A quantitative study of pleochroic haloes III—Thorium

Abstract

This paper gives an account of measurements of thorium haloes in biotite, made with the halo photometer. This apparatus, which has been earlier applied to uranium haloes, gives a permanent record of the structure of a halo, yielding ring diameters and intensity of blackening throughout the halo. Thorium haloes are very rare compared with those due to uranium. Out of more than a hundred biotites examined up to the present, only one gave a reasonable number of clearly defined haloes due to thorium. This material came from the Star Lake district in Manitoba, near the Ontario boundary. It was a brown biotite occurring as easily cleavable crystals a few mm. in diameter. Haloes were not very plentiful in the small amount of material available, but they were in rich variety including thorium, uranium, and compound uranium-thorium, as well as several other types of halo, which will be dealt with in another paper. A few good thorium haloes were also found in a brown biotite from Pierrepont, St. Lawrence Co., N.Y., and one or two more have been recognized in other biotites.