Mechanisms of Pyroxene Alteration to Smectite: Implications for Inferring Elemental Mobility in Martian

Introduction: Orbiter and lander/rover studies of Mars' surface have identified and provided preliminary characterization of surficial materials including igne-ous and sedimentary rocks, possible volcaniclas-tic/pyroclastic rocks, and unconsolidated sediment. Some surfaces expose unaltered anhydrous silicates (militating against extensive interactions with water), but most results (including studies of Mars meteorites) indicate at least minor aqueous alteration of exposed surface materials. The extent of any chemi-cal/mineralogical alteration, the relative importance of physical and chemical processes, the timing of any chemical alteration that may have occurred, and what information the surficial materials contain about the chemical history of Mars' surface and atmosphere are all matters of continuing research interest. This contribution reviews what can be inferred about mineral-environment interactions from the study of parent-mineral/clay-mineral relationships produced by mineral water interactions, emphasizing low-temperature surface phenomena (weathering). Rock and mineral weathering: During weathering , primary rock-forming minerals react with solutions and/or volatiles. Reactants (minerals and mobile species) are consumed through processes governed by interactions among structure, composition, surface properties, and solute composition (the latter itself influenced by the history of the solution, including its origin and other reactions the solution participated in prior to arriving at the current reaction site) [1-5]. Re-goliths produced by weathering contain residual primary rock-forming minerals (remnant reactants) and secondary minerals (weathering products) [1-5]. In many situations of interest, the solutions left the system long ago, and it is from the surviving solids that the former processes must be inferred [1-5]. Although reactant and product solutions and volatiles may no longer exist, considerable insight into their nature, abundance, and properties can be achieved by examining the reactant and product minerals. The composi-tional and textural relationships among reactants and products record mineral stability and elemental mobility , which in turn result from the thermodynamics and kinetics of the weathering reactions. Even on Earth, where high temperatures and abundant water facilitate relatively rapid kinetics (at least in comparison with present conditions on Mars), the mechanisms of weathering reactions often prevent the attainment of thermodynamic equilibrium. Most naturally weathered