Rare-Earth Epidote from Southeastern Altai

A. I. Bazhenov

(Presented by Professor A. M. Kuz’min)

During prospecting and surveying work in the summer of 1955, in the contact zone of one of the intrusive massifs of Southeastern Altai, a dark-colored mineral resembling orthite was encountered; upon detailed study it proved to be epidote containing a small amount of rare earths. In 1956, additional material was collected. The study of the rare-earth epidote was carried out under the supervision of Prof. A. M. Kuz’min.

Conditions of Occurrence of the Rare-Earth Epidote

The mineral described was found in the central part of a complex quartz–carbonate vein, which has a meridional strike and dips eastward at an angle of 30°, and which occurs in the endocontact zone of a granodiorite massif. The vein is composed of dense light-gray quartz. A lens consisting of calcite, ilmenite, and rare-earth epidote is confined to its thickest part (Fig. 1).

Fig. 1. Form of occurrence of the lens containing rare-earth epidote.
1. Calcite lens with rare-earth epidote and ilmenite.
2. Quartz. 3. Granodiorite.

The earliest vein mineral is quartz. Microscopic study reveals an allotriomorphic-granular structure of the aggregate, consisting of quartz grains ranging in size from 0.05 to 1.5 mm, pos-

giving fanciful outlines. Sometimes in large individuals there are poikilitic inclusions of smaller quartz grains, clearly distinguished under crossed nicols. The mineral has blocky (mosaic) extinction, as V. S. Dmitrievskii (1955) proposes to classify it. Such extinction, in V. S. Dmitrievskii’s opinion, is common for vein quartz and is caused by submicroscopic displacement of individual areas under the influence of pressure.

The deposition of ilmenite, rare-earth epidote, and calcite is associated with a later stage of mineral formation; moreover, both macroscopic and microscopic study shows replacement of the vein quartz by these minerals. The earliest mineral of this stage is ilmenite, the crystals of which reach sizes up to \(15 \times 15 \times 2\) mm. Among the crystallographic forms, the most strongly developed are the faces of the basal pinacoid—\((0001)\), which determine the tabular habit of the crystals, and the faces of rhombohedra. The color of the mineral is iron-black; the streak is black. The fracture is uneven, conchoidal. Cleavage is absent. Magnetic properties are weakly expressed. Hardness is 5. The specific gravity was determined by the hydrostatic-weighing method and is 5.0. Chemical tests with hydrogen peroxide (S. A. Yushko and S. S. Borshchanskaya, 1955) gave a positive reaction for titanium. The results of spectral analysis, carried out by V. K. Chistyakov at the Department of Mineralogy of Tomsk State University, are given in Table 1.

In comparison with the data of spectral analysis of ilmenite from Egersund, Norway (I. and V. Noddak, 1935), the difference in trace elements is clearly noticeable: the absence in the Altai ilmenite of Pb and As and the presence of such elements as Sc, Li, Na, Co, Ni.

Fig. 2. Rare-earth epidote and ilmenite in quartz-calcite rock.
Ep—rare-earth epidote. Ilm—ilmenite.

The occurrence of ilmenite in a paragenetic association with rare-earth epidote and calcite deserves attention, since in the mineralogy handbooks of A. K. Boldyrev (1936) and A. G. Betekhtin (1950) we find no indication of the possibility of formation of this mineral in a hydrothermal process. Only Palache Ch. et al. (1946) note that ilmenite “occurs in pegmatites, especially

of intermediate or basic composition and in massive quartz.” Unfortunately, the authors do not elaborate on this observation, and therefore the question of the genesis of the quartz and the conditions of occurrence of the mineral bodies remains unclear.

Table 1

Results of spectral analyses of ilmenite, calcite, and rare-earth epidote

Rare-earth epidote forms distinctive radial-radiating aggregates; in some cases the dimensions of the latter are quite considerable, and the length of individual crystals reaches 50–60 mm (Fig. 2). The position of the mineral in the paragenetic sequence is determined fairly precisely by the fact that its crystals grow upon the surface of ilmenite, while on the other side later calcite encloses well-formed crystals of the epidote described. Only along the periphery of the lens does the mineral form radial-radiating aggregates in vein quartz, but in this case its crystals have an acicular form.

Calcite is represented by a coarse-crystalline variety, white in color and acquiring, upon weathering, a brownish (to brown) tint. In optical properties—No = 1.658; Ne = 1.486 (V. B. Tatarskii, 1955)—it corresponds to ordinary calcite. Spectral analysis, the results of which are given in Table 1, showed the presence of such impurity elements as cobalt, bismuth, titanium, and copper.

