CHAPTER VI

THE PHOSPHATES AND HALIDES

(_a_) THE PHOSPHATES

~Monazite~, Phosphocerite.--Monazite, by far the most important, commercially, of all the rare earth minerals, is essentially an orthophosphate of the ceria earths, of the formula R´´´PO₄.[97] The yttria earths are usually present in small quantities. Silica and thoria, in quantities varying from traces up to 6 per cent. of the former and from 1 to 20 per cent. of the latter, are invariable constituents; it is almost entirely to the percentage of thoria that the mineral owes its commercial value. The following also are common constituents, though usually in very small quantities only--stannic, ferric and manganous oxides, alumina, lime, magnesia, zirconia and water. Helium was observed in it by Tilden, and by Ramsay, Collie and Travers.[98] Boltwood[99] and Zerban[100] found uranium in it; the latter attributed this to impurities, the former regarded it as an essential constituent. Strutt[101] found uranium in a pure monazite. Haitinger and Peters[102] detected radium, their result being confirmed by Boltwood and Strutt.

[97] For the composition of the earths in monazite, see James, _J. Amer. Chem. Soc._ 1913, ~35~, 235.

[98] _Trans. Chem. Soc._ 1895, ~67~, 684.

[99] _Phil. Mag._ 1905, [vi.], ~9~, 599.

[100] _Ber._ 1905, ~38~, 557.

[101] _Proc. Roy. Soc._ 1905, A, ~76~, 88 and 312.

[102] _Sitzungsb. kaiserl. Akad. Wiss. Wien_, May, 1904.

Monazite occurs in small crystals belonging to the monoclinic system. _a_ : _b_ : _c_ = 0·9693 : 1 : 0·9256, β = 76° 20´. These values vary slightly with different specimens. Common forms--Ortho- and clino-pinakoids _a_ {100}, _b_ {010}, hemi-prisms _m_ {110}, and _n_ {120}, hemi-ortho-prisms _w_ {101} and _x_ {1̅01}, hemi-clino-prism _e_ {011}, hemi-pyramid _v_ {1̅11}, etc.; the basal pinakoid _c_ {001} is rare.

Angles--a ∧ _m_ = 43° 17´, _c_ ∧ _w_ = 37° 8´, _c_ ∧ _e_ = 41° 58´.

Habit tabular, parallel to _a_, needle-shaped by elongation parallel to _b_ axis, or prismatic by good development of _v_.

Cleavage ∥ _c_, perfect, ∥ _a_, distinct, ∥ _b_, difficult.

Twin plane _a_ (100). Birefringence moderate, positive; plane of optic axes perpendicular to _b_, nearly parallel to _a_. Acute bisectrix inclined to _c_ at angle of 1°-4°. Dispersion feeble, ρ < υ. Brittle. Hardness 5-5¹⁄₂; sp. gr. 4·9-5·3; conchoidal fracture. Lustre resinous. Colour, red to brown, yellow, yellowish- and greenish-brown. Transparent when pure; more often translucent to opaque.

Monazite is with difficulty soluble in acids; before the blowpipe it is infusible; when moistened with sulphuric acid it colours the flame greenish-blue.

The mineral often occurs massive, yielding angular fragments, but is most common in rolled grains. It occurs in the gneiss of the Carolinas and Georgia, and in sands derived from the gneiss, in Idaho and many of the Pacific States; in Brazil, at various localities in the provinces of Minas Geraes, Bahia, Espirito Santo; in Queensland, Australia; in Madagascar; in Ceylon; near Travancore in India; in the Urals; in Scandinavia, etc. The deposits of commercial value will be treated more fully in the next chapter. It is of wide distribution as an accessory constituent of granites, diorites, and gneisses.

