Tapiolite-(Fe): paragenesis and chemical compositions in pegmatites of the Pampean pegmatite province

Resumen

The paragenesis and chemical compositions of tapiolite-(Fe) present in different pegmatite occurrences from the Pampean pegmatite province are described. Exceptionally, the mineral occurs as an idiomorphic single crystal; more commonly, it is present associated with columbite-group minerals or, rarely, with wodginite-group minerals where usually tapiolite-(Fe) shows textural evidences of replacement by these associated phases. The chemical compositions shows ranges in wt.% of Ta₂O₅ comprised between 86.22 and 72.11, FeO from 13.85 to 9.76, and Fe₂O₃ from 1.73 to 0.00. The minor elements mean contents in wt.% are to Nb₂O₅ (2.4%), TiO₂ (0.73 %), SnO₂ (0.5%) and MnO (1.15 %). The trace elements have contents ≤ 0.5 wt.% or are negligible. The Ta# (Ta / (Ta + Nb) apfu) vary from .99 to .84 with most of the samples showing very good clustering. The associated Ta# numbers are lower in associated columbite-group minerals and wodginite-group minerals, meanwhile the Mn# are always higher than in both group of minerals. The origin of the tapiolite-(Fe) in most of the studied samples is ascribed, as in other occurrences, to the late-stage magmatic crystallization in disequilibrium conditions of a fluxed and highly differentiated evolved pegmatite melt.