Grain Size

The individual microbodies are polydispersed in the rock microstructure so that the size of the microbodies has to be approximated with a size distribution function. The size distribution function can be frequently approximated by a logarithmic normal distribution (LND), which is uniquely characterized by a median d50.3 and a scatter parameter σln [8].

#### Grain Shape

The shape of a microbody corresponds to its outer appearance, which is determined by its grain shape and roughness. The grain shape is a geometric particle characteristic, i.e., a characteristic that makes allowance for all three dimensions [40]. The grain shape of a microbody can be regarded approximately as an ellipsoid with the main axes a, b and c, where a ≥ b ≥ c. The relationship of the main axes to each other describe the grain shape of the microbody. The elongation E describes the "needle shape" of a particle and the flatness F of the particle its "platy shape".

#### Roughness

The shape of severely curved, serrated and similar complex microbodies can be analysed statistically and characterized by the roughness. This parameter can be recorded statistically in thin sections and characterized with indices. The roughness KR is defined as the ratio of the difference between the "real" surface area SV(R) and the "ideal" surface area SV(I) relative to the "real" surface area SV(R) of the individual phases. The calculation of the "ideal" surface area SV(I) is based on the microbody size distribution with consideration of the microbody shape and the phase volume percentage. The "real" surface area SV(R) is calculated from the three-dimensional rose of intersections. The outer shape of the microbodies largely determines the character of the microstructure [8].

#### Orientation

From three 2D-roses of intersections orthogonal to each other, with an approximation, the parameters of a spatial rose of intersections and their orientation angle in the space are calculated. A spatial rose of intersections of an oriented microstructure is the superposition of the roses of intersection of ideal boundary surface systems. From the parameters of the spatial rose of intersections, the orientation degrees (Kis, Klin and Kfl) of a spatial arrangement (percentages of linear- and areal-oriented as well as nonoriented boundary surfaces) can be derived.

#### Distribution

Microbodies of a phase can be evenly distributed in the space, but they can also form clusters in which the microbodies of a phase share boundary surfaces. The degree of clustering C is defined as the quotient of the sum of the boundary surfaces between the grains of the same mineral group and the total boundary surface of this mineral group. The degree of clustering can be calculated with the help of linear analysis [8].

#### Space Filling

Rocks are practically never completely compact, i.e., the aggregate space is not completely filled by solid constituents, but there are cavities (pores) between them, which normally contain gas (e.g., air) and/or aqueous solutions. The space filling degree εVF can vary within wide boundaries for different rocks. The size, distribution and type of pores also show great differences. To assess many properties of the rocks, it is necessary to examine the character of the space filling more closely.

#### *2.2. X-ray Computed Tomography (CT)*

CT evolved as an effective supplement in the complex studies of petrophysical properties of rocks. It includes the definition of porosity, fracturing, cavities and their distribution by diameter, volume, and sphericity degree. Moreover, microtomography is a useful tool for the measurement of mineral phases with equal X-ray density throughout the whole sample. The main advantages of the method include also the wide visualization potentials with comparably low measurement time.

Principles of CT are described in detail in numerous papers [27,29,35]. The method consists in essence in weakening the power of the X-ray beam when passing through a certain volume of the material. During scanning in X-ray beam, the sample is rotated around a vertical axis by 180◦ with a certain step. The result of the scan is fixed by the shadow projection (using Al or Brass filters behind the sample and before the detector). Shadow projections are graphic files where each pixel contains information about the amount of X-ray absorption by the object at a given point. This value is expressed in 256 grey color shades (the darker, the higher the absorption). The processing of the obtained results consists in the mathematical transformation of the shadow vertical projections into a series of horizontal sections of the object using special software CTAn (e.g., Version 1.95, Bruker, Kontich, Belgium). After reconstruction, the most important step of the analysis is the volumetric visualization.

During the binarization of images, e.g., with the help of the CTVox software (Version 2.14, Bruker, Kontich, Belgium) it is possible to create three-dimensional images of the sample for further visual studies and morphometric analysis (Figure 2).

In this paper, the CT studies of typical ore/rock samples were carried out by using a microtomograph "SkyScan-1172" (Bruker, Kontich, Belgium) with resolutions from 19.58 to 32.32 μm, equipped with the certified programs Skyscan1172 μCT, NRecon, DataViewer, CTVox, CTAn, CTVol, SkyScan\_MTS, Gidropora. The shooting parameter set for scanning the samples is shown in Table 2.

**Figure 2.** Basic principles of CT: (**a**) scanning of the sample in X-ray beam, (**b**) mathematical transformation of the shadow vertical projections into a series of horizontal sections of the object, (**c**) creation of three-dimensional images of the sample.


**Table 2.** The shooting parameters set for scanning the samples.
