THE TRANSFORMATION OF ILLITE TO MUSCOVITE IN PELITIC ROCKS - CONSTRAINTS FROM X-RAY-DIFFRACTION

The boundary between diagenesis and metamorphism most likely involves the change of illite into mica. Observations of this change can be mad e using decomposed X-ray diffraction (XRD) spectra of illitic clay min eral assemblages in pelitic sedimentary rocks. XRD analysis of the (00 3) diffraction peak of diagenetic illites indicates that there are 2 c omponents, one of small coherent diffraction domains and another of la rger domain size. Peak width, shape and position define these fraction s. The smaller domain size material in diagenetic rocks is highly illi tic (>95%) but contains some smectite layers and can be best described by Gaussian shapes. The grains with larger diffracting domains show n o expanding layers. Metamorphic illites (probably muscovites) show no smectite interlayers in any fraction. in the transition from sedimenta ry and diagenetic to metamorphic illites, new grains of smectite-free illite are formed at the expense of the older minerals. This suggests that the new metamorphic minerals are recrystallized phases. Metamorph ism of illites then produces new mica phases.