New data on the granite pedestal of the monument to Peter the Great “The Bronze Horseman” in Saint Petersburg

In order to expand and popularize knowledge about the stone decoration of Saint Petersburg, we present new data on the mineralogy and petrography of the famous Thunder-Stone, the parts of which were the basis for the monument to Peter the Great – the legendary “Bronze Horseman”. In the course of studying geological documentation of the monument's granite base, we examined the mineral composition and internal structure of granite, as well as the fragments of a pegmatite vein and veinlets found in it. 25 single-mineral samples were collected from the available microscaled shear fractures within the pedestal surface and studied by electron microscopy, electron probe and X-ray phase analysis. It was established that K-Na feldspar in the granite composition was represented by microcline, whereas micas were represented by annite-siderophyllite and muscovite. Accessory minerals included monazite, xenotime, thorite, zircon, rutile, apatite, fluorite, Ti-, Nb-, Ta-bearing minerals, uranium phosphates. The presence of topaz is characteristic of pegmatites. The revealed structural and textural features of four granite boulders in the monument pedestal, as well as mineralogical and chemical composition of their rock-forming and accessory minerals, showed the similarity of this rock to Precambrian biotite-muscovite granites and topaz-containing pegmatites (stockscheiders) of the late formation phase of the Vyborg rapakivi granite massif. The research results are considered as the basis for further geological and mineralogical study of the Thunder-Stone origin and determining the place of its separation from the primary source.

marked as N 1, the smallest is N 4. One more block, marked as N 5, is 30 cm long and can be seen at the base of the pedestal at the border of blocks N 1 and N 2. Three types of rocks have been identified in these blocks. Firstly, the pedestal is composed mainly (about 90 %) of granite. Secondly, it contains three inclusions (xenoliths) of another rockfine-grained granite (named so conventionally, according to the visual assessment). Inclusions are visible at the very top of block N 1, under the horseman's feet. Their shape is angular, visible dimensions are 50×30, 20×10 and 40×20 cm (Fig.2). Thirdly, in granite of the block N 1, one can see fragments of a pegmatite vein, about 50 cm thick. Not all of it is visible, since its main part is cleaved and covered by block N 2 (Fig.3). In addition, the granite contains thin pegmatite veinlets, 3-5 cm thick. They are visible in block N 1 on the left side of the pedestal, and in block N 2 at the front of the pedestal.
Different shades of color of granite blocks N 1-4 suggest that the color of the Thunder-Stone was not uniform. It is evident that, first of all, the most durable material was chosen and color shades were not taken into account. A detailed study of the orientation of granite trachytoid structure, the position of pegmatite veins and veinlets made it possible to determine from which part of the original boulder these blocks were made, how much they were processed and how exactly they were turned to give the pedestal the appearance of a "single rock" according to Falcone's idea [2]. Three-dimensional computer modeling showed that only 1/3 of the original Thunder-Stone was used for the pedestal, the total volume of which is estimated at 675 m 3 , which corresponds to ca. 1,755 tons of granite [2]. But the authors have not yet been able to identify location of the remaining parts of the original boulder.   [8,13]. The morphology of grains, their intergrowth and inclusions in them were studied in the Resource Center "Microscopy and Microanalysis" using a microscope microanalyzer Hitachi TM3000 (Japan), equipped with an energy dispersive microanalysis attachment OXFORD (UK), a system with focused electron and ion probes QUANTA 200 3D (FEI, Netherlands) and the analytical complex Pegasus 4000 (EDAX, USA). We obtained SE and BSE images of the studied samples. Electron probe microanalysis was carried out on a Pegasus 4000 energy dispersive diffractometer (SDD ApolloX, resolution at 5.9 KeV Mg K -125.7 eV). The measured samples were rough and polished, covered with carbon in a high vacuum at an accelerating voltage of 15 -20 kV. The degree of triclinicity of feldspar was determined by the powder method using an X-ray diffractometer UltimaIV (Rigaku), X-ray tube radiation CuKα1+2; wavelengths CuKα1 = 1.54059 Å and CuKα2 = 1.54443 Å; tube operating mode 40 kV/30 mA; sample rotation speed 20 rpm; temperature 25 °C, atmosphereair.
Results obtained. In total, more than 20 mineral phases have been identified, 15 of them have been reliably diagnosed (Table 1).  Note. In addition to the listed minerals, Fe-, Ti-, Nb-, Ta-bearing minerals, U phosphates, TRR were found in the monument pedestal, but not precisely diagnosed. Inclusions of iron and nickel were not indicated either, as their origin in the rocks of the pedestal has not been identified.
Rock-forming mineralsinclude feldspars and quartz. K-Na feldspar has a light pink color and an almost stoichiometric chemical composition KAlSi3O8 (Table 2). In three analyses, FeO impurity of up to 1-2 % was determined in the mineral. The degree of triclinicity at the two analyzed points was 87.6 % (in granite) and 86.1 % (in a pegmatite veinlet). The mineral is substantially homogeneous and contains rare perthites composed by albite N 5-10. Plagioclase in the granite has a lilac-gray color and is represented by oligoclase N 15-20. Albite was found in pegmatite. Quartz has not been studied in the laboratory.
