Direct observation of reactant-product interfaces formed in natural weathering of exsolved, defective amphibole to smectite: Evidence for episodic, isovolumetric reactions involving structural inheritance
High-resolution transmission electron microscopy (HRTEM) and energy-dispersive X-ray microanalysis (EDS) were employed to characterize grain boundary structures in naturally weathered amphibole. Our observations provide insights into the submicroscopic mineralogic controls on transport of solutions to and from reaction sites. Finely exsolved amphibole (anthophyllite/gedrite) in outcropping, slightly weathered gedrite gneiss transforms isovolumetrically to smectite. Stacking faults and exsolution lamellar boundaries focus weathering reactions, whereas chain width defects decrease the susceptibility of the surrounding amphibole to alteration. Relatively Al-poor anthophyllite lamellae alter slightly more readily than those of Al-rich gedrite. Large quantites of Mg, Fe, Si, and Al are removed from reaction sites. However, smectite compositions directly reflect <0.1 μm-scale variations in Al:Mg:Si of coexisting anthophyllite and gedrite, supporting a transformation mechanism requiring very limited redistribution of elements incorporated into clay products. Topotactic relationships between products and reactants and interface structures suggest that smectite formation requires only partial depolymerization of amphibole structural units. Up to one-third of the amphibole I-beams may be directly inherited by smectite. Grain boundary structures within smectite mimic current amphibole-smectite interfaces and may represent previous reaction fronts. Such interfaces may be the result of episodic reaction, possibly attributable to the balance between rates of consumption of water by reactions and resupply by dewatering of larger fractures (which are themselves episodically supplied by rainfall). In this coherent, slightly weathered rock, transport of solution to and from reaction sites is restricted to diffusion along semi-coherent, subnanometer-wide grain boundaries and smectite interlayers. This contrasts with weathering grains within a soil, more deeply weathered rock, or laboratory mineral dissolution experiments, where a much larger volume of solution is in contact with weathering surfaces. We suggest that although transformation rates are limited by depolymerization and repolymerization at the amphibole-smectite interface, they may be modulated by episodicity in water supply to reaction sites within minerals.