Holmium
Elemental holmium is a relatively soft and malleable silvery-white metal. It is too reactive to be found uncombined in nature, but when isolated, is relatively stable in dry air at room temperature. However, it reacts with water and rusts readily, and will also burn in air when heated.
Holmium is found in the minerals monazite and gadolinite, and is usually commercially extracted from monazite using ion exchange techniques. Its compounds in nature, and in nearly all of its laboratory chemistry, are trivalently oxidized, containing Ho(III) ions. Trivalent holmium ions have fluorescent properties similar to many other rare earth ions (while yielding their own set of unique emission light lines), and holmium ions are thus used in the same way as some other rare earths in certain laser and glass colorant applications.
Holmium has the highest magnetic strength of any element and therefore is used for the polepieces of the strongest static magnets. Because holmium strongly absorbs neutrons, it is also used in nuclear control rods.
Occurrence and production
Like all other rare earths, holmium is not naturally found as a free element. It does occur combined with other elements in gadolinite, monazite, and other rare-earth minerals. The main mining areas are China, United States, Brazil, India, Sri Lanka, and Australia with reserves of holmium estimated as 400,000 tonnes.
It is commercially extracted via ion-exchange from monazite sand (0.05% holmium) but is still difficult to separate from other rare earths. The element has been isolated through the reduction of its anhydrous chloride or fluoride with metallic calcium. Its estimated abundance in the Earth's crust is 1.3 mg/kg. Holmium obeys the Oddo-Harkins rule: as an odd-numbered element, it is less abundant than its immediate even-numbered neighbors, dysprosium and erbium. However, it is the most abundant of the odd-numbered heavy lanthanides. The principal current source are some of the ion-adsorption clays of southern China. Some of these have a rare-earth composition similar to that found in xenotime or gadolinite. Yttrium makes up about two-thirds of the total by weight; holmium is around 1.5%. The original ores themselves are very lean, maybe only 0.1% total lanthanide, but are easily extracted. Holmium is relatively inexpensive for a rare-earth metal with the price about US$ 1000 per kg.
| Symbol | Ho | |
| Atomic Number | 67 | |
| Atomic Weight | 164.93032 | |
| Oxidation States | +3 | |
| Electronegativity, Pauling | 1.24 | |
| State at RT | Solid, Metal | |
| Melting Point, K | 1747 | |
| Boiling Point, K | 2968 |
Appearance and Characteristics
Harmful effects:
Holmium is considered to be of low toxicity.
Characteristics:
- Holmium is a bright, soft, silvery-white, rare earth metal that is both ductile and malleable.
- It does not react in dry air at normal temperatures, but rapidly oxidizes to a yellow oxide (Ho2O3) in moist air or when heated.
- When present in compounds, holmium exists usually in the trivalent state, Ho3+. Most holmium compounds are brownish yellow in color.
- Holmium has unusual magnetic properties, including the highest magnetic moment (10.6 µB) of any naturally occurring element.
Uses of Holmium
- As a result of its special magnetic properties, holmium is used in alloys for the production of magnets and as a flux concentrator for high magnetic fields.
- Holmia (holmium oxide) is used as a yellow or red coloring for glass and cubic zirconia.
- Holmium isotopes are good neutron absorbers and are used in nuclear reactor control rods.
- Holmium is also used in solid-state lasers for non-invasive medical procedures treating cancers and kidney stones.
