Ultramafic Alteration: The Primary Pathway
Most magnesite forms through the carbonation of ultramafic rocks, serpentinite, dunite, and peridotite, rocks rich in magnesium-bearing minerals like olivine and pyroxene. When carbon dioxide-rich groundwater percolates through these rocks, it reacts with the magnesium silicates, dissolving the magnesium and reprecipitating it as magnesium carbonate. This process, called carbonation or listwaenitization, can replace entire rock bodies with magnesite over geological time. The massive white magnesite deposits of Greece, Turkey, and Austria formed this way.
This process is now studied as a potential carbon sequestration strategy. Ultramafic rocks naturally absorb atmospheric CO2 and lock it permanently as magnesite. The same chemistry that creates the crystal in your hand may one day help regulate global carbon levels.
Hydrothermal Crystallization
The finest crystal specimens of magnesite form from hydrothermal fluids in cavities within magnesium-rich host rocks. At Brumado in Bahia, Brazil, magnesium-rich solutions deposited transparent, gem-quality rhombohedral crystals in dolomite and magnesite veins. These crystals can be perfectly clear and facetable -- a form most people never associate with the chalky white magnesite they see in shops. The hydrothermal pathway produces the collector specimens that command museum-level prices.
Sedimentary Evaporite Formation
In arid lake environments with high magnesium content, magnesite precipitates directly from evaporating water. As the lake shrinks, magnesium concentrations increase until magnesite crystallizes on the lakebed. These deposits form horizontal beds of cryptocrystalline magnesite -- the bone-white, porcelain-textured material most commonly found in the crystal market. The grey veining that gives magnesite its characteristic web-like pattern comes from later infiltration of iron-rich or silica-rich fluids along cracks in the deposit.
Mineralogical data: Klein, C. & Dutrow, B. (2007). Manual of Mineral Science. Wiley. | Pohl, W. (1990). Genesis of magnesite deposits. Mineralogy and Petrology, 42, 83-105.