CHAPTER VIII

RADIOACTIVITY OF THE MINERALS

In the present chapter no attempt will be made to give a complete account of all the phenomena of radioactivity which have been observed in the mineral world. There are, however, a few problems of the highest scientific interest which centre about the rare earth minerals, and mention of these can hardly be avoided in a work which professes to give a general account of the rare earth group. It is obvious that a detailed treatment cannot be given without entering into phenomena which would be quite beyond the range of the present work, and an excuse is hardly needed, therefore, for the fragmentary and abbreviated account which follows. The reader’s acquaintance with the general phenomena of radioactivity is of necessity assumed.

Radioactivity (the spontaneous emission of special radiations) was first observed by Becquerel, in 1896, in the case of potassium uranyl sulphate, and was soon found to be common to all uranium compounds, and to the metal itself. Mme. Curie showed that whilst in uranium salts the degree of activity varies directly with the percentage of uranium, in minerals containing the element the same rule does not hold. The observation that pitchblende is considerably more active than the uranium it contains led to the discovery of polonium[125] and radium in 1898. Exactly analogous phenomena were shown to hold for thorium salts and thorium-containing minerals by Mme. Curie and Schmidt in 1898, and in 1905 Hahn separated Radio-thorium from thorianite. In 1899 Debierne discovered that the rare earths precipitated from the solution obtained on treatment of pitchblende are associated with another extremely active body, which he named Actinium; Giesel found that in the separation of the rare earths this remains with lanthanum. In 1903 Ramsay and Soddy experimentally confirmed the prediction of Rutherford and Soddy, that radium would be found to produce helium continuously. The discovery of these remarkable phenomena has modified many fundamental physical conceptions, and has opened up a new field of scientific enquiry, which is being developed with unexampled rapidity.

[125] Polonium, which was named by Mme. Curie in honour of her native country, has been shown to be identical with Marckwald’s ‘Radio-tellurium,’ which was named by Rutherford ‘Radium F’; it is one of the degradation products of radium.

It has been mentioned, in the accounts of the rare earth minerals given above, that almost all these minerals are radioactive, _i.e._ have the property of emitting specific radiations. Moreover, radioactivity, to any considerable extent at least, is, with a few important exceptions, confined to the minerals which have been already described. It has been shown by many investigators, chief amongst whom are Strutt and Boltwood, that the activity is usually due to the presence of uranium or thorium, or both.[126]

[126] Hauser and Wirth (_Ber._ 1910, ~43~, 1807) observed activity in some zirconium minerals containing neither thorium nor uranium.

After the discovery of helium in Cleveite (a variety of pitchblende, _vide_ p. 13) in 1895 by Ramsay, a large number of minerals were examined for this gas, and it was found that almost all the rare earth minerals contain helium. The fact that these minerals are also for the most part radioactive, naturally suggested some relation between the activity and the presence of helium, and led directly to the discovery that radium is continuously producing helium; and it became apparent that helium has been accumulating in these minerals since their formation, by the decay of radioactive elements. The question of the origin of helium in minerals will be touched on again.

In 1904 Boltwood advanced the theory that radium is produced by the degradation of uranium, the parent-element having, however, a much greater half-life period. If uranium continuously produces radium, whilst the latter decays much more rapidly than the former, it must follow that in minerals containing uranium a state of equilibrium is reached between uranium and radium, and the ratio of these two in all minerals should therefore be constant, and independent of the geological age. Boltwood examined a number of the minerals of which descriptions have been given in the preceding chapters, and found the ratio to be surprisingly constant.[127] Strutt also examined a large number of minerals,[128] and whilst on the whole his results seemed to support the theory, his values for the ratio were by no means so constant as those of Boltwood. Strutt included in his examination the interesting radium-containing mineral observed by Danne at Issy l’Evêque.[129] This was a pyromorphite (lead chlorophosphate) containing neither uranium nor thorium. Danne suggested that the radium was not an original constituent, but had been introduced by the action of percolating waters. This view was confirmed by McCoy and Ross,[130] who found that the activity was entirely confined to the surface layer.

[127] _Amer. J. Sci._ 1904, [iv.], ~18~, 97; _Phil. Mag._ 1905, [iv.], ~9~, 599.

[128] _Proc. Roy. Soc._ 1905, A, ~76~, 88 and 312. _Ibid._ 1907, A, ~80~, 56.

[129] _Compt. rend._ 1905, ~140~, 241.

[130] _J. Amer. Chem. Soc._ 1907, ~29~, 1698.

