The Rare Earths: Their Occurrence, Chemistry, and Technology

Thorite contains from 1·4 to 3·1 per cent. of rare earths. According to Nilson and Blomstrand, the uranium is present as uranium dioxide, UO₂ replacing thoria, ThO₂, but Dunstan and Blake state that the two oxides are isomorphous (see under Thorianite, p. 74), and so they might be expected to be vicarious. Thorite was discovered by Esmark in 1828, and first analysed by Berzelius,[48] who announced the discovery of a new earth in it in 1829. The name Thorite is from Thor, the god of Scandinavian mythology.

Thorite is a member of a peculiarly interesting series of isomorphous minerals, which includes Cassiterite (SnO₂), Rutile (TiO₂), Zircon (ZrSiO₄), and most probably the allied silicate Naegite, and the rare earth phosphate Xenotime (q.v.), which are very similar in forms and angles. The oxide TiO₂ is itself trimorphous, being known in the three crystallographically different forms, Rutile, Anatase, and Brookite (q.v.). On account of the isomorphism of cassiterite and rutile with the two silicates, it has been suggested that the oxide formulæ be doubled and written Sn(SnO₄) and Ti(TiO₄) respectively,[49] to show the analogy with Th(SiO₄) and Zr(SiO₄). Consideration of the molecular volumes (obtained by dividing molecular weight by specific gravity, i.e. multiplying by specific volume) lends a certain amount of support to this view. It has often been observed that isomorphous compounds, and many compounds which occur in parallel growth to one another, have nearly equal molecular volumes; there are, however, many exceptions. Taking molecular volumes for the series under consideration, we have, using approximate numbers only—

It will be seen that if the numbers for cassiterite and rutile be doubled, four out of the five show very fair approximation to the constant value 40. The number 60 for thorite is quite irreconcilable with the values obtained from the other members; of course pure silicate of thorium, ThSiO₄, is not known as a mineral, but it is most unlikely that the relatively small amount of impurity in the densest specimens of orangite should have depressed the specific gravity by over two units, as would be required if the molecular volume of thorite were to show even the most approximate semblance of agreement with the others. It cannot be too often remarked, however, that very little indeed is known of the molecular formulas of minerals, and that very little reliance can be placed on such figures as the above. On the contrary, it is hardly conceivable that amphoteric oxides like those of tin and titanium, occurring in the form of heavy crystalline minerals, should have molecular formulæ only double the empirical formulæ. Where agreements of the kind do occur, they must be taken as indicating approximately equal degrees of molecular complexity in the minerals concerned, rather than as affording any real insight into the molecular condition.

Zircon.

—Zircon is a silicate of zirconium, ZrSiO₄, with small quantities of other elements. Most varieties contain ferric oxide and thoria; more rarely small proportions of the yttria earths may be present. All varieties contain traces of a large number of the common metals. Traces of radium are usually present, with helium and neon,[50] and the mineral is strongly radioactive.

It is infusible before the blowpipe, but loses its colour; some varieties glow and increase in density (see p. 38). In some varieties also the colour changes or disappears rapidly on exposure to sunlight, and is often restored on keeping in the dark. These phenomena of colour change have been attributed variously to alteration in the state of oxidation of the iron present, and to the presence of organic matter. It seems probable that either cause or even both may be at the root of the change in particular cases.

On account of the hardness, unalterability, and strong refraction and double refraction, good crystals of zircon are used as gems. The two gem varieties, Hyacinth and Jargon, are found chiefly in the gem gravels of Ceylon. It was in a zircon from Ceylon that Klaproth discovered the new earth, Zirconia, in 1789.[51] In 1795 he found the same earth in hyacinth, and so showed the two to be identical.

Artificial crystals of zircon have been obtained by the action of silicon tetrachloride and silicon tetrafluoride on zirconia, and by the action of zirconium tetrafluoride on silica at high temperatures.

Zircon is one of the most widely distributed minerals known, though usually it occurs in very small quantities. Good crystals have been found in New Zealand, in Ceylon, at Miask in the Urals, and in North Carolina. This last deposit has been worked commercially for the extraction of zirconia for Nernst lamps (vide p. 320). It occurs in a decomposed felspar in a pegmatite dyke in the Archæan gneiss near Zirconia, Henderson Co., and can be easily extracted by picking or washing, after crushing if necessary. Should there ever be a considerable demand for zirconia, it could doubtless be saved as a by-product in the extraction of thoria from monazite sands (q.v.), zircon being very generally found in those sands (see below).

