Micro-World Gems & Gemology, Winter 2016, Vol. 52, No. 4

Sphalerite Inclusions in Namibian Demantoid

Sphalerite inclusion in Namibian demantoid.
Figure 1. A brownish orange sphalerite inclusion in Namibian demantoid with a spheroidal diopside aggregate along its upper left side. Based on its morphology, the rhombohedral crystal on the upper right could be a calcite inclusion. Photomicrograph by Aaron Palke; field of view 0.72 mm.

The inclusion scene of skarn-related demantoid garnets from Namibia and Madagascar is dramatically different from that of serpentinite-hosted demantoid found in the classic locality of the Russian Urals. Reported inclusions in Namibian demantoid include diopside, wollastonite, quartz, calcite, fluid inclusions, and sphalerite (F. Koller et al., “The demantoid garnets of the Green Dragon mine (Tubussi, Erongo Region, Namibia),” Joint 5th Mineral Sciences in the Carpathians Conference and 3rd Central-European Mineralogical Conference, April 19–21, Miskolc, Hungary, 2012). Demantoid from Madagascar is reported to contain inclusions of diopside, wollastonite, fluid inclusions, and growth tubes (F. Pezzotta et al., “Demantoid and topazolite from Antetezambato, northern Madagascar: Review and new data,” Spring 2011 G&G, pp. 2–14). There has been little photomicrographic documentation of these inclusion suites, however.

In this contribution we document relatively rare sphalerite inclusions in a Namibian demantoid crystal. Figure 1 shows a translucent brownish orange sphalerite inclusion with a spheroidal diopside aggregate adhering to it (both identified by Raman spectroscopy). While a clear Raman signal could not be obtained on a nearby crystal, its rhombohedral morphology leads the author to speculate that it is a calcite inclusion. The two sphalerite inclusions seen in figure 2 are larger, so their color is a much darker orangy brown. The oblique fiber-optic illumination used in this photo highlights the highly lustrous surface of these sphalerite inclusions. Further photomicrographic documentation of the inclusion suites in demantoid from Namibia and Madagascar may help to identify inclusion scenes unique to these skarn deposits.

Luster of sphalerite under fiber-optic light.
Figure 2. Oblique fiber-optic light illuminates the surface luster of these sphalerite crystals within a Namibian demantoid. Photomicrograph by Aaron Palke; field of view 0.84 mm.

Aaron Palke is a lecturer in mineralogy at the University of Queensland and senior curator in mineralogy at Queensland Museum in Brisbane, Australia.