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).
Figures & Tables
AAPG Memoir 109 is designed as a practical guide for students and professionals to learn the fundamentals of microscopic examination of sandstones, mudrocks, and associated rocks. With more than 1100 color illustrations, it covers the identification of grains, textures, and structures of clastic terrigenous rocks as well as their diagenetic alteration (compaction, cementation, dissolution, and replacement) and porosity reduction or enhancement. It also provides classification diagrams for formal description of those rocks and their porosity. Although the majority of the outcrop and subsurface examples come from the United States (35 states and Puerto Rico), there are representative photographs from 32 other countries, including many from the offshore areas. The foldout birefringence chart and an included DVD with Powerpoint files of all of the petrographic images provide additional aids for instructors and students.