Smithsonite from Tsumeb, Namibia

--- a non-sulphide ore mineral of zinc

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Figure 1. Pink crystalline smithsonite from the Tsumeb mine. Specimen from Doug Wilson. Right: mineral collectors value provenance, and a set of labels is the classic way of displaying the travels of a specimen from source to current location.


"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.

23 selected zinc minerals

Name Ideal formula
Adamite Zn2AsO4(OH)
Aurichalcite (Zn,Cu)5(CO3)2 (OH)6
Chalcophanite (Zn,Fe,Mn)Mn3O7.3H2O
Descloizite PbZnVO4(OH)
Franklinite (Zn,Mn,Fe)(Fe,Mn)2O4
Gahnite ZnAl2O4
Hemimorphite Zn4Si2O7(OH)2.H2O
Hetaerolite ZnMn2O4
Hydrozincite Zn5(CO3)2(OH)6
Paradamite Zn2AsO4(OH)
Phosphophyllite Zn2(Fe,Mn)(PO4)2.4H2O
Prosperite CaZn2(AsO4)2.H2O
Sauconite Na0.3Zn3(Si,Al4O10)(OH)2.4H2O
Smithsonite ZnCO3
Sphalerite ZnS
Tarbuttite Zn2PO4(OH)
Willemite Zn2SiO4
Wurtzite ZnS
Zinc Zn
Zincite (Zn,Mn)O
Zincochromite ZnCr2O4
Zincroselite Ca2Zn(AsO4)2.2H2O
Zinkosite ZnSO4

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


Smithsonite for mineral collectors

Hughes et al. (2010) offer a multi-author review of fine smithsonite specimens, a topographical mineralogy featuring localities in Italy, Greece, Mexico, the USA (Arizona, Arkansas and New Mexico), Australia and Namibia. One of the attractions is the diversity of colour: minor elements, such as copper (blue-green), cadmium (yellow) and cobalt (pink) are amongst the possible colouring agents. Major mineral collections will often have colourful examples of smithsonite. A review of sources of fine specimens of the mineral was prepared by Moore (2016b, Volume 2, pp.507-513). Some of the most famous mineral localities for smithsonite, in north America and southern Africa, include:

  • Magdalena mine, Kelly, New Mexico - noted for fine, colourful specimens of botryoidal smithsonite (Gibbs, 1989).
  • The Franklin Furnace-Sterling Hill area, New Jersey is a prolific locality. The primary ore at the mines was composed of franklinite, zincite, willemite and tephroite (Palache, 1935).
  • Refugio mine, Sinaloa, Choix, Mexico - botryoidal material in many hues: cream and yellow, orange, pink, mauve, blue-green and green (Wilson, 2011).
  • The Rush Creek district, Marion County, Arkansas - bright yellow "turkey fat" smithsonite (Richards, 1993).
  • Many specimens come from Namibia, whether from the Otavi Mountain land (Tsumeb, Berg Aukas) or elsewhere (e.g., Barlow et al., 1996; Cairncross, 1997). The famed Tsumeb mine is noted for fine crystalline smithsonite (see, e.g., Moore, 2016a,b). Green cuprian smithsonite from Tsumeb is well-prized (Wilson et al., 1992). There is, as noted, a remarkable range of mineral colour in smithsonite from Tsumeb and Berg Aukas (Wilson, 2014, pp.109-126; Moore, 2016a).
  • The Broken Hill Mine, Zambia (Notebaart and Korowski, 1980).

References

Andrew,RL (1980) Supergene alteration and gossan textures of base-metal ores in southern Africa. Min.Sci.Eng. 12, 193-215.

Anon (2006) Firestone extends Torlon zinc high-grade zone. Northern Miner 92 no.20, 11, 07 July

Arfe,G, Mondillo,N, Boni,M, Balassone,G, Joachimski,M, Mormone,A and Di Palma,T (2017) The karst-hosted Mina Grande nonsulfide zinc deposit, Bongari district (Amazonas region, Peru). Econ.Geol. 112, 1089-1110.

Bain,HF (1901) Preliminary report on the lead and zinc deposits of the Ozark region. USGS 22nd Annual Report, Part II - Ore Deposits, 23-227.

