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.
Samarskite from Nellore district
Summary1.Sixty samples of samarskite were collected from a mica pegmatite vein in Nellore district, S. India.2.The physical and chemical properties of six representative samples from this collection were studied.3.Numbers 1 and 6 of the selected samples of samarskite were intergrown with Columbite-Tantalite.4.The specific gravity of these six specimens varies from 5·567 to 5·776. The hardness is between 5 and 6. The colour varies from dark steel grey to velvet-black and the streak is generally brown to dark-brown. They all break with conchoidal fracture.5.Specimen No. 1 which is an intergrowth of samarskite with Columbite-Tantalite was polished and placed in contact with a panchromatic photographic plate. Only the samarskite portion affected the photographic plate.6.The chemical analysis of samarskite shows a variation in UO3 percentage from 4·2 to 10·5% and the Nb2O5+Ta2O5 from 53·4 to 69·7%. Attention is drawn to the fact that the UO3 percentages of samarskite from the Nellore area differ from those of North Carolina.