On the Occurrence of Dark Apatite in some British Rocks

Extract

In a paper (1) by one of us (W. F. F.) reference is made to the occurence in Midland sediments of apatite grains having dark purplish, often pleochroic, cores or nuclei. From an examination of the heavy minerals in Midland rocks ranging from Cambrian to Trias it is evident that these peculiar apatite grains occur at several horizons. Discoloured grains appear in the Cambrian quartzite and apatite of smoky tint in the Old Red Sandstone. In the Carboniferous rocks, dark-banded grains varying in tint from yellowish-brown to smoky-purple and occasionally pleochroic, occur with fair frequency. Similar grains are particularly noticeable in the Gornal Sandstone, which lies in the lower part of the Middle Coal Measures of South Staffordshire. An hexagonal cross section of apatite from this rock, with a dark core, was figured in the paper referred to above. Similar grains occur, though rarely, in the Enville and Bunter Sandstone, but increase in frequency in the lower Keuper Sandstone. The dark-colored “cores” may be sharp-edged or rather indefinite in contour, narrow or broad, and variable as regards depth of tint. Clear-cut bands or stripes of dark colour are exceptional.