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Unusually shaped quartz aggregates from Tirniauz, Russia.

INTRODUCTION

The Russian city of Tirniauz (about 40,000 inhabitants) is situated in the central Caucasus Mountains, in the Baksan River Valley, at an altitude of approximately 1,600 meters, about 40 km northeast of Elbrus Mountain [ILLUSTRATION FOR FIGURE 1 OMITTED]. A well-known molybdenum-tungsten deposit, in which Cu, Bi, and Au mineralizations also occur, lies only a few km northwest of the city. It is exploited by underground as well as open-cast mining. The so-called "upper open mine" is probably the highest active open-mine (altitude of about 3,000 meters) in Europe.

GEOLOGY

The structurally complex Tirniauz deposit is a skarn. In the Tertiary, Paleozoic limestones (Devonian to Carboniferous) were altered by endogenous metamorphism caused by the influence of intruded granitoids, and transformed into marbles. In this process, differently mineralized skarns originated at the contact of hornfelses surrounding the granitoids with the marbles. The skarns usually show layered or stratiform structures. They are often about a hundred meters long, but reach a thickness of only a few meters.

The metamorphism and the formation of younger hydrothermal minerals connected with this process took place in several stages. Many signs of repeated alteration, decomposition, and recrystallization of minerals can be observed. The results of age determination measurements on these minerals vary from 20 to less than 2 million years.

The specimens described here were found in the region of the middle skarn of the "third anomaly." The latter term goes back to the geophysical exploration of the deposit; several magnetic anomalies were found here which were used to name parts of the deposit. The place of discovery lies on the left slope of the valley of the Tirniauzsu River, on the Gitche-Tirniauz mountain ridge.

At an altitude of about 2,800 meters an approximately 20-meter-wide, brecciated skarn-like rock is exposed by a road cut. This breccia adjoins a volcanic-sedimentary conglomerate to the north and a black marble to the south. In this brecciated zone the complex structure of the rock has originated from tectonic torsional movements.

After the marmorization of the limestone, a pyroxene-magnetite-garnet skarn was formed, in which minerals such as epidote, calcite, gold and bismuth tellurides were later deposited by predominantly hydrothermal fluids. This skarn was then fractured by tectonic stresses. The rock fragments were altered metamorphically and cemented into a breccia by newly formed hedenbergite. In the course of this process, cavities were generated, in which actinolite, stilpnomelane, quartz, ilvaite and other minerals crystallized. In subsequent stages arsenopyrite, quartz, a clinochlore-like chlorite, ankerite, dolomite and other rarer minerals were formed. The crystallization of sulfides such as sphalerite and galena, and of manganese-bearing carbonates (manganiferous dolomite and calcite) took place during subsequent hydrothermal stages.

QUARTZ

During a geoscientific excursion organized by the Faculty of Geosciences of Moscow State University in June of 1989, a 1-meter pocket was discovered in the metasomatic skarn-breccia. In association with brownish to grayish yellow rhombohedral, calcite, quartz crystal aggregates up to 4 cm in size were found. Both minerals are overgrown by poorly crystallized secondary minerals, particularly by earthy "limonite" and dark brown manganese oxides and hydroxides which have not yet been identified.

The individual quartz crystals are invariably intergrown in an unusual hexagonal "pine tree" habit [ILLUSTRATION FOR FIGURES 2 AND 3 OMITTED]. Even in publications which specifically deal with the morphology of quartz in great detail we could not find any reference to such completely symmetrical quartz aggregates (see e.g., Hintze, 1915; Rykart, 1989). Starting from the six edges of a hexagonal quartz prism, other quartz crystals are arranged in a six-rayed pattern, their c axes almost perpendicular to the c axis of the host individual in the center [ILLUSTRATION FOR FIGURES 4 AND 5C-D OMITTED].

The regularity of intergrowth in these aggregates first suggested that they might have been formed by repeated twinning: the approximate right angle between main and radial individuals is reminiscent of Japan-law twins (compare Fig. 100 in "Multiple Japanese twins" in Rykart, 1989). However, microsection investigations show that the aggregates have not formed by twinning. Firstly, the main and radial crystals of the quartz aggregates are not identically related, as would be expected for twins. Main and radial individuals represent two different quartz generations, and also the boundaries between them show shapes which are not typical of twins but are typical of overgrowths. Secondly, the angle between main and radial individuals (experimentally determined by extinction between crossed Nicols) is 90 [degrees] [+ or -] 1.5 [degrees] and, consequently, differs greatly from the 84 1/2 [degrees] angle between the two crystals of a (1122) contact twin.

