Podrobnosti záznamu

    Evolution of feldspars in granitic pegmatites
Údaj o odpovědnosti
    Petr Černý
    Černý, Petr
Zdrojový dokument - monografie
    Feldspars and Their Reactions. NATO ASI series, Series C, v. 421
    s. 501-540
    12 obr., 4 fot., 3 tab., 9 s.bibl.
Předmětová skupina
    hydrotermální podmínky
    moldanubikum strážecké
    moldanubikum západní Moravy
    prvky stopové
Geografické jméno
    Biskupice (Třebíč)
    Dolní Bory (Žďár nad Sázavou)
    Radkovice u Hrotovic (Třebíč)
    Rožná (Žďár nad Sázavou)
    Věžná (Žďár nad Sázavou)
Klíčové slovo
Abstrakt (anglicky)
   In most geological categories of granitic pegmatites, the feldspar minerals crystallize under conditions of disequilibrium fractionation facilitated by increasing concentration of some or all of H2O, F, B, P, and Li. This departure from cotectic-to-eutectic or peritectic termination of ordinary granitic crystallization generates zoning in textures and feldspar assemblages, controlled by melt undercooling and variable rates of nucleation and crystal growth: in general, from granitic borders through graphic K-feldspar + quartz in plagioclase + quartz matrix to blocky K-feldspar in the core margins, associated with cleavelandite and followed by saccharoidal albite. - Trends in bulk composition of the feldspars, as it changes throughout their crystallization sequences can be related, to a degree, to the liquidus-solidus-subsolidus relations in the Ab-An-Or or Ab-Or systems.
   Minor elements comprise compatible Ba, Sr and Fe3+ in early generations and incompatible Li, Rb, Cs, Tl, Pb and NH4 which fractionate into late primary feldspars, locally to extreme concentrations not encountered outside the granitic pegmatites. Incompatible behaviour is also typical of P, Ga and possibly B, Be. Low-temperature hydrothermal feldspars are very close to end-member compositions, except local occurrences of Rb- or Ba-rich phases. - Perthite exsolution and recrystallization are common in K-feldspar, whereas peristeritic and antiperthitic breakdown of plagioclase and albite seem to be restricted. Perthitization of the disordered monoclinic (K, Na)-feldspar, as originally crystallized from the melt and/or fluid, is accompanied by diverse degrees of tetrahedral ordering of the K-phase, dependent on the specific local conditions in individual pegmatites.
   In contrast, plagioclase and albite generally achieve very high order, even in low-temperature hydrothermal assemblages which frequently preserve extreme disorder in K-feldspar. - Replacement of K-feldspar by mica + quartz assemblages does not seem to be widespread
    Česká geologická služba
Kód přispěvatele
    ČGS (UNM)
Zdrojový formát
Datum vložení
    17. 8. 2007
Datum importu
    8. 8. 2012