抄録
According to the Soil Contamination Countermeasures Law of Japan (established in 2002), fluoride (chemical compound of F-) is designated to one of the environmental toxic substances, because of the characteristic diseases caused by abundant intake (e.g. spotted teeth and furthermore born accidents). The base line of soil fluoride content is decided by the law to be 4000mg/kg as heat-dried soil composition.
The main reservoir of soil fluoride seems to be industrial sites such as metal mines and filtration plants. However, natural fl uoride can often contributes as the reservoir, especially in some granite areas. Within granite, several % of biotite is contained as usual, and within biotite, less than 1 mass%of F-is contained in general. That is the reason why the natural fl uoride content tends to indicate relatively high in granite areas.
The purpose of this study is to evaluate the expectable scale of soil contamination by the natural fluoride originated from granitic biotite. A detailed electron microprobe study of fluoride content (F-[mass%]) in biotite grains was performed on the 8 different kinds of medium-grained granite samples from 5 investigation sites in Hiroshima prefecture. The 5 investigation sites are Ohtake district (OH-1 and OH-2), Kake district (KA-1 and KA-2), Hiroshima district (HI-1 and HI-2), Kure district (KU-1) and Hongo district (HO-1) respectively. Each elemental analyse of biotites was obtained from polished thin sections using a JEOL JCMA-733 II electron probe microanalyser (EPMA).
As results, fluoride content of biotite in granite is 0.5-1.3 mass%as biotite bulk composition. Converting to the in-soil fl uoride content following to the law, this value corresponds to several hundreds of mg/kg.
