Method for industrial manufacture of pure MgCO.sub.3 comprising providing an olivine containing species of rock, to comminute the olivine containing species of rock to increase its surface, to contact the comminuted olivine containing species of rock with water and CO.sub.2. The process is conducted in at least two steps, namely a first step (R1) at a first pH where a dissolving reacting as represented the equation: Mg.sub.2SiO.sub.4(S)+4H.sup.+=2Mg.sup.2++SiO.sub.2(aq)+2H.sub.2O, takes place. Then a precipitation takes place in the second step (R2) at a higher pH as represented by the equations: Mg.sup.2++HCO.sub.3.sup.-=MgCO.sub.3(S)+H.sup.+, and Mg.sup.2++CO.sub.3.sup.2-=MgCO.sub.3(S), the presence of HCO.sub.3.sup.- and H.sup.+ ions mainly provided by the reaction between CO.sub.2 and water.