Here are examples of chemical formulas of the indicated minerals: Sample N 1 -(K0.84Na0.08)0.92(Si3.00Al1.02O8)microcline. Sample N 9 -(Na0.84Ca0.15K0.01)1.00(Si2.85Al1.15O8)plagioclase N 15. Less abundant minerals. We have found mica (biotite and muscovite), magnetite, topaz, and fluorite. Biotite is represented by annite-siderophyllite with variable values of FeO to MgO ratio (Table 3). Muscovite is significantly enriched in Fe. Li was not detected in any micas. Magnetite was found in two forms. It was identified in the form of flattened crystals with cleavage cracks. Based on a single analysis, it contains Cr (0.77 %) and Ti (1.37 %). Magnetite with a special morphology was found in the form of tiny spheres with a diameter of about 0.08 mm, containing impurities of Mg, Al and Si (about 3 % in total). Such magnetite spheres are encountered worldwide, and there are ongoing discussions about their origin.
Topaz was found in pegmatite veinlets in the form of single grains and in a pegmatite vein in the form of clearly visible clusters, where each individual grain varies in size from 0.3 to 2.5 mm, with an average value of about 1.5 mm. Topaz is transparent, colorless, bluish and greenish; it associates with quartz and feldspar (Fig.6). In two topaz samples we detected impurities of FeO (1.47 and 3.32 %). According to the analysis, the composition of topaz corresponds to formula Al2.01SiO.99O4.00(F1.86OH0.14). Fluorite was found in granite and pegmatite veinlets.
Accessory minerals. Seven minerals were identified in microsamples. In addition, 18 more mineral phases were identified but could not be diagnosed. Zircon, monazite, thorite and apatite were found in granite, xenotimein pegmatite. Their chemical compositions were standard. Rutile (and equally possible anatase) was found in granite and pegmatite veinlets. Some of their grains had an ideal stoichiometric TiO2 composition, but more often they were enriched with isomorphic impurities. The presence of Nb and Ta in undiagnosed phases is worth noting, which indicates either special varieties of rutile or such minerals as titanosilicates.  Uranium and copper phosphates. Five phosphates were identified, four of them were radioactive. They were found in pegmatite and pegmatite veinlets (Table 4) in the form of accumulations of very small (first microns) lamellar, tabular crystals with square or rounded outlines, as well as powder on the surface of feldspar, quartz and topaz grains. Analysis of samples N 59 and N 60 suggests that these can be torbernite and cheralite, respectively. Unambiguous identification of the indicated minerals has not been achieved.   Metals are represented by lamellar inclusions of nickel and iron in microcracks of feldspar crystals. The issue of the origin of such inclusions, whether they are natural or technogenic [13], remains controversial.
Discussion. First, let us pay attention to the ratio of the main rock-forming minerals, as well as structural and textural features of the Thunder-Stone. It is obvious that the volumetric ratios of quartz, feldspars and dark-colored minerals characterize the rock as biotite-muscovite K-feldspar granite. The uniform crystallinity, pronounced trachytoid texture and complete absence of rounded (ovoid) segregations of alkaline feldspar allow to exclude the similarity of this rock to the typical rapakivi granitevyborgite (Baltic-Brown) and peterlite (Carmen-Red)in accordance with the contemporary perception of the rocks from the rapakivi granite formation [17]. The idea of the Thunder-Stone as a "Finnish" rapakivi granite, common among historians and art experts, is apparently explained by its external resemblance to rapakivi, widely represented in the architecture of Saint Petersburg in the 18thearly 20th centuries [18].
In terms of structure and texture, the Thunder-Stone is not identical to the Olginsky boulder, located on the shore of the Gulf of Finland in the area of Konnaya Lakhta, where the former was once discovered. The Olginsky boulder is considered to be a part of the Thunder-Stone. However, this is a misperceptionthe boulder is composed of a completely different rock, ovoidal rapakivi granite. Three boulders on the banks of the Petrovsky Pond can neither be considered fragments of the Thunder-Stone. Observations suggest that this is also ovoidal rapakivi granite.
Secondly, the mineral composition, structure, texture, the presence of fluorite, monazite, apatite and other accessory minerals in the Thunder-Stone, as well as topaz and fluorite in pegmatites, allow us to draw attention to its similarity to coarse-grained trachytoid biotite-muscovite granites of the late magmatism phase, which occurred during the formation of the Vyborg rapakivi granite massif. This refers to biotite-muscovite granites, identified by D.A.Velikoslavinsky (1953) in the study of geological structure of the Vyborg massif and later attributed by A.M.Belyaev [1] to rare-metal biotitemuscovite topaz-bearing granites. The Kymi Massif (southeastern Finland) is given as a regional example of a geological object composed of such rocks. Trachytoid granites of this type, the so-called "even-grained rapakivi granites" of the Lappeenranta region, are described and studied in detail by Finnish researchers [14,15,16]. The age of the rocks in this massif is estimated as the Lower Proterozoic (1.5-1.6 Ga).
