Four types of shungite rocks containing 98.2 wt %, 22.2 wt %, 21.6 wt % and 22.4 wt % C and 416, 311, 78 and 182 ppm V were studied, while the ash from
these samples contained 23120, 400, 100 and 234 ppm V, respectively. The
presence of two vanadium carbides, V
In the past few decades, special attention has been paid to transition metals, in particular vanadium, which belongs to the group of critical elements and is widely used in metallurgy for the production of high-strength steels, in the chemical industry as a catalyst and in materials science for the production of special ceramics and materials for nuclear reactors. However, vanadium and its compounds are also toxic substances (Gustafsson, 2019). Vanadium is extracted only as a byproduct as no independent deposits are known; it is also characterized by having a traceable relationship with organic matter (Parnell, 1988; Bielowicz, 2020). Crude oils and bitumens generally contain up to 2000 ppm V in the form of metalloporphyrin and non-porphyrin species (Filby, 1994). Vanadium metalloporphyrins are assumed to be formed during the early diagenesis of parent rocks and are associated with the sedimentary environment, whereas vanadium non-porphyrin complexes are due to the release of metal complexes from kerogen during catagenesis (Filby, 1994). However, for western Kentucky (USA) coals, it has been shown that the bulk V is a product of volcanic activity, formed mainly as a result of geochemical alterations in volcanic ash (Premovič et al., 1997). In the light of the problem discussed, black Precambrian rocks (shungite rocks) occurring in Karelia, northwestern Russia, are of particular interest (Buseck et al., 1997) for their high V concentrations of up to 3000 ppm (Golubev and Galdobina, 1985), indicating the “geochemical vanadium profile” of shungite rocks (Marakushev and Marakushev, 2008). The main conclusions on the concentrations and form of V in organic matter are generally based on geochemical data and mainly on the results of chemical analysis showing the average content of elements in the rock. To date, no mineralogical studies have actually been carried out on the distinctive accumulation pattern and forms of vanadium in carbon-bearing rocks.
The goal of this contribution is to better understand the characteristics of vanadium-containing mineralization in shungite as an example of organic matter, to study the sources of vanadium and to assess the possible influence of vanadium mineralization on the properties of shungite rocks.
Shungite (Sh1), carbon-rich rock with a carbon content of about 98 wt % from the Shunga locality (Buseck et al., 1997), was used to understand the morphology and composition of micro-sized vanadium-containing minerals. To assess the possible effect of vanadium-bearing mineralization on the properties of shungite rocks, a shungite rock (ShR1) of the second stratigraphic level was selected (Deines et al., 2020), in which micro-sized minerals encapsulated in a carbon shell were previously revealed. This rock consists of rock-forming minerals, such as quartz, albite and chlorite, and contains 22.2 wt % carbon and an elevated vanadium concentration. For comparison with this sample, shungite rocks (ShR2 and ShR3) with a similar content of carbon and rock-forming minerals but with no particles encapsulated in a carbon shell were selected. The amount of carbon in the rock was chosen as a major comparison criterion since it was planned to compare the presence of particles encapsulated in the carbon shell with the electro-physical properties of the shungite rock.
A VEGA II LSH scanning electron microscope (TESCAN, Brno, Czech Republic) equipped with an energy dispersive spectrometry (EDS) Energy 350 system and an SDD X-Act3 detector (Oxford Inca Energy, Oxford, UK) was used to study the sample surface, to perform electron microprobe analysis and to visualize element distribution maps. A JEOL 2000FX electron microscope, operating at 200 kV, was used for high-resolution transmission electron microscopy (HRTEM), selected-area electron diffraction (SAED) and EDS measurements. An EM-125 transmission electron microscope, operating at 100 kV, was used for the routine study of shungite rocks. Powdered samples for transmission electron microscopy were placed onto lacy carbon films that were deposited on copper grids. SAED patterns were recorded photographically and processed, as reported elsewhere (Kovalevski et al., 2001).
Trace element concentrations were determined by mass spectrometry with inductively coupled plasma (ICP-MS). Element abundances were measured on a Thermo Scientific XSeries 2 ICP-MS quadrupole mass spectrometer (Thermo Scientific, Waltham, MA, USA) following the method of Svetov et al. (2015). The analyses were carried out on several subsamples of 100 mg obtained from the ash of shungite samples that were decomposed as a result of treatment with hydrochloric, nitric and hydrofluoric acids in an open system. The measured concentration values are characterized by low relative standard deviation values; the percentages are lower than 7 % for most elements.
