Petrography of Middle-Jurassic carbonate concretions fro the Kardan valley (northern slopes of the Caucasus): Preliminary results

摘要

Carbonate concretions were collected from the black clayshales and mudstones (Bajocian-Bathonian in age) in the Kardan valley (the Caucasus, Russia). Concretions vary in colour from rusty-brown to bright-grey, what suggests differences in mineral composition. Samples were investigated with the use of optical microscope in polarized light, X-ray Powder Diffractometry (XRD) and Scanning Electron Microscope (SEM) coupled with EDS. Three concretions with different macroscopical features were selected for detailed analyses. The XRD and SEM analyses indicate different mineral composition of these concretions: calcite and phosphates; calcite and siderite; carbonate, goethite and phosphates. Phosphates are represented by low-crystalline apatite and collophane.