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Magnesite
Magnesite is a magnesium carbonate mineral, MgCO3, belonging to the calcite group and crystallizing in the trigonal system. It typically forms complete solid solution with siderite (FeCO3), with more limited series toward rhodochrosite and calcite, and shows perfect rhombohedral cleavage with a Mohs hardness of about 3.5 to 4.5. While most magnesite occurs as massive, earthy, granular or cryptocrystalline material, well-formed rhombohedral or hexagonal-prismatic crystals are genuinely rare and highly sought by collectors, ranging in color from colorless and white to pale yellow, brown, gray, or lilac-rose.
The single most important locality for crystallized specimens is the Pomba Pit in the Serra das Éguas district near Brumado, Bahia, Brazil — one of the largest magnesite deposits on earth, and the source of extraordinary transparent, gemmy rhombs reaching several centimeters across, often in classic association with green uvite tourmaline. Historic European occurrences remain benchmark localities in the literature: large crystals from Radenthein in Carinthia and Sunk near Trieben in Styria, Austria; Baldissero Canavese in Piedmont, Italy; and Snarum, Norway. In North America, the Mount Brussilof mine near Radium, British Columbia, produces rhombohedral crystals coated with botryoidal hematite, while notable deposits also occur in Nevada, California, and Washington State. China's Liaoning Province hosts immense commercial magnesite reserves, and additional occurrences are documented in Austria, Greece, India, and Australia, reflecting magnesite's typical genesis as a hydrothermal or metasomatic replacement of dolomite and serpentinized ultramafic rock.
Beyond its appeal to systematic mineral collectors, massive white cryptocrystalline magnesite is widely cut into cabochons, beads, and tumbled stones, and its porous, dye-receptive nature has made it a common — if controversial — substitute for turquoise and howlite in the jewelry trade. Facetable, gem-quality transparent rough is exceptionally rare and comes almost exclusively from the Brumado district, making such pieces prized rarities. Industrially, magnesite is the principal ore of magnesium oxide (periclase), calcined for use in refractory linings for furnaces and kilns. For the specimen collector, however, it is the sharp, lustrous, water-clear rhombs from Brazil and the classic Alpine crystal finds from Austria and Italy that define magnesite's standing as a distinctive and technically demanding carbonate species.
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