Pedogenic manganese oxide coatings (calcium buserite) on fracture surfaces in Tortonian (Upper Miocene) red mudstones, southern Turkey

• The paper investigates the origin of black coatings on fracture surfaces in mudstones.
• The coatings are defined by using XRD, SEM–EDX, DTA–TG, and IR analyses.
• The coatings consist predominantly of Ca-buserite and organic matter.
• The results suggest a pedogenic origin for the coatings.

Fracture surfaces of red mudstones (overbank deposits within the Kuzgun Formation of Late Miocene-Tortonian — age) are coated by thin black colored manganese oxide beneath a thin soil cover in the Mersin area of southern Turkey. These manganese oxide coatings have been investigated by a range of analytical techniques, including X-ray diffraction (XRD), infrared-spectral (IR), differential thermal analysis–thermal gravimetric (DTA–TG), scanning electron microscopy and energy-dispersive X-ray (SEM–EDX) analyses. The basal reflections at 10.00, 7.22, 5.00, 3.35, 2.50, 2.29 and 2.10 Å, sharp infrared spectral lines at 469 and 519 cm− 1 and typical dehydration of interlayer water < 200 °C, together with results from the EDX analysis, indicate the presence of calcium buserite — a manganese oxide mineral. SEM images of buserite crystals show degraded platy pseudo-hexagonal forms suggesting a non-biological origin. Pedogenic processes taking place at the soil material overlying red mudstones, likely induced the reducing or acidic conditions under which Mn2 + was released by the dissolution of Mn-bearing minerals during the wet periods. Organic compounds were also most likely leached during these periods and subsequently, during the dry climatic intervals, manganese oxide minerals precipitated from the infiltrating soil solutions under oxidizing conditions. The nanometer-scale layering in manganese oxide coatings is here attributed to repeated changes in redox conditions. Some trace elements in the coatings, such as Ba, Ni, Co, and Ce have either been absorbed by the Mn oxide minerals or organo-mineral complexes.