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Europium - Sinha S.P.

Sinha S.P. Europium - Springer-Verlag, 1967. - 88 p.
Download (direct link): europium1967.djvu
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Origin La2Oa Ce2Os Pr8Ou Nd2Os Sm2Os Gd2Os Y2Oa Th02
North Carolina [22] 15.4 32.1 3.8 18.8 7.8 3.9 4.8 12.1
Virginia [22] 15.9 36.9 4.4 18.0 4.9 1.6 1.9 15.5
New Mexico [22] 18.4 36.9 4.2 18.0 5.7 2.7 2.4 10.7
California [22] 22.2 43.2 4.5 17.4 2.8 1.1 3.1 7.2
Brazil [22] 13.3 41.4 5.5 23.0 5.0 1.5 1.1 11.3
Madagascar [23] 14.2 56.2 4.1 10.5 3.1 0.2 1.1 10.6
Travancore (India) [22] 19.2 42.0 5.1 17.9 2.8 0.8 0.4 12.4
Pichli (India) [24] 22.2 35.6 5.2 11.0 3.4 4.6 17.9
Western Australia [25] Southern Australia 18.6 40.6 5.0 16.9 4.3 6.3 8.95
(Normanville) [25] Southern Australia 14.6 30.5 4.2 16.3 3.1 5.7 27.0
(Olary) [25] 28.9 42.1 5.1 18.1 3.6 1.93 0.27
Tasmania [25] 21.0 38.9 4.7 16.0 3.3 5.7 10.6
the monazite of Travancore, R. De [26] found evidence for the presence of 208Po, 211 At and Pm. Monazite crystals are always monoclinic, but with variable crystal faces thus giving often prismatic or rhombic appearance. The X-ray patterns of monazite indicate a close similarity in structure to xenotime [27]. Tetragonal xenotime is mainly yttrium phosphate, and monoclinic monazite is largely cerium phosphate, but both minerals contain substituents of either group of rare earths in varying amounts. It has been found [28] that the phosphates of rare earths are dimorphous, one being monoclinic and isostructural with monazite, the other being hexagonal. The hexagonal crystals can easily be converted to the monoclinic form by heating.
Samar shite.—This tantaloniobate mineral has a widely varying composition, and was first discovered in the Urals. The presence of uranium in this mineral renders it radioactive. The presence of lead and helium has been detected. The ratio of lanthanides to yttrium is approximataly 1: 6. In its crystal structure samarskite resembles yttrotantalite. Native crystals are brown, or velvet black, and become yellow orange when crushed showing pleochroism.
8
Introduction
The earth’s crust is again a good source of lanthanides. Although the name rare earths is still used to denote the lanthanide elements, and scandium and yttrium, in the strictest sense of the word “rare” they are more plentiful than many of our common elements. It comes as a surprise to many people when a comparison of the relative abundance of the lanthanides and other elements in the earth’s crust is made. Table 4
Table 4. Natural abundance of the lanthanides and some common elements
Element Abundance (ppm) Element Abundance (ppm)
Lanthanum 18 Carbon 320
Cerium 46 Chromium 200
Praseodymium 5.5 Manganese 1,000
Neodymium 24 Iron 50,000
Samarium 6.5 Cobalt 23
Europium 0.5 Nickel 80
Gadolinium 6.4 Copper 70
Terbium 0.9 Zinc 130
Dysprosium 5.0 Cadmium 0.3
Holmium 1.2 Mercury 1
Erbium 4.0 Silver 0.1
Thulium 0.4 Gold 0.005
Ytterbium 2.7 Platinum 0.005
Lutetium 0.8 Tin 40
Scandium 10 Lead 16
Yttrium 28
shows that the total amount of lanthanides is approximately half that of carbon and is comparable with that of zinc and chromium. Silver, gold and platinum are even scarcer than the lanthanides. Cerium is more abundant in the earth’s crust than tin.
From geochemical considerations some elements e.g., Rb, Hf, or Sc rarely form minerals of their own. These elements belong to the litho-philic category of Goldschmidt’s classification, and usually tend to be accommodated in the structure of the minerals of more abundant elements. Such replacement may only occur when ionic charges, radii and bond type for each pair of elements are very similar. This “fixation” of certain ions in the lattice of the host minerals sometime depends mainly on the ionic size rather than the ionic charge, as for example, in the following replacement series, proposed by Goldschmidt [29]. In these cases the charge balance is maintained by replacements elsewhere in the structure.
Occurrence in Nature
9
Host ion Na+ (0.98) K> (1.33) Fe2+ (0.83) Ca2+ (1.06) Sc3+ (0.83) Ti4+ (0.64)
Impurity Ca2+ (1.06) Ba2+ (1.44) Sc3+ (0.833) Y3+ (1.06) Zr4+ (0.87) Nb5+ (0.64)
The only abundant trivalent element is aluminium, but the radius of Al3+ (0.50 A) is very much smaller than those of the trivalent rare earths (see p. 30), and substitution of rare earths for AY3 ‘ in its minerals does not take place. The radius of Ca2_r (1.01 A) is of the correct magnitude although the charge difference may introduce certain difficulties. In some minerals, ready substitution of Ca2+ by rare earth ions takes place; for example in yttrofluorite the two components, CaF2 and YF3 mix freely. The abundant rock, plagioclase, does not seem to accept trivalent rare earth ions freely, presumably due to the disturbance in charge balance between Ca2+ and M3+. This is one possible reason why the rare earths are enriched in granites and especially in pegmatites.
The following is a summary of the substitution of the rare earth ions in minerals, as commonly observed.
a) Replacement of Ca2+ (or Pb) by rare earths occurs very frequently. Example: yttrofluorite.
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