CHAPTER V

THE OXIDES AND CARBONATES

(_a_) THE OXIDES

~Uraninite~ or Pitchblende.--Uraninite consists essentially of oxides of uranium (UO₂ + UO₃ = 75 to 85 per cent.), associated with thoria, zirconia, rare earths, beryllia, and oxides of lead. Traces of lime, iron oxides, silica, bismuth, and arsenic are also sometimes present, with water in widely varying quantities. Nitrogen and helium are always found in it, and, of course, radium. Groth regards pitchblende as uranous uranate U^{iv}(U^{vi}O₄)₂, the uranium in the acidic radicle being hexavalent and in the basic radicle tetravalent, and in the latter condition partially replaced by lead, thorium, and rare earths.

Szilard[79] regards it rather as a loose compound or even a solid solution of oxides of thorium and uranium,[80] with small quantities of other oxides, he having obtained apparently homogeneous (though non-crystalline) bodies by dissolving thorium hydroxide in solutions of uranium salts and evaporating to dryness.

[79] _Compt. rend._ 1907, ~145~, 463.

[80] See under Thorianite, _infra_.

The cubic form of the crystalline varieties has been taken as indicating that the mineral is really a spinel,[81] but it is difficult to see how the general formula of that group can be considered comparable to the uranyl uranate formula, UO₂,UO₃, for pitchblende.

[81] The Spinels are an isomorphous family of cubic minerals of the general formula R´´O,R´´´₂O₃, where R´´ = Be, Fe, Mg, Ca, etc., and R´´´ = Fe, Al, Cr, etc.

Crystals are rare, and belong to the cubic system, the common forms being the octahedron _o_ {111} and the dodecahedron _d_ {110}; the cube _a_ {100} is sometimes present. The mineral is massive, usually botryoidal. The crystalline or primary form is black, with hardness 5¹⁄₂, sp. gr. 9·0 to 9·7; the altered varieties are grey to greenish- and brownish-black, sp. gr. 5·0 to 6·4.

It is infusible before the blowpipe, but readily soluble in nitric acid.

The mineral occurs both as a primary and secondary constituent of rocks; as a primary mineral it is found in Norway, North Carolina, etc.; as a secondary species it occurs in the massive and hydrated form, with ores of lead, silver, tin, etc., in Saxony and Cornwall, and at the celebrated mine of Joachimsthal, in Bohemia. The latter deposits, consisting of the massive and altered varieties, for which the name Pitchblende is generally reserved, have been much used as a source of radium, especially those at Joachimsthal, and the Cornwall ore.

Several varieties of uraninite have been distinguished by special names. Crystalline varieties from Anneröd and Arendal in Norway are known as Bröggerite and Cleveite respectively; Nivenite is a third form. In these varieties uranium oxides have been replaced to a considerable extent by the rare earths and thoria. An amorphous variety of doubtful composition, produced by alteration, is known as Gummite; Uranosphærite is a similar altered form.

~Thorianite.~[82]--This interesting mineral consists chiefly of thoria, ThO₂ (55-79 per cent.), with oxides of uranium (11-32 per cent.), and ceria oxides (1-8 per cent.); oxides of lead and iron are also present in small quantities, and zirconia with silica, probably due to associated zircon.

[82] Dunstan and Blake, _Proc. Roy. Soc._ 1905, A, ~76~, 253; Dunstan and Jones, _ibid._, 1906, A, ~77~, 546.

Helium is present, and the mineral is strongly radioactive. A careful analysis by Hahn[83] shows traces of many metals; the same chemist has also separated an extremely active component, 250,000 times as active as thorium nitrate, which he calls Radiothorium.

[83] Hahn, _ibid._, 1907, A, ~78~, 385.