Zircon is common in crystalline rocks, limestones, schists, syenites, granites, etc. It is a constant accessory constituent in the acid igneous rocks, especially in the more acid eruptive rocks. It is readily detected under the microscope by the pleochroic haloes with which the tiny crystals are surrounded; these have been shown by Joly to be due to alteration of the surrounding rock by the radiations emitted by the radio-active constituents of the zircon. It also occurs as a constituent of those sands which are formed by the erosion of the igneous rocks in which it is enclosed, and hence it almost invariably accompanies monazite in the so-called monazite sands.

Zircon is one of the least easily altered minerals; by the prolonged action of chalybeate and other waters, during many geological ages, however, it gradually changes, losing silica and gaining lime, oxides of iron, and water. Some of these altered varieties have received special names, as, e.g. Auerbachite, Malacone, Cyrtolite, and Alvite; but none of them is of special interest.

Naegite.

[52]—This rare mineral is a silicate closely related to zircon, but of rather more complex composition. It may be represented as silicate of zirconium, ZrSiO₄ (zirconia = 55·3, silica = 20·6 per cent.), with rare earths (chiefly yttria, 9·1 per cent.), uranium (UO₃ = 3 per cent.), and thorium (ThO₂ = 5·0 per cent.), partly as silicates, partly as columbates and tantalates ((Cb,Ta)₂O₅ = 7·7 per cent.).[53]

It is tetragonal, usually occurring in globular aggregates of crystals. The measurable angles are extremely close to those of zircon, and it is probable that naegite is isomorphous with the series mentioned above under Thorite.

So far it has only been found in the ‘placer’ tin deposits or ‘gravel tin’ of Japan.

The following minerals (see list) are also to be included in this sub-class:

Alvite (Anderbergite or Cyrtolite), Auerbachite, Malacone, Oerstedite and Tachyaphaltite, altered varieties of zircon.

Calciothorite, Eucrasite and Freyalite, altered varieties of Thorite.

Pilbarite, Thorogummite and Yttrogummite, hydrated silicates of thorium with uranium and other metals.

(c) Complex Silicates

Eudialyte (Eucolyte).

—This is a complex silicate of alkalies, lime, ferrous oxide, rare earths, etc., containing chlorine and a high proportion (up to 17 per cent.) of zirconia. The empirical formula is given by Dana as Na₁₃(Ca,Fe)₆Cl(Si,Zr)₂₀O₅₂. Brögger gives the simpler metasilicate formula R´₄R´´₃Zr(SiO₃)₇, where R = (Na,K,H), R´´ = (Ca,Fe,Mn,CeOH), and Zr(OCl) may partly function as an acid in place of SiO₂. The true formula, however, is quite uncertain, as the zirconia may function either as an acidic or basic oxide. The fact that a mineral of such exceedingly complex composition occurs in perfectly well-defined crystals indicates the intricate nature of the problems to be solved in mineral chemistry.

The double refraction is strong, being positive for eudialyte, negative for the Norwegian variety, eucolyte. From careful microscopic examination, Ramsay has found that zones of positive and negative birefringence, as well as isotropic (singly-refracting) zones can occur on the same crystal, and he suggests that the mineral is really composed of two isomorphous compounds forming mixtures. In view of the continuous variation of optical properties in an isomorphous series like the felspars, such an explanation seems doubtful. The optical behaviour of minerals is very often anomalous, and the phenomena in this case are probably due to repeated twinning, with some alteration in the double refraction, or to the lamellar intergrowth of two varieties having slightly different optical properties.

On heating, the mineral evolves moisture and readily fuses. It is easily attacked even by dilute acids, being named by Strohmeyer (1819) on account of this property. The dilute hydrochloric acid solution reddens turmeric paper—a test for the presence of zirconium.

It is found in Greenland, usually embedded in felspar, in Norway, in Lapland and in Arkansas, being generally associated with minerals rich in alkalies, e.g. ægirine, ælæolite, nepheline, sodalite, arfvedsonite, etc.