The contacts of the vein with the enclosing granodiorites are sharp. In the selvages of the vein, flakes of biotite are occasionally observed, usually no larger than 0.06 mm, analogous in optical properties to the biotite of the intrusive rock.

The intrusive massif with which the quartz-carbonate vein containing rare-earth epidote is apparently genetically associated is composed mainly of granodiorite; quartz biotite diorites and other varieties are observed much more rarely. Macroscopically the granodiorite is a medium-grained gray rock.

Microscopic study reveals a granitic texture of the rock, composed of plagioclase corresponding in composition to andesine No. 32, quartz, microcline, and biotite. Accessory minerals are represented by apatite, sphene, hematite, pyrite, magnetite, and monazite. A count of the relative amounts of the rock-forming minerals in several thin sections gave the following values: plagioclase—58%, microcline—11.9%; quartz—14.7%, biotite—15.3%, accessories—0.1%.

The near-vein alteration of the granodiorite was manifested in beresitization, as a result of which the rock was transformed into a quartz-sericite aggregate impregnated with pyrite. The relict granitic texture of the rock

is readily perceived. Quartz is characterized by noticeable cataclasis, the cracks being healed with sericite.

The sericite flakes are no more than 0.008 mm in size. The mineral is biaxial, optically negative, and has a small optic axial angle. \(Ng - Np = 0.028\).

The thickness of the zone of beresitization is about 2.5 m; the intensity of the process gradually dies out with increasing distance from the vein.

Iron hydroxides, which appear in considerable quantity during the decomposition of pyrite, impart a rusty-brown tint to the zone of beresitized rock.

Crystallographic study

To study the crystallographic features of the mineral, the author selected from abundant material the best-formed crystals. Examination of the crystals showed that most of them are either deformed, or else the faces belonging to the zone parallel to the axis \([010]\) bear coarse longitudinal striation, which greatly hinders, and at times makes impossible, goniometric measurements.

In cases where the signals from faces were indistinct, fragments of cover glasses were glued onto such faces.

For measurement, crystals were selected with a cross-section from 0.5–0.8 to 1.0 mm and a length of 3–4 mm.

As a result of the goniometric study, 9 forms were established on the crystals, the designations of which are adopted after I. D. Dana (1900): \(a\)—\((100)\); \(c\)—\((001)\); \(e\)—\((101)\); \(N\)—\((304)\); \(r\)—\((\overline{1}01)\); \(g\)—\((301)\); \(k\)—\((\overline{3}02)\); \(m\)—\(110\); and \(n\)—\((\overline{1}11)\).

The crystals of rare-earth epidote are elongated along the axis \([010]\). The most highly developed are the faces \(a\)—\((100)\), which determine the flattened habit of the crystals (Fig. 3), whereas the other faces are of subordinate importance.

Fig. 3. Crystals of rare-earth epidote.

The face \(a\)—\((100)\) always bears coarse longitudinal combination striation, which causes the appearance of grouped signals, but each of the signals is sufficiently sharp and bright. Sometimes they are indistinct, blurred, and in such cases the signals are multiple.

The face \(c\)—\((001)\) is, as a rule, narrow, even, and devoid of striation. The face is developed on all crystals and determines the habit of the crystal. The signals from it are always single, sufficiently sharp, and bright.

The face N—(\(\bar{3}04\)) is developed on most crystals and, along with the faces c—(001), is a habit face. In its morphological features it is analogous to the face c—(001), but is sometimes faintly striated longitudinally, and in that case a multiplicity of signals is noted, usually dim and blurred.

In the zone parallel to [001] there are always faces m—(110), in most cases also forming the head of the crystal, and only sometimes are they complicated by n—(111).

The faces m and n are morphologically identical and in all cases even, devoid of striation, but their signals are usually dim, though sufficiently distinct.

The faces e—(\(\bar{1}01\)), r—(\(1\bar{0}1\)), g—(\(30\bar{1}\)), k—(\(\bar{3}02\)) are not developed on all crystals and are observed as narrow strips blunting the angles of the habit forms; as a rule, they are distinguished by excellent signals, and only the faces k—(\(\bar{3}02\)) give very weak blurred signals, while from the faces g—(\(30\bar{1}\)) a ray due to a multiplicity of signals is usually observed.

The results of the goniometric study of crystals of rare-earth epidote are given in Table 2.

Table 2

The geometric constants calculated on the basis of the data obtained have the following values: \(\beta = 64^\circ40'\); the axial ratio \(a:b:c = 1.5864:1:1.7996\). The geometric constants for epidote and orthite, according to various investigators, vary within the following limits:

Consequently, in its geometric constants the mineral is closer to epidote.

