Thorium — Element Profile: Chemistry Physics and Natural Occurrence
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Thorium — Element Profile: Chemistry Physics and Natural Occurrence
[edit | edit source]Physical Properties
[edit | edit source]| Property | Value |
|---|---|
| Atomic number | 90 |
| Atomic mass | 232.038 u (monoisotopic for practical purposes) |
| Period | 7 (actinide series) |
| Group | Actinides (f-block) |
| Phase at STP | Solid metal |
| Crystal structure | Face-centred cubic (FCC) |
| Colour | Silvery-white when freshly cut; oxidises to grey/black on air exposure |
| Density | 11.7 g/cm³ at room temperature (denser than lead at 11.3 g/cm³) |
| Melting point | 1,750°C (3,182°F; 2,023 K) |
| Boiling point | 4,788°C (8,650°F; 5,061 K) |
| Thermal conductivity | 54 W/(m·K) — good thermal conductor |
| Electrical resistivity | 147 nΩ·m |
| Hardness (Mohs) | 3 (relatively soft for a metal; machinable) |
| Magnetic ordering | Paramagnetic |
Chemical Properties
[edit | edit source]| Property | Value |
|---|---|
| Oxidation states | +4 (dominant; +2 and +3 in some compounds; +4 in all nuclear-relevant forms) |
| Electronegativity | 1.3 (Pauling scale) |
| Common compounds | Thorium dioxide (ThO2); thorium fluoride (ThF4); thorium nitrate; thorium sulfate |
| ThO2 melting point | 3,350°C — one of the highest melting points of any known oxide; exceptionally refractory |
| ThF4 significance | Thorium tetrafluoride dissolved in FLiBe (lithium fluoride-beryllium fluoride) molten salt is the fuel form in LFTR reactors |
| Chemical stability | Highly stable in the +4 oxidation state; does not corrode in most environments |
| Reactivity | Reacts slowly with oxygen and moisture; burns in air when finely divided; not reactive with dilute acids without oxidising agent |
Radioactive Properties
[edit | edit source]| Isotope | Half-life | Decay mode | Natural abundance | Notes |
|---|---|---|---|---|
| Th-232 | 14.05 billion years | Alpha decay to Ra-228 | Essentially 100% of natural thorium | The primary isotope; fertile (not fissile); the basis of the thorium fuel cycle |
| Th-230 | 75,380 years | Alpha decay | Trace (in uranium ores) | Present as decay product of U-234; not significant for reactor fuel cycle |
| Th-228 | 1.912 years | Alpha decay | Trace | Present in Th-232 decay chain; important in waste assessment |
| Th-234 | 24.1 days | Beta decay | Trace (equilibrium with U-238) | Immediate decay product of U-238 |
Thorium-232 is classified as weakly radioactive — its half-life of 14.05 billion years (approximately three times the current age of the Earth) means it decays extremely slowly. A piece of natural thorium ore emits radiation primarily from its decay daughters, not from the Th-232 itself. Handling unprocessed thorium in bulk requires basic radiation precautions; it is far less radioactive than enriched uranium.
Natural Occurrence and Ore Minerals
[edit | edit source]Thorium is distributed broadly throughout the Earth's crust, found in trace quantities in most rocks and soils. Its average crustal abundance is approximately 6–10 ppm, with the IAEA citing approximately 10.5 ppm. It is:
- Three to four times more abundant than uranium
- Approximately as abundant as lead and gallium
- More geographically distributed than uranium — not concentrated in a small number of countries
Primary ore minerals:
- Monazite — (Ce,La,Nd,Th)PO₄ — the most commercially important; a rare earth element phosphate mineral in which thorium substitutes for some of the REE content; found in placer deposits (beach sands) worldwide; India, Brazil, and Australia have major monazite deposits
- Thorianite — ThO₂ — the purest thorium ore; found in Sri Lanka, Madagascar, and Brazil
- Thorite — ThSiO₄ — a thorium silicate; found in granitic and alkaline igneous rocks
Global Reserves
[edit | edit source]| Country | Estimated Reserves (tonnes Th) | % of World Total | Notes |
|---|---|---|---|
| India | ~846,000–1,000,000 | ~25–30% | Vast monazite beach sand deposits in Kerala, Tamil Nadu, Andhra Pradesh; the primary driver of India's strategic thorium program |
| Brazil | ~632,000 | ~17% | Monazite deposits; significant rare earth element context |
| Australia | ~595,000 | ~16% | Olympic Dam; various mineral sand deposits |
| United States | ~595,000 | ~16% | Idaho, Montana, North Carolina; Thorium Energy Alliance estimates enough US thorium to power the US at current usage for over 1,000 years |
| Egypt | ~380,000 | ~10% | Largely unexploited |
| Turkey | ~344,000 | ~9% | Growing strategic interest |
| Venezuela | ~300,000 | ~8% | |
| Canada | ~172,000 | ~4% | |
| Russia | ~155,000 | ~4% | |
| Other | ~various | ~remaining | Including Norway; China; South Africa; Kazakhstan |
The Rare Earth Element Connection
[edit | edit source]Thorium is chemically and geologically associated with rare earth elements (REEs). Monazite — the primary thorium ore — is also the primary source of several critical REEs including cerium, lanthanum, and neodymium. This creates an important strategic and economic dynamic:
- REE extraction from monazite inevitably produces thorium as a byproduct
- Currently, most thorium recovered from REE processing is stored as waste because there is no established commercial market for it
- If thorium reactors are developed commercially, the existing REE mining industry could supply much of the thorium fuel needed without any new mining — the thorium already being extracted and stored would simply become a valuable energy resource rather than a waste disposal problem
