Nordstrandit z lomu v Děpoltovicích u Karlových Varů (Česká republika)
Jaromír Tvrdý, Bohuslav Bureš, Jakub Plášil, Ivana Jebavá, Jiří Sejkora
The rich occurrence of well-crystalline rare Al-hydroxide, nordstrandite, was found in the Děpoltovice quarry near Karlovy Vary, western Bohemia (Czech Republic). It forms there several morphological types and in the studied association, gibbsite was not found. This is atypical in comparison with other similar localities. Nordstrandite is triclinic, space group P-1 with following unit-cell parameters: a = 6.131(6) Å, b = 6.946(6) Å, c = 5.063(5) Å, α = 95.86(7)˚, β = 99.05(7)˚, γ = 83.26(8)˚, V = 210.7(2) Å3. Chemical analyses yielded the average composition Na2O 0.06, CaO 0.04, MgO 0.08, Al2O3 67.57, SiO2 0.62, H2Ocalc. 36.22, total 104.59 wt. % corresponding to the formula (Al0.99Si0.01)Σ1.00(OH)3 on the basis of 1 atom in cation sites. Nordstrandite was found in the close association of gonnardite and böhmite. Böhmite is orthorhombic, space group Cmcm with following unit-cell parameters: a = 2.82(3), b = 12.17(6), c = 3.71(2) Å, V = 127(2) Å3 and its average chemical composition CaO 0.28, MgO 0.09, Al2O3 78.55, SiO2 0.80, H2Ocalc. 14.06, total 93.77 wt. % corresponding to the formula (Al0.99Si0.01)Σ1.00O(OH) on the basis of 1 atom in cation sites. Gonnardite is tetragonal, space group I-42d with following unit-cell parameters: a = 13.226(3), c = 6.619(3), V = 1157.8(3) and its average chemical composition Na2O 10.83, CaO 5.49, SrO 0.22, Al2O3 29.48, SiO2 40.64, H2Ocalc. 13.48, total 100.15 wt. % corresponding to the formula (Na5.60Ca1.57Sr0.03)Σ7.20[(Al9.27Si10.84)Σ20.11O40] . 12H2O on the basis 40 O. The origin of studied mineral association represented by calcite, gonnardite → nordstrandite → böhmite is interpreted as a product of precipitation from alkaline solutions with high alkali (Ca, Na) concentrations.