3.1.5. Sample OM50

This sample exhibits a characteristic heteroblastic fabric ("mortar-type", Figure 15a). Calcite is the predominant mineralogical constituent. Grain boundaries are usually sutured, but in rare cases, straight-sided grain boundaries, leading to the formation of triple point junctions (120◦) were observed (Figure 15b), perhaps suggesting local equilibrium conditions. Deformation is also traced in this sample, as xenomorphic crystals of calcite in the groundmass often exhibit polysynthetic twinning. Maximum grain sizes in this sample reach up to 1.7 mm. Anhedral apatite and round-shaped pyrite were identified as main accessory phases, while mica group minerals are quite rare. Apatite forms anhedral to subhedral rains, and its size does not exceed 100 μm (Figure 15c). Pyrite forms, fine-grained (up to 20 μm) rounded crystals, scattered in calcite.

**Figure 15.** Transmitted (**a**–**c**) and reflected (**d**) light microphotographs of sample OM50: (**a**) heteroblastic fabric ("mortar") in calcite grains (Cal) most of which exhibit polysynthetic twinning, crossed-polarized light; (**b**) calcite (Cal) crystals forming a triple-junction and exhibiting slightly sutured grain boundaries, crossed-polarized light; (**c**) anhedral apatite (Ap) crystal included in calcite (Cal), plane-polarized light; (**d**) round-shaped pyrite (Py) crystal included in calcite (Cal).
