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