A comparative study of carbon from shungite rocks was carried out using a
Nicolet Almega XR Raman spectrometer with a green laser (532 nm, Nd:YAG).
The spectra were collected at 2 cm
Shungite with a carbon content of about 98 wt % from the greenschist facies (Grew, 1974; Buseck et al., 1997; van Zuilen et al., 2012) is an extreme member in bitumen transformation according to the Van Krevelen diagram (Cornelius, 1987). Carbonaceous matter is a major component of shungite and shungite rocks, but mineral components affect the many properties of rocks and carry genetic information related to the interaction of carbonaceous matter and mineral components (van Zuilen et al., 2012). Shungite is enriched in a variety of trace elements (Golubev and Galdobina, 1985; Ketris and Yudovich, 2009), including vanadium. ICP-MS analysis of the ash of the selected samples revealed a significant content of vanadium in the selected samples (Table 1). The highest content of vanadium was determined in shungite Sh1 and shungite rock ShR1.
The content of some trace elements in the ash of shungite (Sh1) and shungite rock (ShR1, ShR2, ShR3) samples, determined by mass spectrometric analysis with inductively coupled plasma (relative standard deviation values are lower than 7 %).
The study of shungite Sh1 using scanning electron microscopy (SEM) with an energy dispersive spectrometry (EDS) in the element distribution map visualization mode revealed a significant number of V-bearing micro-dimensional inclusions in the carbon matrix (Fig. 1a). The V-bearing inclusions present in shungite are not sulfides, unlike, for example, Zn sulfides shown in Fig. 1b for comparison. It should be noted here that the form of sulfides in the form of micro-spherulites is unusual for inclusions in carbonaceous matter (Rogerson et al., 2017; Baumgartner et al., 2020).
Element distribution maps of the Sh1 regions with V- and Zn-bearing micrometer-sized inclusions.
Further investigation of numerous shungite samples using SEM and EDS
revealed some features of vanadium mineralization directly related to the
carbonaceous substance. Mineral inclusions containing significant vanadium
measure from about 0.1
Scanning electron microscopic images in backscattered electrons of
various types of vanadium-bearing
EDS composition (in atomic %) of various types of vanadium-bearing minerals in shungite.
Type II minerals are aggregates with irregular outlines (Fig. 2b). Their composition could not be determined due to their small size. Total EDS microanalysis of this mineral association revealed the prevalence of carbon and vanadium in the presence of oxygen, as well as traces of aluminum, silicon, sulfur, iron, nickel and arsenic. It was impossible to carry out a separate EDS analysis of the minerals of these associations, and therefore the extreme compositions of these mineral associations are given in Table 2. In this case, the oxygen concentration is too low for the formation of oxides of the elements present in the total spectrum. Associations of clearly separate grains were assigned to Type III of vanadium-containing minerals (Fig. 2c), for which it was possible to conduct a more or less representative EDS analysis (Table 2), as well as to draw conclusions concerning the paragenesis of vanadium carbide and vanadium-containing mica, roscoelite. For this type of vanadium-containing grains, extreme compositions are not given; instead the compositions in one example of this paragenesis are tabulated (Table 2).
With an increase in the size of the vanadium-rich inclusions found during
the EDS study (Type II, for example; Fig. 2b), the contribution from the
carbon matrix decreases (Table 2, mineral association 2), and the
approximate composition of the inclusions can be better constrained; for
example, these inclusions are more likely V
The study of shungite using transmission analytical electron microscopy
equipped with an EDS spectrometer revealed about a dozen individual
vanadium-bearing oxygen-free minerals that were either rounded or had faint
traces of crystallographic faceting. All of those are single crystals as can
be seen from SAED patterns (as, for example, in Fig. 3a, b). All the SAED
patterns obtained were analyzed according to the International Centre for Diffraction Data (ICDD) database of synthetic
vanadium carbides and based on the position of diffraction maxima and
calculated angles between them. The main part of them were unambiguously
interpreted as SAED patterns of vanadium carbide V
TEM images and corresponding SAED patterns, as well as the EDS spectra of
single crystals of compositionally variable vanadium carbides in Sh1
samples:
The most important feature of the vanadium carbides found in shungite is that they are surrounded (encapsulated) by a carbon film (indicated by arrows in Fig. 3), in which the degree of ordering of the carbon layers is greater than in the carbon matrix. Nonetheless, the individual carbon layers in the film are distinguished with difficulty due to the small size of the film compared to the thickness of the material being investigated.
