New Mexico Mineral Symposium — Abstracts

Columbites and tantalites are among the most sought after series of minerals to be collected from the Globe and Harding pegmatite mines. The general chemical formula for the series is (Mn,Fe)(Nb,Ta)206 and individual specimens may have any Mn:Fe or Nb:Ta ratio. Collectors often ask if there are consistent chemical differences between the morphological varieties at either Location, how the compositions of specimens from the Globe and the Harding differ, and what are the ratios present in their specimens (what end-member names are most appropriate). In this preliminary study a few selected specimens were examined in order to answer these questions, to determine the nature of any zoning in the crystals, and to identify any minor elements present (or specifically absent).
An automated electron microprobe was used to qualitatively and quantitatively analyze polished samples from the two mines. Analyses were performed using a variety of natural and synthetic standards of known composition. All analyses were automatically corrected for instrumental effects, and matrix effects were corrected using the Bence-Albee procedures.
The results of the analyses are summarized below:

1. A large crystal from the Globe mine was found to be homogeneous and to have the composition of (Mn.84Fe.15Y.01)(Nb_.86Ta_.11Ti_.03)O₆. This is a manganocolumbite.
2. A "feathery" crystal from the Globe mine was also homogeneous and had a composition of (Mn.63Fe.3Y.02)(Nb_.93Ta_.05Ti_.01). This is a columbite
3. Large alluvial crystals trom the are complexly zoned and a thick Homogeneous zone of (Mn.65Fe.95Y.00)(Nb_.43Ta_.54Ti_.01)O₆(manogano-tantalite) may change to another thick, homogeneous zone of (Mn_.95Fe_.03Y_.01)(Nb_.72Ta_.26Ti_.01)O₆(manganocolumbite) in the space of a few micrometers. This zoning is roughly concentric in some cases and irregular or sector-zoned in others.
4. A small crystal from the interior of the pegmatite was zoned in a similar fashion but was richer in Mn and Nb with one area having the composition (Mn_.99Fe_.01Y_.01)(Nb_.83Ta_.16Ti_.01)O₆
5. Checks for the presence of minor elements revealed that all of the specimens had small amounts of Ti and Y, as listed, and Si. In the Harding specimens Fe is essentially a minor element, varying between 0.12 and 0.50 wt. %. Harding specimens also contain about 0.3 wt. 7 W01. Specifically not detected in any specimens were Mg, Al, Ca, V, Cr, Zr, 811, Sb, La, Ce, Pb, Bi, Th or U.

The major conclusion of this study is that there are consistent chemical differences between morphological varieties of these minerals obtained from the Globe and Harding Mines.

This work supported by the U.S. Department of Energy (DOE) under contract No. DE-AC-O4-75DP00789.A U.S. DOE facility.