Major Minerals

Natural thorium is typically nearly pure 232Th, which is the longest-lived and most stable isotope of thorium, having a half-life comparable to the age of the universe. Its decay accounts for a gradual decrease of Th content of the Earth: the planet these days has about 85% of the amount of Th present at the formation of the Earth. Thorium is only found as a minor constituent of most minerals, and was for this reason earlier supposed to be rare. In nature, Th occurs in the 4+ oxidation state, together with U(IV), Zr(IV), Hf(IV), and Ce(IV), and also with Sc, Y, and the trivalent lanthanides which have similar ionic radii.

Due to Th's radioactivity, minerals containing it are often metamict (amorphous), their crystal structures having been destroyed by the alpha radiation produced during thorium decay. An extreme example is ekanite, (Ca,Fe,Pb)2(Th,U)Si8O20, which nearly never is found in crystalline form because of the Th it contains. Monazite, (Ce,La,Nd,Th,Sm,Gd,Ca)PO4, is the most important economic source of Th as it is found in large deposits worldwide, mainly in India, South Africa, Brazil, Australia, and Malaysia.

About 45 minerals are found in nature to contain structural thorium. Cabvinite (Th2F7(OH)· 3H2O) is the only known halide. The oxide class is represented by 14 different minerals, such as cerianite-(Ce) ((Ce41,Th) O2) and thorianite (ThO2) (Fig. 1). A total of 10 phosphate class minerals have Th, for example, althupite (AlTh(UO2)7(PO4)4(OH)5O2· 15H2O) and cheralite (CaTh(PO4)2). The silicate class contains 16 minerals with Th, such as ciprianiite (Ca4[(Th,U),Ca]Σ2Al(Be0.5&1.5)Σ2[B4Si4O22](OH)2), coutinhoite (ThxBa(1-2x)(UO2)2Si5O13· (H2O)1+y [10 < x < 0.5 and 0 < y < (2 + x)]), ekanite (Ca2ThSi8O20), huttonite (ThSiO4), thorite (ThSiO4) (Fig. 2), and thornasite ((Na,K)12Th3[Si8O19]4· 18H2O).

Chemistry Properties

Thorium is a moderately soft, paramagnetic, bright silvery radioactive actinide metal (Table 7.22). In the periodic table, it lies to the right of actinium, to the left of protactinium, and below cerium. Pure thorium is very ductile and, as normal for metals, can be cold-rolled, swaged, and drawn. At room temperature, thorium metal has a face-centered cubic (fcc) crystal structure; it has two other forms, one at high temperature (above 1360
C; body-centered cubic (bcc)) and one at high pressure (around 100 GPa; body-centerd tetragonal). Thorium is almost half as dense as uranium and plutonium and is harder than either of them.

It becomes superconductive below 2 271.8℃ (1.4K). Its melting point of 1750℃ is above both those of actinium (1227℃) and protactinium (1568℃). At the start of period 7, from francium to thorium, the melting points of the elements increase (as in other periods), because the number of delocalized electrons each atom contributes increases from one in francium to four in thorium, resulting greater attraction between these electrons and the metal ions as their charge increases from one to four. After thorium, there is a new downward trend in melting points from thorium to plutonium, where the number of f electrons increases from about 0.4 to about 6: this is caused by the increasing hybridization of the 5f and 6d orbitals and the formation of directional bonds resulting in more complex crystal structures and weakened metallic bonding (The f electron count for thorium is a noninteger due to a 5f-6d overlap.). Thorium has the highest melting and boiling points and second-lowest density; only actinium is lighter. Thorium’s boiling point of 4788℃ is the fifth highest of all the elements with known boiling points.

A thorium atom has 90 electrons, of which four are valence electrons. Three atomic orbitals are theoretically available for the valence electrons to occupy: 5f, 6d, and 7s. Notwithstanding its position in the f-block of the periodic table, it has an anomalous [Rn]6d2 7s2 electron configuration in the ground state, as the 5f and 6d subshells in the early actinides are very close in energy, even more so than the 4f and 5d subshells of the lanthanides: thorium’s 6d subshells are lower in energy than its 5f subshells, as its 5f subshells are not well shielded by the filled 6s and 6p subshells and are destabilized. This is caused by relativistic effects, which become stronger near the bottom of the periodic table, in particular the relativistic spinorbit interaction. The closeness in energy levels of the 5f, 6d, and 7s energy levels of thorium results in thorium nearly always losing all four valence electrons and occurring in its highest possible oxidation state of 14.  This is different from its lanthanide congener cerium, in which 14 is also the highest possible state, but 13 plays a significant role and is more stable. Thorium is much more like the transition metals zirconium and hafnium than like cerium in its ionization energies and redox potentials, and hence also in its chemistry: this transition-metal-like behavior is the norm in the first half of the actinide series. Notwithstanding the anomalous electron configuration for gaseous thorium atoms, metallic thorium shows substantial 5f involvement.