As is evident from the results presented, the crystals of the mineral described correspond in their forms much more closely to epidote, with which it is also brought together by the circumstance that all the crystalline for-

they coincide completely, whereas for orthite some forms are not known—N—(304), g—(301).

On the other hand, one cannot fail to note also the feature that the obtained values of the angles in almost all cases are intermediate between epidote and orthite, as is clearly seen from Table 2.

It is interesting to note that in the earlier epidote, which is widely distributed in the skarn formations of the near-contact part of the pluton and which does not contain cerium in its composition (in amounts determined by spectral analysis, and in the case of cerium—less than 1%), the crystals bear the same forms; goniometric measurements showed that the following faces are developed on them: a—(100); c—(001); N—(304) and m—(110).

Physical properties and chemical composition of the rare-earth epidote

The crystals of the epidote described are black in color. The cleavage of the mineral is perfect along (001) and considerably less perfect along (100). The luster on fracture is vitreous, but very close to resinous. The fracture is uneven, conchoidal. A high brittleness is very characteristic of it, as a result of which determination of the hardness of the mineral is associated with certain difficulties. Measurement of the latter within the accuracy of the Mohs scale gives a value within the range 5.5–6.0. The increased brittleness of epidote (and orthite) is noted in particular in the well-known handbook of Dana (1900).

The specific gravity of the mineral was determined by the method of hydrostatic weighing and pycnometrically; for this purpose rather large specimens of the mineral were used, which, according to E. M. Bonshtedt-Kupletskaya (1953), gives minimal discrepancies. The obtained values of the specific gravity of epidote are 3.85. This value is rather high for ordinary epidote; for this mineral it does not exceed 3.5. In orthite the specific gravity is 4.1, and only in altered varieties does it fall to 2.7.

Determination of radioactivity, carried out for all specimens, gave negative results. As is known, A. G. Betekhtin (1950) and A. K. Boldyrev (1936) note the radioactivity of orthite, caused by the presence of thorium.

When heated in the flame of a blowpipe, the mineral swells and fuses into dark-brown glass.

The chemical composition of the epidote was determined twice—in the chemical laboratory of the West Siberian Geological Administration and by Z. A. Kim at the Department of Analytical Chemistry of Tomsk State University.

The results of the analyses and the recalculation to a crystallochemical formula according to Vl. Sobolev (1949) are given in Table 3.

Thus, the mineral belongs to a ferruginous variety containing an insignificant amount of rare earths, determined in the analysis as a sum.

Spectral analyses of the mineral, carried out in the laboratory of the Department of Mineralogy of Tomsk State University by V. K. Chistyakov and then repeated in the spectral laboratory of the West Siberian Geological Administration, made it possible to determine the qualitative composition of the rare earths and impurity elements. The results of the analysis are given in Table 1. It follows from the table that the predominant

the rare-earth element is cerium; next, in decreasing order, come yttrium, lanthanum, and ytterbium.

Among the impurity elements, the presence of chromium, arsenic, cobalt, and scandium is of interest.

Table 4 gives analyses of epidotes and orthite that are closest in composition.

It follows from the table that all epidotes are characterized by the absence of rare earths in their composition. A review of published analyses of orthite leads to the conclusion that, despite sharp fluctuations in the content of the various rare-earth elements (especially cerium, yttrium, and lanthanum), their sum does not fall below 14%. A. N. Winchell (1953) gives a diagram of the properties of the clinozoisite–orthite series, on which the results of analyses of various orthites are plotted. Calculation of the cerium content for the varieties poorest in this element gives a figure of 13.2%.

Table 3

Common divisor: \(2687 : 26 = 103.4\). Crystallochemical formula:

\[ (\mathrm{Ca}_{2,46}\ \mathrm{Zn}_{0,01}\ \mathrm{Fe}^{+2}_{1,09}\ \mathrm{Ce}_{0,07}\ \mathrm{Mn}_{0,14}) (\mathrm{Fe}^{+3}_{0,93}\ \mathrm{Al}_{5,37}) [\mathrm{OH}]_{1,78}\ \mathrm{O}_{3,22}\cdot \]

\[ [\mathrm{Al}_{0,29}\ \mathrm{Ti}_{0,06}\ \mathrm{Si}_{6,65}\ \mathrm{O}_{21}]. \]

In this respect, of great interest is the analysis performed by Hermann and cited by Koksharov (1858, 1862) for bagrationite, which represents a variety of orthite. For this mineral only unique occurrences in the Urals are known; moreover, C. Hintze (1913) also emphasizes the exceptional rarity of this variety. In the world mineralogical literature, and in particular in I. D. Dana (1900) and C. Hintze (1913), the description of bagrationite is based exclusively on the data of N. I. Koksharov.

