Illite-smectite structural changes during metamorphism in black Cambrian Alum shales from the Baltic area

Illite-smectite (I-S) from Cambrian black shale of both early diagenetic an d anchimetamorphic grade was investigated to determine the mechanism of the clay transformation. The layer sequences, the distribution of thicknesses of coherent scattering domains (CSDs), and the three-dimensional ordering w ere determined by X-ray diffraction (XRD). The proportions of cis-vacant (c v) and trans-vacant (tv) 2:1 layers were determined by thermal analysis and the proportion and distribution of interlayer ammonium by XRD and by infar ed spectroscopy (IR). The structural formulae were determined from total ch emical analysis, and Mossbauer and Al-27 NMR spectroscopies, and the partic le shape and size investigated by atomic force microscopy (AI;M). In the ea rly diagenetic samples, the I-S is composed of two phases, one of which con tains 0.05 and the other 0.25 smectite (S) interlayers. The first phase doe s not change during metamorphism. In the second phase, 0.20 S are converted to tobelite (T) layers through fixation of NH4+, but the I layers are not changed. Simultaneously, the proportion of cv layers changes from 0.18 to 0 .02, and the tetrahedral substitution of Al for Si is parallel to the incre ase in T layers. All I interlayers contain 0.75K per O-10(OH)(2). Furthermo re, the metamorphism results in increasing mean thickness of CSDs from 5.1- 6.8 nm for the low-diagenetic samples to 6.7-8.4 nm for the anchimetamorphi c samples. We conclude that the tobelitization was accompanied by transform ation of cv to tv 2:1 layers adjacent to the smectite interlayers, and form ation of tv layers adjacent to the newformed tobelite interlayers in otherw ise intact crystallites. This mechanism only partly resembles the tobelitiz ation previously observed in the Upper Jurassic North Sea oil source rocks. I-S in these rocks contained tv 2:1 layers and T interlayers formed throug h solid-state Al for Si substitution in the tetrahedral sheet and by ammoni um fixation in the corresponding interlayers. These different mechanisms ar e probably because the North Sea I-S originated from weathered illite, like the Cambrian high-illitic phase, whereas the Cambrian low-illitic phase un dergoing the transformation originated from cv smectite of volcanic origin. The results indicate that the illitization in oil source rocks is linked t o oil generation, and that it deviates from the illitization in other rocks because of the supply of ammonium formed during oil generation and the fix ation of this ammonium in the former smectite interlayers.