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A NEW MINERAL FROM THE PILBARA GOLDFIELDS, W.A.
By EDWARD S SIMPSON, B.E., F.C.S.

THE mineral described in this paper has recently been found in small quantities above the water level in the main tantalite lode on mineral lease 86 at Wodgina, Pilbara Goldfield. The geology of this district and the nature of the tantalite lode have been described by our President in Bulletins 23 and 40 of the Geological Survey of Western Australia. The tantalite leases are pegged out on hilly country, composed of greenstone schists, which are penetrated by numerous veins of pegmatite, these being at times sufficiently rich in tin and tantalum to constitute "lodes." The main tantalite lode is a strong pegmatite vein, whose outcrop has been traced in a north and south direction over a length of more than half a mile. Practically the whole tantalite yield of Wodgina has been obtained from this lode, or from surface fragments weathered out of it. According to Mr. Maitland's description the chief constituents of this vein are alblte and quartz. Some portions consist almost wholly of albite, other of quartz, whilst occasionally fine scaly lepidolite is a prominent mineral. Orthoclase, manganotantalite, and cassiterite have been recorded from it, and to this list must now be added pilbarite.

A few months ago a few fragments of a bright yellow ochreous mineral from this lode were sent to the author for determination. These were found to be strongly radioactive, and to contain a considerable percentage of uranium | and thorium. Such minerals being of considerable interest and value at the present time, a further supply was asked for, and in response to this request a sample of about two pounds in weight was received. A very full examination of this mineral has, therefore, been possible of accomplishment. The mineral is in nodules, from the size of a pea up to that of a walnut. The largest piece weighs 30 grms., the average size of fragment being about 5 grms. The fragments are subangular in outline, and in one or two instances portions of the surface suggest crystal faces. It is evident that this mineral is a hydrous pseudomorph after a probably anhydrous parent mineral. Examination under the microscope shows that it is amorphous, and of a colloidal gelatinous or gummy texture. Such slight indications of cleavage and external crystalline form as are observed evidently owe their origin to the parent mineral, and are not inherent characteristics of pilbarite itself. The interior of the nodules are of an almost uniform bright canary yellow colour, the exterior is disguised somewhat by a brownish or reddish coating covering a thin slightly bleached zone of mineral. The chief characteristics have been determined on material from the surface of which about 2 or 3 mm. had been removed. The interior of the nodules thus prepared was found in thin slice under the microscope to be homogeneous, except for some streakiness of colour, due probably to slightly different degrees of hydration or to slight staining in places with ferric hydrate. A few minute specks of tantalite were thus seen embedded in the mineral and a little colourless mineral which appeared to be an altered felspar. The mineral is opaque, in layers thicker than one-fifth of a millimetre. Colour, canary yellow. Brittle, earthy, amorphous. Hardness, 25 to 3. Specific gravity, 44 to 47. Strongly radioactive. Composition, hydrated silicate of lead, uranium, and thorium. The analysis of a well scraped fragment weighing 6 grms. was:—

Abstract of a Paper read before the Natural History and Science Society of Western Australia, August 16, 1910.

PbO
CaO

3'50 trace

99'56

283

According to Rutherford and Boltwood's value for the ratio of radium to uranium in such minerals, pilbarite would contain about 7.5 centigrms. of radium per ton; according to Mdme. Gleditsch's values probably somewhat less, being a weathered compound. In calculating a formula for pilbarite the whole of the tantalic oxide and the requisite amount of manganese and iron oxides to form tantalite are rejected, there being abundant evidence in support of this. The traces of alumina and alkalis are also neglected as being certainly present as inclusions of mica or felspar. The phosphoric oxide is probably present as monazite or xenotime. The formula, with these allowances, is PbO.UO3. ThO2.2SiO2.2 H2O+2H2O. The only two minerals with a formula approaching pilbarite are thorogummite and Meckintoshite. Both of these minerals crystallise in the tetragonal system. It is evident that in the presence of lead oxide as an important constituent and also in the relative proportions of uranium oxide, thoria, silica, and water, pilbarite differs essentially from these minerals. In crystalline form it also differs essentially, and one is therefore justified in considering it a new species.

Pilbarite dissolves readily in strong or slightly diluted hydrochloric acid, leaving a flocculent residue of silica. It is completely decomposed by nitric and sulphuric acid, either concentrated or moderately dilute, but with these acids in addition to silica a bulky mass of difficultly soluble thorium and lead salts separates out. During decomposition with hydrochloric acid a fair number of gas bubbles were seen to be given off, which were at first taken to be carbonic acid, but in view of the possibility of helium being present as the result of degradation of uranium, special efforts were made to collect the gas free from air or other gaseous impurities and to examine it. The apparatus used was simply made, but quite effective for the measurement of the helium to one-fiftieth of a cubic centimetre. The helium was determined in both uncrushed and finely crushed mineral. Since Moss has shown that helium escapes on grinding a radio active mineral, and since the generally accepted theory is that helium is only mechanically imprisoned in the mineral, it was to be expected that more helium would be obtained from the uncrushed than from the crushed mineral. The actual results obtained were-Helium per grm. of mineral, crushed mineral, 0.22, 0.26 cc.; uncrushed, o 38, 0.39 cc. Sulphuric acid was found to be the best for the liberation of helium from the powdered mineral, hydrochloric being so rapid in action that the apparatus could not be filled and stoppered without loss of gas. In the case of the uncrushed mineral, which was in masses of about o'5 grm. each, sulphuric acid was found to be useless, as it failed even after two days to penetrate to the centre of the mineral, which became protected by a coating of insoluble sulphate of lead and thorium. Hydrochloric acid of 5E strength, on the con