In the modern literature this variety is almost never mentioned. A. G. Betekhtin only notes bagrationite as a variety of orthite rich in faces. The optical characteristics of the mineral are unknown; one can only suppose that they are analogous to the optical characteristics of orthite.

Table 4

* Including 0.49% SiO₂, 0.09% TiO₂, 0.63% CaO, which enter into the composition of sphene.

The mineral occurs in paragenetic association with white diopside and clinochlore. Its crystals are distinguished by being slightly elongated along the axis \([100]\), and not along the axis \([010]\), as is characteristic of minerals of the epidote group.

Optical properties and X-ray studies

The optical properties of the mineral were studied in thin oriented sections, and the refractive indices were determined in immersion.

As is known, the optical properties of minerals of the epidote group vary sharply. H. G. Tempel (1939) sees the causes of this in changes in the contents of iron, rare earths, and water. In the study of thin sections it is sometimes possible to observe incipient zoning, caused by a somewhat different orientation of the optical indicatrix in such zones. The plane of the optical axes is parallel to \((010)\). The orientation of the optical indicatrix is analogous to that of epidote (Fig. 4). The angle \(cNp = +12^\circ\). The angle \(Ng\) with the cleavage along \((001)\) is \(36^\circ\), which corresponds to the data given by Winchell (1953) for epidote. In longitudinal sections the extinction is straight.

The mineral is characterized by perfect cleavage along \((001)\) and less perfect cleavage along \((100)\). Twinning is manifested rather often; moreover, along with simple twins on \((100)\), polysynthetic twinning in the same direction develops no less frequently.

The mineral is biaxial, optically negative. The angle of the optical axes was measured on the Fedorov stage and is \(-70^\circ\). In thin section, distinct pleochroism is observed according to the following scheme: \(Ng\)—dark yellow-brown, \(Nm\)—dark yellowish-green, \(Np\)—light lemon-yellow. Absorption \(Nm > Ng > Np\).

The refractive indices are \(N_g = 1.778\), \(N_p = 1.736\). The value of \(N_m\) was calculated from A. K. Boldyrev’s diagram (1954), \(N_m = 1.749\). \(N_g - N_p = 0.042\). The mineral is characterized by the variegated anomalous interference color inherent in epidote.

For orthite the optical properties are given by A. N. Winchell (1953). In the case where the plane of the optical axes is parallel to (010), the angle \(cN_p = 30\)—\(40^\circ\). The optic axial angle is large. Birefringence is low, usually 0.010—0.020 (up to 0.036).

In order to refine the characterization of the mineral described, an X-ray investigation was carried out in the X-ray laboratory of the West Siberian Geological Administration; the results, in the form of interplanar spacings, are given in Table 5.

X-ray photography was carried out in cameras 57.3 mm in diameter, using iron radiation at a voltage of about 30 kV and a current of 10 mA. The interplanar spacings were corrected from separate photographs of a mixture with ammonium bromide.

Fig. 4. Optical orientation of rare-earth epidote.

In his conclusion based on the data of this analysis, the head of the X-ray laboratory of the West Siberian Geological Administration, B. G. Erenburg, writes: “The X-ray pattern is very similar to the X-ray patterns (and tabulated data) for epidote that we have, but there is a whole series of differences indicating differences in the lattices. These differences appear, above all, in the values of the interplanar spacings and in the intensities of certain lines. The interplanar spacings are somewhat overestimated.”

Table 5

On the Question of Genesis

The material presented above indicates a hydrothermal origin of the mineral; moreover, judging from the paragenetic association and the nature of the wall-rock alterations, the process took place under conditions of moderate temperatures.

A. E. Fersman (1931) indicates that orthite is a very early mineral and “is quite characteristically associated with two phases: the end of B and phase C... In phase D—E it may form orthite-epidote, and then epidote.” In his mineralogical characterization of pegmatites, A. E. Fersman (1931) limits himself to remarks on the possibility of the formation of orthite-epidote, but gives neither the features of its chemical composition nor the physical properties of this hypothetical mineral.

H. G. Tempel (1939), studying the question of the influence of rare earths on certain properties of epidote and orthite, comes to the conclusion that “at the end of the pegmatite-pneumatolytic phase the rare earths can no longer pass into minerals of the epidote group... Instead of orthite, epidote appears as a hydrothermal secretion, containing no cerium.” Consequently, in the opinion of both A. E. Fersman and H. G. Tempel, epidote is a typically hydrothermal formation, but A. E. Fersman believes that at higher temperatures the appearance of a mineral of mixed composition—orthite-epidote—is possible.