Monazite was first described, under the name Turnerite, by Lévy,[103] in 1823; the specimen was from the collection of the English chemist Turner, who thought it a variety of sphene (titanite), and was named after him at the suggestion of the mineralogist Heuland. The specimen was stated to have been found in Dauphiné, but in spite of considerable examination of the question, the precise locality is still unknown. The resemblance between Turnerite and the mineral later described as monazite (μοναζειν = to be solitary) was pointed out by Dana in 1866, and confirmed by Pisani, 1877. The name Monazite was first used by Breithaupt[104] in describing a mineral found by Menge (1826) accompanying zircon in a granite from Miask in the Urals. Breithaupt concluded, from the high specific gravity, that the mineral contained a heavy metallic oxide. It was again described as Mengite by Brooke[105] in 1831. It was re-discovered by Shephard[106] in South Carolina in 1837, and described by him under the name Edwardsite, a variety from Connecticut being called Eremite. To Shephard belongs the honour of having discovered its true nature; after analysis he described it as a ‘Basic Sesquiphosphate of the Protoxide of Cerium,’ giving the formula (modern notation) 3CeO,2P₂O₅, and finding also zirconia, alumina, and silica in it (his specimen was probably very impure). Gustav Rose[107] showed this to be identical with monazite in 1840. In 1846 Wöhler described, under the name Cryptolite, a variety of tetragonal habit closely resembling zircon. This occurs at Arendal in Norway, enclosed by apatite, in the granite; it may be obtained by treatment with dilute nitric acid, which dissolves the apatite.

[103] _Annals of Philosophy_, 1823, ~21~, 241.

[104] _Schweigg. J._ 1829, ~55~, 30.

[105] _Phil Mag._ 1831, [ii.], ~10~, 139.

[106] _Amer. J. Sci._ 1837, ~32~, 162.

[107] _Pogg. Ann._ 1840, ~49~, 223.

The question of the manner in which the thorium is combined in monazite is of considerable importance, in view of the fact that it is to this element that the mineral owes its commercial value. The amount present varies from traces up to over 20 per cent., but the usual value is between 5 and 7 per cent. The first explanation of its presence was advanced by Dunnington[108] who suggested, on the result of only one analysis, that orangite (ThSiO₄) was present mechanically mixed with the monazite. Penfield[109] supported this suggestion, and stated that in three analyses of pure material he found the ratio of rare earths to phosphorus pentoxide and that of thoria to silica exactly equal to unity, though the actual amounts of thoria varied considerably. He also quotes an analysis made by Rammelsberg in 1877, in which no thoria was found, to show that it is not an essential constituent. In a microscopic examination he found dark resinous particles scattered throughout the section; after moistening with hydrochloric acid, warming, and washing, these dark spots became white, and could be stained with fuchsine, the monazite remaining unaffected throughout. He concluded that these particles were thorite or orangite.

[108] _Amer. Chem. J._ 1882, ~4~, 138.

[109] _Amer. J. Sci._ 1882, [iii.], ~24~, 250; 1888, ~36~, 322.

Blomstrand[110] disputed Penfield’s conclusions. In twelve analyses of monazite from various parts of Scandinavia he never once found either thoria or silica absent. Of these twelve analyses, two give the ratio of thoria to silica, ThO₂ : SiO₂, exactly unity, in seven cases the ratio is not greater than 1·25, in five cases it varies considerably. He summed up his results in three statements:

(_a_) Silica is never absent; its amount depends not on the amount of thoria, but on the amount of phosphorus pentoxide present.

(_b_) The thoria which is always present is combined partly with silica, partly with phosphorus pentoxide.

(_c_) In most cases, the rare earths alone are insufficient to satisfy the ratio R₂O₃ : P₂O₅ = 1.

[110] _J. pr. Chem._ 1890, ~41~, 266.

An exhaustive examination of the question has been made more recently by Kress and Metzger.[111] They made in all over fifty analyses, using thirty different specimens of monazite; they estimated silica both as quartz and as silicate silica, and determined thorium by the fumarate method--the other investigators had used the thiosulphate method of Hermann (_vide_ p. 286). Their results may be summarised as follows:

(i.) Silica is always present.

(ii.) The amount of silica usually increases with the thoria, but not regularly.

(iii.) By far the majority of cases showed insufficient total silica to combine with the thoria present.

(iv.) In about 9 per cent. of the cases, the thoria present was insufficient to combine with the silicate silica, from which it follows that some foreign silicate must be at least occasionally present.

