Fig. 2

Authigenic carbonates at Chaotian. a, b Drill core samples of a calcite crystal bed. In a, the lower and upper parts of the carbonate bed are composed of “randomly oriented” and “bundled” calcite crystals, respectively, and the bed is parallel to the overlying and underlying black mudstone beds. In b, “packed” calcite crystals form a wavy bed. c Photomicrograph of randomly oriented monocrystalline calcite crystals (crossed nicols). The horizontal direction corresponds to the bedding. Note that the interference colors of the elongate crystals (circled in red) are consistently high order (typical of calcite), whereas those of circular crystals (circled in yellow) are low order. d Enlarged photomicrograph of a crystal rotated 45° relative to the bedding (crossed nicols). e Normal velocity curve of an optically negative uniaxial calcite. The optical characteristics of monocrystalline crystals in C show that the crystals are elongate along the c-axis because the interference color of calcite is determined by the angle between the c-axis and the cross section. When the angle is low (e.g., the cross section represented by the red dashed line in e), the apparent shape of the crystals in thin section is elongate and the crystals’ interference colors are high order (the crystals circled in red in c). The opposite is true for high-angle cross sections (the yellow dashed line in e and the circular crystals circled in yellow in c). f, g Photomicrographs of the same sample from a calcite bed. The horizontal direction corresponds to the bedding. The crossed polarizer and analyzer in f and g are parallel/perpendicular to and rotated 45° relative to the bedding, respectively. Note the contrast in brightness between the photos. This difference indicates that the crystals are largely perpendicular to the bedding because calcite exhibits straight extinction. h–j Photomicrographs of the bundled calcites. h (opened nicols) and i (crossed nicols) show the same field of view. The horizontal direction corresponds to the bedding in each photo. Some bundled crystals grow from brachiopod shells (arrows) upward and partly downward. k Photomicrograph of packed calcite crystals (opened nicols). Up to 200 μm long calcite crystals are densely packed in the bed