How to convert wt% MgCO3 to mole% MgCO3

I have geochemistry data for limestone samples consists of oxides and REE. I want to find out the mole% MgCO3 for the samples. I understand that I can use conversion factor of 3.4683 to convert MgO wt% to MgCO3 wt%. But from there I don't know how to proceed. Do I need to convert all of my data to moles first and proceed to calculate the mole%. And if all do I need to convert both oxides and REE or just the oxides

Would be really grateful for the help