(v.) A careful microscopic examination showed conclusively that no thorite (ThSiO₄) was present, the silicate being biaxial; quartz is present as such.

[111] _J. Amer. Chem. Soc._ 1909, ~31~, 640.

They conclude that thorium is present as phosphate, and is an essential constituent, but that there is always some admixed silicate, most probably a felspar.

~Xenotime.~--Chemically this mineral is closely allied to monazite, being an orthophosphate of rare earths, containing silica and thoria; whereas, however, in monazite the content of yttria earths does not rise above 4 per cent., in xenotime these constitute by far the greater part of the bases, the content of ceria earths ranging from 8·2 to 11 per cent. The yttria earths, chiefly oxides of yttrium and the erbium group, vary from 54·1 to 64·7 per cent. There are traces of zirconia; Ramsay, Collie and Travers detected helium, whilst Boltwood, and also Strutt, found uranium and radium. It also appears to contain traces of sulphuric anhydride.

The crystals are tetragonal, holosymmetric. _c_ = 0·6187; (001) ∧ (101) = 31° 45´.

Common forms are the prisms _a_ {100} and _m_ {110}, the basal pinakoid _c_ {001}, the pyramids _e_ {101}, _f_ {201}, _z_ {111}, etc.

Cleavage ∥ _m_, perfect. Uniaxial, double refraction strong, positive. Transparent to opaque. Colour, brown to reddish-brown and yellow. Hardness 4-5; sp. gr. 4·45-4·56.

It is insoluble in acids, and infusible before the blowpipe; when moistened with sulphuric acid, however, it turns the flame bluish-green, like most mineral phosphates (_vide_ monazite).

It is not so widely distributed as monazite, but is not uncommon. It often occurs with zircon--to which it is very closely allied in crystal form, if the two are not actually isomorphous--in parallel growth, in granitic rocks. The diamond sands of Diamantina, Brazil, form the richest source of the mineral, but it is also found in Scandinavia, at Hitterö, Åro, etc.

The mineral is of considerable importance, chemically, on account of the high percentage of erbia earths.

In the works of Bauer, Rosenbusch, Weinschenk, Schilling and Iddings will be found accounts of a mineral named ‘Hussakite.’ These accounts rested on the work of Kraus and Reitinger,[112] who in 1901 announced the discovery of a new species. The crystals were obtained as a specimen of xenotime by Prof. Muthmann from Dr. E. Hussak, in São Paulo, and had the crystallographic properties of that mineral. On analysis, the amount of sulphur trioxide present was found to be remarkably high (6·3 per cent.), and Kraus and Reitinger concluded that the substance was distinct from xenotime. They announced it as a new mineral, with the name Hussakite, and the formula 3R₂O₃,3P₂O₅,SO₃ or 6RPO₄,SO₃, and stated that by the action of dilute alkalies the sulphur trioxide could be easily and completely removed. They therefore regarded xenotime as a pseudomorph[113] after hussakite, the sulphur trioxide having been removed from the latter by the action of the alkaline waters of the earth’s crust. In support of this view, they gave analyses of opaque crystals from a Bahia sand represented as containing 2·6 to 2·7 per cent. of sulphur trioxide, and so as being intermediate forms produced during the change.

[112] _Zeitsch. Kryst. Min._ 1901, ~34~, 268.

[113] One mineral is said to be pseudomorphous after another when the first is produced from the second by a chemical change which proceeds so slowly that the original structure and crystalline form are unaltered (_i.e._ a change proceeding molecule by molecule). The pseudomorph is usually opaque and shows clear signs of the alteration.

The latter conclusion was quickly challenged by Brögger, who found no sulphur trioxide in a perfectly fresh and transparent xenotime from Åro in Scandinavia. Brögger concluded that the Hussakite of Kraus and Reitinger was an independent species of the formula 5YPO₄,(YSO₄)PO₃, and that xenotime was not derived from it.

Basing his work on the barium chloride test given by Kraus and Reitinger (see below) Rösler[114] declared that ‘Hussakite’ was a common accessory constituent of igneous rocks, having been previously mistaken for zircon, which it resembles in appearance and optical properties.

[114] _Zeitsch. Kryst. Min._ 1902, ~36~, 258.

