| Abstrakt: |
Cobalt-blue spinel from two localities (Qila and Trailside) on Baffin Island, Nunavut, Canada, are compared with other spinels from 12 occurrences in the same geological terrane and to Co-bearing blue spinel from other localities worldwide. Qila spinel is suitable for cutting cabochons, which commonly incorporate small amounts of associated calcite, dolomite, corundum, fluorapatite, pargasite and other silicates, as well as secondary cobaltpentlandite and fluid inclusions. Trailside spinel can be faceted into small gemstones (generally <0.2 ct), which rarely contain magnesite, fluorapatite, muscovite and CO2-rich fluid inclusions. The bright blue colour of the spinels from Qila and Trailside is due to TCo2+, although they also contain other chromophoric elements such as Fe and traces of Cr, Mn and V. Compared with other Mg-Al spinels from Baffin Island, those from Qila and Trailside are generally enriched in Co, Ni, Mn, Li and Be, and depleted in Ti, V and Cr. In addition, the Qila and Trailside Co-blue spinels are fairly chemically distinct from those of other world localities: they are depleted in V and Ti, fairly poor in Cr and Zn, and considerably enriched in Co and Mn. A new approach for the preliminary chemical discrimination of gem spinels is proposed by plotting the weighted chromophore concentrations in a ternary diagram (i.e. Cr+V vs 40×Co vs Fe/8). These weighted percentage values are also plotted against the ppmw concentrations of the chromophoric elements in X-Y plots, which provide further correlations of high Co with vivid Co-blue spinel and elevated Fe with overly dark non-gem spinel. This method can be applied to samples that are difficult or inefficient to characterise using spectroscopy, such as spinel grains in heavy-mineral concentrates obtained during regional exploration programmes. [ABSTRACT FROM AUTHOR] |
