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 00l X-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
K0.90 Ca0.06 Na0.49 [Al3.52, Fe0.18
3+ Mg0.27 Ti0.03 (Al1.24 Si6.76) 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.