$\alpha$-Radioactivity of Individual Crystals of Accessory Zircon from Rocks of the Alenguy Massif (Eastern Transbaikalia)

V. A. Ermolaev, A. M. Kuzmin, A. A. Yatis

The article presents the results of studying the $\alpha$-radioactivity of individual crystals of accessory zircon from rocks of the Alenguy massif (Eastern Transbaikalia). According to data from geologists of the Chita Geological Administration, E. V. Barabashev and N. A. Trushcheva, two intrusive phases are distinguished in the indicated massif. In the first phase, predominantly coarse- and medium-grained, sometimes weakly porphyritic, leucocratic granites with pink feldspar and smoky quartz were formed. In the second phase, more alkaline, in composition, morion granites were formed.

By detailed crystallomorphological analysis in the rocks of the above-mentioned phases of the Alenguy intrusion, several morphological types of zircon crystals were established (3). The ideal forms of the isolated crystals are shown in Fig. 1. Thus the coarse- and medium-grained granites of the first phase, both of the main and of the marginal intrusive facies, are characterized, with a few exceptions, by zircon crystals of the 9th (frequency of occurrence 60–70%) morphological type. Crystals of the 8th (frequency of occurrence 14–9%) and other types are also of minor distribution here. Between crystals of the 8th and 9th types, intermediate forms are very often observed, caused by gradual transitions in the development of the faces $\{311\}$ and $\{111\}$. Most crystals are colorless or have a slightly yellowish tint. The average total $\beta$-radioactivity of the studied zircon types for rocks of the first phase is 55.6 imp/min*).

Fig. 1. Crystals of accessory zircon from rocks of the Alenguy granitoid massif. Simple forms: $a\{100\}$, $m\{110\}$, $s\{111\}$, $\lambda\{311\}$.

The morion granites of the second intrusive phase compose several separated massifs, which, despite the great distances between them (tens of kilometers), contain predominantly brown and dark-brown zircon crystals of the 10th type, with a frequency of occurrence equal to 80–90%. Here crystals of the 8th and 9th types are observed with an average frequency of occurrence of 10 and 5%, respectively. Crystals

of these types are yellow, light yellow in color, and rarely colorless. The total $\beta$-radioactivity of zircons from rocks of the second phase averages 88 imp/min. The crystal sizes, both in rocks of the first and of the second phase, vary from hundredths to tenths of a millimeter. It should be noted that in rocks of the second phase they are somewhat larger than in the first.

The presence of several morphological types of zircon crystals in one and the same rock within each phase indicates that the physicochemical conditions during their crystallization did not remain constant, but changed in a regular manner. This is evidenced by the geometric relationship in the development of crystallographic forms among the identified types; the presence of zoning in zircon crystals and the features of their intergrowths point to the same conclusion. In a number of cases the following sequence of crystallization of the above-mentioned morphological types of zircon crystals is established quite clearly. Thus, colorless and water-clear crystals of type 12, which have a minor distribution (about 1%), are linked by gradual transitions (intermediate crystal types) with colorless and light-yellow crystals of type 8. In turn, type 8 is replaced by the 9th morphological type, of browner colors. This sequence from type 12 to type 9 is also emphasized by a decrease in the role of the faces {311} and a gradual increase in the significance of the faces {111}, which is observed very clearly in the study of concentrates. In the same direction the isometric character of the crystals also increases. The position of crystals of the 10th morphological type in this sequence has not been established with sufficient precision.

As for the radioactive elements, especially uranium and, apparently, thorium, the latter, as is known, are capable of accumulating in later acidic differentiates during the differentiation of magmas of granitoid composition [5, 6]. It should also be noted that the uranium content, as shown by L. V. Tauson [5], E. S. Larsen and others [4], does not remain constant in different rock-forming and accessory minerals. Interesting data on this subject are given by A. I. Bazhenov [2], who established that the $\beta$-radioactivity of zircons in the youngest phases of an intrusion is higher than in the ancient ones.

Thus, it may be assumed that the different morphological types of zircon crystals, separated during the crystallization process at different times, will contain unequal amounts of radioactive matter. In this connection the authors attempted to investigate the $\alpha$-radioactivity of individual crystals of accessory zircon from rocks of the different phases of the Alenguy intrusion.

The determination of the $\alpha$-radioactivity of zircon was carried out with the aid of nuclear plates with an emulsion-layer thickness of 100 microns. The density of the emulsion was 3.6 g/cm$^3$. The $\alpha$-tracks over the entire irradiated area were counted under a microscope at a magnification of $900\times$ (Fig. 2). For irradiation, crystals were placed on the plates, on their emulsion layer, on one of the best-developed and smooth faces of the prismatic belt. The exposure time for irradiation was 30 days. This method for studying $\alpha$-radioactivity has already been used repeatedly by many researchers [1, 5, 7, 8, 9]. It should be noted that, for the study of $\alpha$-radioactivity, crystals from one sample were selected, as far as possible, of identical size and with equally lustrous faces, since earlier Zaghlool [9] had established that the specific $\alpha$-radioactivity depends on the size of the crystals.

For the study of the α-radioactivity of individual crystals from unaltered rocks of the first and second phases of the Alenguy intrusion, two of the most representative samples (10000, 4587) were selected, respectively.

Fig. 2. Microradiograph of a crystal of accessory zircon. Magnified 900 times.

The results of the crystallomorphological analysis of zircon in the indicated samples are summarized in Table No. 1.

Table 1

58 — maximum frequency of occurrence, in %.

