Chapter 4 Surface Textures and Dissolution Processes of Heavy Minerals in the Sedimentary Cycle: Examples from Pyroxenes and Amphiboles

Abstract

Abstract Natural weathering of pyroxenes and amphiboles corrodes them in a crystallographically controlled manner, resulting in ubiquitous etch pits on grain surfaces and eventually denticulated margins. Most denticles originate at fractures, but others form without fractures, along arrays of laterally adjacent dislocations. The range of corrosion morphologies is identical between pyroxenes and amphiboles. Weathered pyroxenes and amphiboles at numerous locations and in a wide variety of weathering environments exhibit the same types and ranges of weathering morphologies across a wide range of mineral compositions, regolith types and pedogenic/geochemical environments. Similar ranges of dissolution textures also occur on pyriboles (a short single term for single- and double-chain silicates) in modern sediments and on intrastratally dissolved pyriboles in clastic sedimentary rocks. Mineral grains are exposed to varying physical and chemical conditions as they move through the sedimentary cycle. Grain surfaces respond to these changing conditions; thus, surface textures on detrital heavy-mineral grains are used to infer weathering processes in soils and weathered regoliths, provenance and sedimentary environments, and intrastratal dissolution during burial diagenesis. The ability to arrive at useful interpretations from sand grain morphology and/or grain-surface textures is based on specific relationships (which differ among different heavy minerals) between rates of surface-texture modification in a given environment and the length of time that the grain is exposed to modifying processes in that part of the sedimentary cycle. For chemically produced features, the timescale for the development of new grain-surface textures depends on the geochemical kinetics of the chemical texture-modifying reactions, which are in turn related to the persistence of each mineral in weathering and the sedimentary cycle. Pyroxenes and amphiboles are minerals with intermediate rates of dissolution and intermediate persistence among heavy minerals; they corrode gradually and survive long enough for the degree of corrosion to be a scientifically useful soil relative-age and environmental indicator. The significance of etch pits and other surface textures on heavy minerals in a soil or regolith depends on the rates at which they form in that regolith, and how long the grains have been subject to such reactions (i.e., the age of the soil) in the weathering environment. Similar relationships are to be expected in other compartments of the sedimentary cycle.