HEMATITE IN THE SHALES OF THE TRIASSIC CULPEPER BASIN OF MARYLAND

A mineralogical examination of the reddish brown (mainly 5YR 4/4) shal es from the Triassic Culpeper Basin of Maryland was performed to deter mine the mineralogic composition of the dominant rock type of this bas in. Results by x-ray diffraction (XRD) indicated that the silicate min eralogy consisted predominantly of quartz and mica, with lesser amount s of chlorite and feldspars. Results by XRD following the 5M NaOH trea tment 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 hematitite as the only iron oxide in the shales. Aluminum substitution of the hematite in the shales ra nged from 0.2 to 6.5 mol % and averaged 2.8 mol %. Mean crystallite di mensions (MCD) using the (110) peak of the hematite averaged 77 nm, wh ich is consistent with MCD values of hematite from other Triassic red beds and terra rossa samples. Because thermodynamics favor the formati on of goethite over hematite, and because hematite forms only rarely r elative to geothite in Maryland soils, this monomineralic iron oxide p arent material provides an opportunity to study Fe (hydr) oxide transf ormations in the soils derived from these hematitic parent materials.