Fig. 6

Composition and texture of displacive layers in carbonate concretions of Xiamaling Formation (Jizhentun section). a Small sized calcite crystals (< 20–100 μm) in a displacive layer; b Calcite crystals with rod-like morphology, with their width and length up to ~ 50 μm and ~ 200 μm, respectively; c A calcite geode, showing that the rod-like minerals have their long axis toward the center of the geode (arrows), and the geode center is filled with green-yellow clay and later diagenetic subhedral to euhedral carbonate minerals; d BSE image of the geode center in panel c, showing that the clay minerals are mainly of chamosite (Ch) with minor illite (Ill); the carbonate minerals have dolomite (Dol) nuclei and ankerites (Ank) rim; with some pyrite (Py) grains; e A calcite geode, showing that the rod-like minerals in the geode have higher length-width ratio and larger size (arrow) than those of surrounding minerals; f BSE image showing a calcite crystal surrounded by clay minerals (arrow); g and h SEM image and the CL image of the same area, showing that some areas display no luminescence, while the others show weak orange luminescence