Charlesite was found in a calcite vein that developed along the boundary between crystalline limestone and gehlenite-spurrite skarns at the Fuka mine in Okayama Prefecture. It occurred as a flattened hexagonal dipyramid up to 6 mm across in association with calcite. An electron microprobe analysis of the charlesite showed,a marked variation in its composition from the core to the rim. The average chemical composition indicated the empirical formula (Ca5.77Na0.02K0.07)(Sigma 5.86)(Al1.23Si0.79Mg0.01Mn0.01Fe0.01)(Sigma 2.05)[(CO3)(1.16)(SO4)(0.93)](Sigma 2.09)[B(OH)(4)](0.98) [(OH)(10.62)O-1.38](Sigma 12.00)25.41H(2)O based on a total of 11 cations (anhydrous part). The unit cell parameters of charlesite are a = 11.097(5) and c = 21.22(3) (A) over circle. The mineral is optically uniaxial negative, with the indices of refraction being omega = 1.498(2) and epsilon = 1.462(2). Its Vickers microhardness is 96.0 kg/cm(2) (10-g load), and its Mohs hardness number is 2.5. The measured density is 1.84 g/cm(3). It is likely that charlesite in the Fuka mine was formed primarily under a low-temperature hydrothermal condition.