Beckelite.

—This is a mineral similar in composition to eudialyte, though not so complex, and of more recent discovery.[54] It is a silicate of ceria earths and lime, in which zirconia replaces silica; the oxygen ratio (i.e. ratio of oxygen in basic oxides to oxygen in acid oxides) is 3 : 1, and the formula Ca₃R´´´₄(Si,Zr)₃O₁₅, where R = rare earth metals, chiefly of the cerium group. It is thus a salt of an acid H₁₈Si₃O₁₅ [= 3H₆SiO₅ = 3(3H₂O,SiO₂)] with zirconium and silicon vicarious.

It is soluble in hot hydrochloric acid, even after ignition; the solution gives the turmeric test for zirconium.

It was found in a dyke in an ælæolite syenite, near the Sea of Azov.

The following minerals (see list) are also to be placed in the class of mixed silicates:

Arfvedsonite and cataplejite, complex zircono-silicates.

Hiortdahlite (Guarinite) and Lavenite, zircono-silicates with fluorine.

Caryocerite, Melanocerite and Steenstrupine, complex fluosilicates.

Auerlite, Britholite, Erikite and Florencite, phospho-silicates.

Cappelenite, Homilite and Tritomite, boro-silicates.

CHAPTER III
THE TITANO-SILICATES AND TITANATES

(a) The Titano-Silicates

Yttrotitanite or Keilhauite.

—A titano-silicate of calcium, aluminium, iron and yttrium metals. The mineral is isomorphous with titanite, CaO,TiO₂,SiO₂ (q.v.), and is itself probably an isomorphous mixture of titanite with the silicate (Y,Al,Fe)₂SiO₅, where Y = yttrium metals. Its composition will then be represented by the formula m (Y,Al,Fe)₂(SiO₅) + n CaTi(SiO₅).

The mineral is fusible before the blowpipe, and is decomposed by hydrochloric acid.

It was named by Scheerer in 1844 from its composition, and by Ekeberg in the same year in honour of the Norwegian geologist Keilhau.

Titanite or Sphene.

—This species, important as an accessory mineral of many rocks, is a titano-silicate of calcium, generally containing small quantities of aluminium and iron. The approximate formula usually given, CaTiSiO₅, is unsatisfactory; some specimens contain as much as 7 per cent. of ferric oxide, others up to 2 per cent. of manganese, whilst the percentage of titanium oxide, TiO₂, varies very considerably (30 to 45 per cent.). Zambonini and Nickolan have independently analysed specimens for which no satisfactory formulæ could be deduced. For specimens containing trivalent metals, Groth considers the mineral to be an isomorphous mixture of CaTiSiO₅ and R´´´₂SiO₅ (see under Yttrotitanite, above); Blomstrand, however, advances the formula 2(R´´R´´´₂O₂,TiO)O,SiO₂, where TiO is basic, and the trivalent metals occur in the divalent group R´´´₂O₂; this formula is also supported by Zambonini.

More recently the problem of the constitution has been attacked by Bruckmoser, using Tschermak’s method of determining the nature of the salts present in silicates. In this method, the mineral is digested with hydrochloric acid, at a temperature not greater than 60°, until decomposition is complete; the silicic acid formed is washed by decantation, and dried in air at a constant temperature; it is weighed at regular intervals until the weight is constant. It is stated that if a curve of times and weights be plotted, a break is observed at the point where drying ceases (for the acid is of course wet) and decomposition begins; the composition at this point, which is taken as the composition of the acid required, can be determined from the weight of the acid, and the weight of anhydrous silica present, which is determined by ignition after the weight has become constant.

Employing this method in the case of titanite, Bruckmoser claims to have obtained the acids H₂Si₂O₅ and H₂Ti₂O₅. He therefore concludes that the constitution of the mineral is represented by the formula Si₂O₅,Ti₂O₅Ca, which presumably may be written Ca(Ti,Si)₂O₅.

It is fusible with difficulty before the blowpipe. Hot concentrated hydrochloric acid decomposes it partially, with separation of silica; boiling sulphuric acid, or, better, fused potassium hydrogen sulphate, decomposes it completely.

