wurtzite

Wurtzite is the hexagonal polymorph of sphalerite; several polytypes are known (Back 2022).

The earliest published description of wurtzite from Tsumeb was by Hurlbut (1957a) who studied several specimens provided by TCL courtesy of Drs Söhnge and Geier.

Although very rare at Tsumeb, concentrations of wurtzite were encountered:

Hurlbut (1957b) described and analysed three colour variants (green, brown and orange) of Zn-Cd sulfide from ores occurring between 17 and 20 levels and showed them to be intermediate compositional members of the wurtzite – greenockite series.

Söhnge (in Hurlbut 1957a) noted that ore from this zone averaged 13 wt % Cd (compared with an average of 0.2 % in mill-heads from run-of-mine ore).

Hurlbut’s (1957a) analysis showed that the green sulphide contained 39.87 wt % Cd (41.5 mol %); the brown sulphide contained 49.34 wt % Cd (54.2 mol %) and the orange 49.75 wt % Cd (57.6 mol %). Refractive index, specific gravity and unit cell size were all shown to increase as a function of Cd content in a comparable fashion to synthetic members of the series. On the basis of Hurlbut’s analyses the brown and orange varieties are probably better described as zinc-rich greenockite (rather than wurtzite).

Geier (1973/74) noted that cadmium-bearing wurtzite was the main carrier of cadmium in the Tsumeb ore.

Pinch and Wilson (1977) reiterated that "Cd-wurtzite … with a Zn:Cd ratio of 1:1 has been reported" and that: "Orange, brown and green massive wurtzite and microscopic crystals of wurtzite occur rarely between the 18 and 19 levels."

According to Keller (1984) wurtzite occurs as lustrous orange-red grains that fluoresce orange under UV light.

Hughes (1987) merely restated the findings of Hurlbut (1957b), describing "cadmium-wurtzite" as "…moderately abundant at the western end of the ore body between 17 and 20 levels". His graphic (Hughes 1987; Fig. 71 on page 136) shows wurtzite as occurring between levels 17 and 22.

Gebhard (1999) recorded that crystals of orange, cadmium-bearing wurtzite to 5 mm were found between levels 18 and 19.

A specimen in the collection of the University of Canberra (catalogue number UC3248), now housed at Geoscience Australia, displays a deep carmine-red fluorescence under long wavelength UV radiation. The specimen was donated to the university in the early 1970s, courtesy of TCL mineralogist Bob Schedler, labelled as "cadmium wurtzite" but unfortunately the level from which it was collected was not recorded. A recent (2023) re-examination of this specimen by SEM/EDS has indicated that much of the "wurtzite" is in fact a Zn-rich greenockite (F. Keutsch and M. Southwood, unpublished data); further investigation is required.

bornite; calcite; chalcocite; digenite; galena; greenockite; pyrite; quartz; sphalerite; tennantite-(Zn)