Bruce Cairncross

The Bivalve Megadesmus from the Permian Volksrust Shale Formation (Karoo Supergroup), northeastern Karoo Basin, South Africa: implications for late Permian Basin development

The bivalve Megadesmus is described from the Late Permian Volksrust Shale Formation in the northeastern Karoo Basin, South Africa; this is the first reported discovery of this genus in Africa. The fossil is large, 9 cm dorsally and 8.4 cm laterally, and both valves are articulated indicating minimum transport after death. The bivalve was encased in interbedded siltstone-shale that constitutes the distal sediments of a prograding delta, at the Beaufort Group–Ecca Group boundary. Megadesmus is known from other continents (Australia, India, Siberia, South America and Tasmania) where its presence indicates exclusively marine conditions. The implication for the northeastern Karoo Basin during the Late Permian is that a marine enclave still existed in this geographic area, and that terrestrial conditions did not yet prevail as in the southern basin region.

The Minerals of Palabora, Limpopo Province, South Africa

Ancient copper workings on the hill called Loolekop (at the core of the PIC) attracted early European explorers to the area, and mines were established in the 1930s and 1940s to recover apatite and vermiculite from the complex. In 1956 the Palabora Mining Company was set up to develop the large low-grade copper resource hosted by the carbonatite and foskorite (or phoscorite†) rocks of the PIC. Large-scale open-pit copper mining commenced in 1964 (fig. 4), and a diverse range of minerals and metals including copper, apatite, vermiculite, magnetite, baddeleyite, uranium, nickel, gold, silver, platinum, and palladium have subsequently been recovered (http://www.palabora.com/palabora.asp; accessed February 2016; Roux et al. 1989). Limpopo Province South Africa

South African Diamonds: A Photographic Personal Perspective

Dr. Bruce Cairncross is head of the Department of Geology at the University of Johannesburg and a consulting editor of Rocks & Minerals. Diamonds, together with gold, are inextricably woven into the social, economic, and political fabric of South Africa. For more than 140 years, these two commodities have given rise to industries that have sustained the economic development of the region. Diamonds were originally the mainstay of the South African mining industry, and although South Africa no longer leads the world rankings, diamonds still provide employment and money for many individuals and companies.

The Mineral Collection of Desmond Sacco, Johannesburg, South Africa

collections of southern African minerals in the world. Many knowledgeable individuals, including major museum curators, experienced collectors, prominent mineral dealers, and others, have viewed the collection, and all unanimously agree that it is outstanding. Although the collection focuses primarily on the Tsumeb mine and the Kalahari manganese field, other “classic” southern African localities are also represented, such as Klein Spitzkoppe, Kombat, Tschudi, Erongo, Onganja, Berg Aukas, Rosh Pinah, the Alto Ligonha district of Mozambique, the Goboboseb Mountains, and the Democratic Republic of the Congo. Despite this emphasis on southern Africa, there are also outstanding specimens from other classic worldwide localities, but because this issue of Rocks & Minerals is celebrating Africa and its minerals, these will be featured here and not the non-African specimens. Suffice it to say that the Sacco collection also contains outstanding Russian and Moroccan minerals as well as subsuites from China, Pakistan, and other overseas localities. Sacco was born in Johannesburg and has lived his entire life in this city. His father, Guido Sacco, was an Italian mining engineer who came to South Africa in 1926 and who played a pivotal role in the discovery and development of the Kalahari manganese field. The elder Sacco’s activities in mining and exploration prompted his son’s early interest in geology and minerals that led to his pursuit of degrees in geology and also to his collecting minerals. BRUCE CAIRNCROSS Department of Geology University of Johannesburg PO Box 524, Auckland Park 2006 Gauteng, South Africa brucec@uj.ac.za

