St Petersburg mineralogists determine the approximate age of the Thunder Stone – the pedestal for the Bronze Horseman
A group of researchers from St Petersburg University and St Petersburg Mining University have studied the mineral composition and structure of the famous Thunder Stone. Its four blocks of complex configuration make up the pedestal for the statue of Peter the Great – the Bronze Horseman. For the first time, scientists have been able to estimate the approximate age of the rock – about 1.5–1.6 billion years. Besides, they have studied its structure. It is granite composed of biotite, muscovite, and potassium-feldspar.
The research team leader was Andrei Bulakh (1933–2020), Honorary Professor of St Petersburg University, a great connoisseur of the natural stone used in the construction and decoration of St Petersburg. The scientists examined more than 20 mineral samples taken without harm to the monument. The samples were collected over the period between 2003 and 2018 from surface micro-scaled spalls in the statue’s pedestal. Spalls are rock fragments that have apparent cracks.
Map of the sampling sites (2018)
‘The mineral composition and internal structure of the Thunder Stone draw attention to its similarity with the biotite-muscovite granites of the Vyborg massif, one of the largest granite massifs in the world. The age of these rocks is estimated as Palaeoproterozoic (1.5–1.6 Ga). Genetic similarity of the Thunder Stone to such rocks is also indicated by the presence of pegmatite veinlets. These are veinlets of igneous rock showing a characteristic texture, with large interlocking crystals,’ the researchers explained.
The issue of topaz presence in the Thunder Stone has to be addressed. According to the scientists, topaz is found in biotite-muscovite granites of the Vyborg massif. It would therefore be important to check the similarity of the Thunder Stone to such rock in this respect. For this purpose, thin section petrography of the statue’s pedestal is necessary. Thin section petrographic analysis may provide qualitative description and quantitative estimation of rock properties. For obvious reasons, however, this research method cannot be used in this case. Thus, the question regarding the place of origin of the Thunder Stone has not been settled yet.
‘Despite the very small amounts of the obtained material, modern research equipment of the Research Park of St Petersburg University enabled us to obtain significant, sometimes unexpected findings, reported in a number of research papers. Interest in the history of the Thunder Stone has not faded away because there are still a few blank spots in it. For example, the question of its age remains hanging. This research paper only gives an approximate age of the boulder. For more accurate answer, it is necessary to apply advanced methods of research, which requires rock material,’ said Svetlana Janson, Candidate of Geology and Mineralogy, Deputy Director of the Centre for Microscopy and Microanalysis at the St Petersburg University Research Park and co-author of the paper.
Svetlana Janson also mentioned another blank spot in the history of the Thunder Stone. The rock fragments that were separated from the boulder during its transformation into a statue’s pedestal have not been tracked yet. Honorary Professor of St Petersburg University Andrei Bulakh made numerous attempts to find them, but to no avail. ‘These fragments could facilitate further research, including accurate determination of the Thunder Stone’s age,’ concluded Svetlana Janson.
We should not fail to mention that 2022 is the year of the 350th birth anniversary of the Russian Emperor Peter the Great. As part of the celebrations, St Petersburg University holds an all-Russian creative works contest ‘Peter the Great’ for students enrolled in higher education programmes in art. Young painters, designers, sculptors and art historians are invited to take part in the competition. The contest entries are to reflect the themes of the contest: accomplishments of Peter the Great, his associates and followers; preservation of historical heritage; and honouring the traditions of the Petrine era in our time. For more information about the contest, please, visit the project webpage.