Mlle. Gleditsch has also examined the question of the uranium-radium ratio in minerals. Her earlier work[131] gave ratios which, whilst constant for each mineral species, varied in much the same manner as Strutt’s for different species, and afforded very little support to Boltwood’s theory. Her more recent results,[132] however, are much more closely in accord with the theory, which has been still further strengthened by the work of Pirret and Soddy[133] and of Marckwald and Russell.[134] It may now be regarded as firmly established that radium is in the line of direct descent from uranium.

[131] _Compt. rend._ 1909, ~149~, 267; _Le Radium_, 1909, ~6~, 165.

[132] _Le Radium_, 1911, ~8~, 256.

[133] _Phil. Mag._ 1911, [vi.], ~21~, 652.

[134] _Ber._ 1911, ~44~, 777.

Boltwood had assumed that the helium in radioactive minerals is produced from the uranium, during its disintegration. Strutt, however, disputed this; his experiments showed that very little helium is found even in the richest radium-uranium minerals unless thorium is also present. Thus pitchblende contains a very high percentage of uranium, but relatively little helium (there is usually a considerable thorium percentage here too, so that nothing conclusive can be deduced from this). Adams[135] found that carnotite, a mineral very rich in uranium, but containing no thorium, contains no helium at all; he explained its absence by the very loose texture and permeability of the mineral, which would allow the gas to escape. Strutt concluded that whilst helium is undoubtedly produced by disintegration in the uranium series, in minerals it is produced more by thorium or, as more recent work indicates, by radio-thorium, than by uranium.

[135] _Amer. J. Sci._ 1905, [iv.], ~20~, 256.

The question of the origin of helium in minerals is, however, not definitely settled, for several anomalous cases are known. Thus the yttria silicate, Thalénite (_q.v._), contains quantities of helium, but no uranium or thorium is given in the analyses. Similarly, Risörite contains a relatively large quantity of helium, but only traces of uranium and thorium. In the last mineral, the active constituent is precipitated with the lead, so that no radio-thorium appears to be present. Further, Thomsen analysed a fluorspar from Ivitgut in Greenland which he found to contain 27 c.c. of helium per kilogram. This specimen contains no uranium, but gives off the thorium emanation in quantities which suggest the presence of radio-thorium; moderate quantities of thorium are also present. Since the α particle has been definitely identified as a positively charged helium atom, it appears certain that disintegration in all three series (uranium, actinium, and thorium series) produces helium, and a mineral containing a member of any of these series (which gives α rays or α ray-giving products) would also contain helium.

Even so, there is a case in which the helium content is anomalous, if not altogether beyond explanation at the present stage. In examining a large number of minerals for helium, Strutt[136] found that some samples of beryl, a beryllium aluminium silicate, contain a relatively very large amount of helium, but only traces of thorium, and was altogether inactive. The absence of any active constituent renders untenable the ordinary explanations of the presence of such a surprising quantity of helium. Boltwood has put forward a suggestion which in the present state of our knowledge must be regarded as a provisional explanation. He conceives that in the concentration of beryllium from the parent magma, it may have become associated with some short-lived intermediate radioactive element, which had been altogether separated from its long-lived parent element in the process of concentration; this intermediate element, having collected in the crystallised beryl, decayed completely in the course of the great period which must have elapsed, leaving the helium to which it had given rise during its disintegration enclosed in the mineral. It is difficult to see how two substances which must be so intimately connected as a parent-element and its product could be completely separated in the process of cooling of a magma; but since so little is known of the process of crystallisation of minerals, the suggestion can hardly be rejected on geological grounds. In any case, we have here only one strongly marked exception to the very definite rule that in all cases in which helium occurs in minerals, it is accompanied by and undoubtedly produced from, a radioactive element or elements; and in the majority of cases, the helium in minerals is produced by disintegration of uranium or thorium and their products.

[136] _Proc. Roy. Soc._ 1908, A, ~80~, 572.

Strutt found that traces of helium are universal in the mineral world. His method of determining helium was approximate only. He obtained the gas content by heating the powdered mineral--a method which, as Wood has shown,[137] will only give all the gas when very high temperatures (up to 1000°C.) are employed. The gases were freed from oxygen and hydrogen by passing over a heated, partially oxidised, copper spiral, and from carbon dioxide by means of potash. Nitrogen was removed by sparking with excess of oxygen and shaking over potash; the excess of oxygen was removed by melted phosphorus. The inert gases so obtained were freed from all impurities by the use of the liquid alloy of sodium and potassium for the electrodes of the spectrum tube in which the gases were examined spectroscopically.[138] Argon, if present--it seems to be a universal constituent of igneous rocks, into which it may have been absorbed from the air--was removed by charcoal at a temperature of -80°C. The helium so left was examined spectroscopically, and measured in a MacLeod gauge.