In the light of experimental evidence the following genetic explanation can be offered. First a thin, slender prismatic crystal grew which now is the main individual. The edges of the prism were starting points for the crystallization of numerous other quartz individuals. Most of these secondary crystals show a preferential orientation the reason for which is not yet clear. The secondary crystals are oriented in such a way that the directions of their c axes are roughly parallel to one of the a axes of the central host individual and that one of their a axes is nearly parallel to the c axis of the central individual [ILLUSTRATION FOR FIGURE 2 OMITTED]. Therefore, in microsections perpendicular to the c axis of the main crystal it can often be observed that the secondary crystal growth started at an edge of the host individual in three main directions (see e.g. central part of [ILLUSTRATION FOR FIGURE 5D OMITTED]). However, the growth of most of these crystals was restrained in favor of the growth of a few crystals [ILLUSTRATION FOR FIGURE 5A OMITTED] which now are the centers of the radial quartz individuals.

According to cathodoluminescence investigations [ILLUSTRATION FOR FIGURE 5B AND 5D OMITTED], crystallization of the framework of the aggregates (i.e., the first two steps of their formation) may have occurred from an undersaturated hydrothermal solution in a relatively slow, steady process. After a superficial decomposition of these first quartz generations (which was sometimes connected with a slight overcrusting by iron oxides), the aggregates continued to grow, in which the crystallographic orientation of every individual was preserved. Cathodoluminescence photographs of this newly formed quartz suggested that it originated from an oversaturated, gel or briny solution, at relatively low temperatures. During further crystallization of quartz, this solution must have been diluted and passed into an undersaturated state. In addition to the central host crystal, the outermost tops of the radial individuals occasionally show a short-lived greenish blue cathodoluminescence color which is characteristic of quartz formed from an undersaturated hydrothermal solution (compare e.g., Ramseyer, et al., 1988; Ramseyer and Mullis, 1990).

The aggregation of quartz by growth of a second generation of crystals starting at the edges of an earlier central crystal suggests that the external forms of some cubic minerals (such as sal ammoniac or gold), which are very similar to that of quartz, need not necessarily be interpreted as crystallographically continuous skeletal growth. Possibly some of them are likewise formed by crystallization and growth beginning at the edges or comers of a central crystal. However, in the case of an isotropic mineral it is certainly more difficult to check experimentally whether the single individuals in those aggregates are crystallographically uniform or not, as compared to the aggregates described here. Perhaps the use of different luminescence methods will help to solve this problem.

ACKNOWLEDGMENTS

The authors wish to thank O. V. Kononov (Moscow State University) for detailed discussions and comments on geological development and mineralogy of the Tirniauz deposit. The cathodoluminescence photos were kindly taken by J. Gotze (Freiberg Mining Academy). We are grateful to R. B. Heimann and J. Kreher (Freiberg Mining Academy) for constructively reviewing the manuscript.

REFERENCES

HINTZE, C. (1915) Handbuch der Mineralogie. Leipzig, Verlag von Veit & Comp., 1, 2nd part, 1266-1445.

RAMSEYER, K. BAUMANN, J., MATTER, A., and MULLIS, J. (1988) Cathodoluminescence colours of [Alpha]-quartz. Mineralogical Magazine, 52, 669-677.

RAMSEYER K., and MULLIS, J. (1990) Factors influencing short-lived blue cathodoluminescence of [Alpha]-quartz. American Mineralogist, 75, 791-800.

RYKART, R. (1989) Quarz-Monographie. Die Eigenheiten von Bergkristall, Rauch-quarz, Amethyst und anderen Varietaten. Thun, Ott Verlag, 413 p.
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Author:Nasdala, Lutz; Pekov, Igor V.; Bode, Rainier
Publication:The Mineralogical Record
Date:May 1, 1996
Words:1359
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