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.

[59] In this and all similar minerals, columbium (niobium) and tantalum are to be regarded as vicarious; they replace each other in all proportions. It seldom happens that a pure columbate is found free from tantalum, or _vice versa_; one or other may predominate, but the two are almost always found together.

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.

Crystal system--orthorhombic; _a_ : _b_ : _c_ = 0·5456 : 1 : 0·5178.

Forms--macro- and brachy-pinakoids _a_ {100} and _b_ {010}; prisms _m_ {110} and _h_ {120}, the macrodome _e_ {101}, and pyramids _p_ {111} and _v_ {231}.

Angles--(100) ∧ (110) = 28° 37´; (001) ∧ (101) = 43° 30´; (001) ∧ (011) = 27° 22¹⁄₂´.

Habit usually prismatic, with _e_ prominent; sometimes tabular parallel to _a_ or _b_. Cleavage ∥ _b_, imperfect. The faces are usually rough. The mineral commonly occurs massive, and in flattened grains embedded in granite. Conchoidal fracture. Brittle. Hardness 5 to 6; sp. gr. 5·6 to 5·8.

Colour velvet-black, streak reddish-brown. Opaque even in thin films.

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.

[60] _Pogg. Ann._ 1839, ~48~, 555.

[61] _Ibid._, 1847, ~71~, 157.

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.

[62] _Abstr. Chem. Soc._, 1907, ~92~, ii. 885.

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.

[63] Hauser u. Finch, _Ber._ 1909, ~42~, 2270; Hauser, _ibid._, 1910, ~43~, 417.

[64] It is to be understood that small quantities of columbium are replaced by tantalum.

The mineral is massive, with some indication of crystalline structure. It is dark brown to black, transparent in flakes, and under the microscope is seen to be isotropic, with doubly-refracting inclusions, undoubtedly of a secondary nature. Hardness 5 to 5¹⁄₂; sp. gr. 4·80 to 4·81. Unlike samarskite, it does not glow on ignition.

It occurs with mica and pitchblende in pegmatite veins in granite, at Morogoro, in the Uluguru Mountains, German East Africa.

~Yttrotantalite.~--This is a tantalo-columbate similar in composition to Samarskite, and isomorphous with it; though, as the name implies, the acidic oxide is chiefly tantalum pentoxide, the percentage of columbic anhydride being much lower than in the latter mineral. It is a pyro-salt of the formula R´´R´´´₂(Cb,Ta)₄O₁₄ + 4H₂O,[65] where R´´ = (Fe,Ca) and R´´´ = rare earth (chiefly yttrium) metals (Rammelsberg). Strutt found thorium and radium in it. The manner in which the water is combined in this, as in many other minerals, is at present undetermined.

[65] Dana gives R´´R´´´₂(Cb,Ta)₄O₁₅ + 4H₂O; this appears to be an error.

Crystal system--orthorhombic; _a_ : _b_ : _c_ = 0·5411 : 1 : 1·1330. Common forms--pinakoids _b_ {010} and _c_ {001}, prisms _m_ {110}, _o_ {210}, _p_ {120}, domes _s_ {201} and β {011}. Habit, prismatic with _m_ and _b_ prominent, or tabular parallel to _b_. Colour yellow to black, white after strong ignition.

It is found at Ytterby in Sweden, and in South Norway.

~Fergusonite~, Tyrite, or Bragite.--A columbate and tantalate of the rare earth metals, with uranium, iron, calcium, etc. The general formula is that of an ortho-compound, R₂O₃,(Cb,Ta)₂O₅ or R(Cb,Ta)O₄, where R = metals of the rare earths, chiefly of the yttrium group. Brögger includes the other constituents in the more complex formula (Th,U)(Si,Sn)O₄ + 12R(Cb,Ta)O₄; but the simpler formula agrees quite well with specimens from the most widely separated localities, and is usually adopted. The mineral is radio-active and contains helium.