On account of the high dispersion and refractive index, clear specimens of sphene make very beautiful gems, but the stone is not sufficiently hard to stand much wear.

The mineral was discovered in Chamouni by Pictet in 1787, and was named Pictite by Delamètherie (1797). In 1795 Klaproth analysed a specimen from Passau, and, observing the presence of titanium (which he had just discovered in rutile), proposed the name Titanite. The mineral described by de Saussure (1796) as ‘Schorl rayonnante,’ and afterwards by Hauy (1801) as Sphene (σφήν = a wedge), was shown to be identical in composition with titanite by Cordier, and also by Klaproth (1810); the crystallographic identity was proved by G. Rose (1820).

On account of the difference in colour and composition, a large number of varieties are distinguished. The ordinary yellow and brown varieties are known indifferently as sphene or titanite. Ligurite has an apple-green colour; Semeline is a greenish form named from a fancied resemblance to flax seed. Lederite is a brown variety of tabular habit; Greenovite is rose-coloured, and contains manganese. Alshedite and Eucolite-Titanite are rich in the trivalent metals; Grothite is a brown variety containing a considerable percentage of ferric iron. Yttrotitanite, which contains a high proportion of rare earths, is usually treated as a separate species (see above). Titanomorphite and Leucoxene are white amorphous varieties chiefly produced by alteration of rutile and ilmenite.

Titanite is a fairly widespread mineral; as an accessory rock constituent it is common in the massive plutonic rocks in tiny crystals, readily distinguished under the microscope by the high refraction and birefringence, whilst in large embedded crystals it occurs in many granular limestones, and in plutonic acid, as well as in some metamorphic rocks. In good crystals it is found in many parts of Switzerland and the Alps, in Dauphiné, the Tyrol, Piedmont, the Urals, South Norway, and other European localities; it is also widely distributed in the United States and Canada.

The mineral is important as a valuable source of titanium.

The class of Titano-silicates is a very large one, and might be extended almost at will by the inclusion of the numerous silicates which contain titanium. Owing to the frequency with which small quantities of silica are replaced by titanium dioxide, almost all the commoner silicate minerals contain the latter oxide, so that titanium is one of the most widely distributed of the elements. Relatively very few, however, of the titanium-bearing minerals contain the element in considerable quantities, and only two or three have any importance as commercial sources of titanium compounds.

Only those additional titano-silicates which contain titanium as an important constituent are mentioned below; short accounts will be found in the alphabetical list.

Johnstrupite, Mosandrite, Rinkite, Rosenbuschite and Tscheffkinite are complex titano-silicates containing yttrium or cerium metals.

Astrophyllite, Leucosphenite, Molengraafite, Neptunite and Rhönite are complex titano-silicates free from rare earth elements.

Benitoite is a simple titano-silicate of barium; Ænigmatite and Narsarsukite contain iron and sodium; Lorenzenite has sodium and zirconium. Schorlomite is a titaniferous garnet. A variety of olivine rich in titanium (Titanium Olivine) is also known.

(b) The Titanates

Yttrocrasite.

[55]—This is a complex titanate of rare earths (chiefly yttria earths) with lime, thoria, and oxides of lead, iron, uranium, etc.; it has a considerable water content. An approximate formula is R´´O,R^ivO₂,3R´´´₂O₃,16TiO₂,6H₂O, where R´´ = (Ca,Pb,Fe), R^iv = (Th,U), and R´´´₂O₃ = rare earths. No constitutional formula can be given; it will be noticed that the amount of titanium dioxide is considerably more than is required to combine with the bases present (cf. also Delorenzite below). It is radioactive.

It is infusible, and not easily soluble in acids. Hydrofluoric acid decomposes it, and the powdered mineral is also slowly attacked by boiling concentrated sulphuric acid.

It was found in 1904 by Barringer, in Burnet Co., Texas.

Delorenzite.

