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.

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.

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.

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:

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.

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.

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.

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.

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.

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.

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:

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:

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.

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.

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.

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.

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).

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.

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).

CHAPTER VII
THE MONAZITE SANDS

It has been stated that monazite is a not uncommon accessory constituent of many rocks, particularly of granites, gneisses, diorites, etc. The crystalline material, of which an account has been given, is found sometimes in veins in these rocks, more often in tiny crystals disseminated throughout the mass. Most of these monazite-bearing rocks are extremely old, belonging to the Archæan or pre-Cambrian age, and probably none are of secondary (Mesozoic) or later age. It follows, then, that they have been subjected to erosion during practically the whole immense period of which geology can give us any detailed knowledge. Heat, frost, wind, the action of vegetation and of percolating water, the innumerable weathering agents known to the geologist, have been at work on them during countless ages, breaking, crushing, dissolving; rains, brooks, rivers, even ocean-waves have dissolved or washed away the fragments, sorted them out unerringly according to density, and re-deposited them, now in a river-bed, now at the base of some sea cliff, now in a wide alluvial plain from which the water has long since retired. It is in deposits of this nature that the monazite has been concentrated. Its relatively high specific gravity (about 5·0) has secured its separation from the lighter mica, quartz, and felspar of the parent-rock; but the heavier vein or accessory minerals have, of course, been concentrated with it. Zircon is an invariable constituent of these ‘monazite sands,’ as such deposits are called; and others almost as frequently found are rutile, ilmenite, sphene (titanite), and apatite. Common, too, are the characteristic minerals of the metamorphic rocks, garnet, epidote, sillimanite, tourmaline, etc. Rare earth minerals found in the monazite sands include xenotime, fergusonite, samarskite, gadolinite, and allanite. The remaining minerals are oxides of iron and tin, with, of course, a considerable amount of quartz.

It is apparent, from what has been said above, that monazite will be concentrated with the heaviest constituents of the rocks from which it is derived. Very often, indeed usually, these rocks are precisely those in which gold occurs, disseminated sometimes in tiny particles, sometimes collected into nuggets in veins of quartz and pegmatitic minerals. The erosion of these rocks concentrates the gold with the heaviest minerals; and hence it happens that monazite is an almost universal constituent of the gold- and gem-bearing sands and gravels. In the Carolinas and in Brazil, monazite is found in the gold washings; and though in the past the two have always been extracted separately, the gold first and the monazite from the washings or tailings, there appears to be no reason why a system calculated to extract both—where, of course, the content is high enough—should not be put into operation in the future.

A chemical test affords the only reliable method of detecting monazite in a sand. A little of the sand is washed with water to remove the lighter minerals and warmed with concentrated sulphuric acid. A few drops of the liquid are poured off, evaporated to small bulk, and one drop placed on a glass plate. This is placed under a microscope and one drop of a concentrated solution of sodium acetate is added. If monazite is present in the sand, tiny pointed oval crystals of sodium cerium sulphate will separate.

On the commercial scale, monazite is extracted from the sands only, in the manner described below. An effort was made in North Carolina in 1906 by the British Monazite Company, representing the South Metropolitan Gas Light Company of London, to extract monazite from the rock in which it occurs disseminated. The rock was crushed and powdered, and the monazite separated by washing off the lighter particles on concentration tables (see below). In the same year, however, the price of thorium nitrate was suddenly lowered 50 per cent. by the German Thorium Syndicate, which largely controls the Brazilian output of monazite, and the British company stopped operations in 1907. At present it may be said that only the sands are available for profitable extraction.

Up to 1895, the Carolina deposits, which were worked chiefly by the Welsbach Light Company of New York, either directly or indirectly, supplied all the demand, but in that year the Brazilian sands were first worked, and a keen struggle commenced for the market. The American companies, after keeping up a considerable output for some years, were forced to suspend operations in May 1910. The Brazil deposits, worked by the German Thorium Syndicate and the Austrian Welsbach Company, which have an agreement, now meet practically the whole demand. The Brazilian sand occurs chiefly along the shores of the southern provinces, having been concentrated by the action of the tides from the products of erosion of the cliffs; it is very uniform and considerably richer than the Carolina sand, and owing to its occurrence on the sea-shore, the cost of transporting it is very low. It is exported chiefly to Germany, recently also to the United States, and to a small extent lately to England. The method of working it is similar to that employed in Carolina—namely, concentration by washing and magnetic separation.

The North American Deposits[119]

There are two important regions in North America within which monazite sands occur; one extends over the Carolinas, and the north-western part of Georgia, the other over the Idaho basin and neighbouring counties of the Pacific Slope. It will be best to treat these separately, as the deposits are somewhat different in character.

(a) The Carolina Deposits

, including the unimportant Georgia deposits, which belong to the same field, occur over an area approaching 4000 square miles. The area is occupied chiefly by the Piedmont plateau, which is drained by a number of streams rising in the South Mountains, an eastern outlier of the Blue Ridge; it is in the basins and valleys of these streams, particularly at the head-waters, that the monazite is chiefly found. The geology of the district is very complicated,[120] the rocks being very highly altered granites. The chief bed is known as the Carolina gneiss, and includes several types of gneiss, usually very much weathered. The sands, which average about 1 per cent. of monazite, are worked in and near the stream beds; they occur in the beds, and in layers 1 to 2 feet in thickness a few feet below the surface of the surrounding soil.

Concentration was formerly effected chiefly by a crude process of washing. In this process the sand is thrown on to a sort of sieve, fixed over the upper end of a long wooden trough, by one workman; a jet of water is directed on to the sieve, washing the sand through it. The heavier particles fall to the bottom of the trough, whilst the lighter are washed right through. A second workman continually turns over the sand left in the box and on the sieve; at the end of a day’s work the ‘concentrate’ is collected. This averages from 15 to 70 per cent. of monazite, according to the nature and amount of the heavy minerals accompanying it in the sand. The concentrate is dried either on rubber or oiled cloths in the sun, or on an iron plate covering a trough in which a fire is lighted. The iron minerals are then picked out by means of a magnet, and the sand filled into sacks for transport.

Before treatment for thorium nitrate, the sand is at the present day further concentrated by powerful magnetic separators. In a few cases the older method of concentration by hand-washing has been abandoned for machine concentration, the Wilfley table being sometimes employed. The principle here is exactly the same, the sand being fed into a hopper by means of a moving belt and thence on to a machine-shaken table from which running water constantly removes the particles, sorting them according to their specific gravity.

Further separation of the dried concentrate has been effected by three kinds of separators.[121] The first was of the Edison, or fall-and-deflection type; in this the sand is allowed to flow in a thin vertical stream past a horizontal magnet, which deflects the minerals containing iron; these fall on one side of a partition, the part richer in monazite on the other. The second was an electrostatic machine; the heated sand is borne on a moving belt underneath a rotating vulcanite cylinder, excited by felt-covered rubbers; the lighter particles are attracted to the cylinder, and dropped on one side, the heavier passing on. Neither of these machines is of much value in effecting concentration, and neither is in general use.