Barlow,FJ, Jones,RW and LaBerge,GL (editors) (1996) The F. John Barlow Mineral Collection. Sanco Publishing, Appleton, WI, 408pp.

Boni,M and Large,D (2003) Nonsulfide zinc mineralization in Europe: an overview. Econ.Geol. 98, 715-729.

Boni,M, Gilg,HA, Aversa,G and Balassone,G (2003) The "calamine" of southwest Sardinia: geology, mineralogy, and stable isotope geochemistry of supergene Zn mineralization. Econ.Geol. 98, 731-748.

Cairncross,B (1997) The Otavi Mountain land Cu-Pb-Zn-V deposits, Namibia. Mineral.Record 28, 109-130,157.

Emmons,WH (1917) The Enrichment of Ore Deposits. USGS Bull. 625, 530pp.

Gibbs,RB (1989) The Magdalena district, Kelly, New Mexico. Mineral.Record 20 no.1 (New Mexico issue, pp. 1-96), 13-24.

Goodfellow,WD (1989) Interpretation of stream geochemistry leading to the discovery of a secondary zinc deposit, Pelly River, Nahanni map area, Yukon. GSC Pap. 89-1E (Cordillera and Pacific Margin), 279pp., 31-50.

Hayward,CL (1998) Cathodoluminescence of ore and gangue minerals and its application in the minerals industry. In `Modern Approaches to Ore and Environmental Mineralogy' (Cabri,LJ and Vaughan,DJ editors), MAC Short Course 27, 428pp., 269-325.

Heyl,AV and Bozion,CN (1962) Oxidized Zinc Deposits of the United States. Part 1. General Geology. USGS Bull. 1135-A, 52pp. plus map.

Hughes,T, Liebetrau,S and Staebler,G (editors) (2010) Smithsonite: Think Zinc! ExtraLapis, 104pp.

Loughlin,GF (1918) The oxidized zinc ores of Leadville, Colorado. USGS Bull. 681, 91pp.

McConachy,TF, Yang,K, Boni,M and Evans,NJ (2007) Spectral reflectance: preliminary data on a new technique with potential for non-sulphide base metal exploration. Geochemistry: Exploration, Environment, Analysis 7, 139-151.

Megaw,PKM (2009) Evaluation of oxidized Pb-Zn-Ag carbonate replacement deposits of Mexico in light of supergene zinc and residual lead enrichment processes. In `Supergene Environments, Processes, and Products' (Titley,SR editor), SEG Spec.Publ. 14, 149pp., 51-58.

Moore,TP (2016a) Collector profile: John Schneider, Tsumeb collector. Mineral.Record 47 no.2, 171-185.

Moore,TP (2016b) Moore's Compendium of Mineral Discoveries, 1960-2015. Mineralogical Record, Inc., Tucson, 2 volumes, 809+813pp.

Notebaart,CW and Korowski,SP (1980) Famous mineral localities:the Broken Hill Mine, Zambia. Mineral.Record 11 no.6, 339-347 .

Palache,C (1935) The minerals of Franklin and Sterling Hill, Sussex county, New Jersey. USGS Prof.Pap. 180, 135pp.

Richards,DA (1993) Famous mineral localities: the Rush Creek district, Marion County, Arkansas. Mineral.Record 24, 285-299.

Salter,JD and Jones,MP (1986) Galena alteration in a bicarbonate environment. TIMM C95, 95-106.

Schneider,S (2006) The World of Fluorescent Minerals. Schiffer Publishing Ltd, Atglen, PA, 192pp.

Wilson,WE (2011) The Refugio mine, municipio de Choix, Sinaloa, Mexico. Mineral.Record 42, 453-461.

Wilson,WE (editor) (2014) Des Sacco: A Few African Favorites. Mineral.Record 45 no.1, supplement, 136pp.

Wilson,WE, Bartsch,JA, Van Pelt,H and Van Pelt,E (1992) Minerals of the Houston Museum of Natural Science. Mineral.Record 23 no.1, supplement bound into issue, 34pp.

Graham Wilson, 29 August 2017-01 September 2017

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