Tetragonal, polar (with tetrad axis of symmetry only) _c_ = 1·4643. (001) ∧ (101) = 55° 40´. Common forms--Basal pinakoid _c_ {001}, tetragonal prism _g_ {320}, pyramids _s_ {111}, _z_ {321}. Brittle. Hardness 5 to 6; sp. gr. 5·84, decreasing on hydration. Lustre dull, brilliantly vitreous on broken surfaces. Colour brownish-black. Translucent to opaque.

Fergusonite was discovered by Hartwell. It occurs with samarskite, and often with gadolinite and allanite, in Norway and Sweden, the Carolinas, Texas, the Urals, W. Australia, etc.

On heating it glows suddenly between 500° and 600°C.,[66] losing all its helium, and with decrease in density (5·619 to 5·375). At the same time it gives out a considerable amount of heat--8·09 C.[67] for 1 gm. (see p. 38).

[66] Ramsay and Travers, _Zeitsch. physikal. Chem._ 1898, ~25~, 568.

[67] The heat of combustion of a gram of hydrogen is 342 K.

~Sipylite.~--Essentially a columbate of rare earth metals, with oxides of tantalum, tungsten, zirconium, uranium, iron and calcium, and some water. Mallet, the discoverer, gives the formula as R₂O₃,Cb₂O₅, the basic oxides including, besides the rare earths, Cb₂O₅ with Ta₂O₅ and WO₃, and some water. An alternative formula, making it a complex pyro-salt, is also given, but from its great similarity in form and angles to fergusonite, the first formula is preferred. Strutt finds that it contains not only uranium, radium and helium, but also thorium in considerable quantity (ThO₂ = 4·9 per cent.), a fact which had been overlooked by Mallet. The rare earths contain a high proportion of erbia.

It is tetragonal, _c_ = 1·4767, (001) ∧ (101) = 55° 54´. The crystals are octahedral, with the form _p_ {111}; _p_ ∧ _p_´ = 79° 15´, _p_ ∧ _p_´´ = 128° 50´. Cleavage distinct ∥ _p_. It is usually granular and amorphous. Colour brownish-black to brownish-red, lustre resinous. Brittle. Hardness 6; sp. gr. 4·89. Translucent.

Its behaviour on heating has been already mentioned (see p. 39); it is infusible. Boiling hydrochloric acid partially dissolves it; the solution gives the turmeric test for zirconium, and on diluting and adding metallic tin a sapphire-blue colour is developed, due to the columbium present. Boiling concentrated sulphuric acid decomposes it slowly.

It is found in Amhurst Co., Virginia, adherent to the allanite which occurs there in large quantities. It was discovered there by Mallet in 1877, who named it, on account of the columbium (niobium) present, from Sipylus, one of the sons of Niobe.[68]

[68] See Mallet, _Amer. J. Sci._ 1877, [iii.], ~14~, 397.

* * * * *

In this class, also, are to be included the following minerals (see list):

_Nohlite_ and _Vietinghofite_, varieties of Samarskite.

_Hjelmite_ and _Kochelite_, minerals closely related to Yttrotantalite and Fergusonite respectively.

_Koppite_, _Loranskite_, _Microlite_ and _Rogersite_, complex tantalo-columbates containing elements of the cerium or yttrium groups.

(_b_) TANTALO-COLUMBATES CONTAINING TITANIUM DIOXIDE

~Æschynite.~--A columbate and titanate of the cerium metals, with thorium, calcium, iron, etc. From the results of an analysis on a specimen from Hitterö, Norway, Tschernik proposed the rather formidable formula

2(2Ce₂O₃,3TiO₂),4(ThO₂,TiO₂),Y₂(CbO₃)₆,3(CaO,TiO₂),3Fe(CbO₃)₂, Fe(TaO₃)₂,6TiO₂.

This can be simplified to Y(CbO₃)₃ + ThTiO₄ + ³⁄₂TiO₂, in which Y represents rare earth metals partially replaced (2 atoms) by ferrous iron (3 atoms), whilst thorium can be partially replaced by (2 atoms of) ferrous iron or calcium. Strutt found it to contain the uranium-radium combination and helium.

