Reaction Kinetics of Primary Rock-forming Minerals under Ambient Conditions

Mineral dissolution kinetics influence such phenomena as development of soil fertility, amelioration of the effects of acid rain, formation of karst, acid mine drainage, transport and sequestration of contaminants, sequestration of carbon dioxide at depth in the earth, ore deposition, and metamorphism. On a global basis, mineral weathering kinetics are also involved in the long-term sink for CO₂ in the atmosphere: (Formula Presented) CaSiO₃ þ CO₂ ¼ CaCO₃ þ SiO₂ ð1Þ MgSiO₃ þ CO₂ ¼ MgCO₃ þ SiO₂ ð2Þ These reactions (Urey, 1952) describe the processes that balance the volcanic and metamorphic CO₂ production to maintain relatively constant levels of atmospheric CO₂ over 10⁵–10⁶ yr timescales. In these equations, Ca- and MgSiO₃ represent all calcium- and magnesium-containing silicates. Calcium- and magnesium-silicates at the Earth’s surface are predominantly plagioclase feldspars, Ca–Mg-pyroxenes, amphiboles, and phyllosilicates, Ca–Mg orthosilicates. Although dissolution of the other main rock–forming mineral class, carbonate minerals, does not draw down CO₂ from the atmosphere over geologic timescales, carbonate dissolution is globally important in controlling river and ground water chemistry. Despite the importance of mineral dissolution, field weathering rates are generally observed to be up to five orders of magnitude slower than laboratory dissolution rates (White, 1995), and the reason for this discrepancy remains a puzzle. For example, mean lifetimes of 1 mm spheres of rock-forming minerals calculated from measured rate data following Lasaga (1984) are much smaller than the mean half-life of sedimentary rocks (600 My, Garrels and Mackenzie, 1971). As pointed out by others (Velbel,....