varieties, the basal reflections observed in experimental patterns from the oriented ethylene glycol-saturated samples were subjected to the following procedures: (1) determination of the half-width (βcosθ)e of each 00l reflection of the analyzed sample and plotting Q dependence on (βcosθ)e for the observed series of basal reflections, with its further approximation by the linear regression equation and the corresponding straight line; (2) calculation of the slope angle of this straight line α and determination of the content of smectite layers in the sample structure using the equation relating α and WSm. Application of the described procedure showed that the content of smectite layers in the studied samples varied from 5.2 to 12.6%. The content of smectite layers was determined accurate to a few tenths of a percent. Since the half-width of reflection 003 does not depend on the content of smectite layers, its value was used to determine the thickness and number of layers in the averaged CSD of the studied sample. XRD patterns calculated for mixed-layer structures with experimentally determined WSm, WMc, R, T, and N parameters are in a good agreement with experimental patterns of the studied samples.