In 1907 Hussak[115] published a paper in which he showed that the mineral named after him was not a new species at all, but a xenotime of prismatic habit. Analyses made at his request by Florence in Brazil, G. T. Prior in London, and Tschernik in St. Petersburg, confirmed the original values given by Gorceix (sulphur trioxide up to 0·25 per cent.). He mentions Brögger’s analysis of the Norwegian specimen in which Kraus and Reitinger had found 2-3 per cent. of sulphur trioxide, but in which Brögger found none. He explains the results of Kraus and Reitinger as due to the addition of barium chloride to the acidified solution of the carbonate fusion of the mineral, by which barium phosphate was precipitated; this was dried and weighed as barium sulphate. Rösler’s tests are declared doubtful; xenotime is not a widely spread rock constituent, the mineral in question being really zircon.

[115] _Centr. Min._ 1907, 533.

In face of these results, there can be little doubt that the name ‘hussakite’ is unnecessary and undesirable, since the mineral to which it was applied is proved to be xenotime.

* * * * *

In the alphabetical list, particulars of the following rare earth phosphates will be found:

_Castelnaudite_, a variety of xenotime containing zirconia.

_Churchite_ and _Rhabdophane_ (Scovillite), hydrated phosphates.

_Gorceixite_, an alumino-phosphate of alkaline and ceria earths.

_Retzian_, an hydrated arsenate of manganese, calcium and rare earth metals.

(_b_) THE HALIDES

~Yttrocerite.~--This mineral is a fluoride of calcium and rare earth metals, with water. A recent analysis by Tschernik[116] gives the formula Ce₂F₆,2Y₂F₆,9CaF₂,2H₂O. Putting the rare earth metals together, this gives 6RF₃,9CaF₂,2H₂O, or R₂Ca₃F₁₂,²⁄₃H₂O. Yttrocerite is of interest since it was probably in the analysis of this mineral by the discoverers, Berzelius and Gahn, that the double sulphate method of separating the yttria from the ceria earths was first employed[117] (_vide_ p. 156).

[116] _Abstr. Chem. Soc._ 1907, ~92~, ii. 362.

[117] _Schweigg. J._ 1816, ~16~, 244.

It is found only massive or granular. Colour usually white to violet-blue, sometimes reddish-brown. Hardness 4¹⁄₂; sp. gr. 3·45. Infusible, but loses colour before the blowpipe. When powdered, it dissolves completely in boiling hydrochloric acid, and readily in sulphuric acid with evolution of heat. It has been found at various localities in Scandinavia.

~Yttrofluorite.~[118]--This is a fluoride of varying composition, very similar to yttrocerite, but characterised by the absence of water, and the very small ceria content (1·7 per cent.). It is thus a fluoride of calcium and the yttrium metals.

[118] T. Vogt, _Centr. Min._ 1911, 373.

Cubic, with poor octahedral cleavage. Colour, yellow to brown and yellowish-green; transparent to translucent, bleached by weathering. Very brittle. Hardness 4¹⁄₂; sp. gr. 3·54-3·56.

It is very similar to fluorspar (except that the octahedral cleavage of the latter is very good), and is regarded by Vogt as an isomorphous mixture of the latter with yttrium fluoride (or with a double yttrium calcium fluoride, which is less probable). This view would account for the variations in composition, and also for the remarkable frequency with which traces of rare earths are found in fluorspar (_vide_ p. 2). Yttrocerite is regarded as a similar isomorphous mixture, but containing cerium metals in addition to the yttrium group.

Yttrofluorite occurs in pegmatite veins in granite in Northern Norway, with gadolinite, fergusonite, allanite, fluorspar, and the usual vein minerals.

* * * * *

The other members of this family (see list) are:

_Fluocerite_, a basic fluoride of yttrium and cerium metals.

_Tysonite_, a hydrated fluoride containing carbonates.

It is to be noticed that fluorine is the only member of the halogen family which occurs in nature in combination with rare earth elements. This fact is possibly connected with the great age of the rare earth minerals, and their formation during pneumatolytic metamorphism of plutonic rocks (_vide_ Chapter I).