In addition to micro-sized single crystals of vanadium carbide, parageneses vanadium carbide and roscoelite were also found in shungite (Fig. 4). Between vanadium carbide and mica mineral, roscoelite was revealed in three nano-sized regions with an increased vanadium content (Fig. 4, arrowed). The dark regions (indicated by an arrow) with greater electron scattering in the absence of diffraction contrast correspond to a substance with a higher atomic weight against carbon. These regions extend from the vanadium carbide to a new formation, which is probably due to the diffusion transfer of vanadium to the site of roscoelite formation. Therefore, roscoelite in shungite may be a secondary mineral formed during the decomposition of vanadium carbide.
Paragenesis of vanadium carbide and roscoelite with corresponding EDS spectra in Sh1 samples. Three nano-sized regions with an increased vanadium content are indicated by an arrow.
Roscoelite is associated with a special feature, namely individual fragments of a carbon film revealed at its surface (Fig. 5). Similar films of partially graphitized carbon were encountered at the surface of chlorite and quartz, which are believed to have triggered localized partial graphitization of sedimentary organic material during metamorphism in shungite rocks (van Zuilen et al., 2012). In our case, only a fragment of a carbon film is observed, which cannot be called a graphite film due to both the absence of three-dimensional reflections of graphite on the SAED patterns and a greater interlayer distance with respect to graphite at 0.35 to 0.36 nm.
Fragment of an partially ordered carbon film at the surface of roscoelite and its corresponding SAED pattern (ICDD – 00-010-0496) in Sh1 samples.
Based on the described features of vanadium-containing mineralization in shungite and its possible influence on the electro-physical properties, a comparative study of shungite rocks selected from different levels of the Zaonezhskaya structure was carried out (Deines et al., 2020). One of the samples (ShR1, Table 1) was purposefully taken as a rock in which previously routine transmission electron microscopic (TEM) studies revealed the presence of a significant number of micro- and nano-scale inclusions encapsulated by carbon shells (Fig. 6). Samples ShR2 and ShR3 with the same carbon content, which primarily determines the electrical conductivity and shielding effectiveness, were selected for comparative analysis. Since the electro-physical properties are also significantly affected by the ordering of carbon, Raman spectroscopy was used to estimate it (Deines et al., 2020).
TEM images of micro-and nano-scale particles encapsulated by carbon shells (arrows) that were found in ShR1.
The Raman spectra for all selected samples are consistent with poorly
ordered carbonaceous material (Beyssac et al., 2002, 2003;
van Zuilen et al., 2012). The first-order region of the shungite Raman
spectra shows two bands: D1 at about 1350 cm
To assess vanadium distribution in shungite rocks, element distribution maps were compiled using scanning electron microscopy and EDS (Fig. 7). The elements of greatest interest are (1) carbon, which shows the distribution of the most abundant constituent of shungite, (2) oxygen, which is mainly associated with mineral components, (3) vanadium, and (4) chromium. The zirconium distribution is presented as a control for noise estimation. From the distribution of carbon and oxygen, it can be seen (Fig. 7) that a shungite rock is a combination of interpenetrating carbon and mineral components. Carbon does not form separate regions when its concentration of 22 wt % provides good electrical conductivity of the rock.
Element distribution maps of the region shown in the backscattered electron (BSE) image of ShR1.
The distribution of all chemical elements is associated with rock-forming
and accessory minerals varying in size from a few micrometers to less than 1
One of the most essential electro-physical properties is the shielding effectiveness that is currently relevant for the search for and creation of materials used for reducing the influence of anthropogenic electromagnetic background on electronics and biological objects. The shielding effectiveness was determined in a wide frequency range for selected shungite rocks (Fig. 8). The data obtained showed no correlation between carbon ordering and shielding effectiveness for samples at the same carbon concentration. However, shielding effectiveness is highest for sample ShR1 with elevated vanadium concentration and the presence of particles encapsulated in carbon shells. In this regard, it has also been shown that vanadium carbides enclosed in a carbon shell have impressive electrochemical characteristics and clear prospects for practical application as the most stable materials for batteries (Mahajan et al., 2020).
Dependence of the shielding effectiveness (SE) of shungite rocks on the electromagnetic field frequency (in MHz). SE is in decibels (dB), a relative unit of measurement that expresses the ratio of two values of a power quantity on a logarithmic scale.
Shungite-bearing deposits have attracted considerable interest because of
their complex (volcanogenic sedimentary) conditions of formation, huge
reserves, a unique structure of carbonaceous matter and multi-directional
practical application (Grew, 1974; Buseck et al., 1997; Melezhik et al.,
1999; van Zuilen et al., 2012; Strauss et al., 2013; Deines et al., 2020).
