Pressure effect on magnesian calcite coating calcite and synthetic magnesian calcite seeds added to seawater in a closed system

Abstract

When pure crystalline calcite seeds are added to supersaturated seawater, precipitation results in a coating which with time equilibrates at atmospheric pressure with seawater and corresponds to a calcite containing probably only 2 or 3% of MgCO3 (mole fraction).If synthetic crystalline magnesian calcite is added, the surface layer equilibrates not only with respect to seawater but also in relation with the crystalline sites initiating precipitation. Adding Mg0.03Ca0.97CO3 results in a coating with a solubility close to that of calcite. This confirms that the surface coating on pure calcite seeds contains about 2 or 3% MgCO3 (K'sp = 10−6.30).The surface layer precipitated on a synthetic Mg0.08Ca0.92CO3 equilibrates finally with a carbonate more soluble than calcite (K'sp = 10−5.94) corresponding to the seeds composition.Experiments at 1000 kg cm t-2 imply that when magnesian calcites are precipitated at the surface of calcite or magnesian calcite seeds, the precipitate must be hydrated, otherwise pressure accelerated recrystallization or rearrangement with loss of Mg would thermodynamically be impossible.By changing the pressure of a seawater sample originally saturated with a solid carbonate phase, changes in pH result from the effect of pressure on the dissociation constants of carbonic acid and boric acid causing either undersaturation or supersaturation with respect to the solid. By changing pressure we can show whether precipitation, dissolution and recrystallization are reversible processes if pH is taken as criteria of reversibility.