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 (Ca
5.77Na
0.02K
0.07)
Σ5.86(Al
1.23Si
0.79Mg
0.01Mn
0.01Fe
0.01)
Σ2.05[(CO
3)
1.16(SO
4)
0.93]
Σ2.09[B(OH)
4]
0.98[(OH)
10.62O
1.38]
Σ12.00·25.41H
2O 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/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.
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