A NEW CHEMICAL INDEX OF WEATHERING FOR METAMORPHIC SILICATE ROCKS IN TROPICAL REGIONS - A STUDY FROM SRI-LANKA

Weathered and fresh samples of metamorphic rocks from Sri Lanka were c ollected from various localities and analysed for major elements by XR F method (RIGAKU, KG-X System, Japan). The content of water was determ ined by the ignition method. The XRF results, obtained from these samp les form the basis of a new index of chemical weathering, particularly for Sri Lanka, which is named the Silica-Titania Index, and is calcul ated as follows: Silica-Titania Index = {(SiO2/TiO2)/[(SiO2/TiO2) x (A l2O3/TiO2) X (SiO2/Al2O3)]} x 100(molecular proportions). The index ca n be used to determine the degree of weathering in chemically weathere d silicate rocks of other countries in tropical regions. A triangular diagram plots the position of this index. The point load strengths of fresh rocks and weathered rocks with different degrees of weathering w ere correlated with the values of this new chemical index. The relativ e variation in strengths of fresh rocks and weathered rocks clearly in dicates its suitability and usefulness for engineering geologists.