Toward a Methodology for Mass Crystallomorphological Analysis of Accessory Zircon

V. A. Ermolaev

(Presented by Professor A. M. Kuzmin)

Recently many researchers [1, 3, 5, 7] have been attempting to use accessory minerals, and in particular zircon, as a corresponding indicator for determining the petrogenetic features of an igneous rock. Especially great attention in this connection is paid to the study of the morphology and physical properties of the given mineral. However, the method of detailed investigation of the form of individual crystals of accessory zircon, whose dimensions range from tenths and hundredths to thousandths of a millimeter, is specific and differs from the methods used to study large crystals [6]. In connection with this, the methodology set forth below was used in studying the morphology of accessory zircon from igneous and sedimentary rocks.

The selection of typical crystals characteristic of the rock complex under study is carried out under a Min-4 or Min-5 microscope. They are then measured on a GD-1 goniometer. On the basis of the measurements, stereographic and orthogonal projections of the crystals are drawn and are compiled into a consolidated reference table. The mass study of individual crystals is carried out under the microscope by further comparison with the reference table. For this purpose, each sample, in the form of a more or less uniform layer, is placed on a slide. Beforehand, a square grid with a mesh size of \(1 \times 1\) mm is applied to the latter with a diamond, which makes it possible to avoid repeated measurement of the same crystals. The study of each individual crystal is conducted in the following order:

1. With the aid of the divisions of the scale of the microscope ocular micrometer \((20\times\)—objective and \(8\times\)—ocular), first the length \((d)\) along the fourth-order axis \((G_4)\) is measured, and then the width \((\sh)\) and thickness \((\tau)\) along two mutually perpendicular second-order axes \((G_2)\) of the zircon crystal (Fig. 1).

The asymmetry coefficient is determined:

\[ K = \frac{\sh}{\tau} \]

for the most highly developed faces of the prismatic zone. It should be noted here that only whole crystals having both vertices are subjected to study.

An important genetic index is calculated—the ratio of length to width \(\left(\frac{L}{W}\right)\).

The width of the faces \((100)\), \((110)\), and \((010)\) is measured. Faces lying parallel to the surface of the microscope slide are measured directly. The width of the inclined faces \((100)\) and \((010)\), as is clear from Fig. 1, is calculated by the following formula:

\[ S=\sqrt{a^{2}+a^{2}}=a\sqrt{2}, \]

where \(S\) is the width of the face in divisions of the scale of the microscope ocular micrometer;

\(a\) is the measured visible projection of the face in divisions of the scale of the microscope ocular micrometer.

1. The crystal type is determined. For this purpose, instead of an \(8\times\) eyepiece, a photo eyepiece of \(10\times\) or \(15\times\) is inserted into the microscope tube; instead of the \(20\times\) objective, a special OSF-22 objective with an iris diaphragm is mounted, making it possible to obtain a greater depth of field. Then the lens of the lasomicroscope is switched on. Sometimes, in addition, lateral illumination through a narrow slit is used. As a result of the optical effect caused by the faceting of the crystal, it is possible to make out clearly which simple forms enter into the combination (Fig. 2). If necessary, the crystal is removed from the sample and measured on the GD-1 goniometer.

The degree of development of the dipyramidal faces is estimated approximately by comparison with standard drawings of crystals.

The faces \(\{001\}\) present a special difficulty in determination. Faces of this simple form occur extremely rarely on crystals of accessory zircon. However, in individual cases the number of crystals in a sample having faces \(\{001\}\) reaches 5%. In such cases an additional study of the crystals under a binocular magnifier is carried out, where these faces are detected more easily than by the above-described method under the microscope.

Fig. 1. Section of a zircon crystal perpendicular to the fourth-order axis.

Fig. 2. Crystal of accessory zircon of type 8. Magnification \(250\times\). \(a\{100\}\), \(m\{110\}\), \(\lambda\{311\}\), \(S\{111\}\).

Then the color of the crystal is studied in transmitted and reflected light. All the data obtained are entered in the journal of crystallomorphological analysis in the form of the following table (see Table 1).

In the notes, the additional dimensions of individual faces and the degree of asymmetry of their development are indicated, as well as features of their sculptural structure, the presence or absence of zoning, the nature and individual dimensions of inclusions, and also other information reflecting the actual appearance of the crystal and a number of its properties and features.

In all, 100 crystals in each sample are studied in such detail. This number of crystals, proceeding from Student’s statistical distribution law [4], makes it possible to state with a confidence of 0.95 that the true frequency of the corresponding conditional types of crystals fluctuates within ±5%. All the data obtained from measurements of accessory-zircon crystals are processed using methods of mathematical statistics.

Table 1

Note: The crystal is slightly flattened along [110]. \(K = 0.8\). The faces \(\{111\}\) and \(\{311\}\) are developed asymmetrically, and the latter are observed in full number. On the faces, overgrowths are clearly visible, apparently belonging to the smallest zircon crystallites. The dimensions of the faces of the prismatic belt are as follows: \((110)=0.0116\) mm, \((1\bar{1}0)=0.1504\) mm, \((0\bar{1}0)=0.0235\) mm. Gas–liquid inclusions of round and rounded-tubular form are observed.

After the study of each sample, a conclusion is given, in which, along with the general information indicated above, especially detailed attention is devoted to crystals that are not recorded in the journal of crystallomorphological analysis (intergrowths of crystals, rounded crystals, etc.).

On the basis of analysis of accessory-zircon samples by the above-described method, the following summary tables are compiled for the rocks of a given magmatic complex:

1. Prevalence of morphological types of accessory-zircon crystals.
2. Dimensions and elongation of accessory-zircon crystals.
3. Color of accessory-zircon crystals.

In addition, data on impurity elements, microhardness, \(\beta\)- and \(\alpha\)-radioactivity are also considered additionally.

Such a detailed analysis of samples makes it possible to reveal the most general and particular features of the morphology and physical properties of the mineral being described, characteristic of the given complex as a whole, and also of its individual phases and facies.

The results obtained make it possible, to one degree or another, to judge the petrogenetic features of the magmatic complex, and also to solve practical problems in connection with their subdivision.

LITERATURE

1. T. N. Agafonova. Morphology of distorted zircon crystals as an indicator of their genesis. Reports of the Academy of Sciences of the USSR, vol. 140, No. 2, 1961.

1. V. V. Lyakhovich, D. A. Rodionov. Toward a methodology for the study of accessory minerals in igneous rocks. Proceedings of the Institute of Mineralogy, Geochemistry, and Crystal Chemistry of Rare Elements, issue 6, 1961.

2. A. M. Dlin. Mathematical Statistics in Engineering. Moscow, 1958.

3. S. D. Turovskii. On the morphological characteristics of zircon from igneous rocks of Northern Kirghizia. Proceedings of the Academy of Sciences of the Kirghiz SSR, issue VI, 1955.

4. I. I. Shafranovskii. On the crystallomorphological description of microcrystalline minerals. Mineralogical Collection of the Lvov Geological Society, No. 13, 1959.

5. A. Poldervaart. Zircon in rocks. 2. Igneous rocks. Amer. J. Science, 254, No. 9, 1956.