Note on magnesite formation (Studies on irreversible geochemical reactions N° 9)

摘要

In recent laboratory experiments magnesite (MgCO3) has been synthesized at a temperature of 313°K (= 40°C). The experiments have demonstrated that irreversible reactions are involved in the low-temperature formation of magnesite. Fundamental to such irreversible reactions is a requirement for fluctuations, i.e., alternations between precipitation and dissolution. But unequivocal evidence for the necessity for fluctuations in order to produce such irreversible geochemical reactions can be demonstrated only by static control experiments. The present note describes several static control experiments on the low-temperature synthesis of magnesite. The first experiment consisted of adding the total amount of ammonia (used in the original experiment in 14 different titration steps) in a single action: only magnesium hydroxide carbonate formed, not magnesite. In the second experiment the possible reaction between magnesium chloride, ammonia and carbon dioxide in solution was studied at 318°K (= 45°C): magnesium hydroxide formed, not magnesite. The third static control experiment involved the reaction between magnesium chloride and ammonium carbamate; this time nesquehonite formed, not magnesite. The implications of these static control experiments in relation to the low-temperature formation of magnesite and dolomite in the sedimentary environment are discussed.