[137] _Proc. Roy. Soc._ 1910, A, ~84~, 70.

[138] As soon as the discharge is started in such a tube, all the gases present other than those of the helium family are absorbed by these electrodes.

As stated, helium was found in traces in nearly all minerals, and its presence is to be attributed to traces of radium, which also appears universal. In minerals containing uranium or thorium, or rare earths (the latter are almost always accompanied by uranium and thorium), helium is found to a much greater extent, and Ramsay considers it possible that some fraction of the helium content may arise from the rare earth metals. There is, however, no positive evidence to support the conjecture. He found that the helium ratio, _i.e._ the volume of helium per gram of uranous oxide, UO₂, varies with the amount of thoria present; but where the latter is absent the variations are much less marked. If helium were produced in a mineral from uranium alone, and none escaped, it is obvious that the helium ratio would depend only on the age of the mineral. For minerals of about the same age, and containing no thorium, the helium ratio would be roughly constant, if no disturbing factor required consideration.

In 1905 Strutt pointed out that in all the minerals he had examined, thorium was never present unless accompanied by uranium and radium, whilst uranium and radium often occurred without thorium. He suggested that the present atomic weight of thorium, 232·5, was too low, and that it was really the parent of uranium (at. weight 238·5); he further supposed that the next permanent member in the line of descent was one of the cerium metals. These suggestions have been negatived by later work of Boltwood and Holmes. The former pointed out[139] that it was far more likely that thorium is a disintegration product of uranium of considerably longer life. On the whole, however, there is very little positive evidence to connect thorium with uranium.

[139] Boltwood, _Amer. J. Sci._ 1905, [iv.], ~20~, 256.

In the same year Boltwood (_loc. cit._) drew attention to the persistent appearance of traces of lead, bismuth, barium, etc., in the radioactive minerals, and also pointed out that the variations of the ratio of helium to uranium in pitchblende might be used to determine the age of the mineral. In 1907 he suggested[140] that lead was the final product of the degradation of uranium, from which it follows that the ratio of uranium to lead should be constant for minerals of the same age (since, lead decays, if at all, at an infinitely slower rate than uranium). He collected all the available analyses, and classified the minerals dealt with into six groups according to the value of the ratio. The order given by the ratio was declared to be in accordance with the order of age as given by geological evidence.

[140] _Amer. J. Sci._ 1907, [iv.], ~23~, 77.

Holmes[141] has further extended this work. He examined a number of rare earth and allied minerals from the Christiania district, which Brögger considers to be of approximately Lower Devonian age, and found the ratio of lead to uranium to approximate quite closely, for almost all the minerals examined, to 0·045. Representing the change in the usual way as

U → 8He + Pb

238·5 → 31·92 + 207·1

and using the data calculated by Rutherford and others for the rates of decay, he gives the age of Lower Devonian strata as about 370 million years. This figure is about twice as great as that deduced by palæontologists from the flora and fauna, and greater still than the times based on physical data, _e.g._ rates of cooling, precession and nutation, etc. His figures for pre-Cambrian rocks, based on the same ratio, range between 1000 and 1640 million years, the later being deduced from a thorianite from the Archæan rocks of Ceylon. Strutt’s figure for Archæan rocks is about 700 million years; this was derived from work on the helium ratio, which must now be considered.[142]

[141] _Proc. Roy. Soc._ 1911, A, ~85~, 248.

[142] See Strutt, _Proc. Roy. Soc._ 1908, A, ~82~, 166; 1909, ~83~, 96; 1909, ~83~, 298; 1910, ~84~, 194.

In 1898 Travers[143] had examined the effect of heat on cleveite and fergusonite, and found that about half the total helium, together with hydrogen, is given off at a bright red heat. He considered it likely that the helium was combined with a metal (though he recognised no distinction between occlusion and combination) and remarked: ‘The results of such experiments cannot therefore serve as a basis for speculation as to the origin or history of the substances in question.’ The chemical inactivity of helium, however, as well as the experiments of Moss and Gray, who showed that helium was evolved on grinding the materials,[144] indicate that the gas is mechanically bound only. This, however, introduces the difficulty, if an attempt be made to use the helium-uranium ratio to calculate the age of minerals, that the gas would be expected to escape from a porous material, so that its amount is never so great as it should be. Strutt himself found that helium escapes rapidly from powdered monazite, whilst even the solid mineral was found to evolve helium at a rate much in excess of the probable rate of production by radioactive changes. Similar results were found with thorianite, and the only conclusion, since helium is found in the minerals, is that under the conditions under which these minerals exist in the earth’s crust, this escape is checked or altogether prevented. It follows, however, that any age determined from the helium ratio must be a minimum age, since there is always the chance of loss; this of course is not the case--except where the minerals have suffered chemical changes--with the lead ratio, and may account for the discrepancies observed.