The crystals are orthorhombic, holosymmetric; _a_ : _b_ : _c_ = 0·4866 : 1 : 0·6737.

Common forms--brachy- and basal pinakoids _b_ {010} and _c_ {001}, prisms _m_ {110} and _r_ {120}, domes _d_ {101} and _v_ {021}, with pyramid _o_ {111}.

(100) ∧ (110) = 25° 57´; (001) ∧ (101) = 54° 9´; (001) ∧ (011) = 33° 58´.

Habit prismatic, vertically striated, or tabular parallel to b with horizontal striations. Brittle. Hardness 5 to 6; sp. gr. 4·9 to 5·7. Colour nearly black. Nearly opaque.

It occurs at Miask, in the Urals, at Hitterö in Norway, and at Fredriksvarn. The variety from the last locality is called Polymignite; it was shown by Rose to be probably identical with Æschynite. Æschynite was discovered by Berzelius at Miask and named by him from the Greek αίσχύνη, shame, from the fact that its composition could not at that time be determined.

If the ceria earths be largely replaced by yttria earths, a variety very similar in appearance and angles, but approximating to polycrase (_q.v._) in composition, is obtained. This mineral was found in 1879, and referred to Æschynite; analysis subsequently showed its true composition, and it was named Blomstrandine (_q.v._) by Brögger in 1907.

_The Isodimorphous Series Euxenite, Polycrase, Blomstrandine, and Priorite._

Euxenite and Polycrase are members of an isomorphous series and vary considerably in composition. The composition of the series is that of mixed columbates and titanates of yttria earths (with, as usual, some ceria earths), with uranium and zirconium, and water. Before the isomorphous relation was recognised, Rammelsberg gave for Euxenite the formula R´´´(CbO₃)₃,R´´´₂(TiO₃)₃,1¹⁄₂H₂O. The ratio of the acidic oxides, Cb₂O₅ : TiO₂, is here 1 : 2. This is the greatest value of the ratio, which varies for the series between 1 : 2 and 1 : 5.[69] The end members, the pure metacolumbate and pure metatitanate respectively, are unknown; all the members occurring in nature are to be regarded as mixtures of these within the limits set by the ratios ¹⁄₂ and ¹⁄₅. Brögger[70] suggests that the name Euxenite be retained for all members for which the ratio is between ¹⁄₂ and ¹⁄₃, whilst for those minerals in which it is less than ¹⁄₄ the name Polycrase be kept; these views have been supported by Lange, who has analysed members of the series.

[69] Lange (_Abstr. Chem. Soc. 1911_, ~100~, ii. 499) gives the limits ¹⁄₂ and ¹⁄₆.

[70] _Abstr. Chem. Soc. 1907_, ~92~, ii. 885.

The members of this isomorphous series, however, are themselves dimorphous, that is, can each crystallise in two different ways. The second form corresponding to the Euxenites is known as Priorite, whilst that corresponding to Polycrase is known as Blomstrandine; and these second forms are themselves members of a parallel isomorphous series of the same chemical composition, of course, as the first series. It is, perhaps, undesirable to cite this as a typical example of an isodimorphous series, since no end members of unmixed composition are known. A perfect example of such a series is furnished by the oxides of antimony and arsenic. Each of these compounds exists in two distinct crystalline varieties, antimony trioxide, Sb₂O₃, as Valentinite (orthorhombic) and Senarmontite (cubic), arsenic trioxide, As₂O₃, as Claudetite (orthorhombic) and Arsenolite (cubic); and these two modifications are isomorphous with one another, senarmontite with arsenolite, and valentinite with claudetite.

In the case we are considering, the name Euxenite is applied to one crystalline modification (A) of a number of isomorphous compounds within certain limits of composition, the name Priorite to the second crystalline modification (B) of the same compounds; the name Polycrase is applied to compounds having the crystal form A, and a composition varying within a second set of limits in the same chemical series, whilst this second set of compounds in the crystalline form B is known as Blomstrandine.

