The ratio of the absolute intensities of a 001 phyllosilicate X-ray diffraction peak measured on cleavage (C)- and bedding (B)-parallel slabs, or the ratios of the relative intensities of a 001 lattice plane to that of a hkO or hkl lattice plane, reflects the relative orientation of the phyllosilicate mineral in elastic rocks showing incipient slaty cleavage. The I-10A and I-10A/I-4.46A-19.9 degrees C/B ratios are used as mica-orientation criteria. The dependence of these ratios on slab sizes below ca. 20 mm is minimized when the peaks are measured using suitable divergence/scatter slit pairs. These X-ray orientation criteria were measured on a series of Lower Paleozoic elastic rocks showing incipient slaty-cleavage development from the external Caledonides of Jamtland, western central Sweden. In the external low-grade areas the I-10A C/B and I-10A/I-4.46A-19.9 degrees C/B ratios are smaller than unity in the 'diagenetic' (par)autochthon, and around unity in the low-anchimetamorphic lower thrust sheet. In the more internal anchimetamorphic areas many rocks show a combination of low X-ray orientation (I-10A C/B = 0.3-0.9) with high db cleavage/bedding fissility ratios. This reflects cleavage-domain development without appreciable mica re-orientation, whereas the increase in the X-ray CiB ratios for essentially similar db cleavage/bedding fissility ratios reflects increased mica orientation in the cleavage domains and the microlithons at the expense of bedding-parallel micas, particularly in fold hinges, It appears that, at these incipient stages of slaty-cleavage development, the phyllosilicate fabric as expressed by the X-ray orientation is a more sensitive indicator of local variations in strain than the c/b fissility ratio - apparently the expression of cleavage-domain development -, which tends to be more uniform over a given area.
