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Mineralogical Almanac  

Cluster of topaz crystals

Cluster of topaz crystals.
Specimen: 12 x 11.5 x 7.2 cm.
Alabashka pegmatite field. St.-Petersburg Mining Museum collection.
Bought from Count L.A. Perovsky in 1847
Photo Michael B. Leibov

Murzinka.
Mineralogy.
Topaz.

Topaz Al2SiO4(F,OH)2
Topaz is a main treasure of Mokrusha. It occurs also in noticeable quantities in the Golodnaya, Kazennitsa and Bolshaya Tyazhelovesnitsa veins and rarely in the Krutorechenskaya and Mys-II veins. According to A.N. Karnozhitskiy and A.E. Fersman, topaz was recorded in the other pegmatites of the Alabashka field.

A.E. Fersman devoted 17 text pages to the Murzinka topaz, the most voluminous description in comparison with the other Alabashka minerals. He wrote: «…Despite exclusive beauty and attractiveness of Murzinka topaz for crystallographic investigation, the literature about this mineral from Murzinka is relatively rare (43 works). A distinct definition of the habits of Murzinka topaz was given by Levi, whose illustrations provided the most complete and correct images, which characterized 20 simple forms in total. The works by N.I. Koksharov, who obviously did not know the works by Levi, are of perfect accuracy, but he added nothing new to this subject. The scheme by G. Rose showed two types of Murzinka topaz, and it was later confirmed by N.I. Koksharov. It is necessary to mention the work by P.V. Eremeev, who confirmed a prism {210}, recorded on the illustration by Levi, and found a new bipyramid {338}» (Fersman, 1959).

In 1914, Fersman summarized the data of various researchers, who earlier recognized simple forms of topaz crystals from Murzinka (Fersman, 1959). 24 forms were recognized as the growth forms, and 6 were recognized as the solution forms. Fersman noted that topaz crystals from Murzinka are poor in diversity of the growth forms. We can only suggest that previously studied «topaz from Murzinka» originated mostly from Mokrusha, because we confirmed presence of these forms during our goniometric measurements of topaz crystals from Mokrusha and added some new information (Table 6). In the other veins of the Alabashka field the topaz faceting is less diverse in forms.

Lepidolite rosettes on topaz

Lepidolite rosettes on topaz.
Specimen: 8 x 8 x 6 cm.
Mokrusha, Alabashka pegmatite field. Fersman Mineralogical Museum collection.
Collected by Neiva GPP in 1987
Photo Michael B. Leibov

Fersman distinguished four morphological types of topaz crystals from Murzinka. However, his third type is analogous to the second and differs only in solution forms. If to exclude the solution forms, only three types of topaz crystals can be identified (Fersman, 1962): (1) crystals of «almost cubic form» with the habit facets of basopinacoid and almost square prism l{120}; (2) crystals «with hexagonal habit resulted from the dominance of the prism m{110}; basopinacoid is very narrow due to bipyramids; there are numerous facets, and this type is similar to Ilmeny»; (3) crystals «reminding the postal envelopes» (the prism y{021} develops instead of escaping basopynacoid). Later, the new mining works and new finds increased a morphological variety of the topaz crystals (Fig. 24 and 25).

Topaz of the first generation («raw» topaz) occurs in the quartz-felsdpar graphics and in the blocky zone of pegmatite (second generation), but the individual surfaces are inductive, e.g., «compromise» surfaces with pseudofacets and pseudoridges. In the druse cavities, coarse topaz crystals of the third generation (with «square» section) represent the first (Murzinka) morphological type by Fersman (Fig. 24a-c; Fig. 25a-b). In Mokrusha, the growth periphery zones of these crystals are of intensive blue color, and they are especially effective. The central zones of these crystals are colorless or slightly pinkish (Popov, 1982). Sometimes, these crystals display colored zoning: the growth pyramids [120] are blue, whereas the other forms are pinkish, yellowish and colorless. Topaz of the first type hosts finest solid inclusions of blue-gray tourmaline (schorl). The primary gas-liquid inclusions reveal a solid phase represented by the borax (Na2B4O7•10H2O) crystals (Yakubova, 1952).

Fine pink crystals of the second type by Fersman (Fig. 24e,f,i; Fig. 25i) are the most widespread in Mokrusha. They often contain the finest ingrowths of split needle-like crystals of columbite- tantalite and ilmenorutile, whereas schorl crystallized together with the periphery zones of topaz. Crystals of the second type developed later than crystals of the first type.

Crystals of the third type precipitated after the second type crystals. They are envelope-shaped and are blue or colorless. (Fig. 24g,h; Fig. 25c,e,f,n). In Mokrusha, they crystallized together with lithium mica, albite-cleavelandite and polychromatic tourmaline. Then, fine-grained aggregates of mica, chlorite and clay have been formed.

Besides the previously mentioned types of topaz crystals, there are morphologically intermediate types and crystals of the «Ilmeny» type (Fig. 25d,g,j,k,n). For instance, in Mokrusha the pseudocubic topaz crystals, up to 2 cm in size, occur together with pink elbaite. Such topaz crystals are morphologically similar to early topaz, which developed together with lithium mica. In the clayey material of the cavities, there were single finds of yellow topaz of the «Ilmeny» type. Its relative age is not clear, but it is probably close to the crystals of the third type. Small (up to 1–3 cm or rarely greater) colorless, pale-blue and pale-pink topaz crystals (Fig. 25n- p) weighing between 1 to 150 g occur in Kazennitsa. Colorless prismatic crystals are relatively rich in facets (Fig. 25n) and form paragenesis with smoky quartz, schorl, microcline and beryl. The growth pyramids [120] of these crystals are often blue. The blue short-prismatic topaz crystals grew together with smoky quartz, cleavelandite and muscovite. The pseudocubic habit is typical for the pale-pink crystals of the elbaite paragenesis.

In the cavities of the Golodnaya vein, topaz crystals of the early nucleation are colorless and subisometric (similar to Fig. 24d), whereas the next stage blue crystals with cassiterite and tantalo-niobate ingrowths produced more complicated faceting (Fig. 25k,l); the latest generation consists of fine (1–2 cm) colorless long-prismatic topaz crystals (Fig. 24e) in paragenesis with fine-tabular colorless muscovite.

Surprizing variety of the sculpture of topaz facets in the Alabashka pegmatite is a result of multiple morphological changes during its growth, dissolution and regeneration (Fersman, 1940; Popov, 1982).

In various cavities and even sometimes in various parts of the same cavity, the intensity of dissolution and regeneration of topaz is very diverse. Dissolution varies from slight etching up to complete destruction of crystals. At the same time, the other minerals of druse cavities are less dissolved or even were not touched by the process of dissolution. The relationships between these minerals show that the processes of growth and dissolution of topaz in the cavities changed many times, displaying the earliest and the latest stages.

The fluorine content in topaz from the early Mokrusha cavity, containing gray muscovite, from the late cavity, containing rauchtopaz and schorl, and in topaz from Kazennitsa is almost identical, being about 20 wt. % as estimated by V. Popova by value of the cell parameter b0 = 8.780–8.784 A°.

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Last update: May, 5, 2007