Mineralogy and occurrence of glauconite in central Taiwan

The occurrence of glauconite in Oligocene and Miocene sedimentary rocks remains obscure. This study focused on the mineralogy and occurrence of glauconite at Kuohsing (KH) and Chungliao (CL) villages, Nantou Prefecture in central Taiwan. The glauconite pellets occur mainly in siltstones and mudstones that contain abundant shell and fossils, respectively. X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM) with petrography microscopic investigations, and electron probe microanalysis (EPMA) were employed for this study. The glauconitic pellets show two main types of morphology. (1) Spheroidal or ovoidal glauconitic pellets consist of microcrystals of phyllosilicates in random orientation. The HRTEM study showed that the spheroidal or ovoidal pellets consist mainly of glauconitic packets, iron-rich chlorite, muscovite, biotite and low amounts of smectites. The glauconite and chlorite packets were generally arranged in random orientation in the spheroidal or ovoidal pellet. There are also low concentrations of submicroscopic quartz, apatite, and monazite in the pellets. (2) Vermicular pellets in packet sizes of glauconite ranging from several to 50 nm in thickness. Vermicular glauconitic pellets occur like a single crystal (i.e., oriented aggregation of polycrystalline glauconites). The basal planes of vermicular glauconitic pellets show cleavages, which are approximately perpendicular to the long axis of the vermicular pellets. The XRD analysis of vermicular glauconitic pellets showed glauconite and chlorite present. The EPMA indicated that vermicular glauconitic pellets contained more K and Fe than spheroidal or ovoidal glauconitic pellets. The spheroidal or ovoidal glauconitic pellets contained more glauconitic/smectite mixed-layer clays than the vermicular glauconitic pellets. The morphology, mineral constituents and chemical compositions, and submicroscopic textures, indicated that spheroidal or ovoidal and vermicular glauconitic pellets are formed from faecal pellets through neoformation.