Journal of

GEOsciences

  (Formerly Journal of the Czech Geological Society)

Original Paper

Novák

Minerals of spinel group from pyrope-bearing gravels of the České středohoří Mountains, Czech Republic

Journal of the Czech Geological Society, volume 46 (2001), issue 1-2, 45 - 52

Spinel, ferroan spinel (pleonaste), chromian spinel (picotite), hercynite, magnetite and titanomagnetic were identified in a heavy mineral concentrate after industrial extraction of pyrope from pyrope-bearing gravels near Podsedice in the České středohoří Mts. Very rare spinel occurs as dark grey grains exhibiting relatively enhanced content of FeO and Fe₂O₃. Ferroan spinel is the most abundant mineral of the spinel group, and amounts together with picotite to 17.5 wt. %. The average composition is Mg 72, Fe²⁺ 28 (at. %) and atomic ratio Al/Fe³⁺ = 11.9. Chromian variety of pleonaste with 10.3 wt. % Cr2O3 (11.2 at. %) is very rare. Pleonaste has a = 8.146(2) 10^-1nm and D_calc. = 3.848. Picotite composes about 30 wt. % of pleonaste grains. Picotite grains are smaller (≤ 4 mm): Mg (72 at. %) predominates over Fe²⁺ (30 at. %) in its tetrahedral sites, and Al (58.5 at. %) predominates over Cr (36.5 at. %) and Fe³⁺ (5.0 at. %) in octahedral sites. The content of Cr₂O₃ in picotite varies between 26.89 and 37.35 wt. %. Hercynite is very rare. Magnetite plus titanomagnetite constitute 5.6 wt. % of the concentrate. Magnetite contains ≤ 5.4 wt. % TiO₂ and ≤ 4.8 wt. % MnO. Relatively higher concentration of MnO and the absence of MgO separate this mineral from titanomagnetite which has 10.90-12.07 wt. % TiO₂. It is more abundant in the concentrate than magnetite. Its cell parameter a = 8.436 10^-1nm and D_calc. = 4.845. Various types of basalt and its tuffs, basaltic pipe breccias and particularly xenoliths of various ultramafic rocks brought to the surface through the pipe vents are thought to be the primary source of spinel-group minerals in pyrope-bearing gravels. The mode of occurrence, abundance and chemical composition of the spinel-group minerals in pyrope-bearing gravels indicate that they are genetically similar to placers at the Jizerská louka in the Jizerské hory Mts and at Seifengründel in Saxony at the northern border with Czech Republic.