[56]—A compound similar to the above, but even richer in titanium dioxide, which amounts to 66 per cent. Tin dioxide is also present, with traces of columbic anhydride. The bases are the yttria earths (almost free from ceria earths), uranium dioxide, and some ferrous oxide, the formula being 2FeO,UO₂,2Y₂O₃,24TiO₂, with a little SnO₂ replacing TiO₂. It is strongly radioactive. Its closest chemical neighbour is yttrocrasite, but in appearance and angles it closely resembles polycrase (q.v.). Its discoverer, Zambonini, therefore formulates it as a metatitanate with titanium acting also as a base—polycrase is a mixed metatitanate and metacolumbate—thus, 2FeTiO₃ + U(TiO₃)₂ + 2Y₂(TiO₃)₃ + 7(TiO)TiO₃.

Ilmenite or Menaccanite (Specular Iron Ore, Titaniferous Ironstone, etc.).

—This is a titanate of iron, usually written FeTiO₃. Its constitution has given rise to very considerable discussion[57]; not only do the relative proportions of iron and titanium vary greatly, but the iron is undoubtedly present in both the ferrous and the ferric states, and in the former state is partly replaced in some specimens by manganese and magnesium. In 1829 Mosander put forward the view that the mineral consisted of FeTiO₃, ferrous titanate, with varying proportions of ferric oxide, the forms and angles of ilmenite being very similar to those of hæmatite, Fe₂O₃. This view was disputed by H. Rose, who concluded that the mineral must have been originally an isomorphous mixture of ferric oxide, Fe₂O₃, and titanic oxide, Ti₂O₃, which on exposure to high temperature in the earth’s crust would change according to the equation

Fe₂O₃ + Ti₂O₃ = 2TiO₂ + 2FeO

so that the proportion of ferrous iron increases with the proportion of titanium dioxide, as is actually found to be the case. This condition, however, is also satisfied by Mosander’s view. The latter view was also supported by Rammelsberg, who pointed out that the presence of magnesium indicated the existence of ferrous iron as a primary constituent. Additional support is lent to this view by the discovery of Pyrophanite, MnTiO₃ (see list), which is found to be isomorphous with ilmenite, so that there can be little doubt that MgTiO₃, which can be only a titanate, would, if it existed in the crystalline form (see Geikielite in list), also be isomorphous with ilmenite. Friedel and Guérin (1876) prepared artificial titanium sesquioxide, Ti₂O₃, and found it to be isomorphous with hæmatite, Fe₂O₃; they concluded that FeFeO₃, FeTiO₃ and TiTiO₃ formed an isomorphous series, and that ilmenite was a mixture of the second with the other two. In 1890 Hamberg pointed out that there was no reason to suppose that hæmatite, Fe₂O₃, contains ferrous iron, i.e. has the constitution Fe´´Fe^ivO₃, analogous to Fe´´Ti^ivO₃, since in corundum, the analogous compound of aluminium, Al₂O₃, divalent aluminium can hardly exist; nevertheless, strict analogy of constitution is not necessary for isomorphism, as shown by the case of potassium nitrate, KNO₃, and aragonite, CaCO₃, so that hæmatite, Fe₂O₃, and ferrous titanate, FeTiO₃, might form solid solutions in varying proportions without the strictly analogous formula FeFeO₃ being true for the former. The balance of opinion inclines to the constitution (mFeTiO₃ + nFe₂O₃ in isomorphous mixture) originally proposed by Mosander. The evidence in support of this view has been greatly strengthened by the recent work of Manchot,[58] which has proved the absence of titanium sesquioxide, Ti₂O₃; the mineral is therefore to be regarded as a titanate.

The mineral is infusible; when powdered, it dissolves slowly in boiling hydrochloric acid, the filtered yellow solution giving the characteristic blue colouration of titanium salts on addition of tinfoil. In fused potassium hydrogen sulphate it dissolves readily. The variation in composition can be judged from the following limits:

Ilmenite is a widely distributed mineral. In crystals it occurs chiefly at Kragerö and Arendal in Norway, at Miask in the Ilmen mountains, in Dauphiné, the St. Gothard, etc.; in the massive form at Bay St. Paul, Quebec, and other localities in America; and in sands at Menaccan in Cornwall, Iserwiese in Bohemia, Puy de Dôme, dép. Haute Loire, France, and in Brazil, Australia, and New Zealand.

The mineral was discovered at Menaccan in Cornwall by McGregor, about 1790. He described it as containing iron and a new oxide; the unknown oxide was obtained in 1795 from rutile by Klaproth, who gave the name Titanium to the new metal it contained.

