In Rangpur, the value of skewness for all rainfall data are positive that indicate the data are skewed to the right. The increasing trend of annual rainfall suggests the climate change in Rangpur within the investigated period of time. Pearson correlation index shows that the mean daily temperature is significantly negatively correlated with daily rainfall which implies that fall of daily mean temperature with the daily rainfall. The analysis shows that it rained in every year in the month of June as the minimum monthly value for June is greater than zero. The total amount of annual rainfall is strongly significantly positively correlated with the total monthly rainfall of the months of June, July August, September and October and significantly positively correlated with the total monthly rainfall of the month April. The rainfall trend can be expressed as monsoon rainfall > pre-monsoon rainfall> post monsoon rainfall. The highest amount of total monthly rainfall was recorded in July, 1987 and measured as 1314 mm. Heavy rainfall occurs in the month of July of the year. The highest amount of annual rainfall was recorded in 1984. The analysis of rainfall data reveals that the average annual rainfall in Rangpur was 2099.25 mm that varies from 427 mm to 3748 mm within the investigated period of time. The study area lies in Bangladesh that is in the tropical area. Keywords: Clay Mechanical Weathering Illite XRD and Lavendulan. The dissolution of copper arsenate mineral, lavendulan might be a source of arsenic in the sediments of the study area. The accessory minerals like enstatite, sekanianite and ferrierite are derived from basic igneous rocks. Glauconite forms in the sediments of continental shelf in the marine environment. The presence of illite and kaolinite suggests their derivation from the crystalline rocks that contain feldspar and mica, as well as from the pre-existing soils and sedimentary rocks. The values of the illite crystallinity index varies from 0.228 to 0.345, indicating that the illites are relatively well crystallized and derived from the mechanical weathering of pre-existing rocks. Illite constitutes 36% of the total minerals of the Tista River basin. The minor to trace amounts of lavendulan, glauconite lepidolite, enstatite, sekaninaite and ferrierite minerals were identified in the investigated area. The X-ray Diffraction (XRD) of the clay sized sediments of the Tista River reveals that illite (and/or mica), chlorite, kaolinite, quartz and feldspar are the principal mineral constituents. The deposits that exposed along the both banks of the Tista River are characterized mainly by sand, sand laden with gravel and pebble with minor amounts of silt and clay. The Tista River is a tributary of the Brahmaputra River.
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