Uraninite and Thorianite
WASSERSTEIN1 suggested that the generation by radioactive decay of Pb4+ of smaller ionic radii within the crystal lattice of uraninite caused a shrinkage in the unit cell proportionate to the geological age and the shrinkage was of the order of 0.0025–0.0041 Å. per hundred million years. The anomaly in the relation of cell dimensions of Bisundni and Gaya uraninites with the age, reported by Holmes2, was explained by Wasserstein3 by classifying uraninites into two different types—α-type for Bisundni uraninite and γ-type for Gaya uraninite, the latter being apparently anomalous and requiring heat treatment under oxidizing conditions in order to reduce the cell size to correspond to that of an α-type analogue. Wasserstein did not, however, state the temperature and period of heating and the conditions under which the cell size of γ-uraninite was reduced to correspond to age values consistent with those of the α-type.
Cube-Edges of Uraninites as a Criterion of Age
I HAVE suggested that there is a progressive reduction in the size of the unit cell of uraninite-thorianites with time1. The ionic radius of Pb4+ is appreciably smaller than those of U4+ and Th4+. The decay of the parent element causes its substitution by radiogenic lead within the crystal lattice; because of the differences in sizes of the ions, a shrinkage results. It is therefore likely that this factor can be utilized for indicating the relative ages of uraninites after these have been subjected to an X-ray powder examination. No great accuracy can be envisaged, but such a method would be more accessible to geologists than the highly refined mass-spectrographic one, which can be practised in very few laboratories in the world.
The lead ratio of a crystal of monazite from the Gaya district, Bihar
THE desirability of determining the `lead ratio' of a radioactive mineral to estimate its age, and hence that of the formation where it occurs, need not be enlarged upon. So far no precise determination of the lead ratio of any Indian mineral has been made. W. R. Criperl in 1902 analysed a specimen of pitchblende of unknown quality from Singar, Gaya district, Bihar, but his data cannot be used in calculating the lead ratio, because the thorium content of the mineral was not determined, thorium being present in many pitchblendes and sometimes in quantities up to 10 per cent. or more. Further the sample analysed by Criper contained 0.35 per cent. P20 5 and this suggests that perhaps the sample was contaminated with some of the alteration product of the mineral; for Tipper 2 has observed that the alteration product of the pitchblende of the locality reacts strongly for phosphates, whereas usually phosphates are present in pitchblendes only in traces, if at all. 3 Later Holmes4 analysed a good sample from the same locality, but his analysis also suffers from the defect that thorium was not estimated.