The mechanism of rock-weathering in tropical dry areas is important in studies on desertification. The present studies dealt with the changes in the chemical and physical properties of standard rocks during experimental weathering. The formation of secondary minerals and the behavior of chemical species in aqueous solution were studied by geochemical simulation which is an analytical method of rock decomposition. The physical weatherability of rocks as a cause of rock disintegration can be determined by the relation between the TMA (Thermo-Mechanical Analysis)-values and pore ratio.
The process of chemical weathering can be simulated as a reaction occurring between rock-forming minerals and aqueous solution. The changes in the amount (grams) of chemical species/1000g of solution(M）and the degree of saturation of secondary minerals, shown in Fig. 3 , were calculated as increments of the reaction progress(ξ) in the hydrolysis of standard granite as an example. The changes in the values of the chemica 1 parameters were complex. The formation of saturated mineral zoning by weathering is represented by the reaction area (10 cm²) in the profile (Fig.4 ). Satisfactory results were obtained in the alteration to laterite.
Physical weathering takes place when rocks on the earth surface are broken into fragments or grains. Weathering by thermal expansion only occurs under very hot and very dry climatic conditions. The behavior of thermal expansion and shrinkage of 14 standard rocks in the temperature ranges of 0 to + 100℃ and - 125 to+ 550℃ was examined by TMA. The rocks characterize dy a large thermal expansion contain a large amount of quartz. The samples of granite were fractured during the experiment (Fig. 8 ). A new stability series of physical rock-weathering can be determined based on the thermal expansion-shrinkage coefficient and the pore ratio in each rock type (Fig. 11). This order reflects the geomorphological characteristics in a desert area.
Although there is no natural water at present, many hydrous minerals (e.g. goethite, gibbsite, kaolinite etc.) occur on the earth surface in a desert area. The formation of these secondary minerals can be explained by geochemical simulation that describes the water-rock interaction. As natural water is essentially involved in soil genesis, it is considered that the secondary minerals forming soils in a tropical dry area have not been formed under the present climatic conditions. The rock stability series in physical weathering indicates that the rocks characterized by a large thermal expansion are not prone to weathering, because the pores in rocks play an important role in the alleviation of various environmental stresses. The importance of the thermal expansion coefficient of rocks on the earth surface in tropical dry areas has been well documented. The physical weatherability of rocks can be studied by the determination of thermal coefficients and the physical properties of each rock type.