Experimental studies on Magnesite Genesis - Application to the Magnesite Deposits Associated with Ultramafic Rocks

Previous experimental studies on magnesite formation which are confined to temperatures greater than 250°C, do not explain all the field and petrographic observations in natural situations where magnesite is associated with ultramafic complexes, as at Dodkanya, Mysore district, Karnataka.

A series of experiments were conducted using natural samples (rock and mineral powders) as starting materials. The experimental temperatures were varied from 35° to 300°C at intervals of approximately 50°C and normal and elevated pressures (500 and 1000 bars). At higher pressures, the X_CO2 in the fluid phase was also varied.

These studies have shown that magnesite and quartz can be formed directly by the interaction of CO₂-bearing water vapour and ferromagnesian minerals (olivine, orthopyroxene) without passing through the intermediate serpentine stage. The temperature conditions at which these reactions take place is between 150 and 250 °C at 500 bars total pressure with X_CO2 in the fluid phase greater than 0.015. The presence of Na⁺ salts enhances these reactions.

The presence of calcite-aragonite-quartz-aluminous hydroxide gel-limonite (birbirite) cap rock intersected by a network of magnesite veins in Dodkanya and surrounding areas has been explained on the basis of experiments with natural hornblende, the major Ca²⁺-bearing mineral in hornblende-peridotites. Hornblende, under hydrothermal conditions, breaks down to calcium silicate hydrate (CSH). CSH, being unstable in the presence of CO₂, inverts to CaCO₃ (calcite/aragonite) and quartz. Mg²⁺ in trace quantities stabilises aragonite.