rar-, rare- +

(Latin: rarus, rare, thin, loose, sparse; airy, porous, infrequent, unusual)

bone rarefaction
A decrease in density of part of a bone; especially, when this causes increased radiolucence (almost entirely invisible in x-ray photographs and under fluoroscopy).
rare
1. Occurring infrequently; an uncommon quality; such as, a rare sports event.
2. Excellent; extraordinary: "His neighbor has a rare sense of humor."
3. Not widely known; especially, valued for its uncommonness.
4. Thin in density or having a low density: rarefied; such as, rare air.
5. Etymology: originally "few in number and widely separate"; from Old French rere, "sparse" (14th century); from Latin rarus, "thinly sown, having a loose texture".
rare species
1. In biology, a species which is found infrequently; especially, one that is missing or which is minimally present in a sampling of a given geographical area.
2. In ecology, a species of an animal or a plant which is not in immediate danger of becoming extinct, but that is the topic of protective regulations or conservation measures because of its limited population and distribution.
rare, scarce
rare (RAIR) (adjective)
1. A reference to describe the degree of cooking for a piece of meat allowing the center of the piece to remain red: Cara always orders her steak rare when she goes out for dinner.
2. Distinguished by unusual merit or appeal: At the gallery, Mark viewed the rare collection of ivory miniatures.
3. Superlative, seldom occurring: Tabitha and Shelby saw a rare bird while they were out on a hike.
scarce (SKAIRS) (adjective)
1. Intentionally not present: The children were told to make themselves scarce when the parents were decorating the house.
2. Limited quantity in comparison to the interest or demand; so, not easy to procure: The grocer reported that lemons were scarce at this time of year.
3. Hard to find; absent or rare: U.S. steel pennies are scarce now except in coin shops.

In Kevin's art store, there is a scarce supply of copies of the rare print made by the Italian print master of the last century.

rare-earth alloy
Any metal alloy containing appreciable amounts of one or more rare-earth elements; for example, mischmetal.

Mischmetal is an alloy consisting of a crude mixture of cerium, lanthanum, and other rare-earth metals obtained by electrolysis of the mixed chlorides of the metals dissolved in fused sodium chloride.

They are used in making aluminum alloys, in some steels and irons, and in coating the cathodes of glow-type voltage regulator tubes.

rare-earth elements, REE, critical resources for high technology
The rare-earth elements (REE) form the largest chemically coherent group in the chemical periodic table.

Although they are generally unfamiliar, the rare-earth elements are essential for many hundreds of applications.

The versatility and specificity of the rare-earth elements have given them a level of technological, environmental, and economic importance considerably greater than might have been expected from their relative obscurity.

As technological applications of rare-earth elements have multiplied over the past several decades, demand for several of the less abundant (and formerly quite obscure) REE has increased dramatically.

Some of the Applications of the Rare-Earth Elements

  • Color cathode-ray tubes and liquid-crystal displays used in computer monitors and televisions employ europium as the red phosphor and no substitute is currently known.
  • Fiber-optic telecommunication cables provide much greater bandwidth than the copper wires and cables they have largely replaced.
  • Fiber-optic cables can transmit signals over long distances because they incorporate periodically spaced lengths of erbium-doped fiber that function as laser amplifiers because it alone possesses the required optical properties.
  • Permanent magnet technology has been revolutionized by alloys containing neodymium, samarium, gadolinium, dysprosium, or praseodymium.
  • Small, lightweight, high-strength rare-earth element magnets have allowed miniaturization of numerous electrical and electronic components used in appliances, audio and video equipment, computers, automobiles, communications systems, and military gear.
  • Several rare-earth elements are essential constituents of both petroleum fluid cracking catalysts and automotive pollution-control catalytic converters.
  • Although more expensive, lanthanum-nickel-hydride batteries offer greater energy density, better charge-discharge characteristics, and fewer environmental problems when they are recycled or disposed of.
  • The rare earth elements are essential for a diverse and expanding array of high-technology applications, which constitute an important part of the industrial economy of the United States.
  • Long-term shortages or unavailability of rare-earth elements would force significant changes in many technological aspects of American life.
  • State-run Chinese firms sharply expanded production and slashed prices of rare earths in the 1990's, forcing producers in the United States (previously the world’s leading producer and exporter) and elsewhere out of the market which no doubt will change now that China has restricted its exports of rare-earth minerals.
—Compiled primarily from information located at the
U.S. Geological Survey web site.
rare-earth magnet
A magnet which is manufactured with a rare-earth element; such as,a rare=earth cobalt magnet.

It can have as much as ten times more coercive force than a typical magnet.

rare-earth mineral
Any mineral that is composed of a high percentage of rare-earth elements; such as, ytterbium and cerium.
rare-earth minerals, rare earth minerals; rare earth elements
1. A collective term for a series of fifteen related metallic elements having atomic numbers ranging from 57 to 71, and placed in a special row on the periodic table.

    The group consists of the following elements which are not earths and are not literally rare; however, they are called "rare earth minerals" because they were associated with more familiar substances known as "common earth".

