HEMATITE IN THE SHALES OF THE TRIASSIC CULPEPER BASIN OF MARYLAND

Abstract

A mineralogical examination of the reddish brown (mainly 5YR 4/4) shales from the Triassic Culpeper Basin of Maryland was performed to determine the mineralogic composition of the dominant rock type of this basin. Results by x-ray diffraction (XRD) indicated that the silicate mineralogy consisted predominantly of quartz and mica, with lesser amounts of chlorite and feldspars. Results by XRD following the 5M NaOH treatment showed hematite was the only iron oxide identified in the shales. The low ratio of oxalate-extractable to dithionite-extractable iron is consistent with the occurrence of hematite as the only iron oxide in the shales. Aluminum substitution of the hematite in the shales ranged from 0.2 to 6.5 mol % and averaged 2.8 mol %. Mean crystallite dimensions (MCD) using the (110) peak of the hematite averaged 77 nm, which is consistent with MCD values of hematite from other Triassic red beds and terra rossa samples. Because thermodynamics favor the formation of goethite over hematite, and because hematite forms only rarely relative to goethite in Maryland soils, this monomineralic iron oxide parent material provides an opportunity to study Fe (hydr) oxide transformations in the soils derived from these hematitic parent materials