The composition has been accounted for (Dunstan and Jones, _loc. cit._) on the hypothesis that thoria (ThO₂) and uranous oxide (UO₂) are isomorphous, the mineral being really a solid solution. Whilst, however, the crystal system of the natural body is really rhombohedral (_vide infra_) the two pure oxides appear to be cubic. Thus Troost and Ouvrard[84] obtained artificial thoria in minute octahedra; and, similarly, Hillebrand[85] obtained uranous oxide in octahedra by reduction of uranyl chloride, UO₂Cl₂, though his work seems to be open to objection. On the other hand, the same author[86] found that uranous oxide and thoria, fused together in almost any proportions, gave a homogeneous body crystallising in octahedra (cf. Szilard, _Compt. rend._ 1907, ~145~, 463, quoted under Uraninite). The probability of the isomorphism of the oxides is strengthened by the observation of isomorphism in the sulphates. As early as 1886, Rammelsberg showed that uranous sulphate, U(SO₄)₂, crystallises with nine molecules of water and is isomorphous with the corresponding thorium sulphate, Th(SO₄)₂,9H₂O; and six years later, Hillebrand and Melville[87] obtained mixed crystals of the two sulphates which were exceedingly close in forms and angles to those of pure uranous sulphate. It is then at least probable that the two oxides are isomorphous, though the point cannot be regarded as satisfactorily proved, by reason of the anomalous crystal forms of the naturally occurring mixtures, thorianite and uraninite. The recent results of Kobayashi[88] point to the conclusion that different varieties of thorianite may exist, in each of which the oxides of thorium and uranium bear definite simple ratios to one another.

[84] _Compt. rend._ 1882, ~102~, 1422.

[85] _Zeitsch. anorg. Chem._ 1893, ~3~, 243.

[86] _Bull. U.S. Geol. Surv._ No. 113, 1893.

[87] _Ibid._ No. 90, 1892, p. 30.

[88] _Abstr. Chem. Soc._ 1912, ~102~, ii. 1181.

Thorianite occurs in jet-black crystals with a bright resinous lustre. They are pseudocubic, and the twinning resembles that of the cubic mineral fluorspar--interpenetrant cubes, twin axis a cube diagonal. Close examination shows, however, that twinning can only take place about one of the four diagonals, and an optical examination makes it clear that the symmetry is really rhombohedral. The case is exactly analogous to that of the mineral chabazite, a zeolite which occurs in rhombohedra of which the angles differ but little from those of the cube, and which also forms the interpenetrant twins. In view of the fact that both uranous oxide and thoria have been obtained as octahedra, whilst a fused mixture of the two on cooling forms cubic crystals, it seems not unlikely that at high temperatures the pseudocubic thorianite would become truly cubic; but no experiments in this direction seem to have been tried.

The crystals are brittle; hardness 7; sp. gr. 8·0-9·7.

Thorianite is infusible, incandescing before the blowpipe. When powdered, it dissolves readily in nitric and sulphuric acids, with evolution of helium. Gray[89] has shown that the helium content can be reduced by 28 per cent. by fine grinding, thus showing that part at least of the gas must be mechanically held.

[89] _Proc. Roy. Soc._ 1908, A, ~82~, 306.

Thorianite was found in Ceylon, being originally mistaken for pitchblende. A sample was supplied by the discoverer, Mr. Holland, to the officers of the Mineral Survey, by whom it was sent to London for examination. Its composition was determined by Dunstan, who named it. It was found in the river gravels (gem-gravels), the matrix being a pegmatite granite. It is a valuable source of thorium nitrate for incandescent mantles, one ton of the mineral (with thoria content of 70 per cent.) having been sold for £1500; but the supply is small and unreliable.

~Baddeleyite.~[90]-- Baddeleyite consists of almost pure zirconia (ZrO₂ = 96·5 per cent.) with small quantities of ferric oxide, alumina, lime, magnesia, alkalies and silica. Thoria and rare earths are present in traces, uranium is absent; the mineral is not radioactive, and contains only traces of helium.

[90] _Vide_ Fletcher, _Min. Mag._ 1893, 46, ~10~, 148; Hussak, _Zeitsch. Kryst. Min._ 1895, ~24~, 164, and ~25~, 298.

Monoclinic--_a_ : _b_ : _c_ = 0·9871 : 1 : 0·5114. β = 98° 45¹⁄₂´.

Common forms--all three pinakoids, _a_ {100}, _b_ {010}, and _c_ {001}, with the hemi-prisms _m_ {110}, _k_ {120}, and _l_ {230}, and various pyramids and domes.

Angles--(100) ∧ (110) = 44° 17¹⁄₂´; (100) ∧ (001) = 81° 14¹⁄₂´; (100) ∧ (101) = 55° 33¹⁄₂´.

Cleavage ∥ _c_ and ∥ _b_, parting ∥ _m_ due to repeated twinning. Twinning is exceedingly common; of many hundred crystals examined by Hussak, only three were found untwinned. Twin planes _m_ (110), _a_ (100), and _x_ (201).