Connoisseur's Choice: Hydrocerussite, Tsumeb Mine, Namibia

White minerals are generally considered to be less collectible than the more vividly colored varieties; however, when they assume the morphology of other world-famous species, such as reticulated cerussite, their status becomes somewhat elevated. Combine this aspect with a degree of rarity, and white minerals such as hydrocerussite become more desirable. The Tsumeb mine, in particular, is world famous for its magnificent cerussite specimens, especially the twinned honeycomb variety. These occur in dinner plate–sized specimens, but even smaller ones are highly sought by collectors. When hydrocerussite coats or pseudomorphically replaces these crystals, it imparts an attractive pearly white or snow-white coating providing beautiful contrast to the cerussite specimens. For this reason, Tsumeb hydrocerussite has been chosen as the featured specimen in this Connoisseur’s Choice column. Tsumeb hydrocerussite is somewhat of a “Cinderella” to its closely related cousin. As an example, Mindat.org (accessed June 2017) has 715 images of cerussite specimens, but only 18 of hydrocerussite. This is because Tsumeb mine, Namibia, hydrocerussite occurs as an alteration product of cerussite rather than as spectacular, large primary crystals and also because hydrocerussite is less common than cerussite. Hydrocerussite, Pb3(CO3)2(OH)2, is hexagonal and forms pyramidal to tabular crystals, typically in scaly aggregates and cleavable masses (Anthony et al. 2003). It has perfect {0001} cleavage and a hardness of 3.5. The crystals are transparent to translucent white to gray, with a pearly to adamantine luster. The Connoisseur’s Choice specimen of hydrocerussite chosen here (figs. 1 and 2), from the Tsumeb mine, was formed by the alteration of cerussite. However, primary hydrocerussite crystals do occur here, as well as at such other places as the United Kingdom and Arizona in the United States, as described further on. Hydrocerussite is chemically equivalent to white lead, an artificial compound formerly used as an additive to a cosmetic, Venetian Ceruse, because of the opacity and satinsmooth property that it imbued when mixed with oils. White lead was also used in lead paint. Nowadays these applications are prohibited for health reasons. Synthetic hydrocerussite can easily be manufactured by the action of carbon dioxide and water on either lead or litharge at pH 4–5 (Martinetto et al. 2002). Keller (1977) was the first to positively identify Tsumeb, Namibia, hydrocerussite (figs. 1–10). The presence of hydrocerussite indicates highly alkaline conditions of pH 10– 13; therefore, the mineral is an indicator of such chemical environments. Keller (1977) described Tsumeb hydrocerussite from three different mineral assemblages: (1) tiny (to 0.3 mm), tabular, iridescent to pearly white crystals forming scaly aggregates, associated with 3-mm azurite crystals, acicular malachite, mimetite, and calcite (the hydrocerussite occurs only on the azurite); (2) hydrocerussite either intergrown with or overgrown on cerussite, associated with pale green arsentsumebite, azurite, malachite, pale yellow mimetite, and 2–4-mm phosgenite crystals; and (3) pseudomorphs of hydrocerussite after cerussite, in crystals to 2 mm. Some of these crystals still contain unaltered cores. These are associated with small radiating sprays of mimetite, duftite, and white dolomite crystals. Keller (1977) concluded that the highly alkaline weathering solutions that produced the hydrocerussite were rare because hydrocerussite is relatively rare at Tsumeb. He speculates that more might be found at depth because the formation of hydrocerussite requires a constant and stable Hydrocerussite BRUCE CAIRNCROSS Department of Geology University of Johannesburg PO Box 524, Auckland Park 2006 Johannesburg, South Africa brucec@uj.ac.za

The Where of Mineral Names: Skorpionite, Skorpion Mine, Lüderitz District, Karas Region, Namibia

Skorpionite, Ca3Zn2(PO4)2CO3(OH)2·H2O, forms acicular, vitreous, colorless monoclinic crystals a few millimeters long and seldom more than several millimeters in length (fig. 1). These are elongate...

The Where of Mineral Names: Namibite, Namib Desert, Copper Valley, Khorixas District and Region, Namibia

NAMIBITE, Cu(BiO)2VO4(OH), was discovered on the farm 504 Mesopotamia located in Copper Valley in northwestern Namibia (von Knorring and Sahama 1981) where it occurs in a polymetallic mineralized hydrothermal quartz vein (fig. 3). Subsequent to its description as a new species, its crystal structure and symmetry were revised by Kolitsch and Giester (2000). Namibite is found as small (less than 2-mm), translucent to transparent, olivegreen to dark green crystals (fig. 2) with a pistachio-green streak (https://www.mindat.org/min-2836.html; accessed April 2017). It is triclinic-pseudomonoclinic (Kolitsch and Giester 2000), has good cleavage on {001}, and forms in various habits including platy crystals, radiating aggregates, and rounded masses (Anthony et al. 2000). Twinning is common on {011}. At the type locality, namibite is associated with beyerite, brochantite, khorixasite, and quartz. Since its discovery in Namibia, namibite has been found in other global localities (Dunning and Cooper 1998), and Mindat currently lists twenty-three known localities. The mineral is named after the Namib Desert (fig. 1).

Who's Who in Mineral Names: Edgar Donald Mountain (1901–1985)