[143] _Proc. Roy. Soc._ 1898-99, ~64~, 140.

[144] _Vide_ Gray, _Proc. Roy. Soc._ 1908, A, ~82~, 306.

Strutt’s earlier work on the helium ratio was made with phosphate minerals (coprolites and fossil bones) of known ages. The ratios found were not in order of age, the minerals being very permeable, so that helium had probably been lost. He next turned his attention to igneous rocks, and selected zircon for the work. Here he obtained some sort of regularity in the order of age and the order given by the ratio, and assumed that if helium were lost at all, it must be lost in roughly proportional amounts by reason of the similarity in conditions. Geological criticism tends to lessen the trustworthiness of the conclusions; it is pointed out that the age of a specimen of zircon is not necessarily that of the rock in which it occurs, for zircon is an extremely stable mineral, and might survive unchanged several fusions and re-crystallisations of the magma. Strutt replies to this that at the temperature of fusion of a rock, zircon would certainly give up its accumulated helium, so that the age determined from the helium content would be that of the last fusion, _i.e._ the age as given by geological data. On the other hand, our ignorance of the real mechanism of the crystallisation of a magma, and especially of the amount and effect of the pressures obtaining, robs this reply of its force, and the objection must be counted valid.

In still later work Strutt used sphene and thorianite, and his results agree as well as can be expected. The sphenes used were all from Archæan rocks, except one, which was from a Tertiary volcanic deposit of the Laacher See, near Coblenz (the lake is in the crater of an extinct volcano). In this case the helium ratio was very much smaller (about ¹⁄₄₀₀₀ of the values for Archæan rocks) indicating the (comparatively) extremely recent formation of the deposit.

The most recent results in the study of radioactivity point to the conclusion that elements which differ in atomic weight and radioactive properties may be chemically identical, or at least chemically inseparable; such elements have been termed isotopes. The end product of the thorium series of radio-elements should have an atomic weight of about 208·4, and it has been suggested that the element actually produced in this series of changes may be bismuth. The latest results, however, rather point to the conclusion that disintegration in the thorium series gives rise to an isotope of lead. If this hypothesis be true, the lead derived from a mineral rich in thorium and poor in uranium should have an atomic weight appreciably higher than that of ordinary lead. Experiments to test this conclusion have recently been carried out by Soddy and Hyman.[145]

[145] _Proc. Chem. Soc._ 1914, ~30~, 134.

These authors have made analyses of Ceylon thorite, which they find to contain 0·35 per cent. of lead; from the ratio of thorium to uranium in the mineral, they calculate that the lead should have an atomic weight of 208·2, that of ordinary lead being 207·1. Preliminary comparative experiments on 1 gram of pure lead chloride extracted from the mineral point to an atomic weight for the thorite lead of 208·4, a result surprisingly in accord with theory. More extended experiments on this most interesting question are in progress.

The present chapter would be incomplete without a reference to the interesting work of Goldsmidt on radioactivity as an aid in identifying mineral species.[146] He describes a simple method by which the activity of a mineral may be rapidly and easily measured to a sufficient degree of approximation, and shows how the determination enables a line to be drawn on a diagram already mapped out; this line will intersect an area on the diagram which corresponds to the particular mineral. Owing to lack of analytical data, and to the great difficulty of determining with accuracy small quantities of uranium and thorium, the method is at present of scientific interest only; but it is capable of development, and its development would be of undoubted value in the further study of this branch of radioactivity.

[146] _Zeitsch. Kryst. Min._ 1907-8, ~44~, 545; _ibid._ 1908, ~45~, 490.

In order to make this part of the subject as clear as possible, the chief points in this chapter are summarised as follows:

1. Radioactivity is only observed to an appreciable extent in some rather rare minerals. These minerals as a rule contain radium, uranium, thorium, rare earths, and helium.

2. The helium has been produced during geological time by the degradation of one or more members of the three series of active elements (the Uranium, Actinium, and Thorium series).

3. Radium is a degradation product of uranium, and itself is degraded continuously; the final product of degradation is probably lead.

4. The age of minerals has been calculated from the ratio of lead to uranium; the figures obtained are much greater than those put forward by geologists and physicists.

5. The helium ratio has also been used, but appears less trustworthy, owing to escape of helium, and uncertainty as to geological age of the minerals employed.

6. Some connection between radioactivity and the presence of the yttrium or cerium metals appears highly probable, but no satisfactory theories have been advanced on this point; it has been shown that actinium is very closely allied to lanthanum.