New mineralogical data for khademite (orthorhombic AlSO4F·5H2O) and the story of rostite (orthorhombic AlSO4OH·5H2O) from Libušín near Kladno, the Czech Republic
Abstract
Khademite and rostite are orthorhombic minerals Al(SO4)F.5H2O and Al(SO4)OH.5H2O, respectively. Libušín is a type locality of rostite but recent studies have only confirmed the presence of khademite.
New electron microprobe analyses of this mineral from Libušín revealed a fluorine concentration of 8.12-8.18 wt%, which is the highest ever found for khademite. The empirical formula is: Al(1.01-1.03)(SO4)(0.98-0.99)(F(0.99-1.00)OH(0.00-0.10)).5H2O, and lattice parameters for Pcab symmetry are a = 11.182(3), b = 13.051(4), c = 10.889(5) Å, V = 1589.0(7) Å(3).
The intense Raman lines are 990 cm(-1) (ν(1)); 1081 and 1131 cm(-1) (ν(3)); 420 and 504 cm(-1) (ν(2)); 589 and 632 cm(-1) (ν(4)). Therefore, rostite has not been proven in Libušín and its existence at other localities is not credibly documented.
The presented data of fluorine khademite are of high importance for geochemical studies and precise discrimination of structurally very similar sulfates. This information can be also used for better understanding of planetary surface and subsurface processes.