The relative intensities of selected "near prismatic" and "near basal" calcite X-ray reflections diverge from those of randomly oriented calcite aggregates; this divergence can be used effectively as a detector of lattice-preferred orientation (LPO) of calcite. Since the "prismatic" reflection 03.0 at 64.6° 2⊖ and the "basal" reflection 00.12 at 65.6° 2⊖ (at Cu Kα radiation) are weak, the X-ray reflections used for this purpose are the "prismatic" 11.0 at 36.0° 2⊖, the "acute-rhombohedral" 20.2 at 43.2° 2⊖, and the "flat-rhombohedral" 01.8 at 47.5° 2⊖. The peak intensities are measured on bedding-parallel and two bedding-normal slabs, and are normalized against the intensity of the reflection at 48.6° 2⊖, which is virtually unaffected by the existence of preferred c-axis fabrics, and divided by their intensities in randomly oriented calcite aggregates. In the case of a preferred c-axis fabric that is subnormal to a slab, the intensity of the "flat-rhombohedral" 01.8 at 47.5° 2⊖ is larger, and that of the "prismatic" 11.0 at 36.0° 2⊖ and the "acute-rhombohedral" 20.2 at 43.2° 2⊖ are smaller than unity; in the case of slab-subparallel c-axis fabrics, these divergences from unity are reversed. The relative intensities of the less acute "rhombohedral" reflections 01.2 at 23.1° 2⊖ and 11.3 at 39.4° 2⊖ can be used to qualitatively assess the "obliqueness" of c-axis fabrics to the slabs. The method successfully detects the existence of different calcite c-axis LPO patterns-subnormal and subparallel to the bedding-in Senonian and Eocene chalks. The chalks are unlikely to have been buried below more than some 300 m of cover, and the LPOs are considered to reflect preferential c-axis orientation normal to microfossil fragments.
ASJC Scopus subject areas
- Earth and Planetary Sciences (all)