Primary oxide minerals in the system WO3-Nb2O5-TiO2-Fe2O3-FeO and their breakdown products from the pegmatite No. 3 at Dolní Bory-Hatě, Czech Republic
Résumé
Symmetrically zoned barren pegmatite dike No. 3 cuts granulite at Dolní Bory-Hatě in the Czech Republic. It contains minor to accessory biotite, schorl, muscovite, sekaninaite, andalusite, diaspore, apatite and several rare accessory minerals. Black, tabular crystals of ferberite-"wolframoixiolite" occur almost exclusively in an andalusite-diaspore aggregate with sequential accumulations of late pyrophyllite, kaolinite, and muscoviteillite. Complex zoning of the individual crystals of ferberite-"wolframoixiolite" shows primary, coarse oscillatory zoning consisting of narrow zones of ferberite (5.01-6.00 apfu W) and dominant niobian ferberite I (5.00-3.01 apfu W) to "wolframoixiolite" (< 3.00 apfu W), the latter two phases with very fine oscillatory internal zoning. The primary phases were locally replaced by a fine-grained mixture of ferberite to niobian ferberite II > tungstenian-titanian ferrocolumbite >> niobian and tungstenian rutile > ScPO4 phase > scheelite. Primary and secondary phases are characterized by large variation in W/(W+Nb+Ta) and calculated Fe2+/(Fe2++Fe3+) but low and almost constant Mn/(Mn+Fetot) and Ta/(Ta+Nb). The contents of P, U, Ti, Zr, Si, Sc, Al and Ca vary from negligible in ferberite to minor in "wolframoixiolite" and its U-rich variety: UO2 2.71 (19.82 U-rich variety), P2O5 0.30, TiO2 4.81, ZrO2 2.84, SiO2 0.93, Sc2O3 4.36, Al2O3 0.87, CaO 1.02 (all in wt.%). The combined exchange vector: (R3+)8(R4+)6(R5+)16(R2+)−13(R6+)−17, where R2+ = Fe2+ > Mn, Ca; R3+ = Fe3+ > Sc, Al; R4+ = Ti > Zr, Si, U; R5+ = Nb > Ta, P; R6+ = W, seems to be the best expression of the actual substitution mechanism from ferberite to "wolframoixiolite" with the theoretical end composition (R2+1.1R3+3.4R4+1.9R5+5.6)∑12O24. The homovalent substitutions expressed by the exchange vectors: Fe2+Mn−1, TaNb−1 and ScFe3+−1 are rather negligible. Formation of primary W, Nb, Fe-oxide minerals is closely related to the assemblage andalusite+diaspore, formed at T < ~ 400 °C for P = 2 kbar. The breakdown process probably proceeded at slightly lower temperatures of about 350-300 °C. Textural relations of the breakdown products characterized by the total absence of any depleted primary phase in the BSE images indicate that the secondary assemblage did not originated from an exsolution-type process. Complete recrystallization and reconstitution of the primary minerals seems to be responsible for its origin. The chemical composition of the primary niobian ferberite to "wolframoixiolite" from Dolní Bory-Hatě is distinct from all other W-rich Nb,Ta-oxide minerals described to date except niobian wolframite from the granitic pegmatite at Nuaparra, Mozambique.