Genetic similarity of the Thunder-Stone to such rocks is also indicated by the presence of pegmatite veinlets. In terms of mineral composition and structure, they are similar to the so-called stockscheiderstopaz-bearing pegmatites, developed in the marginal zones of topaz-bearing granitoid bodies of the Vyborg massif [1]. Both in stockscheiders and in pegmatites, topaz is represented by intergrowths of tiny crystals.
In particular, the issue of topaz presence in the granite that composes the Thunder-Stone remains open. This mineral is found in biotite-muscovite granites of the Vyborg massif, and it would be important to check the similarity of the Thunder-Stone to such rock in this respect. For this purpose, it is necessary to study the stone pedestal of "The Bronze Horseman" in petrographic thin sections. For obvious reasons, it is not yet possible to carry out such study in practice.
The noted similarity of the rocks does not settle the questions of Thunder-Stone origin, but definitely directs on the way to further search for their solution. Of course, the natural outcrops of rapakivi granite in the Vyborg massif are located relatively close to the place of Thunder-Stone discovery, and the conclusion about their spatial proximity seems to suggest a conclusion about their genetic affinity. Nevertheless, it is wrong to exclude from consideration other rapakivi granite massifs, known in the mainland of Finland [20], on the Aland Islands and in other regions of Northwest Russia and neighboring countries. Moreover, the processes of destruction and movement of crystalline basement rock fragments on the Baltic shield during the last glaciation covered vast territories of Northern Europe [6,10]. So, the noted mineralogical and petrographic similarity of rocks should be considered only as a precondition for determining the desired direction in further research. It can clearly be stated that in the future there is a need for a detailed comparative analysis between the rocks in the monument stone base, the rocks of the rapakivi granite formation and the granites of other formations in the northwestern region of Russia, Finland and Sweden [7,9]. Examples of rapakivi granite studies [11,12] show that for the correlation of this type of rocks it is especially important to take into account individual typomorphic features of accessory minerals.
For obvious reasons, petrochemical comparison of the rocks is impossible. Therefore, further study of "The Bronze Horseman" pedestal should focus on physiographic analysis of the compared rock structure and texture, as well as on electron-probe analysis of chemical (including isotopic) composition and age of accessory minerals, specifically, zircon. The authors recommend taking into account the following mineralogical features of the Thunder-Stone: the K content according to the results of radiometry is 6.4 % * , which in terms of K2O corresponds to 7.4 %; the K/Na ratio is greater than 1, which is evident from the ratio of the main rock-forming minerals; the content of Fe and Mg in granite is low, which follows from a small amount of dark-colored minerals; the Fe/Mg ratio is high, which is evident from the chemical composition of biotite; the gamma activity of granite varies between 31and 38 mcR/h [8]; the content of radioactive elements is 10 -4 for Ra and 10 -4 for 65 Th. In granites and pegmatites of the Thunder-Stone, biotite is represented by annite-siderophyllite. At the same time, in mica from granite (Table 3) the Fe 2+ /(Fe 2+ + Mg) ratio is 0.96, the Fe 2+ /(Fe 3+ + Mg) ratio is 94.5/4.2/1.2, the annite/phlogopite/muscovite ratio is 65.4/2.7/31.9. The authors believe that these indicators are typical for micas from the late formation phases of the Vyborg rapakivi granite massif, the chemical composition of which is well studied [15,19].
Conclusions. Thus, the famous Thunder-Stone, the four main parts of which make up the pedestal for "The Bronze Horseman", is a boulder of biotite-muscovite K-feldspar granite. The structure of the granite boulder is complicated by the inclusions of xenoliths of other finer-grained granites. On one edge, it is composed of pegmatite lying in granites in the form of a vein, half a meter in thickness, which is only partially preserved. Numerous pegmatite veinlets are also developed in granites. The structural and textural features of granite, its mineral and chemical composition of the rockforming and accessory minerals, together with the same characteristics of the pegmatites developed in it, indicate their similarity to trachytoid biotite-muscovite granites and topaz-bearing pegmatites (stockscheiders) of the late formation phase of the Vyborg rapakivi granite massif, hard rock outcrops of which are known in southern Finland. The presumed age of the Thunder-Stone formation of granite and pegmatites, as well as the rocks of the Vyborg massif, is estimated as 1.5-1.6 Ga.
The article explores Thunder-Stone origin, as well as other completely unexplored aspects of its history, in particular, how it was separated from the primary rocks, weathered and moved to the coast of the Gulf of Finland, where it was found in 1768. All these questions are undoubtedly interesting for both professional historians of Saint Petersburg and for a wide range of curious and inquisitive enthusiasts. After all, the Thunder-Stone and "The Bronze Horseman" monument to Peter the Great in Saint Petersburg are a part of the historical and cultural heritage of Russia.