    This Lanthanide series is shown with their atomic numbers, their symbols, and their names with links to much more detailed information about the history, who and where they were discovered, terms in four other languages, etc. for each of the listed elements as shown in the Periodic Table of Chemical Elements:

  • 57 La, Lanthanum
  • 58 Ce, Cerium
  • 59 Pr, Praseodymium
  • 60 Nd, Neodymium
  • 61 Pm, Promethium
  • 62 Sm, Samarium
  • 63 Eu, Europium
  • 64 Gd, Gadolinium
  • 65 Tb, Terbium
  • 66 Dy, Dysprosium
  • 67 Ho, Holmium
  • 68 Er, Erbium
  • 69 Tm, Thulium
  • 70 Yb, Ytterbium
  • 71 Lu, Lutetium
  • The elements range in crustal abundance (igneous crust or outer layer of the earth) from cerium, the 25th most abundant element of the 78 common elements in the earth's crust at 60 parts per million, to thulium and lutetium, the least abundant rare-earth elements at about 0.5 part per million.

2. Minerals which contain one or more rare-earth elements as major metal constituents.

Seventeen rare-earth minerals are used in a wide variety of commercial and military applications ranging from precision guided smart bombs, to efficient light bulbs, car batteries, sophisticated radar systems, mobile phones, clean energy technology, DVDs, very large wind turbines, phosphors for monitors, televisions, lighting, catalytic converters, glass polishing, petroleum refining; plus other modern applications.

Rare-earth minerals are also used in computer display screens, motherboards, hard drives, chips, and other-related elements in computers; rare metals like indium is used in liquid-crystal display screens, antimony is used in silicon wafers for semiconductors, neodymium is a vital element in industrial batteries which are used in electric motors and it is found in parts used in the speakers of cellphones, and dysprosium is used in laser materials.

Cerium is needed in such high-profile and sensitive applications as optical sensors used in F-15 fighter aircraft, and the windows and domes at the National Ignition Facility (NIF) which explores the world of high-energy-density physics.

Over 50 pounds of rare earth metal can be found in each Toyota Prius automobile and Japan is the world’s largest importer of rare earths for such products.

Hybrid vehicles use a special neodym magnet made with neodymium to help produce the energy they require to offset their usage of gas and oil.

On April 20, 2010, neodymium was priced at about $46.50 a kilogram (2.20 pounds). Since China made export rules and regulations in July, 2010, prices went up to $92 a kilogram (2.20 pounds).

Rare earth production outside of China by other countries has been limited by higher costs of mining (compared to those of China) and by concerns regarding environmental pollutions from mining wastes by other nations.

The United States previously produced all stages of the rare earth material supply chain, but now most rare earth materials processing is performed in China, giving it a dominant position that can affect the worldwide supply and prices of rare earth minerals.

According to the United States Geological Survey (USGS), the name “rare” earth elements is an “historical misnomer”, reflecting the elements’ unfamiliarity, rather than their true rarity.

Even the most scarce of rare earths, lutetium and thulium, are 200 times more abundant than gold in the earth’s crust.

China has about fifty-seven percent of the world’s known reserves, according to the United States Geological Survey. The United States has nine percent of global reserves, Australia has four percent, and Russia has fourteen percent.

Also, according to figures from the U.S. Geological Survey published earlier this year, production from Chinese mines accounted for 120,000 of the 124,000 tons of rare-earth oxides produced globally in 2009; which is more than 97 percent of the available supply; while India, Brazil, and Malaysia made up the rest of the supply or just three percent of the total.

Molycorp, the United States company that owns the Mountain Pass mine in the Mojave Desert of California, announced its intention to raise rare-earths production to meet about a sixth of global demand by 2012, and the company indicated that it would double that output if circumstances justify such an increase of production.

—Compiled from information located in various sources; especially:
The U.S. Geological Survey web site, and
The Chemical Elements List at this Word Info web site.
Additional valuable information is available at this
Rare Earth Elements and their Uses web site.
rare-earth salt
Any of the various salts of the rare-earth elements from the lanthanide series; especially, a mixture derived from the mineral monazite (a reddish brown phosphate mineral which contains cerium, lanthanum, and some thorium).
rare-earth screen
An x-ray-intensifying screen made of rare-earth elements; such as, yttrium and gadolinium.

These screens make it possible for lower radiation doses to be used while producing acceptable film densities.

rarefaction
1. A decrease in density.
2. In acoustics, an area of expansion in a sound wave, the portion of a three0dimensional sound wave in which the medium compression that is due to acoustic energy is minimal and which, if measured as a standing waving of pressure versus time, would be a point at which the signal amplitude is minimum.
3. The process of decreasing in density and weight, as of air.

The farther from the surface of the earth, the less dense the atmosphere becomes.

rarefaction of bone
The process by which bone becomes more porous because of absorption of mineral substances.

This may be caused by a disturbed calcium-phosphorus metabolism possibly resulting from excess parathyroid hormone.

rarefied gas
A very low density gas.
rarefier
Something that becomes thinner or less dense.