Systematic review of forsterite dissolution rate data

This paper demonstrates a method for systematic analysis of published mineral dissolution rate data using forsterite dissolution as an example. The steps of the method are: (1) identify the data sources, (2) select the data, (3) tabulate the data, (4) analyze the data to produce a model, and (5) report the results. This method allows for a combination of critical selection of data, based on expert knowledge of theoretical expectations and experimental pitfalls, and meta-analysis of the data using statistical methods. Application of this method to all currently available forsterite dissolution rates (0 < pH < 14, and 0 < T < 150 degrees C) normalized to geometric surface area produced the following rate equations: For pH < 5.6 and 0 degrees < T < 150 degrees C, based on 519 data log r(geo) = 6.05(0.22) -0.46(0.02)pH - 3683.0(63.6)1/T (R-2 = 0.88) For pH > 5.6 and 0 degrees < T < 150 degrees C, based on 125 data log r(geo) -4: 07(0.38) -0.256(0.023)pH -3465(139)1/T (R-2-0.92) The R-2 values show that similar to 10% of the variance in r is not explained by variation in 1/T and pH. Although the experimental error for rate measurements should be +/-similar to 30%, the observed error associated with the log r values is similar to 0.5 log units (+/- 300% relative error). The unexplained variance and the large error associated with the reported rates likely arises from the assumption that the rates are directly proportional to the mineral surface area (geometric or BET) when the rate is actually controlled by the concentration and relative reactivity of surface sites, which may be a function of duration of reaction. Related to these surface area terms are other likely sources of error that include composition and preparation of mineral starting material. Similar rate equations were produced from BET surface area normalized rates. Comparison of rate models based on geometric and BET normalized rates offers no support for choosing one normalization method over the other. However, practical considerations support the use of geometric surface area normalization. Comparison of Mg and Si release rates showed that they produced statistically indistinguishable dissolution rates because dissolution was stoichiometric in the experiments over the entire pH range even though the surface concentrations of Mg and Si are known to change with pH. Comparison of rates from experiments with added carbonate, either from CO2 partial pressures greater than atmospheric or added carbonate salts, showed that the existing data set is not sufficient to quantify any effect of dissolved carbonate species on forsterite dissolution rates. (C) 2012 Elsevier Ltd. All rights reserved.