Colour brown, varying in zones by twinning, with distinct pleochroism. Hardness; sp. gr. varies from 4·4 to 6·0, being about 5·5 to 5·6 for fairly pure material. Double refraction negative, 2 E = 70-75°. Acute bisectrix nearly coincident with _c_ axis, plane of the optic axes _b_, (010).

The mineral is insoluble in acids, readily soluble in fused potassium hydrogen sulphate. Before the blowpipe it is almost infusible; it dissolves in the fused borax bead, rapid cooling causing separation of crystals. If a bead containing zirconia be heated until the borax is partially volatilised, zirconia crystallises on cooling in tetragonal crystals, isomorphous with those of rutile.[91]

[91] Nordenskiöld, _Pogg. Ann._ 1861, ~114~, 625; for tetragonal zirconia see also Troost and Ouvrard, _Compt. rend._ 1888, ~102~, 1422.

The mineral was discovered in 1892 by Hussak and L. Fletcher independently. The former, who obtained it from the pyroxenite sand of São Paulo, South Brazil, believed it to be a tantalo-columbate, and called it Brasilite. Fletcher found it in a gem-gravel from Rakwana, Ceylon, and named it Baddeleyite. An analysis by Blomstrand of Hussak’s mineral showed it to be identical with the Ceylon mineral, and Hussak withdrew his name and accepted Fletcher’s. It has recently been found[92] in a corundum-syenite, near Bozeman, Montana, U.S.A.

[92] Rogers, _Amer. J. Sci._ 1912, [iv.], ~33~, 54.

The mineral now comes on the market in commercial quantities; pure zirconia almost entirely free from iron can be obtained by leaching with acids. The pure oxide is extraordinarily refractory, and promises to be of great use for crucibles, furnace linings, etc. (_vide_ p. 324).

~Rutile.~--Titanium dioxide, TiO₂, occurs crystallised in nature in the three minerals Rutile, Brookite, and Anatase (Octahedrite), which therefore form a trimorphous series. They are all stable minerals, though rutile appears the most stable, being occasionally found in pseudomorphs after the other two. The family is remarkable in that it is not unusual to find two of them occurring together--an uncommon phenomenon with polymorphous minerals.

Rutile often contains small quantities of iron and chromium. The ferriferous varieties are distinguished as Nigrine, which is black, with 2-3 per cent. ferric oxide, and Ilmenorutile, with up to 10 per cent. of ferric oxide, and specific gravity up to 5·13.

Crystal system--tetragonal, holosymmetric; _c_ = 0·6442; (001) ∧ (101) = 32° 47´.

Common forms--prisms _a_ {100}, _m_ {110}, and _l_ {310}; pyramids _e_ {101}, _s_ {111}, and many others. The basal pinakoid _c_ {001} is very rare. Habit, prismatic, with vertical striations; or in slender needles. Twinning very common and varied; usually on the cassiterite law--twin plane _e_ (101)--forming the knee-shaped twins, and irregular rosettes by repetition, and many contact twins. Contact twins on the law--twin plane _v_ (301) are less common.

Cleavage ∥ _a_ (100) and _m_ (110), distinct. Hardness 6-6¹⁄₂; sp. gr. 4·18-4·25, and up to 5·2 if much iron is present. Colour reddish-brown to black, with good metallic lustre; transparent to opaque. The refraction and double refraction are very high--ω = 2·6158, ε = 2·9029 for sodium light--and allow the crystals to be readily distinguished in rock-sections.

The mineral is insoluble in acids, but can be dissolved after fusion with alkalies or alkali carbonates.

Rutile is a member of the isomorphous series, cassiterite, zircon, etc. (see under Thorite), and in particular it has the colour, appearance, and twinning of cassiterite, from which, however, it is readily distinguished by its lower specific gravity. In this connection it is interesting to note that an apparently pure specimen, quite free from inclusions, was found (1904) to contain 1·7 per cent. of tin dioxide.[93]

[93] Friedel et Grandjean, _Bull. Soc. franc. Min._ 1909, ~32~, 52.

