RARE EARTHS, in chemistry, the name given to a group of metallic oxides originally called by this title since they resemble the substances then known as earths (lime, magnesia, etc.). The corresponding elements are unique in that they are all very similar in their chemical properties. The group has been roughly sub-divided into three portions more for convenience than from theoretical considerations. The elements, together with their main characteristics, are given in the sub-groups shown below. The atomic number precedes the name and the atomic weight is given after the symbol. For details see separate article on each element.
(57) Lanthanum (La 138.9). Colourless salts; solutions show no absorption spectrum. White, strongly basic oxide, which is diamagnetic.
(58) Cerium (Ce 140.2). Forms two series of salts, cerous and ceric, derived from the oxides and Ce02. Cerous salts are colourless and show no absorption spectrum. Ceric salts are yellow or orange-red, and are rapidly hydrolyzed by water. The stable oxide, Ce02, is insoluble in hydrochloric and nitric acids when pure.
(59) Praseodymium (Pr 140.9). Forms leek-green coloured salts whose solutions show a very characteristic and intense ab sorption spectrum. The stable oxide is black.
(6o) Neodymium (Nd 144.3)• Amethyst-coloured salts, the solutions of which give a characteristic and intense absorption spectrum. The oxide, is blue when carefully ignited.
(6i) Illinium (II ?). Solutions said to give an absorption spectrum which is obliterated by the presence of neodymium or samarium. Also called florentium.
(62) Samarium (Sa 150.4). Very pale yellowish oxide, Sa203, giving topaz-yellow salts, showing a characteristic absorption spectrum.
(63) Europium (Eu 152.0). Pale pink oxide, and pale rose-tinted salts; solutions give weak but characteristic absorp tion spectrum and brilliant and very strong spark spectrum.
(64) Gadolinium (Gd 157.3). White oxide, colourless salts; solutions show absorption spectrum in the ultra-violet only.
(65) Terbium (Tb 159.2). Stable oxide, almost black.
Salts and solutions almost colourless, giving one absorption band in the blue.
(66) Dysprosium (Dy 162.5). Oxide, white, forms greenish-yellow salts; solutions characterized by an absorption spectrum. Compounds possess a very high magnetic susceptibility.
(67) Holmium (Ho 163.5). Oxide, yellow, giving orange-yellow salts ; solutions show characteristic absorption spectrum.
(39) Yttrium (Y 89.o). This element is placed here for con venience. Its atomic weight is out of order, and many do not consider it a true member of the series. Oxide, white and diamagnetic, gives colourless salts whose solutions give no absorp tion.
(68) Erbium (Er 167.7). Oxide, rose, gives rose coloured salts showing very characteristic absorption spectrum.
(69) Thulium (Tm 169.4). Oxide, greenish-white, forms pale bluish-green salts. Solution characterized by both ab sorption and bright line spectra.
(7o) Ytterbium (Yb 173.5). Oxide, white, gives colourless salts and solutions showing no absorption spectrum. Spark spectrum, however, is very characteristic. Differs from and since the oxide is a much weaker base and is paramagnetic.
(71) Lutecium (Lu 175.0). Oxide, white; almost identical in its properties with the preceding element except that it has a lower magnetic susceptibility and a characteristic spark spectrum.
Of the elements listed above, cerium, lanthanum and neodymium are comparatively common, while europium, terbium and thulium are very rare. While it is true that certain minerals are richer in one sub-group than in others, nevertheless if a sufficient amount of any mineral is examined, the presence of every member of the series can be detected. Monazite, cerite, allanite, etc., are the best sources for the elements of sub-group 1; samarskite and certain varieties of xenotime for sub-group 2; gadolinite, xenotime, euxenite, fergusonite, etc., for sub-group 3. These minerals are found chiefly in Norway, U.S.A., Brazil, India and Australia.