Reactions

Thorium is a highly reactive and electropositive metal. With a standard reduction potential of 21.90 V for the Th41/Th couple, it is slightly more electropositive than zirconium or aluminum. Finely divided thorium metal can show pyrophoricity, spontaneously igniting in air. When heated in air, thorium turnings ignite and burn with a brilliant white light to produce the dioxide ThO2, also called thoria or thorina, which has the fluorite (CaF2) structure. In bulk, the reaction of pure thorium with air is slow, though corrosion may take place after several months; the majority of thorium samples are contaminated with varying degrees of the dioxide, which significantly accelerates corrosion.

At standard temperature and pressure, thorium is slowly attacked by water, but does not readily dissolve in most common acids, with the exception of hydrochloric acid, in which it dissolves leaving a black insoluble residue of ThO(OH,Cl)H. It dissolves in concentrated nitric acid with a small amount of catalytic fluoride or fluorosilicate ions; if these are absent, passivation by the nitrate can take place, as with uranium and plutonium. Thorium hydroxide, Th(OH)4, can be formed by adding a hydroxide of ammonium or an alkali metal to a thorium salt solution, where it emerges as a gelatinous precipitate that will dissolve in dilute acids, among other substances.

When heated in air, thorium dioxide emits intense blue light; the light turns to white when ThO2 is mixed with its lighter homolog cerium dioxide (CeO2, ceria): this is the basis for its previously common application in gas mantles. The light emitted by thorium dioxide is higher in wavelength than the blackbody emission expected from incandescence at the same temperature, an effect called candoluminescence.

A number of binary thorium chalcogenides and oxychalcogenides are known with sulfur, selenium, and tellurium. As well as several binary compounds, the oxychalcogenides ThOS (yellow), ThOSe, and ThOTe also exist. The five binary thorium sulfides—ThS (lustrous metallic), Th2S3 (brown metallic), Th7S12 (black), ThS2 (purple-brown), and Th2S5 (orange-brown)—can be formed by reacting hydrogen sulfide with thorium, its halides, or thoria (the last if carbon is present): they all hydrolyze in acidic solutions.

All four thorium tetrahalides are known, as are some low-valent bromides and iodides: the tetrahalides are all 8-coordinated hygroscopic compounds that dissolve easily in polar solvents such as water. Many associated polyhalide ions are also known. Thorium tetrafluoride (ThF4, white, m.p. 1068℃) is most easily produced by reacting various thorium salts, thoria, or thorium hydroxide with hydrogen fluoride: procedures that involve steps in the aqueous phase are more difficult as they result in hydroxide and oxide fluorides that must be reduced with hydrogen fluoride or fluorine gas.

Major Uses

Most Th uses employ its dioxide (ThO2, occasionally called “thoria” in the industry), instead of the metal. This compound has a melting point of 3300℃, the highest of all known oxides; only some compounds have higher melting points. This improves the compound to stay solid in a flame, and it substantially improves the brightness of the flame; this is the most important reason why Th is used in gas mantles. All materials emit energy (glow) at high temperatures, but the light emitted by Th is almost completely in the visible spectrum, hence the brightness of Th mantles.

ThO2 is employed in GTAW (gas tungsten arc welding) to improve the high-temperature strength of W electrodes and increase arc stability. ThO2 is being substituted in this application with other oxides, for example, those of Zr, Ce, and La. ThO2 can be found in heat-resistant ceramics, for example, high-temperature laboratory crucibles, either as the principal compound or as an addition to ZrO2.

ThO2 can be found in heat-resistant ceramics, for example, high-temperature laboratory crucibles, either as the principal compound or as an addition to ZrO2. An alloy of 90% Pt and 10% Th is an efficient catalyst for oxidizing ammonia to nitrogen oxides, but this has been substituted by an alloy of 95% Pt and 5% Rh due to its better mechanical properties and durability. When added to glass, ThO2 increases its refractive index and decreases dispersion. This type of glass is used in high-quality lenses for cameras and scientific instruments. The radiation from these lenses can darken them over time and turn them yellow over a period of years and degrade film, though the health risks are negligible.