『Abstract
　Mineralogy and chemical properties of the fine earth and particle size fractions were examined with total chemical analysis, selective chemical analyses and X-ray diffraction. Clay minerals were investigated using microprobe and transmission electron microscopy. The chemical changes during soil formation were calculated from results of the chemical weathering mass balance. The study soil is of the brown acid type, unsaturated and poor in short-range ordered products. All soil fractions sized between 0 and 1 cm originate from transformation of the weathered rind developed around tuff stones, where chemical weathering is intense. The same weathering reactions which were initiated in the early stages of tuff weathered rind were continued in the soil with strong dissolution of plagioclases and transformation of biotite. Two chemical weathering reactions led to the formation of clay minerals in the soil: the precipitation of kaolinite into biotite pseudomorphs and into the small white mica and the transformation of the biotite (BI) into hydroxy-interlayered vermiculite (HIV) with intermediary stages shown in HIV-BI interstratified minerals, which were defined (95-75％ of biotite layers). Pure vermiculite or V-BI interstratified minerals were hydoxy-interlayered, even in the weathered rind of the tuff. The chemical and mineralogical mass balance between the weathered rind and the soil shows a heavy dissolution of andesine (90％) and to a lesser extent of albite (55％) and of apatite (60％), a precipitation of kaolinite and a transformation of biotite into HIV. The plagioclase dissolution in weathered tuff was almost complete, while there were only traces remaining in silt. The biotite has almost disappeared in the fine earth fractions due to dissolution and transformation into HIV.

Keywords: acid brown soil; mineral evolution; hydroxy-interlayered vermiculite; kaolinite; chemical mass balance』

1. Introduction
2. Material and methods
　2.1. Study area
　2.2. Methods
3. Results
　3.1. General soil characteristics (Table 2)
　3.2. Mineralogy
　　3.2.1. Clay fractions
　　3.2.2. Coarse sand fractions
　　3.2.3. Chemistry of the secondary minerals
4. Discussion
　4.1. Mineralogy of particle size fractions
　　4.1.1. Short-range ordered minerals
　　4.1.2. Weathering processes of primary minerals into crystalline clay minerals in the soil
　　4.1.3. Inherited minerals and their evolution in the soil
　4.2. Formation of soil from weathered rind
5. Conclusion
Acknowledgements
References