Mineralogy of magnesite from the Pčinja River Area-North Macedonia

Magnesite from the Pčinja river area most often occurs in the form of veins of varying thickness and length, with their orientation being determined by the direction of faults and cracks. In certain parts of the terrain, the appearance of multiple subparallel veins is observed, while in other places they cross, branch, or locally disappear and reappear. It appears as microcrystalline mass resembling unglazed porcelain. Its fracture is conchoidal, the hardness is 3½ – 4½, its streak is white and the density is 3.0–3.2 g/cm3. Based on the XRD pattern magnesite is the dominant crystalline phase. The unit cell parameters obtained using the main reflection lines of X-ray diffraction are: a = b = 4.63±0.002 Å, c = 15.02±0.003 Å, V = 279.35 Å3. In addition to magnesite, the presence of serpentine, dolomite and talc has also been determined. The absence of a pronounced basal reflection near 12° indicates that serpentine is not the dominant phase. Dolomite fits into the genesis as an indicator of the early phase of carbonization, and this is genetically very significant. During the initial carbonization of serpentinite, CO₂-rich fluids react with serpentine which also contains Ca (taken from pyroxenes). While Ca is still available in the system, Ca-Mg carbonate is formed dolomite. This occurs at relatively higher temperatures (~350°C). When Ca is depleted from the system (excreted by fluids), reactions continue only with Mg pure magnesite is formed. At the same time, Si released from the serpentine reacts with Mg and water to give talc. This occurs at lower temperatures (~280°C). The dolomite in the examined samples indicates that carbonization has proceeded quite far. Most of it has already been converted to magnesite, and only a small relict amount remains. The serpentine is present but nearly consumed. The Pčinja river area represents an advanced stage of carbonization, where CO₂-rich hydrothermal fluids have converted ~85–90% of the original serpentinite to magnesite, with only isolated relic patches of serpentine surviving. This is entirely consistent with Type I cryptocrystalline magnesite deposits of the Balkan ophiolitic belt.