Magnesite growth rates as a function of temperature and saturation state
Geochimica Et Cosmochimica Acta 73(19): 5646-5657
Magnesite growth rates and step velocities have been measured systematically as a function of temperature from 80 to 105 °C and saturation state in 0.1 M NaCl solutions using hydrothermal atomic force microscopy (HAFM). The observations indicate that at these conditions magnesite precipitation is dominated by the coupling of step generation via spiral growth at screw dislocations and step advancement away from these dislocations. As these two processes occur in series the slowest of these dominates precipitation rates. At 100 °C magnesite growth rates (r) determined by HAFM are consistent with r=k([Omega]-1)2, where k is a constant equal to 6.5 × 10-16 mol/cm2/s and [Omega] is the saturation index with respect to magnesite. This equation is consistent with spiral growth step generation controlling magnesite precipitation rates. Corresponding magnesite precipitation rates measured using mixed-flow reactors are shown to be consistent with both the rates measured by HAFM and the spiral growth theory, confirming the rate limiting mechanism. Step advancement, however, is observed to slow far faster than step generation with decreasing temperature; the activation energy for step advancement is 159 kJ/mol whereas step generation rates have an estimated activation energy of ~60 kJ/mol. As such, it seems likely that at ambient temperatures magnesite growth is limited by very slow step advancement rates.