As an accessory rock mineral, and also as an important constituent of many sands, rutile is of very wide distribution. It occurs, usually imbedded in quartz or felspar, in many granites, syenites, gneisses, slates, and allied rocks; in acicular crystals penetrating quartz it forms the ‘Veneris Crinis’ of Pliny. At Risör and other localities in Norway, it is found in the massive form, and it is largely worked at Risör as a source of titanium. It occurs in all the countries of Europe, and largely in America. Arendal, Kragerö, and Risör, in Norway, the Binnenthal, the Urals, the St. Gothard, Castile, Magnet Cove in Arkansas, Alexander Co. in N. Carolina, Barre and Shelburne in Massachusetts, and Chester Co. in Pennsylvania are the chief localities.

It was in this mineral that the element titanium was first recognised by Klaproth (1795).

~Anatase~ (Octahedrite) is the second crystalline modification of titanium dioxide.

Tetragonal _c_ = 1·7771. (001) ∧ (101) = 60° 38´, (111) ∧ (11̅1) = 82° 9´.

Common forms--Prisms _a_ {100} and _m_ {110}, pyramids _p_ {111}, _e_ {101}, and many other complex forms; the basal plane _c_ {001} is occasionally found. Habit usually octahedral, with _p_ or _v_ prominent; sometimes tabular with _c_, more rarely prismatic with _a_ well developed. Cleavage ∥ _c_ and _p_ perfect. Hardness 5¹⁄₂-6; sp. gr. 3·82-3·95, usually increasing after heating. Lustre adamantine, so splendent that in Brazil detached crystals have been mistaken for diamonds. Colour, some shade of bluish-black to brown; by transmitted light, greenish-yellow. Transparent to opaque. Double refraction negative, strong; for sodium light ω = 2·554, ε = 2·493.

It is found at Bourg d’Oisans in Dauphiné, and in Norway, the Urals, Brazil, etc. In Switzerland it occurs as the variety Wiserine, which was at one time believed to be xenotime. It was named Octahedrite by de Saussure, in 1796, from the prevailing habit, and Oisanite, from its occurrence in Dauphiné, by Delamètherie, in 1797. The name anatase (ανατασις = erection) was proposed by Haüy, being intended to denote that the vertical axis (_c_ : _a_) is greater than that of rutile, the other tetragonal modification of the dioxide.

~Brookite~, the third form of this compound, is orthorhombic.

_a_ : _b_ : _c_ = 0·8416 : 1 : 0·9444.

Common forms--the three pinakoids _a_ {100}, _b_ {010}, and _c_ {001}, prisms _m_ {110}, _l_ {210}, pyramids _e_ {122}, _z_ {122}, and numerous others.

Angles--(100) ∧ (110) = 40° 5´; (001) ∧ (100) = 48° 18´; (001) ∧ (011) = 43° 22´.

The habit is varied; it occurs usually in bipyramids with _e_ and _m_ or prismatic with _m_, _a_, and terminating pyramids. Cleavage ∥ _m_ indistinct, ∥ _c_ very poor.

Hardness 5¹⁄₂-6; sp. gr. 3·87-4·01. Lustre metallic. Colour brown to reddish- and yellowish-brown and black. The optical behaviour is interesting. The acute bisectrix is perpendicular to _a_ (100), but while for red light the plane of the optic axes is (001), for blue it is (010); for an intermediate light, therefore (λ = 5550 µµ), the mineral appears uniaxial.

The chief localities are Bourg d’Oisans, Miask, the St. Gothard, the Tyrol, Magnet Cove in Arkansas, and Tremadoc in Wales.

Titanium dioxide can be obtained crystalline by the action of steam on titanium tetrafluoride, TiF₄, at high temperatures; it is stated that by varying the temperature of the reaction, any one of the three crystalline modifications can be obtained.

* * * * *

The only other minerals which need be mentioned in this class (see list) are:

_Zirkelite_, a complicated mixture of oxides, in which thoria, zirconia, and titanium dioxide act as acidic oxides, and

_Mackintoshite_, a mixture of several oxides, of which those of thorium and uranium are the most important.

(_b_) THE CARBONATES

~Lanthanite~, Hydrocerite.--This mineral is a carbonate of ceria earths, chiefly lanthana, of the formula La₂(CO₃)₃,9H₂O.

Orthorhombic; _a_ : _b_ : _c_ = 0·9528 : 1 : 0·9023. Common forms--the pinakoids _a_ {100} and _c_ {001}, with _m_ {110} and _o_ {111}.

