Abstract:
Three rectorite samples from the Beatrix Gold Mine, South Africa were characterized. Scanning electron microscopy
revealed a layered morphology. High resolution transmission microscopy showed well distinguished light
and dark layers of about 2.20 nm consistent with the 1:1 interstratified mica-smectite nature. X-ray diffraction
measurements confirmed the basal spacing d001 of 2.20 nm consistent with a one-water-layer structure. Unit
cell parameters, for a monoclinic unit cell with primitive lattice, refined to a = 5.177 Å; b = 8.980 Å; c =
22.489 Å and β = 97.335° with mean crystallite size around 14 nm and calculated cell volume of 1045 Å3. The
Greene-Kelly test suggested that the expandable smectite layers have montmorillonite-beidellite composition.
Nuclearmagnetic resonance spectroscopy indicated a high degree of Al substitution and the presence of two different
Al sites corresponding to six- and four-fold octahedral and tetrahedral aluminumrespectively. The chemical
composition and diffraction data suggest that the mica is Na-Ca-rich, i.e. of paragonite-margarite series. The
fixed interlayer regions (mica interlayers) contains proportionally dominant Na+ and Ca2+ and minor amounts
of K+. The exchangeable smectitic interlayers contain almost equal amounts of Na+ and Ca2+ ions. The distribution
of the interlayer Na+ ions was quantified by 23Na solid-state NMR spectroscopy. It points to a three component
mixed-layer structure with considerable variation in the composition of the mica layer of the different
samples.