Data from the study of the $\alpha$-radioactivity of individual zircon crystals, without separating them into morphological types, from these samples are given in Fig. 3.

From the curves in Fig. 3 it is quite clear that the $\alpha$-radioactivity of zircon crystals from the morion granites of the second phase is 2.5 times higher than that from rocks of the first phase. The same difference is also established from

Fig. 3. $\alpha$-radioactivity of zircon crystals from rocks of the first (curve 1) and second (curve 2) phases of the Alenguy granitoid massif

$\beta$-radioactivity (Table 1). It should also be noted that the maximum $\alpha$-radioactivity of zircons from rocks of the first phase does not exceed 330 $\alpha$-tracks per unit area, whereas for the second phase, over the same area, the maximum number of $\alpha$-tracks ranges from 330 to 480, and in individual cases these values reach 600–720.

On the basis of measurements of the maximum lengths of $\alpha$-tracks, an energy spectrum of the $\alpha$-radiation of radioactive elements in zircon from rocks of the first phase was constructed (Fig. 4). Study of this spectrum shows that, in the zircon investigated, radioactive elements of the uranium and thorium families are mainly observed.

From the curves in Fig. 3 it is readily apparent that individual zircon crystals in both phases have differing $\alpha$-radioactivity. Since several morphological types with different colors are observed in the samples, the former fact, as well as this circumstance, prompted us to study the $\alpha$-radioactivity of crystals of known morphological types from rock of one and the same phase.

For the study of $\alpha$-radioactivity, a representative sample No. 66 from unaltered granite of the first phase was selected. The results of its detailed crystallomorphological analysis are given in Table 1. From this sample, 300 colorless crystals of type 8, 300 crystals of type 9 ranging from light-yellow to light-brown in color, and the same number of crystals of type 10 of light-brown and brown colors were selected. It should be noted that the coloration of most crystals is uniform, and only rarely can zoned crystals be observed among them. Under an MBS-1 binocular microscope, crystals of a single color were selected, which required great effort and attention, since gradual transitions exist between the colors of the different morphological types of zircon crystals. Therefore, among the 300

crystals of more or less the same color, but with one or another shade, fell into every 300 crystals. Each of these groups (300 crystals each) was placed on a nuclear plate, and the $\alpha$-radioactivity was studied by the method described above.

Results of the study of individual zircon crystals of definite morphological types are shown in Fig. 5. From these curves it follows that $\alpha$-radioactivity increases from the 12th and 8th morphological types to the 10th; moreover, for the last two curves (9 and 10) a large scatter of values is observed. Here, on the plates, a zonal distribution of $\alpha$-tracks was encountered; in the central part of their grains the density was significantly lower than at the periphery.

Fig. 4. Energy spectrum of $\alpha$-particles of the radioactive substance in zircon

As already noted earlier, among the distinguished morphological types of zircon crystals from granite of one phase, a genetic connection is observed in the geometric development of forms, closely related to the conditions in the sequence of their development during crystallization. The same connection is observed in the features of the change in crystal color: from the colorless and water-transparent 12th and 8th types to the 10th, it changes through light yellow and light brown to brown and dark brown. In the same direction, as is evident from Fig. 5,

Fig. 5. Change in $\alpha$-radioactivity of crystals of different morphological types of accessory zircon from rocks of the first phase of the Alenguy intrusion

the $\alpha$-radioactivity of zircons from granites of the first phase increases and changes on average from 20 $\alpha$-tracks/unit area (8th type) to 111 $\alpha$-tracks/unit area (9th type) and to 146 $\alpha$-tracks/unit area for the 10th morphological type.

Conclusions

1. The α-radioactivity of individual crystals of accessory zircon from the granites of the Alenguy Massif increases by approximately 2.5 times in rocks of the second phase, which is connected with the accumulation of uranium and thorium in more acidic differentiates. This circumstance has repeatedly been emphasized by many investigators [2, 5, 6].

2. The regularities in changes in the morphology, color, and α-radioactivity of individual crystals of accessory zircon of one and the same phase give us grounds to suppose the following:

a) In the process of zircon crystallization, in the direction from the 12th and 8th to the 10th morphological type from rock of one and the same phase, accumulation of radioactive elements likewise occurs.

b) In connection with this, it may be supposed that during granite crystallization the 12th type is the first to separate out; it is replaced by the 8th and 9th, and then by the 10th morphological type. In general, zircon crystallization apparently takes place over a prolonged period of time.

In conclusion, the authors take this opportunity to express their gratitude to E. V. Barabashev and N. A. Trushcheva for providing the heavy-mineral concentrates, and also to the staff of the research institute at TPI for their extensive work in counting α-tracks in nuclear plates.

Literature

1. V. I. Baranov, D’u-le-t’yan’. Geochemistry, No. 11, 1961.
2. A. I. Bazhenov. Materials on the Mineralogy, Petrography, and Mineral Resources of Western Siberia. Tomsk, 1962.
3. V. A. Ermolaev. Proceedings of TPI, vol. 122, 1963.
4. E. S. Larsen, Feyer et al. In: Geology of Atomic Raw Materials. GNTI, 1956.
5. L. V. Tauson. Geochemistry of Rare Elements in Granitoids. Publishing House of the USSR Academy of Sciences, 1961.
6. D. Neuerberg. In: Geology of Atomic Raw Materials. GNTI, 1956.
7. A. R. Gindy. Economic Geology, vol. 56, no. 2, 1961.
8. I. H. Ford, C. D. Ollier. Nature, 176, 1955.
9. L. M. Zaghloul. Egypt. G. Geol., nos. 2, 3, 1959.