Diagenesis: Iron Sulfide, Oxide & Hydroxide Cements
2015. "Diagenesis: Iron Sulfide, Oxide & Hydroxide Cements", A Color Guide to the Petrography of Sandstones, Siltstones, Shales and Associated Rocks, Dana S. Ulmer-Scholle, Peter A. Scholle, Juergen Schieber, Robert J. Raine
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The type and abundance of minerals that we observe in the earth’s crust and its sedimentary cover is governed by elemental abundances and thermodynamic mineral equilibria. Iron is fourth in abundance (e.g., Mason, 1966) by weight (∽ 5%) after oxygen, silica, and aluminum (∽82.5% cumulatively), and whereas that relationship readily explains the preponderance of silica and clay minerals as cements in sedimentary rocks, the story for iron is a bit more complicated. Due to its multiple redox states, iron can form (or be part of) minerals in oxidizing as well as reducing environments, and the main “sinks” in the sedimentary rock record are Precambrian banded iron formations, Phanerozoic ironstones and continental red beds. Iron is generally supplied to sedimentary basins in the form of iron hydroxide coatings on fine particles (Carroll, 1958) and as iron silicates in the sand fraction (Walker, 1967). In fluvial sediments, the commonly oxidizing pore water conditions result in intrastratal alteration of detrital iron silicates (pyroxene, hornblende, biotite) and in the precipitation of iron hydroxides (limonite, goethite; yellow-brownish color) within pore spaces. Over time, iron hydroxides in both fine and coarse sediments are converted to hematite that gives the rocks their characteristic red color (Walker, 1967). The timing of hematite formation can, under some circumstance, be dated using paleomagnetic information (e.g., Lu et al., 1994) or geochronologic data (e.g., Reiners et al., 2014).