"Rock of the Month #195, posted for September 2017" ---

Smithsonite

is zinc carbonate, ZnCO₃. It is perhaps the quintessential "oxide zinc" mineral, a non-sulphide zinc ore mineral with a non-metallic lustre, which can be found in well-formed crystals but which more often occurs as massive to pulverulent crusts in supergene alteration zones atop sulphide ores. Because of its appearance and its allochromatic (many colours) nature, smithsonite is easy to overlook amongst other secondary minerals (goethite, jarosite, etc) in weathered outcrops.

Smithsonite was named for James Smithson (circa 1765 to 1829), a high-born English chemist and mineralogist, who became, posthumously, the founding donor of the renowned Smithsonian Institution in Washington, D.C. The mineral calamine (as in calamine lotion, a soothing skin cream - the term was also used for other Zn minerals) was renamed smithsonite in his honour.

Smithsonite is rhombohedral, with vitreous lustre and widely varying colour. The formula indicates a Zn metal content of 52 weight percent. Mohs hardness 4-4.5, specific gravity circa 4.2. Smithsonite may exhibit pale yellow fluorescence in both long-wave and short-wave ultraviolet light. Smithsonite from some localities fluoresces hot pink to blue in SW UV light (Schneider, 2006). The carbonate is well-known, being described and illustrated in 173 records in the MINLIB database, 1901-2017. 75 (43%) of these are in collector-oriented publications, mostly the Mineralogical Record, and 60 of those in the past 20 years, a sure index of the recent popularity of this colourful mineral species with well-heeled collectors.

Smithsonite as an ore mineral

Sphalerite alters during weathering, and one alteration product is a boxwork of smithsonite and hematite (Andrew, 1980). Smithsonite has long been recognized as a product of the weathering zone of ore deposits (e.g., Emmons, 1917,pp.372-290; Loughlin, 1918). Secondary minerals may form especially quickly in artificial settings, such as old mine workings and mine dumps. In one study of a limestone and base-metal sulphide ore dump, alteration of galena, chalcopyrite and sphalerite to cerussite, goethite/malachite and smithsonite, respectively was observed. The alteration rate of sphalerite was much faster than that of galena, which was similar or somewhat faster than that of chalcopyrite (Salter and Jones, 1986).

Heyl and Bozion (1962) wrote a 3-part review of oxidized zinc deposits in the USA. The carbonate, silicate and oxide deposits are mostly of supergene origin. Smithsonite and hemimorphite are the main minerals of economic interest: others are hydrozincite, willemite, aurichalcite, descloizite, chalcophanite and sauconite. Hypogene deposits are uncommon, but the unique franklinite- willemite- zincite deposits of Franklin Furnace and Sterling Hill in New Jersey have been the most important source of oxide Zn ores in America.

In Mexico, oxidized manto and chimney deposits were exploited as compact bodies of silver-rich cerussite and anglesite, initially ignoring associated Zn -rich oxide deposits containing Fe- Mn- Zn oxides, smithsonite, hemimorphite, willemite and hydrozincite (Megaw, 2009). Some zinc deposits are dominated by smithsonite, and example being the Torlon Hill deposit in the Santa Rosa corridor of western Guatemala (Anon, 2006). The historically important mines of the Upper Mississippi Valley are dominated by galena, sphalerite and smithsonite, the important ore minerals, associated with calcite and dolomite, chalcopyrite, bitumen and barite (Bain, 1901). European deposits may host blue cuprian smithsonite (Laurion [Laurium] in Greece) or cadmium-rich varieties (Sardinia: Boni and Large, 2003; Boni et al., 2003). In a recent study, Arfe et al. (2017) describe the Mina Grande deposit in Peru. At this deposit karst cavities, derived from the weathering of a Mississippi-Valley-type (MVT) deposit in Mesozoic carbonate rocks, are filled with non-sulphide minerals such as hydrozincite, smithsonite and hemimorphite. In this example, karst formation and supergene mineralization were restricted to the late Miocene and early Pliocene. The karst cavities formed along northwest-southeast faults, and locally along joints in the stratification. Production to date is high-grade (20-30% Zn). The authors illustrate their description with details of the ore textures and paragenesis of the deposit.

In geochemical terms, it is important to note that Pb is fixed whereas Zn is very mobile under alkaline conditions, leading to a high-Zn and low-Pb stream anomaly. This knowledge can be used to find zinc mineralization (e.g., Goodfellow, 1989). Besides UV fluorescence, there are other properties of secondary zinc minerals that may be useful in exploration, on scales from the microscope to airborne and satellite-based surveys. The cathodoluminescence of smithsonite and many other species was reviewed by Hayward (1998). In a detailed review, McConachy et al. (2007) compiled spectral reflectance properties for over 200 non-sulphide Zn and Pb minerals . Tarbuttite, a Zn phosphate, has a distinct spectral reflectance signature. It is an important Zn mineral in the Skorpion Zn deposit in Namibia.

The following table shows a selection of 23 zinc minerals and their ideal formulae. Note that cubic sphalerite and its hexagonal polymorph, wurtzite-2H, alpha- and beta-ZnS, are the only sulphides on the list. The rest, besides the rare native zinc and the silicates hemimorphite, sauconite and willemite, are a variety of oxidized species, such as oxides, carbonates, sulphates, arsenates, vanadates and phosphates. A few, such as the spinels gahnite and franklinite plus willemite, are found in metamorphosed deposits and their trains of indicator minerals. The rest are typically found in weathered and oxidized zones.

Figure 2 (left). Lustrous grey smithsonite rhombs on a thin bed of fine-grained, blue-black chalcocite from Tsumeb.Specimen is from David K. Joyce.

Figure 3 (right). Another "oxide zinc" mineral, the orthorhombic silicate hemimorphite, as clear crystals on a substrate of goethite-coloured material speckled with bright yellow mimetite. Ojuela mine, Mapimi, Durango, Mexico. Specimen: Roger's Minerals.

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