Partially-ordered mixed-layer mica-montmorillonite from Maitland, New South Wales

A mixed-layer clay mineral from a Permian sandstone at Maitland, New South Wales has been identified as 2:1 mica-montmorillonite structure with ‘imperfectly regular’ interstratification. The results from Fourier transform analysis and Fourier synthesis of 00lX-ray diffraction data have not fully elucidated the interlayering patterns but have indicated that there is complete alteration in the stacking and that the 1:1 (allevardite type) layer sequence relationship is strongly developed. X-ray diffraction, differential thermal, thermogravimetric, chemical, cation exchange and electron microscopic data for the mineral are given. The chemical analysis for the Na+-saturated material gives the structural formula K0.90Ca0.06Na0.49[Al3.52, Fe0.183+Mg0.27Ti0.03(Al1.24Si6.76) O20(OH)4] H2O It is considered that most of the fixed K+and Ca++ions are probably held in the mica interlayers, while the exchangeable components are largely accommodated in the expanded montmorillonite zones.