Shungite rocks form a large, diverse group of black Lower Proterozoic rocks,
2.0–2.1 Ga in age, all of which contain free carbon. The formation of
shungite rocks was associated with synchronous magmatic activity in addition
to providing metasomatic fluids during rifting and associated magmatism
(Buseck et al., 1997). The shungite rocks were deformed and underwent
greenschist-facies metamorphism during the 1.8 Ga Svecofennian orogeny. The
paragenesis chlorite–actinolite–epidote reflects a temperature of
300–350
The difference in the forms of vanadium carbide in shungite is difficult to
explain, assuming that they were formed simultaneously under the same
conditions, for example, from vanadium-containing metalloporphyrins and/or
non-porphyrins. According to the two more recent phase diagrams calculated
for the V–C binary system (Hu et al., 2006; Okamoto, 2010), V
The presence of micro-sized particles of vanadium carbides encapsulated in the carbon shell in shungite is very unexpected for a rock formed under greenschist-facies conditions and directly raises the question of determining the source paths of vanadium. If we assume that vanadium was derived from organo-vanadium complexes of organic matter, then we need to identify a mechanism explaining how vanadium was sufficiently concentrated to crystallize into micro-sized particles of V carbide. One possible mechanism is suggested by the processes involved in concentrating and encapsulating V carbide in a carbon shell during mechanical and chemical synthesis in a planetary mill (Mahajan et al., 2020). In the case of shungite, it is unlikely that during the migration of the parent carbonaceous matter, conditions of mechanochemical synthesis, comparable to vigorous processes in a planetary mill, could be achieved. In addition, the morphostructures of carbon differ in shungite; the formation of an ordered layer occurs only on the surface of the particles, whereas during mechanochemical synthesis, carbon consists entirely of such more ordered layers. Due to this difference we cannot regard the above mechanism as the cause of the formation of vanadium carbides in shungite.
Given that the nano-sized grains of V
It is noted in the literature that carbon in the form of closed shells is
one of the most chemically resistant substances in nature, even at high
temperatures (Saito, 1995). At the same time, the carbon network of atoms is
impervious to most light and other atoms, which prevents the diffusion of
atoms both from and into a carbon shell. Thus, carbon-coated materials (or
encapsulated materials) are reliably isolated from the external environment
and protected from diffusion changes. In particular, when studying
reactively active crystals of lanthanum carbide (LaC
This mechanism involves the introduction of high-temperature particles with
a high content of vanadium into the parent carbonaceous substance. In this
regard, according to some authors, the source of vanadium in many settings
is associated with volcanic events (Nahar, 2017; White, 2010). In these
events, the behavior of related redox-sensitive elements such as Fe, V and
Cr is controlled by redox conditions during magmatic differentiation
(Nicklas et al., 2016). Recently, in the course of experiments on the
segregation of metals in a basalt melt during its interaction with hydrogen,
the formation of initially small (several micrometers) spheres of metal,
which then coalesce (merge) during the experiment, has been established
(Persikov et al., 2020). In this case, metal segregations in the basalt melt
are formed at a temperature significantly lower than the melting point of
the metal phase and correspond to real magmatic temperatures in nature (
Four types of shungite rocks containing 98.2 wt % C, 22.2 wt % C, 21.6 wt % C and 22.4 wt % C
and 416, 311, 78 and 182 ppm V were studied. The presence of two vanadium
carbides, V
All data derived from this research are presented in the enclosed tables and figures in the main text and the Supplement.
The supplement related to this article is available online at:
VVK developed the idea, conducted TEM and HRTEM experiments, performed data analysis, and supervised the preparation of the paper. IAM conducted EM experiments and subjected the samples to SEM and electron microprobe analysis. Both authors contributed equally to the discussion.
The contact author has declared that neither they nor their co-author has any competing interests.
Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
The authors thank Peter R. Buseck for the possibility to use high-resolution electron microscopy at ASU, Arizona, USA. We are grateful to Edward S. Grew for the discussion and the assumption that nanocrystals of vanadium carbides maybe sublimate, as well as for comments, recommendations and edits in English that helped to improve this paper. We also thank Ferraris Giovanni for his valuable comments and edits. The authors also thank Gregory N. Sokolov for correcting the English manuscript. TEM, SEM and Raman spectroscopy studies were performed at the Analytical Center, Karelian Research Center.
The work was funded from the Federal budget for the accomplishment of KarRC RAS (IG KarRC RAS) Research Project AAAA-A18-118020690238-0).
This paper was edited by Cristiano Ferraris and reviewed by Edward Grew and Ferraris Giovanni.