It is common practice to use clay for coal-burning in fluorosis areas on the border of Yunnan, Guizhou, where coal-bearing strata is widely outcropped, and clay for coal-burning is usually considered to be collected from efflorescent clay in coal-bearing strata by some scholars. However, the relevant chemical properties of the clay, the mechanism that causes the fluorosis are not clear and the chemical forms of fluorine species present during transfer from the clay to the human body remain unclear. In this study, approximately 71 samples of efflorescent clay of coal-bearing strata were collected from seven counties in this area, and the results showed that those samples had a high-fluorine content with an average of 751 μg•g -1 (237-1,764 μg•g −1 , n = 71). The clays were rich in acid with an average pH of 5.81 (2.39-8.25, n = 71) and the acidic clays accounted for 52% of the total clay samples. The sulfate (SO 4
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