PHASE-RELATIONS AMONG SMECTITE, R1 ILLITE-SMECTITE, AND ILLITE

A variety of smectitic and illitic clays were studied by TEM and AEM, following expansion by L.R. white resin, to define phase relations for clay minerals undergoing digenesis and low-grade metamorphism. Sample s included a prograde shale sequence from the Gulf Coast, a hydrotherm al bentonite from Zempleni, Hungary, and shales from the Nankai Trough , Japan, Michigan, and the Welsh sedimentary basin. All samples were d ominated by various proportions of only three kinds of clay minerals: smectite having no 10 W layers, R1 illite-smectite (I/S) (50% illite), and illite with only small proportions of smectite-like interlayers; mixed-layer phases with intermediate ratios of VS were observed only a s minor components of other layer sequences. Lattice fringe images sho w that the common layer spacing of R1 I/S is 21 Angstrom and that it h as 0.7-0.8 K pfu; its properties are not an average of those of smecti te and illite, which is consistent with the uniqueness of the R1 I/S s tructure. A prograde sequence of clay mineral transitions in the studi ed samples can be characterized by five stages with different combinat ions of the three major phases (i.e., smectite, R1 US, and illite) cor responding to different grades. The sequence from low to high grade is (1) pure smectite, (2) smectite with small proportions of discrete R1 VS and illite, (3) R1 I/S with small proportions of smectite and illi te, (4) illite with some R1 I/S and smectite, and (5) illite. Common e xceptions to this scheme are inferred to be caused by the inherent met astability of all phases, in occurrences that are rate or path depende nt.