Compositional variations in ferrotapiolite plus tantalite pairs from the beryl-columbite pegmatite at Moravany nad Vahom, Slovakia

Three texturally and compositionally distinct pairs of ferrotapiolite+tanta lite, all spatially related to a cleavelandite unit, were distinguished in the zoned beryl-columbite pegmatite at Moravany nad Vahom, Povazsky Inovec Mts., Slovakia. (1) Inclusions of ferrotapiolite I (similar to 200 mu m) in zoned ferrotantalite I exhibit crosscutting tielines of coexisting composi tions in the columbite-tantalite quadrilateral. (2) Large ferrotapiolite II grains and adjacent, compositionally variable grains of ferro- to manganot antalite II (similar to 100 mu m) display remarkably diverse tielines, not yet observed in a single sample or locality. (3) Sn-depleted and slightly M n,Ta-enriched narrow rims (similar to 20 mu m) of ferrotapiolite III with s mall inclusions of manganotantalite III (less than or equal to 5 mu m) are located along a late, hydrothermal, fracture-filling microlite vein cross-c utting ferrotapiolite II. Changes in fO(2) derived from calculated Fe2O3 as well as variations in Ti, Sn, W concentrations through the ferrotapiolitetantalite pairs I to III are negligible. Consequently, it seems plausible t o explain the broadening of the two-phase field and the enrichment of ferro tapiolite and tantalite in Mn and Ta particularly by decreasing temperature . A disequilibrium crystallization from highly evolved residual melt and/or fluids is suggested for all textural types.