TECHNICAL NOTE A Comparative Quantitative Study of an Omani Soil Using X-Ray Diffraction Technique AMER ALI AL-RAWAS 1 , ABDULWAHID HAGO 1 , HECTOR H. SUTHERLAND 2 , ALI A. YOUSIF 2 , MOHAMMED AL-SHIHI 1 and BADR AL-SHIHI 1 1 Department of Civil Engineering, Sultan Qaboos University, P.O. Box 33, Al-Khod 123, Sultanate of Oman, E-mail: ameraar@squ.edu.om 2 Department of Physics, Sultan Qaboos University, P.O. Box 36, Al-Khod 123, Sultanate of Oman. (Received 6 March 2000; revised 8 December 2000; accepted 26 February 2001) Abstract. Two methods for the quanti¢cation of clay minerals using X-ray diffraction techniques: the Constant Mineral Standards Method and the Constant Clay Method were applied to soil samples from Al-Khod (Northern Oman).The ¢rst method was based on the addition of different concentrations of clay to constant mineral standards while the latter was based on the additions of known internal standards to the clay sample. The clay investigated in this study contained montmorillonite, palygorskite, illite and kaolinite. The areas of the re£ection peaks were found to relate linearly with additions of different proportions of clay or mineral standards. Using the two methods, the concentrations of the different components and their experimental errors were determined. From the errors and self consistency of the results obtained, and the practicality of sample preparation, it is proposed to use the Constant Mineral Standards Method. Key words: quantitative analysis, clays, X-ray di¡raction, internal mineral standards. 1. Introduction Clays are de¢ned as soils ¢ner than 0.002 mm, and are usually plastic when they are in contact with water but hard when they are dry. Expansive clays are composed of expanding lattice type minerals such as montmorillonite, illite, palygorskite, etc., and exhibit swelling behaviour upon wetting and shrinkage upon drying, which result in volumetric changes. Consequently, they cause distress and damage to engin- eering structures founded on them. Quantitative analysis of clay minerals is import- ant in the evaluation of the swelling potential of expansive soils. Qualitative and semi-quantitative methods are used worldwide for the analysis of clay minerals. The semi-quantitative methods include: Peak-height Ratio Method, Biscay Method (Biscay, 1965) and the Chemical Analysis Method (Imbrie and Poldervaart, 1959). However, these methods provide only indirect and rough esti- mates and thus, cannot be considered reliable. Direct estimation of clay minerals through quantitative X-ray diffraction methods is therefore needed in order to Geotechnical and Geological Engineering 19: 69^84, 2001. 69 # 2001 Kluwer Academic Publishers. Printed in the Netherlands.