This conclusion of A. E. Fersman and H. G. Tempel concerning the conditions of occurrence of orthite proved not entirely accurate, and A. G. Betekhtin (1950) points to the possibility of the formation of orthite in contact-metasomatic deposits. The Altai cerium epidote described here, corresponding in its composition and properties to A. E. Fersman’s orthite-epidote, extends still further the limits of the geochemical migration of rare-earth elements, including also the field of hydrothermal mineral formation.

Conclusion

In the mineralogical literature, epidote and orthite are regarded as two mineral species, sufficiently well distinguished by their physico-optical properties and chemical composition. A distinctive feature of the composition of orthite is its considerable content of rare earths (especially cerium), which also determines the physico-optical characteristics of the latter. In all known analyses of the mineral, the content of the sum of rare earths does not fall below 14%; the exception is the unique analysis of bagrationite, containing 3.60% rare earths. Therefore H. G. Tempel (1939), in studying the minerals of the epidote group, in all cases deals with varieties enriched in rare earths, as a result of which he was unable to compare the properties of epidote and orthite.

The mineral described in this article belongs to that variety of epidote in which the content of rare earths is small. A feature of its physico-optical properties is that they give intermediate values between epidote proper and orthite, approaching the former in optical and structural indices. On this basis the mineral cannot be assigned to bagrationite, with which its chemical composition brings it closer, but their optical and crystallographic characteristics are different. Unfortunately, the literature lacks data on an X-ray study of bagrationite; but since it belongs to orthite, it should be assumed that its structural features must also be analogous to, or at least close to, the latter.

Thus, the Altai rare-earth epidote should occupy, in the epidote—orthite series, a position closer to epidote than all known varieties of orthite.

The study of this variety of epidote confirms H. G. Tempel’s (1939) conclusion regarding the great influence of the rare earths on the properties of minerals of this group, since an insignificant content of cerium causes a noticeable increase in the mineral’s specific gravity and a change in the parameters of the crystal lattice.

REFERENCES

1. Betekhtin A. G. — Mineralogy. Gosgeolizdat, 1950.

2. Boldyrev A. K. — Course of Mineralogy. ONTI, 1936.

3. Boldyrev A. K. — Diagrams of the magnitude of birefringence and the angle of the optic axes. In: The Universal Stage of E. S. Fedorov. Publishing House of the Academy of Sciences of the USSR, 1953.

4. Winchell A. N. and Winchell H. — Optical Mineralogy. Publishing House of Foreign Literature, 1953.

5. Dmitrievsky V. S. — On the undulatory extinction of quartz in rocks and the petrogenetic significance of this phenomenon. Transactions of the Geological Faculty of Voronezh State University, vol. 39, 1955.

6. Noddack V. and Noddack I. — Geochemistry of rhenium. In: Fundamental Ideas of Geochemistry, issue 3, 1935.

7. Sobolev Vl. — Introduction to the Mineralogy of Silicates. Publishing House of Lvov State University, 1949.

8. Solodovnikova L. L. — Epidotes from contact deposits of the eastern spurs of the Kuznetsk Alatau within Minusinsk Uezd, Yenisei Governorate. Transactions of the Geological and Mineralogical Museum of the Academy of Sciences of the USSR, vol. 5, issue 8, 1926.

9. Tempel H. G. — The influence of rare earths and certain other components on the physicochemical properties of minerals of the epidote group. Central Geological Library, issue 132, 1939.

10. Tatarsky V. B. — Microscopic Determination of Carbonates of the Calcite and Aragonite Groups. Gostoptekhizdat, 1955.

11. Fersman A. E. — Pegmatites. Publishing House of the Academy of Sciences of the USSR, vol. 1, 1931.

12. Yushko S. A. and Borishanskaya S. S. — Table of Diagnostic Features of Minerals in Heavy-Mineral Concentrates. Gosgeoltekhizdat, 1954.

13. Dana E. D. — The System of Mineralogy of J. D. Dana. 1900.

14. Hintze C. — Handbuch der Mineralogie. 1897–1913.

15. Kokscharow N. — Materialen zur Mineralogie Russlands. Vol. 3 — 1858, Vol. 4 — 1862.

16. Palache Ch., Berman H. and Frondell C. — Dana’s System of Mineralogy, v. 1, 1946.

Corrections and Noted Misprints

In S. A. Stroitelev’s article “Investigation of the Crystallization of Epsomite and Mirabilite,” for technical reasons braces have been replaced by square brackets, for example: [100] instead of {100}, etc.