3.1.4. Sample OM49

This sample is characterized by heteroblastic fabric (Figure 14a) and the presence of a fine-grained band, with a maximum thickness of about 500 μm (Figure 14b). This band, which is also remarked as a grey-colored stripe in macroscopic scale, is composed of anhedral calcite with maximum MGS up to 0.6 mm. These MGS values are much smaller compared to the MGS values of calcite grains in the rest of the rock, where they reach up to almost 2 mm. Bent twinning lines are common, along the band and the rest of the rock as well. Grain boundaries inside the band are commonly embayed, while in the rest of the sample, the most common grain boundary shape is sutured, although embayments were observed here as well. Minor mineralogical components like apatite and micas (phlogopite and/or muscovite) were identified (Figure 14c), with the latter being commonly found in the band, oriented parallel to its major dimension. Rarely, round-shaped pyrite was observed included in calcite (Figure 14d).

**Figure 14.** Transmitted (**a**–**c**) and reflected (**d**) light microphotographs of sample OM49: (**a**) heteroblastic fabric ("mortar") in calcite grains (Cal) most of which exhibit polysynthetic twinning, crossed-polarized light; (**b**) fine-grained calcite (Cal) crystals composing a band, which is grey-colored in macroscopic scale, crossed-polarized light; (**c**) anhedral apatite (Ap) crystal included in calcite (Cal), crossed-polarized light; (**d**) round-shaped pyrite (Py) crystals included in calcite (Cal).
