Serpentine Alteration by Hydrothermal Processes

Deposits of massive cryptocrystalline magnesite occur in serpentized ultra-basic rock that has undergone a hydrothermal leaching of magnesium from the serpentine see reaction (2.5). The hydrothermal solution 

The magnesium carbonate is deposited in veins and the silica carried away in solution. 

Magnesite may also form through the serpentization of ultrabasic rock, under low-temperature and low-pressure conditions, which favor the exosolution of hydrothermal cryptocrystalline magnesite. Reaction (2.6) shows serpentization of olivine, (Mg, Fe)2Si04, to form antigorite, H4(Mg, Fe)2Si209, and magnesite  

The magnesite occurs as massive bodies, lenticular (lens-shaped) masses, and veins of varying thickness. 

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