Basaltic (or generically MgO-rich igneous rocks) have olivine phenocrysts commonly replaced by calcite, expecially if found in submarine environments. Of course this is not the only way to transform olivine in a secondary mineral aggregate, but I am asking expressely the olivine-calcite transformation.
My question is: why olivines and less so commonly clinopyroxenes are transformed into calcite?
The classical carbonation process is:
Mg2SiO4 + 2CO2 = 2MgCO3 + SiO2,
or
Mg3Si2O5(OH)4+ 3CO2 = 3MgCO3 + 2SiO2 + 2H2O
but in these cases we have magnesite, not calcite formation.
Hello,
I would also be interested in this. We have many subaquatic picritic rocks (teschenite-picrite association, for example) with olivine phenocrysts (even around 1 cm) completely transformed into a mixture of calcite and quartz. The groundmass is usually at least partially argillized and carbonatized as well. The alteration is almost certainly related to (post-)volcanic hydrothermal transformations, deep burial and also tectonic influence. But I don't know why dolomite or magnesite does not form dominantly. Some dolomites and small magnesites were also found. Both, however, rather in amygdaloid fillings and veins than in pseudomorphoses after olivine.
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