Stated as concisely as possible, the relationship is as follows: Each member of this chemical series of continuously varying composition can crystallise in two forms, which are the same for every member. The two varieties at one end of the series are called euxenite and priorite, at the other end polycrase and blomstrandine.

Thus, whilst euxenite and priorite, at the one end, and polycrase and blomstrandine at the other, have the same compositions, euxenite and polycrase have the same crystalline form, whilst priorite and blomstrandine have the same second crystalline form.

All four minerals have the same bright black appearance, and bright conchoidal fracture; they are all four isotropic, probably as a result of hydration. All are orthorhombic, but the measurements for euxenite and polycrase are different from those for blomstrandine and priorite. The two latter are not so widely distributed as the two former. Blomstrandine occurs at Hitterö, Arendal, and other localities in Norway; priorite is found in Swaziland, South Africa.

The crystal system of the Polycrase-Euxenite series is orthorhombic, but Dana gives slightly different axial ratios for the two minerals. This, though Brögger gives the same values for both, is by no means incompatible with isomorphism, as a glance at the axial ratios for the minerals aragonite, strontianite, witherite, etc., of the series of the orthorhombic carbonates, will show.

Brögger’s ratios for the two are _a_ : _b_ : _c_ = 0·3789 : 1 : 0·3527; Dana gives for polycrase 0·3462 : 1 : 0·3124, for euxenite 0·364 : 1 : 0·303.

~Euxenite.~

This species occurs usually in the massive form as a bright brownish-black mineral, of hardness 6¹⁄₂, and sp. gr. 4·6 to 5·0. The crystals are prismatic in habit; the common forms are the pinakoids _a_ {100} and _b_ {010}, the prism _m_ {110}, the unit pyramid _p_ {111}, and the dome {201}. Ramsay, Collie and Travers found no helium in it; Boltwood found uranium, radium and helium, and Strutt found in addition to these thorium. As early as 1879, Blomstrand had observed zirconium in euxenite.

The mineral is infusible and with difficulty soluble in acids. It occurs in many localities in Scandinavia (Hitterö, Arendal, Brevig, etc.), in North Carolina, South Australia, etc. It was discovered by Scheerer at Jölster, in Norway, in 1839.

The Euxenite-Polycrase series was studied by Hauser and Wirth in 1909,[71] in an endeavour to establish their theory that the proportions in which the various earths and acids occur in this group of minerals is subject to definite laws beyond the ordinary laws of combination. Thus of the erbia earths they state that the proportion of holmia and dysprosia increases relatively to erbia as titanium dioxide increases, _i.e._ as we pass from the euxenites to the polycrases; at the same time scandia and yttria increase relatively to the other yttria earths (the terbia group), whilst in the ceria group samaria and praseodymia decrease relatively to the others. Thus samaria is found in appreciable quantities only when the titanium content is low. The original paper must be consulted for full details.

[71] _Ber._ 1909, ~42~, 4443.

It was stated above that zirconium was found in euxenite in 1879. In 1901 Hofmann and Prandtl[72] declared that zirconia was an unfailing constituent of the mineral, and that it was always accompanied by a new oxide, which they named Euxenia (‘Euxenerde’). This was characterised by the solubility of its oxalate in acid solutions, the insolubility of the precipitated hydroxide in excess of alkali, and the gradual precipitation by hydrogen peroxide from a slightly acid solution of its salts. In their paper quoted above, Hauser and Wirth state that zirconia is never present in typical euxenites. In a second paper[73] they state that after exhaustive treatment of every known zirconia mineral, they can find no trace whatever of the ‘new earth,’ and conclude that Hofmann and Prandtl must have made some experimental error. During this examination, they observed radioactivity in some minerals which contained no traces of uranium or thorium.

[72] _Ibid._ 1901, ~34~, 1064.

[73] _Ber._ 1910, ~43~, 1807.

