Apophyllite of the Kaltarovskoye Deposit (Khakassia)
Abstract Generated abstract
This paper reports the identification and description of apophyllite from the Kaltarovskoye skarn deposit in Khakassia, where it occurs as small milky-white dipyramidal crystals in calcite and quartz-calcite veinlets cutting garnet skarns and quartz-pyroxene rocks. The study uses physical and chemical tests, blowpipe observations, optical examination, and goniometric measurements to confirm the mineral as apophyllite and to compare its crystallographic constants with published values. The author describes its morphology, cleavage, density, hardness, optical behavior, zoning, and alteration, and argues that it formed during a late hydrothermal stage, probably contemporaneously with small rhombohedral calcite crystals. Its recognition at another Khakassian deposit suggests that apophyllite may be more widespread in the region’s skarn deposits than previously recorded.
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Apophyllite from the Kaltarovskoye Deposit
(Khakassia)
E. N. Zykov
In the course of mineralogical investigations in the area of the skarn deposits of the Ulen group (Khakassia) in the summer of 1947, specimens with an unknown mineral were found at the Kaltarovskoye deposit. The mineral occurred as small, fairly well-formed, milky-white dipyramidal crystals resembling those of apophyllite.
A review of the relevant literature on this district revealed no information about apophyllite as a mineral of skarn deposits. In connection with this, it became necessary to verify and clarify the assumption that had been made. For this purpose, the physical and chemical properties of this mineral were studied, its behavior before the blowpipe was examined, and goniometric measurements of the crystals were carried out. The results obtained thereby definitively confirmed the initial supposition that the mineral found at the deposit is apophyllite. The work on the study of this mineral was carried out under the direction of Prof. A. M. Kuz’min.
At the same time, apophyllite was also identified at the Dar’inskoye deposit of the Karysh group. The discovery of this mineral at two spatially separated deposits can only indicate that it is not an exceptional rarity in the Khakassian skarn deposits. Evidently, this inconspicuous mineral was overlooked by previous investigators; therefore, the possibility of finding it also at a number of other deposits is not excluded.
Forms of Occurrence of Apophyllite
At the Kaltarovskoye deposit, apophyllite occurs in calcite and quartz–calcite veinlets cutting garnet skarns and quartz–pyroxene formations. One of the specimens with apophyllite is a dense garnet skarn composed mainly of dark-green andradite, individual crystals of which have a zonal structure. The narrow peripheral zones of such crystals are composed of a brown and brownish-brown variety of garnet. In the mass of garnet, disseminations of chalcopyrite and molybdenite are observed; calcite is present in small amounts. Along the cleavage of garnet crystals, pyrite “mirrors” can often be observed.
The garnet skarn is cut by a calcite veinlet up to 0.8–1.0 cm thick, with included single crystals of pyrite. The central part of the calcite veinlet is filled with drusy, translucent, yellowish calcite, individual crystals of which have a rhombohedral habit, and with apophyllite crystals reaching 1.5–3.0 mm in cross section.
In two other specimens, apophyllite, in the form of numerous small crystals and druse-like crystalline intergrowths, together with crystals of semitransparent calcite, fills cavities in the central part of a quartz–calcite vein, forming peculiar rhombohedron-like cells, the mechanism of formation of which will be considered below. The peripheral part of the vein is composed mainly of semitransparent quartz, while its central part is composed of white calcite. The quartz–calcite vein cuts a dense greenish-gray rock of quartz–pyroxene composition. Microscopic study showed that this rock consists mainly of quartz and diopside, the grains of the latter, usually having a regular crystallographic outline, being intensely replaced by calcite, up to the formation of pseudomorphs. Remnants of diopside grains that have escaped replacement can often be observed among the calcite.
Both in the quartz–pyroxene rock and in the quartz–calcite vein cutting it—and especially along their contact—molybdenite develops in the form of separate scales and jet-like aggregates. In addition, a fine dissemination of pyrrhotite and chalcopyrite is observed in the quartz–pyroxene rock.
Crystallographic measurements
In the specimens found at the deposit, apophyllite forms small dipyramidal crystals, representing a combination of the following simple forms: (111), (001), and (100) (Fig. 1), of which the latter two are very weakly developed and are sometimes detected only under a binocular microscope.
Fig. 1. Apophyllite crystal. P—(111); a—(100); c—(001).
The crystals were measured on a one-circle reflecting goniometer. Although at first glance the apophyllite crystals seem comparatively well formed, on closer examination it was established that the crystal faces very often bear traces of dissolution and prove to be far from even, being uneven and knobby; as a result, the signals obtained from individual faces were, as a rule, diffuse and weak, which could not fail to affect the results of measurement. An attempt to use cover glasses glued to the crystal faces during measurement did not give good results: the readings obtained after repeated gluing of the glasses to different parts of one and the same crystal face showed variations within wide limits, and individual angle values obtained in measuring the crystals showed significant deviations from their calculated values; therefore these measurements were not taken into account.
