Iron oxide mineralogy of placic horizons

SUMMARY The iron oxide mineralogy of 15 placic horizons from Germany, New Zealand, Belgium and U.K. was determined by differential X‐ray diffraction (DXRD). All samples examined contained ferrihydrite and goethite, lepidocrocite occurred in 10, and six contained non‐DCB‐reducible, lithogenic hematite. Ferrihydrite was most readily identified in ‘untreated minus oxalate‐treated’ DXRD patterns. Al‐substitution of goethites ranged from 4 to 13 mole %. Mean crystallite dimensions varied from 11 to 21 nm and from 6 to 21 nm for the goethite (110) and (111) lines respectively, from 7 to 51 nm for the lepidocrocite (020) line (lath thickness), and from 55 to 134 nm for the hematite (104) line. These values are typical for goethites from carbon‐rich environments with poor crystallization conditions, and for well crystalline lithogenic hematites. The DXRD method is ideally suited to the evaluation of the many selective dissolution techniques used in soil chemistry and mineralogy.