| Popis |
Two distinct types of sekaninaite were found in zoned, miarolitic NYF pegmatites at Zimnik, Strzegom-Sobotka granitic massif: (1) graphic intergrowths of sekaninaite I (Sek-I) with quartz, up to similar to 5 cm in size, from a graphic unit are completely replaced by a fine-grained mixture of muscovite, chlorite, minor K-feldspar, albite, calcite, and rare secondary beryl; (2) columnar to conic subhedral crystals and irregular grains, up to 6 cm in size, of grey-violet to brownish sekaninaite II (Sek-II) occur in a blocky unit. Sek-II is partly replaced by biotite, muscovite, chlorite, schorl, and minor to trace zinnwaldite, beryl, albite, fluorapatite, calcite, topaz, and several accessory Nb, Ta, Ti-oxides and sulfides, and late smectite. Rather heterogeneous Sek-II (Sek(71-75)MnCrd(23-28)Crd(1-2)) has high contents of Mn (up to 0.54 apfu; 5.76 wt.% MnO) and Na (0.28-0.35 apfu; 1.32-1.63 wt.% Na2O). It is significantly enriched in Li (0.12-0.18 apfu; 0.27-0.40 wt.% Li2O) and Be (0.15-0.25 apfu; 0.55-0.94 wt.% BeO) via the substitutions (Ch)(Na+K+Rb) + Li-VI = (Ch)(sic) + R-VI(2+) and (Ch)(Na+K+Rb) + Be-IV = (Ch)(sic) + Al-IV. Sek-II has the highest content of Mn ever found in natural cordierite-group minerals, along with almost no Mg (similar to 0.01 apfu) and also high concentrations of Li and Be. The composition of Sek-II suggests low-pressure conditions of crystallization at similar to 1-2 kbar.
|
