Thorium minerals in Sri Lanka, history of radioactivity and thorium as a future energy source: a compendium to commemorate the International Year of Chemistry 2011

Abstract

The discovery of radioactivity more than a century ago and vigorous research on the subject thereafter has transformed the world scientifically and technologically through a deeper understanding of the nature of matter. Two stalwarts among the illustrious pioneers in this field were Lord Ernest Rutherford and Madame Marie Curie. Both were physicists by training and both won the Nobel Prize for Chemistry. While Rutherford's work was more inclined towards physics, enabling elucidation of the structure and mechanics of the atom, Curie's discoveries instantly catalyzed a revolution in chemistry. The year 2011, designated as the International Year of Chemistry, celebrates the centennial anniversary of award of the Chemistry Nobel Prize to Marie Curie. When Marie Curie separated radium from pitchblende, the curious researchers and amateurs started turning stones around looking for pitchblende and other radioactive minerals. In this endeavour, a British planter in Sri Lanka (then Ceylon) collected heavy rock samples, similar in appearance to pitchblende, and dispatched them to two eminent researchers pursuing studies on radioactivity in Britain. Analysis by chemists in Britain and subsequent prospecting by the then Ceylon Geological Survey enabled identification of thorium oxide thorium and silicate minerals, thorianite and thorite, containing a very high percentage of thorium. A number of major discoveries in radioactivity by outstanding pioneers in this field have been conducted using Ceylon thorianite and thorite. Most reputed physics and chemistry journals in the first three decades of the twentieth century carry articles on these discoveries making conspicuous references to Ceylon thorium minerals. The International Year of Chemistry is an appropriate moment to review the history of radioactivity in relation to thorium minerals of Sri Lanka. Coincidently, Sri Lanka and several other countries are currently focussing much attention on the use of thorium as a safe, environmentally benign and lasting source of energy. This work examines discoveries in radioactivity made by pioneers of this field, using Ceylon thorium minerals, giving historical anecdotes on use of Sri Lankan minerals in understanding fundamental properties of matter. The important aspects of uranium-thorium mineral chemistry, formation of radioactive mineral sand placer deposits and generation of thorium based nuclear energy are also discussed, because of current relevance of these topics to Sri Lanka.

Combined X-ray absorption and SEM–EDX spectroscopic analysis for the speciation of thorium in soil

“…Pure forms of natural ThO 2 and ThSiO 4 crystals are rare due to their common association with monazite, uraninite, and other major phases. Thorium can also be present in trace amounts in mixed phases of phosphate, oxide and silicate minerals 15 , 16 . Notably, the possible mobility of thorium upon release from mineral phases is of interest, potentially contributing to the transport into groundwater reservoirs or the food chain 16 .…”

“…Due to its relative simplicity and non-destructive character, µ-XRF can be used to determine elemental composition and in principle distribution (elemental mapping) for a given particle. Besides Th, lanthanides were included in the study given the fact that they are abundant in Sri Lankan soils and commonly form mixed phases with Th-bearing minerals 15 . Even though SEM–EDX has higher spatial resolution than µ-XRF, particularly in this study due to the effect of beam size vs particle sizes, µ-XRF has certain advantages over SEM–EDX regarding the elemental analysis.…”