Iron in sandstones, siltstones and claystones occurs as detrital iron oxide minerals, authigenic iron mineral crystals, iron oxide grain coatings, iron oxide pigment in matrix materials, in clay minerals, and in trace amounts in other minerals. Generally the more oxide present, the higher the proportion of ferric to ferrous iron. Iron oxide content increases as particle size decreases in both red and drab rocks. Hematite is the dominant red iron oxide mineral. Red rocks are bleached by chemical reduction of iron oxide and leaching by bicarbonate solutions. Red color in rocks of the Chugwater is a product of diagenesis.

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