~Risörite.~[74]--A columbate of yttria earths, with titanium; ferric oxide, alumina, lime and lead monoxide are present in small quantities. It resembles fergusonite in composition, but differs in the almost complete absence of uranium, the high loss on ignition, and the amount of titanium present, which is here considerable (TiO₂ = 6·5 per cent.). Hauser regards it as an orthocolumbate, R´´´(Cb,Ta)O₄, with an isomorphous admixture of metatitanate, R´´´₂(TiO₃)₃.

[74] Hauser, _Ber._ 1907, ~40~, 3118; _Zeitsch. anorg. Chem._ 1908, ~60~, 230.

The rare earths are chiefly yttria, with some erbia earths and a little terbia; ceria, lanthana and didymia are also present. The mineral contains a considerable amount of helium, which is remarkable in view of the very small content of uranium and thorium (cf. Thalenite). It is radioactive, the active constituent being precipitated with the lead (and to a very small extent with the rare earths).

It is infusible, but at a red heat it loses much water, and becomes very brittle, with increase of specific gravity; no glowing is observed. It is attacked by boiling concentrated sulphuric acid, and by fused potassium bisulphate; also by hydrofluoric acid (40 per cent.), with separation of the insoluble rare earth fluorides.

No good crystals have been found, and no crystallographic data are known; examined by polarised light it appears isotropic, but this may be due to alteration. Colour, yellowish- to greenish-brown. Streak, yellowish-white. Hardness 5¹⁄₂; sp. gr. 4·179, increasing to 4·678 after ignition (cf. p. 38).

The mineral was found in a granite-pegmatite at Risör, South Norway.

~Wiikite.~[75]--A mineral of very complex composition, for which no definite formula can be assigned. Its chemical nature may be understood from the following analytical data:

Columbic and tantalic anhydrides = 16·0; Dioxides of titanium and zirconium = 23·4; Silica = 17·0; Ceria = 2·5; Yttria = 7·6; Scandia = 1·2; Thoria = 5·5; Ferrous oxide = 15·5; Uranic oxide = 3·6; water (and gas) = 5·8 per cent.

[75] Crookes, _Phil. Trans._ 1908, A, ~209~, 15.

Traces of lime, magnesia, stannic oxide and sulphur are also present.

The mineral is infusible; on heating, helium, sulphuretted hydrogen and water vapour are given off, and a white sublimate is formed. The evolution of gas is almost explosive, the mineral breaking with a curious fracture.

It is black and perfectly amorphous, showing no trace of crystalline structure or action on polarised light. Hardness, 6; sp. gr. 4·85.

Wiikite is partially attacked by acids, readily by fused potassium bisulphate. It is radioactive.

The mineral was found with monazite in a felspar quarry at Impilaks, Lake Ladoga, Finland. It is important as the source of scandium used by Sir William Crookes in his investigations of that element; some specimens of the mineral contain over 1 per cent. of the oxide (see p. 44).

* * * * *

The following related minerals, of which descriptions are given in the alphabetical list, are to be included here:

_Arrhenite_, _Chalcolamprite_, _Endeiolite_ and _Wöhlerite_, are complex tantalo-columbates containing silica.

_Hainite_ contains both silicon and titanium.

_Dysanalyte_ is a titano-columbate believed by Hauser[76] to be merely an impure form of perovskite (see p. 14).

[76] Vide _Zeitsch. anorg. Chem._ 1908, ~60~, 237.

_Ilmenorutile_ and _Strüverite_ are closely allied minerals believed by Prior[77] and Schaller[78] to be isomorphous mixtures of rutile with Tapiolite or Mossite (ferrous tantalo-columbates).

[77] _Min. Mag._ 1908, ~15~, 78.

[78] _Abstr. Chem. Soc._ 1912, ~102~, ii. 773.

_Pyrochlore_ is a complex titano-columbate containing elements of the cerium or yttrium groups.

_Blomstrandite_ is an hydrated titano-columbate of rare earth elements, with calcium and uranium; it must not be confused with blomstrandine.