It seems appropriate that a geologist bearing the surname of mountain should have a mineral named after him, and so it is with Edgar Donald Mountain, geologist, academic, and Olympic athlete. He was born in 1901 in Camberwell, Great Britain, graduated from Corpus Christi College, Cambridge, and died in 1985 in Grahamstown, Eastern Cape Province, South Africa. Apart from his prowess as a geologist, Mountain was an accomplished athlete and a Cambridge blue (honored athlete). At the age of nineteen he competed in the 1920 Antwerp Olympic Games in the 800-meter final. He finished fourth, setting a new United Kingdom junior record time (http://www.sports-ref erence.com/olympics/athletes/mo/edgar-mountain-1.html; accessed January 2017). In addition, he set a world record for the 500 meter at Stockholm in 1921 and competed a second time in the 1924 Paris Olympic Games where he also ran the 800 meter. Conflicting documentation states that he qualified for the final race, but some sources state he was eliminated in the semifinal heats. In June 1922, at the age of twenty-one, Mountain was employed by the Natural History Museum, London as an assistant in the Department of Mineralogy (http://www. nhm.ac.uk/research-curation/library/archives/catalogue; accessed January 2017). He worked at the museum for four years before resigning due to ill health. He and his wife then immigrated to South Africa where he took up an academic post as Lecturer in Mineralogy at the Rhodes University College, now Rhodes University, Grahamstown. In 1929, when Ernest H. L. Schwarz, head of the department, was killed during a field trip to Senegal, Mountain took over the position. He spent the following forty-three years at Rhodes University, until his retirement in 1969 (http://www.ru.ac.za/geology/geology/rhodesgeology department; accessed January 2017). Mountain was an accomplished field geologist, and during his tenure at Grahamstown, he undertook extensive field mapping in Eastern Cape Province producing the Geological Survey maps for areas east of Grahamstown, Keiskamma Hoek (1952), the Border region (1962), Port Alfred (1962), and East London, Kidd’s Beach, and Kei Mouth (all in 1974). He had a particular interest in dolerites, which are common in the Eastern Cape and neighboring KwaZulu-Natal Province. Apart from these geological mapping expeditions, BRUCE CAIRNCROSS Department of Geology University of Johannesburg PO Box 524, Auckland Park 2006 Johannesburg, South Africa brucec@uj.ac.za

The Where of Mineral Names: Iowaite, Sioux County, Iowa

I T HAPPENS FROM TIME TO TIME IN THE MINERALOGICAL WORLD that a mineral species gets discovered in one locality and, sometime thereafter, superior examples are found in a new far-flung area. Such is the case with iowaite. It was originally described from a drill core that penetrated Precambrian serpentinites 1,000–1,500 feet below the surface in Sioux County, Iowa (Kohls and Rodda 1967); approximately thirty-five years later, some of the finest examples known were collected at the Palabora mine, Limpopo Province, South Africa (Southwood and Cairncross 2017). One of these specimens is illustrated here. Iowaite, Mg6Fe2 (OH)16Cl2·4H2O, is a member of the hydrotalcite group and crystallizes in the trigonal system. It is typically blue-green to pale green and becomes an amber (dull orange) color when exposed to air, reputed to be as a result of alteration to pyroaurite (Anthony et al. 1997). It has perfect cleavage on {0001}, a waxy to resinous luster, a hardness of 1.5–2.5, and a greasy feel. At the type occurrence, crystals reached 2–3 mm in veinlets 1–20 mm wide in the serpentinized olivine-rich rocks. Associated with iowaite were post-serpentinization chrysotile, dolomite, brucite, calcite, magnesite, and pyrite. Since the description of iowaite in 1967, its structure has been established by Allmann and Donnay (1969). Braithwaite et al. (1994) further refined the formula of iowaite using crystals from the Palabora mine. Sioux County, Iowa

Connoisseur's Choice: Hausmannite, N'Chwaning II Mine, Kalahari Manganese Field, Northern Cape Province, South Africa

Black minerals are generally not considered highly desirable among collectors, although there are exceptions. This bias is unfortunate because some black minerals crystallize in aesthetic forms and can be associated with other species that do add color and interest. Such a mineral is hausmannite. N’Chwaning II hausmannite is the featured species of this Connoisseur’s Choice column, chosen because it forms large, highly lustrous black crystals and is frequently associated with colorful species such as andradite, datolite, and calcite. Hausmannite is widespread in manganese deposits globally. Mindat.org (accessed October 2017) lists forty-one countries with 338 individual localities that are reported to have produced hausmannite. This attests to the mineral’s relatively common distribution, yet fine connoisseur collector specimens are rare compared to the mineral’s abundance. Hausmannite, MnMn2 O4, crystallizes in the tetragonal system and can be considered a tetragonally deformed spinel type (Jarosch 1987). The most common habit is as dipyramidal pseudo-octahedral crystals. It forms resinous, metallic, submetallic to dull black, frequently striated crystals. It has a dark brown streak and a hardness of 5.5; it displays perfect {001} cleavage (Anthony et al. 1997). Depending upon the amount of iron present, some hausmannite is magnetic (Gutzmer et al. 1995; Baron et al. 1998), a physical property known since the 1950s (Kasper 1959). The type locality for hausmannite is Öhrenstock, Langewiesen, Ilmenau district, Thuringian Forest, Thuringia, Germany. Originally named “flaky black-brown-stone” (blättricher Schwarz-Braunstein) in 1813 by Johann Friedrich Ludwig Hausmann, it was renamed hausmannite in his honor by Wilhelm Karl Haidinger in 1827 (Haidinger 1827), although Haidinger states the renaming was a joint proposal with a Dr.Turner (Haidinger 1827, p. 128):