Dolomite (CaMg(CO3)2) is a ubiquitous carbonate mineral. Geologists have been debating exactly how it formed for more than two centuries. Because it's a sedimentary rock, it's clear that it was deposited in vast amounts on prehistoric seabeds.
The 'dolomitization' reaction has been resolved as :
2CaCO3 + Mg2+ → CaMg(CO3)2 + Ca2+
But can only take place in a very limited way in modern-day seawater - so the vast deposits of dolomite cannot currently be explained. The so-called 'Dolomite Problem'.
It's proposed by some research groups that ancient seawater composition was very significantly different to today's. Or, that some biological process was involved (e.g. sulfate-reducing bacteria living in oxygen-free conditions may have precipitated dolomite).
Further reading Wikipedia