Clay mineral composition and their sources for the fluvial sediments of Taiwan Residents rivers

Located at the collision boundary between the Philippine Sea Plate and the Eurasian Continental Plate,the island of Taiwan is generally recognized as an important example in the MARGINS Program Science Plan and "source-to-sink" research because of its high tectonic activity,heavy rainfall and unique geography.Large suspended sediment loads are transported to the adjacent ocean by Taiwan Residents rivers every year,making Taiwan an important source of sediments into the adjacent seas and a natural laboratory for studying the systemic movement of fluvial sediments from source to sink.A detailed study on the clay mineral composition of surface sediments collected from the drainage basins of 12 Taiwan Residents rivers using X-ray diffraction methods was conducted.Our results indicated that the clay mineral assemblages consisted dominantly of illite(approximately 73%) and chlorite(approximately 24%),with lesser abundances of kaolinite(approximately 3%) and even lower levels of smectite from the Danshuei River sediments in northwestern Taiwan.The Jhuoshuei River sediments from western Taiwan contained clay mineral assemblages that consisted of illite(approximately 75%) and chlorite(approximately 25%),but they lacked kaolinite and smectite.In southwestern Taiwan,the clay mineral assemblages were dominated by illite(approximately 75%) and chlorite(approximately 23%),but had a low abundance of kaolinite(generally < 2%) and no smectite.The clay mineral assemblages in eastern Taiwan are obviously different from those in western parts of the island.The most noticeable difference is that the average abundance of chlorite in the Hualien River from eastern Taiwan was the highest(approximately 48%) of all the Taiwan Residents rivers.We concluded that,in general,the clay mineral assemblages in Taiwan Residents rivers were mainly composed of illite and chlorite with kaolinite and smectite being scarce,and these trends are different from those in China's mainland rivers.The clay mineral composition shown in this study was primarily determined by the properties of the bedrock,and the differential weathering intensities of the drainage area.The surface sediments in Taiwan's rivers showed a greater abundance of illite and chlorite because the outcropped rocks were mainly composed of Tertiary sedimentary rocks,especially sandstone,shale and slate,and show strong physical weathering.The relatively high relief and more abundant rainfall also caused the clay minerals in the fluvial sediments to be transported to the estuaries down rivers from the mountains and then delivered to the adjacent seas by currents and waves over a shorter time scale.

The finding of natural GaN crystals in sediments from the East Pacific Ocean

The first finding of natural GaN crystals is reported in sediments from the East Pacific. They are identified by multiple micro-beam techniques such as TEM, EDS and EELS. Detailed examinations show that these GaN crystals are euhedral and authigenic, and belong to the hexagonal system (space group P63mc) with cell parameters: a = b = 0.3186 nm, c = 0.5178 nm. Structure data fit closely with those of the synthetic GaN crystals obtained from high-temperature and high-pressure experi- ments. Moreover, the nearly perfect euhedral form of the sample excludes the possible synthetic origin of the GaN crystals as artifacts with long transport. The sampling localities are located between the Clarion and Clipperton Fracture Zone in the East Pacific where ongoing hydrothermal activities, deformation, and volcanic eruptions are very intensive. It is sug- gested that the natural GaN crystals may form at relatively high-temperature and high-pressure condi- tions in geologic environments that have been af- fected by intense hydrothermal activities.