THE WEATHERING OF QUARTZ-DIORITE IN SENMAYA DISTRICȚ IWATE PREFECTURE, JAPAN The weathering process of quartz-diorite mass in Senmaya district, Iwate Prefecture, was studied from chemical and mineralogical angles. Especially the relationship between the compositional changes of the weathered rocks and their main constituent minerals was detailedly discussed. The non-altered quartz-diorite is chiefly composed of quartz, plagioclase, hornblende and biotite, together with few apatite and magnetite. As weathering proceeds, most of the primary minerals, plaigoclase, hornblende and biotite, are gradually eliminated and secondary minerals such as vermiculite, kaolin etc. appear in stead. The change in chemical composition of the weathered rocks is almost controlled by the solubility of each chemical component under netural conditions. Among the primary minerals, biotite is most sensitive for weathering, the sequence of the alteration being as follows: biotite-hydrobiotite-vermiculite-kaolin mineral In the earlier stage of the weathering, the alteration process of biotite was mostly affected by its original crystallographic structure, so that the change in chemical composi tion of that mineral differs markedly from that of the host rock. But it is also controlled by the solubility of each chemical component, thus the compositional change becomes to be similar to that of the host rock in the later stage.