"Rock of the Month #198, posted for December 2017" ---

Willemite

is a zinc silicate, Zn₂SiO₄. As a graduate student I was not familiar with the mineral as a natural component of rather unusual ores. Rather, willemite appeared to me on many days (and nights) as a tiny, bright green spot in the cross-hairs of an electron microprobe constructed and run by Jim Long and Stephen Reed, my supervisors at Cambridge. The green light was cathodoluminescence, produced when the mineral was struck by a focused beam of electrons,the colour the same as that produced by ultraviolet irradiation, as discussed further below. The willemite made sure that the probing beam was tightly collimated, and aligned on the target when the surface of the sample- to- be- analysed was in focus.

These samples are from a famous, prolific mineral locality and former major producer of zinc ore, a notable distinction along the Atlantic coast of the U.S.A. The ore deposits are developed in the Mesoproterozoic rocks of the Grenville province, and differ from more familiar zinc deposits in that the ores are strongly metamorphosed and recrystallized (Figs. 1-2). An orangey sphalerite is present (and may be strongly fluorescent in pink, in LW-UV: Figs. 3-4) but the important ore minerals are willemite and franklinite. The latter is a black Zn-rich spinel.

Willemite may be green (Balmat), blue-green (Tsumeb) or orange to red (as at Franklin). In so-called black willemite ore, the willemite owes its colour to tiny franklinite inclusions <10 microns across (Makovicky and Skinner, 1990). The distinctive colour of the "red" Franklin willemite has been ascribed to rodlike inclusions 5-20 microns in diameter, aligned parallel to the c-axis of the host crystals. Inclusions in willemite include franklinite and friedelite, zincian kutnohorite, serpentine and rhodonite. The rods may have formed as fluid inclusions or melt inclusions that then crystallized into minute, randomly-oriented crystallites inside the rods (Johnson and Griffen, 2007).

The fluorescence and cathodoluminescence of willemite are well-known (see the review by Robbins, 1994, and also the photographic catalogue by Schneider, 2006). Willemite in this sample glows strong green in short-wave UV, without significant response to long-wave UV. As seen in Figure 1, the franklinite does not fluoresce in the SW UV, while some of the coarse white calcite glows a bright red.

Willemite is an uncommon mineral (94 MINLIB records, 1914-2017), whether in primary or secondary (supergene) occurrences in "oxide" (non-sulphide) zinc deposits.

More on the Franklin / Sterling Hill district

The Franklin-Sterling Hill district has long been a favourite of mineral collectors (e.g., Bostwick, 1988). The Franklin and Sterling Hill orebodies contain >330 mineral species, and are the type localities for 67 species (Palache, 1935; Dunn and Kozykowski, 1991). More than 70 mineral species display UV fluorescence.

The Franklin and Sterling Hill mine workings are now inaccessible, being flooded and/or backfilled with tailings. More than 800 publications have appeared since the first paper, an 1810 description of a new mineral species, zincite (Frondel, 1990). In this "rock of the month", even though it is a relatively detailed presentation, we are still just skimming the surface, without - I hope - too many sins of (c)omission!

The New Jersey deposits occur in an area of sillimanite- grade regional metamorphism. Most of the many mineral species occur in small to minute amounts, and are unrelated to the primary mineralization. The coarse granular ores, such as our primary sample for this month, are composed largely of Mn-bearing spinel (the magnetite- franklinite series), plus willemite, zincite and tephroite in a gangue of manganoan calcite. Associated calc-silicate bodies are mostly manganoan andradite garnet, K-feldspars and other minerals such as manganoan calcite, rhodonite, diopsidic pyroxenes and amphiboles (Pinger, 1950; Frondel, 1990).

The deposit has a complex history, including perhaps a stratiform sedex deposit (Sclar, 1990), a "protore" later metamorphosed and deformed (folding at elevated temperature and pressure allowed the dense orebody to sink as an inverted diapir through the marble host rocks) with later formation of a rubble breccia body, evidence of likely Cambrian-Ordovician karstification (Metsger, 2001). The ore textures can be interpreted such that the franklinite, willemite and zincite may represent precursor sedimentary mineralogy, with exsolution textures indicative of a long period of slow cooling following the peak of regional metamorphism (Sclar, 1990).

Other Occurrences and Deposits of Willemite

Thus, metamorphic terranes may contain the economic concentrations of willemite. The Grenville metamorphism, dated at circa 1000 Ma, reached granulite facies: similar supracustal terranes may contain similar deposits (Johnson et al., 1990). Willemite and franklinite also occur in skarns and marbles of the Bergslagen district of central Sweden (Holtstam, 2002).

Willemite can form in hydrothermal settings too, as at the Beltana deposit in South Australia (Elliott, 1991; Groves et al., 2003; Brugger et al., 2003). In practice, large deposits develop, more often than not, over a long span of time and a range of geological regimes, from high temperatures to ambient (surface) conditions, interacting with a variety of fluids. This goes a long way to explaining the long lists of minerals identified at Sterling Hill, Tsumeb, Broken Hill (New South Wales, Australia) and other major deposits.

The Otavi Mountain Land of northeast Namibia contains, besides the Tsumeb and Kombat mines, the Berg Aukas (Grootfontein) mine, noted for kinerals such as descloizite, willemite, dioptase and vanadinite. Berg Aukas ceased operation in 1978, and was hosted in dolomite. The Central orebody was a brecciated pipelike body, oxidized, with cavities lined by massive willemite and/or descloizite (a vanadate of Pb: Cairncross, 1997; see also Verbeek and Bostwick, 1997).

Vein deposits may contain willemite. Willemite and native silver occur in calcite veins cutting Precambrian James River Formation argillite and greywacke in the Antigonish Highlands of Nova Scotia (Sangster, 1986). In the Bolanos silver district, Jalisco, Mexico, veins are related to rhyolitic, tin-anomalous, rhyolitic magmatism. Oxidized fractures contain native Ag with cerussite, willemite, Cu oxides and native Cu (Lyons, 1988).

In addition to zinc deposits subjected to regional metamorphism in the upper amphibolite to granulite facies, willemite is reported as a minor mineral in various alkaline intrusive complexes. Examples are Mont St-Hilaire in Quebec (Fisher and Glenn, 1989). Strange Lake in Labrador (Mariano and Birkett, 1988) and albitite of the Ilimaussaq complex in the Gardar province of southwest Greenland (Metcalf-Johansen, 1977).

Willemite also occurs in certain zinc-bearing metallurgical slags (e.g., Bachmann, 1982, and, in the case of the Harz in Germany, Chaudhuri and Newesely, 1993).

Figures 3-4. Left: Sample in daylight, from the Sterling Hill mine. Orangey sphalerite occurs with willemite (a slightly darker- orangey, tabular crystal can be seen at the centre of this view, long axis trending from upper left to lower right: not fluorescent in LW UV), white calcite and black franklinite. Specimen, circa5 x 5 x 2 cm, from Charles Gould. Right: in LW UV light (sphalerite shows no response in SW UV). Photos provisional. Sphalerite is quite rare in association with ores at Sterling Hill, but does form a component of the matrix of rubble breccias, especially at greater depths (Metsger, 1990). It does occur in much larger amounts at some other Grenville localities, such as the Long Lake mine in southeast Ontario.