friable, powdery mineral aggregates consisting mainly of disperse clay particles with metacolloidal oxides and hydroxides of iron (goethite and lepidocrocite, FeOOH; hydrogoethite, FeOOH x nH2O; hematite, Fe2O3; and hydrohematite, Fe2O3. nH2O). They usually contain traces of pulverized quartz and opal, hydrated aluminum oxide, and so on.
According to the mineralogical composition and total Fe2O3 content, a distinction is made between yellow ochers (goethite, lepidocrocite, and hydrogoethite), which contain 15–50 percent Fe2O3, and red ocher and iron miniums (hematite and hydrohematite), which contain 20–80 percent Fe2O3. Iron ochers form soft, sometimes plastic aggregates that are easily ground to powders and become soil readily. Their density varies greatly according to composition (2,000–3,500 kg/m3). Iron ochers are formed by the weathering and surface oxidation of rocks and ore formations that are rich in iron minerals (silicates, carbonates, and sulfides). They are also often associated with the accumulation of iron hydroxides in sediment formation processes (sedimentary marine deposits of iron ores and ochers; ferric lacustrine and marsh sediments). They are formed in the deposition products of mineral springs, and in the process of formation of oxidation zones in pyrite deposits. In industry, after removal of impurities consisting of rock chips and grinding to fine fractions, iron ochers are used extensively as inorganic pigments.
G. P. BARSANOV