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 (SO₄^2–) content of the clay samples were also high and ranged from 295 to 13086μg·g⁻¹ (average 1851μg·g⁻¹, n = 71). The pH value of the acidic efflorescent clay was positively correlated with –lgC [SO₄^2-] (R = 0.75), which indicated that the acid may exist in the form of acidic sulfate such as KHSO₄ or NaHSO₄. Further research using time-of-flight secondary ion mass spectrometry (TOF-SIMS) found that the surface of the clay samples was rich in characteristic positive and negative ions such as HSO₄^-, SO₄^-, FeSO₄⁺, FeO⁺, and F^-. This indicated that the clay samples contained pyrite and the pyrite in clay had been weathered and produced acid sulfate as a result of long-term natural exposure to coal strata. By heating the clay samples, hydrogen fluoride (HF) was released from the clay and it was verified quantitatively without exception, ranging from 28 to 302 ppb (average 105.67 ppb), which was significantly higher than the HF background concentration in the laboratory. The result provided direct evidence of HF release during the heating process. The possible reaction mechanism was that a chemical reaction between the acid (HSO₄^–) and fluorine in the clay occurs, thereby producing HF, which is the chemical form of fluorine released from clay under relatively mild conditions. The unique chemical and physical properties of HF may provide new insights into the pathogenic mechanisms of coal-burning induced endemic fluorosis.