The results of measuring the crystals are presented in Table 1. Geometrical constants, calculated on the basis of those obtained in the measurements
Table 1
| Names of faces | Variation in angle values, from—to | Mean value | Calculated value | Deviation | Number of measurements |
|---|---|---|---|---|---|
| (111) : (11$\overline{1}$) | 58°35′—59°34′ | 59°00′ | 58°56′ | +4′ | 5 |
| (111) : (001) | 60°11′—60°33′ | 60°21′ | 60°32′ | −11′ | 5 |
| (111) : ($\overline{1}$11) | 75°49′—76°15′ | 76°04′ | 76°00′ | +4′ | 4 |
| (111) : (100) | 52°01′—52°41′ | 52°21′ | 52°00′ | +21′ | 2 |
data, have the following values: \(a:c = 1:1.2498\). The constants accepted in the literature for crystals of apophyllite (according to Miller) are \(a:c = 1:1.2515\) (2).
Ploner, in his survey of apophyllites, gives for them geometrical constants ranging from \(a:c = 1:1.2057\) to \(1:1.2530\) (3). The axial ratio \(a:c\) for apophyllite from the Kaltarovskoye deposit is thus close to the accepted values.
The most strongly developed face on the crystals, (111), as a rule with traces of dissolution, and sometimes also with natural etch figures in the form of truncated tetragonal pyramids, usually gives diffuse, sometimes multiple, signals. The face (100) also gives very weak and indistinct signals, and only from the face (001) are somewhat clearer signals observed.
Physical properties and behavior of the mineral before the blowpipe
Apophyllite crystals have a milky-white color, sometimes with a clearly expressed pale pink tint. Along the plane (001) the mineral has very perfect cleavage and weaker cleavage along (110). The luster is vitreous, on cleavage planes up to pearly. It is brittle, easily splitting along the planes of very perfect cleavage; much more rarely it gives cleavage fragments along the plane (110). The crystals are opaque, but transmit light well at the edges. The hardness of the mineral, determined with the precision allowed by the Mohs scale, is 4.5–5.0. Specific gravity is 2.33–2.38. Determination of the specific gravity was carried out with a pycnometer (capacity 5 ml, mineral sample 0.702 g).
On heating, the mineral fuses comparatively readily to a white bubbly mass. On heating in a closed glass tube it becomes cloudy and releases water. After ignition, and also without it, the mineral powder shows an alkaline reaction. The mineral is readily decomposed by hydrochloric acid, with formation of gelatinous silica.
Optical properties
To study the optical properties, transparent thin sections were prepared from individual apophyllite crystals. The study of the optical properties was considerably complicated by the fact that the mineral is contaminated by abundant alteration products, which at times make it completely opaque. This alteration of the apophyllite may be caused by a change in the content of zeolitic water in it.
Under the microscope, apophyllite crystals show a zonal structure, which is further emphasized by the arrangement of the mineral’s alteration products, oriented perpendicular to the bounding forms of the crystals. In some cases the zoning is expressed in the fact that the mineral has a transparent core and, sharply separated from it, an almost completely opaque peripheral zone, abounding in products of alteration of the apophyllite (Fig. 2). In other cases, with crossed nicols, one can also detect the presence of a number of narrower zones in the marginal parts of the crystals. Similar zoning is noted by P. P. Pilipenko for apophyllites from the Nizhnyaya Tersa River [1].
Fig. 2. Zonal structure of an apophyllite crystal. Magn. ×20.
The refractive index of apophyllite is very close to that of Canada balsam, as a result of which the boundary between them is barely perceptible.
Determination of the optical sign was carried out in four crystals of apophyllite; three of them proved to be optically positive and only one negative. The optical anomalies often noted for many apophyllites are weakly expressed or are not observed at all in the apophyllites of the Kaltarovskoye deposit.
On the question of the genesis and paragenesis of apophyllite
The apophyllite of the Kaltarovskoye deposit is spatially confined to calcite and quartz-calcite veinlets, which are cutting with respect to the garnet skarns and quartz-pyroxene rocks that are undoubtedly earlier in time of formation. It is quite obvious that the deposition of apophyllite took place from hydrothermal solutions, for which the calcite and quartz-calcite veinlets apparently served as convenient channels of circulation; in these veinlets the crystals of apophyllite were deposited. The small accompanying rhombohedral crystals of translucent yellowish calcite are, in all likelihood, formations contemporaneous with it. Confirmation of their close genetic connection may be provided by the character of their occurrence. Both occur as small crystals on the walls of cavities in quartz-calcite veinlets; in some cases the calcite crystals are overgrown on apophyllite, while in others the reverse relations can be observed.
The formation of cavities in quartz-calcite veinlets, with characteristic cells in them, having in rough outline a rhombohedral form and composed of apophyllite, may be regarded as the result of deposition of apophyllite along cracks probably coinciding with the directions of cleavage of the calcite of the veinlets, followed by leaching of part of the calcite material.
Thus, the formation of apophyllite took place in the final stages of the hydrothermal process, in a later phase than the deposition of the material of the quartz-calcite and calcite veinlets.
LITERATURE
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Pilipenko I. P. — On apophyllite from the river Nizhnyaya Tersa, a right tributary of the Tom. Yearbook on Geology and Mineralogy of Russia, vol. 10, p. 189, 1908.
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Goldschmidt V. Krystallographische Winkeltabellen. 1897, S. 51.
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Ploner P. I. Ueber die Kristallformen des Apophyllits der Seiseralpe. Zeitschr. f. Kryst., 18 Bd., 1891, S. 338.