Short descriptions of the following titanates are also given (see list):

Davidite and Knopite; these are complex titanates containing elements of the cerium and yttrium groups.

Arizonite and Pseudobrookite—ferric titanates.

Perovskite, calcium titanate, and its variety Hydrotitanite.

Pyrophanite, a manganese titanate isomorphous with ilmenite, and Senaite, a species intermediate in composition between these two.

Geikielite, the magnesium analogue of ilmenite, with the variety Picroilmenite, which is rich in iron.

Uhligite, a titanate of zirconium, calcium and aluminium.

Derbylite, Lewisite and Mauzeliite, an interesting series of titano-antimonates.

Warwickite, a boro-titanate.

CHAPTER IV
THE TANTALO-COLUMBATES

(a) Tantalo-Columbates containing no Titanium Dioxide

Samarskite, Yttro-ilmenite or Eytlandite (Urano-tantalite).

—Samarskite is a tantalo-columbate[59] of the rare earth metals, with iron, calcium, and uranium.

Rammelsberg gives the formula R´´₃R´´´₂(Cb,Ta)₆O₂₁, where R´´ = (Fe´´,Ca,UO₂), and R´´´ = rare earth metals. Groth regards it as essentially a pyrocolumbate (tantalate) of rare earth metals R₄[(Cb,Ta)₂O₇]₃ the iron, calcium and uranium being more or less accessory constituents. Des Cloizeaux considers the formula indefinite. The mineral has also been found to contain tin, thorium, germanium, and helium. The yttria earths usually predominate (11·9 to 18·9 per cent.), the percentage of ceria earths being low (2·4 to 5·2 per cent.). The yttria earths contain the very rare oxide samaria.

The mineral is radio-active.

Before the blowpipe it fuses at the edges; with borax it gives an iron bead. It is decomposed by boiling concentrated sulphuric acid, better by fusion with potassium hydrogen sulphate, and leaching the residue with dilute hydrochloric acid—this leaves the insoluble oxides Cb₂O₅ and Ta₂O₅. On heating it glows, with decrease in specific gravity (cf. p. 38).

Samarskite occurs with other columbo-tantalates in felspar, or in veins in granite, near Miask in the Urals, near Quebec in Canada, and in Mitchell County, North Carolina. From the last-named locality, masses up to twenty pounds in weight have been obtained.

The mineral was first discovered in the Urals by Ewreinoff, captain of a corps of Russian mountain engineers. He sent a specimen for identification to the mineralogist Gustave Rose, who pronounced it to be a tantalate of uranium containing manganese, and called it Urano-tantalite.[60] In 1847 the chemist Heinrich Rose, brother of Gustave, in the course of his researches on tantalic ‘acid’ (oxide), analysed a specimen. He found the composition given above, and renamed it Samarskite,[61] in honour of the Russian engineer who furnished him with the specimen for analysis.

In 1907, Brögger[62] announced that Annerödite, of which he had published an account as a new species in 1881, was a parallel growth of the mineral columbite, (Fe,Mn)Cb₂O₆, on samarskite.

Both minerals are orthorhombic, but they are not isomorphous. The mistake was due to the fact that whilst the crystallographic data were determined from the upper crystals of columbite, the crystals of samarskite were used for analysis.

Plumboniobite.

[63]—This is a recently discovered mineral closely related to samarskite and yttrotantalite (q.v.). It is essentially a columbate[64] of yttrium metals, lead and uranium, with water, ferrous oxide, titanium dioxide, stannic oxide, alumina, lime, and cuprous oxide. The formula given is R´´₂Cb₂O₇,R´´´´₄(Cb₂O₇)₃, where R´´ = (Fe,Pb,Ca,UO), and R´´´ = Al and yttria metals, with isomorphous (?) metatitanate. The mineral is radio-active, and gives considerable quantities of gas on being heated with sulphuric acid (carbon dioxide 0·19, helium and nitrogen 0·22 per cent.). The yttria earths are rich in the oxides of gadolinium and samarium, and the mineral should prove a valuable source of these elements. It is remarkable that the ceria earths are almost entirely absent.