Abstract
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)Σ5.86(Al1.23Si0.79Mg0.01Mn0.01Fe0.01)Σ2.05[(CO3)1.16(SO4)0.93]Σ2.09[B(OH)4]0.98[(OH)10.62O1.38]Σ12.00·25.41H2O 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) Å. The mineral is optically uniaxial negative, with the indices of refraction being ω = 1.498(2) and ε = 1.462(2). Its Vickers microhardness is 96.0 kg/cm2 (10-g load), and its Mohs hardness number is 2.5. The measured density is 1.84 g/cm3. It is likely that charlesite in the Fuka mine was formed primarily under a low-temperature hydrothermal condition.
