Scientists from the Frank Laboratory of Neutron Physics, JINR, carried out investigations to gain an insight into the mechanism of deterioration of rapakivi granites, from which the famous architectural monuments of the City on the Neva were created.

 

Weathering of rapakivi granites: how to preserve monuments?

Magnificent St. Petersburg, a granite city! How many poets and writers have glorified its gorgeous embankments, palaces and monuments! But relentless time erodes the famous rapakivi granites, from which many symbols of the city have been created since the end of the eighteenth century. The pedestal of the Bronze Horseman is gradually crumbling, new cracks and fractures appear on the Alexander Column and the vases of the Summer Garden, they become visible on the colonnades of St. Isaac's and Kazan Cathedrals, on granites of embankments, bridges and plinths of buildings.
On the one hand, rapakivi granite is a very beautiful and easily processed building material, which is quarried in the giant Vyborg massif (such granite is called vyborgite). On the other hand, the structure and mineral composition of the rock determine its special properties, so it is not for nothing that "rapakivi" is translated from Finnish as “rotten, crumbling stone” because of its susceptibility to rapid weathering. Coarse granite consists of ovoid feldspar structures surrounded by a matrix of quartz, mica and plagioclase. Usually pinkish-brown in color, ovoids cement minerals of different shades and colors, which creates a unique amazing pattern of the rock. The degradation of granite is usually explained by the climatic conditions of St. Petersburg with its high degree of precipitation and humidity, and constant changes in internal stresses in the rock due to temperature fluctuations and frost weathering.

Fig. 1. Rapakivi rock specimens: (a) sphere, (b) cylinder.

Regardless of the causes of cracks and fractures (physical, chemical, biogenic or anthropogenic), specialists from different fields of research are trying to determine the deterioration mechanism. This information will make it possible not only to develop methods of granite restoration and preserve the history captured in stone for future generations, but also to select the appropriate material for future construction.

Neutron diffraction and ultrasonic studies

FLNP scientists within the joint international research team studied* the mechanisms of the initial deterioration of rapakivi granite (vyborgite) under repeated freeze-thaw cycles, i.e. in the weather conditions usual for St. Petersburg. The resulting stresses lead to changes in the properties of the rock, the development of microcracks, and, as the scientists have found out, an increase in the anisotropy of the structure and values of porosity and permeability.
To understand how rapakivi granite specimens behave under repeated freeze-thaw cycles, the scientists carried out neutron diffraction and 3D ultrasonic sounding experiments, measured the permeability and porosity of rapakivi granite (dry and saturated with water) at different pressures, and also simulated the elastic properties of the rock.
Neutron diffraction studies of rapakivi granite using the SKAT texture diffractometer at the IBR-2 pulsed reactor in Dubna (FLNP JINR) provided insights into its crystallographic structure. The ultrasonic method made it possible to measure elastic properties and, as a result, to come to the conclusion that at atmospheric pressure, rapakivi granite is expected to be a nearly isotropic rock (as suggested by both its weakly pronounced texture and almost isotropic distribution of elastic velocities at atmospheric pressure).
“We cannot trace the development of cracks in granite during freezing and thawing, since there are no nondestructive methods to do this for a bulky sample. But we can simulate its elastic properties by adding various defects (cracks, as well as their sizes and orientation) to the elastic properties of a polycrystalline sample calculated using data from neutron diffraction measurements,” explains one of the research authors, Tatyana Ivankina.

Fig. 2. Decrease in elastic wave velocities and increase in crack development and anisotropy (four freeze-thaw cycles of water saturated rapakivi granite specimens).

The obtained experimental and simulation data point to the formation of microcracks, a decrease in elastic velocities, and an increase in porosity and elastic anisotropy in the specimens subjected to repeated freeze-thaw cycles. These differences in the elastic properties of the rock can be explained by the development of one of the two detected systems of preferentially oriented microcracks with specific orientation. Further weathering is likely to contribute to their development and coalescence into larger cracks.

Properties and practical significance of granites: looking into the future

The results obtained by the scientists highlight the importance of taking into account the anisotropic properties of rocks when it comes to studying objects of cultural and historical heritage that are sensitive to weathering (for example, rapakivi granites), construction or restoration of granites, especially in cold regions.
Information about rapakivi granites is also significance for geologists, since these rocks are considered promising for the search for signs of tin-tungsten-beryllium-sulfide mineralization. The Vyborg massif might also contain topaz-bearing biotite granites.
Rock deterioration is typical not only for rapakivi granites, but also for many other types of rocks, therefore, the study may be of interest for oil and gas geology. With the depletion of shallow hydrocarbon deposits, more and more attention is paid to the search for hydrocarbons in crystalline basements. Such deposits often occur in the so-called brecciated granites composed of angular cemented small fragments, the description of the properties of which contributes to the understanding of the conditions of occurrence of ore bodies and their prospects
*Ivankina T.I., Zel I.Y., Petruzalek M., Rodkin M.V., Matveev M.A., Lokajicek T. Elastic anisotropy, permeability, and freeze-thaw cycling of rapakivi granite // International Journal of Rock Mechanics and Mining Sciences. 2020. V. 136. Article 104541. DOI:10.1016/j.ijrmms.2020.104541.

Olga Baklitskaya-Kameneva