Angles--(100) ∧ (110) = 43° 37´; (001) ∧ (101) = 43° 26¹⁄₂´; (001) ∧ (011) = 42° 3¹⁄₂´.

Habit tabular, parallel to _c_; cleavage perfect, ∥ _c_.

Double refraction negative; optic axis plane _a_ (100).

Usually amorphous, being probably an alteration product of a mineral rich in lanthanum. Hardness 2; sp. gr. 2·6-2·7.

Colour white to yellowish-white, usually opaque; infusible before the blowpipe (being converted to the oxide), readily soluble in acids.

Lanthanite occurs with cerite at Bastnäs, and at Bethlehem, Pennsylvania.

Morton[94] states that he prepared a crystalline didymium carbonate in the laboratory, of the formula Di₂(CO₃)₃,8H₂O, which was isomorphous with lanthanite; he concluded that the latter had only eight instead of nine molecules of water.

[94] See abstract in _Zeitsch. Kryst. Min._ 1886-87, ~12~, 518.

~Parisite~ (~Synchisite~), and ~Cordylite~.--_Parisite_ is a fluocarbonate of calcium and cerium metals; _Cordylite_ is an analogous compound in which barium replaces calcium, and is isomorphous with Parisite. The formula of Parisite is CaR₂F₂(CO₃)₃, where R = cerium metals. Groth formulates this as (CaF)(RF)R(CO₃)₃, Penfield and Warren as (RF)₂Ca(CO₃)₃, whilst Schilling gives Ce₂(CO₃)₃,CaF₂. Analogous formulæ may be proposed for Cordylite, BaR₂F₂(CO₃)₃. Since the two minerals are very similar in crystallographic properties, one description will be sufficient for both. The following are Dana’s data for Parisite:

Hexagonal, _c_ = 3·2891. (0001) ∧ (101̅1) = 75° 15´.

Forms are extremely numerous, and have remarkably high indices. Among the simplest are the base _c_ {0001}, the prism _m_ {101̅0}, pyramids _q_ {101̅2}, and _h_ {112̅2}; the other forms are chiefly rhombohedra and pyramids. The usual habit is that of an acute double hexagonal pyramid, with form _o_ {202̅1}, terminated by _c_. Cleavage ∥ _c_, perfect.

It is brownish-yellow to red. Hardness 4¹⁄₂; sp. gr. 4·36.

The double refraction is strong, positive. Soluble in hydrochloric acid with effervescence.

Both minerals are characteristic pneumatolytic species of the riebeckite-ægirine rocks. Parisite was discovered by Paris in the emerald mines of the Muso valley, Colombia, in 1835, and first correctly analysed by Bunsen in 1845. Before the blowpipe it glows, remaining infusible (the glow does not appear to have been investigated in this case).

_Cordylite_ was discovered by Flink in 1900, in Greenland.

It is yellow to brownish-yellow and colourless. Hardness 4¹⁄₂; sp. gr. 4·31. Before the blowpipe it decrepitates, and is infusible; moistened with hydrochloric acid, it gives the characteristic barium flame.

The so-called Synchisite was discovered by Nordenskiöld who correctly described it as Parisite. Flink found it in Greenland, and announced it as a new species, with the formula R₂F₂Ca₂(CO₃)₄, _i.e._ the formula for parisite plus one molecule of calcium carbonate, CaCO₃. From its extraordinary resemblance to parisite in physical and crystallographic properties, Palache and Warren[95] believe that the specimens selected by Flink for analysis must have consisted, in reality, of parisite with admixed calcium carbonate. This conclusion has now been confirmed by Quercigh, by a careful comparison of the optical properties.[96] The minerals are usually found together, the chief localities being S. Norway, the gold districts of the Urals, Narsarsuk in S. Greenland, and Montana, U.S.A.

[95] _Amer. J. Sci._ 1911, [iv.], ~31~, 533.

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

* * * * *

The following rare earth carbonates are described in the alphabetical list:

_Ancylite_, a basic hydrated carbonate.

_Tengerite_, a hydrated carbonate formed by the weathering of gadolinite.

_Kischtimite_, a fluo-carbonate related to parisite.

_Bastnäsite_ (Harmatite) and _Weibyite_, hydrated fluocarbonates of the cerium elements.