-   Bruce Cairncross

Dept of Geology, RAU    P O Box 524    Auckland Park, 2006, RSA    Email:bc@na.rau.ac.za

Poldervaartite (Ca(Ca_0.5Mn_0.5)(SiO₃OH)(OH) is a type-locality acid nesosilicate species discovered at the Wessels mine in the Kalahari manganese field South Africa (Dai et al., 1993). It is named after Arie Poldervaart (1918-1964) who was professor of petrology at Columbia University. He was an expert on the dolerites and Archaean rocks in the Northern Cape Province in South Africa. Although the species was described in 1993, specimens in private collections in South Africa are known to predate the type material. At least five previous finds have been made of this rare mineral. During 1979, one of the first specimens was collected at the Wessels mine. In the following year, 1980, pale-pink colloform crystal aggregates were collected at the Wessels mine. Some of these were found together with bultfonteinite. At the time, these were recognised as being an oddity from the area, but were never quantitatively identified. Later, in mid-1995, some of the finest examples of poldervaartite known at the time were collected (Cairncross et al., 1997). These were highly lustrous pink to peach coloured crystals, most occurring as stellate groups. The majority of these were small thumbnail to miniature specimens, but a few spectacular specimens had crystals on matrix (see the illustrations on pages 121-122 in Cairncross et al., 1997). Then in 1996 beautiful amber-coloured, euhedral micro-crystals up to 4 mm were found at the Wessels mine. All of the poldervaartite was recovered from the Wessels mine and most of these finds yielded relatively small and relatively few specimens. This changed dramatically late last year, during the period from October 2001 through to late February 2002. During this period, specimens of poldervaartite were discovered in the hanging wall at the neighbouring N’Chwanning II mine. During this 4-5 month period, an estimated 5,000 specimens were recovered, a remarkable mineralogical discovery for a rare type-locality species that is still only known from this one region. Some of the top-end specimens from this discovery are now, undoubtedly, the finest known examples of poldervaartite.

2001 Discovery.

During the mining of the orebody, an alteration zone was encountered in the hanging wall. This consisted of oxidised, brown, altered hematite and manganese ore. This zone of alteration ran parallel to the hanging wall for some distance and frequently had to be blasted down for safety reasons. The poldervaartite mineralisation was in a fissure approximately 30 cm wide and about 40 m long that was striking in a north-south direction, and approximately 16 m wide in an east-west direction. The stratigraphic location of the poldervaartite zone was in the contact between the lowermost manganese orebody and the overlying banded-iron formation. The exact location in the mine is in the 45 and 46 South Section of N’Chwaning II, at a vertical depth of 480 metres below the surface. The first “pocket” of poldervaartite was discovered due to the fact that roofbolts in the hanging wall failed to anchor properly as they had penetrated the friable mineral-bearing zone. The cream variety of poldervaartite was first discovered and shortly thereafter, the orange-red variety was found about 25 m into the area.

The first hint of poldervaartites to appear in the collector market was when I saw some specimens late September 2001, when the Kimberley collector Dr Ludi von Bezing showed me a few minute crystals that he had recently obtained from N’Chwanning II mine in a parcel of other minerals. It was not until several weeks later, during November, that the size of the discovery became apparent.

Specimen sizes range from thumbnails to museum sized pieces. Two distinct varieties of poldervaartite were collected. The first and most common type has individual crystals that are opaque and cream, pale flesh-pink to off-white in colour. Many specimens have crystals bundled together into radiating colloform aggregates. Some are bowtie-shaped, with the crystal aggregates tapering into a central point. A lot were collected on matrix, with the cream-coloured poldervaartite providing an attractive colour contrast on the black to ochre-brown matrix. Then some specimens have freestanding individual crystals with a high lustre. These occur randomly scattered on matrix or in groups of crystals. A third distinctive habit consists of spherical balls, up to several centimetres in diameter. Scanning electron microscopic imaging reveals that these spherical balls are composed of numerous stacked stubby crystals, with a myriad of sharply defined crystal faces on the outer surface. Some of these spheres are paired, resembling dumbbells; others have several spheres aggregated together. The terminations of some of the crystals display an interesting feature. The ends of the crystals, particularly the spherical varieties have what appear to be damaged surfaces. On some specimens, all of the crystals display this “damaged” surface, while on others, “damage” appears to be restricted to isolated occurrences. On closer inspection, however, the terminations are seen to have no damage but have a corroded or resorbed texture; instead of pinacoidal crystal surfaces, the ends are partly cavernous and reveal what may be interpreted as concentric compositional zoning of the crystals, with some zones being more susceptible to corrosion than others.

The second variety of poldervaartite is very different, and visually stunning; either perfect spheres of translucent crystals or “wheat-sheaf” like bundles of crystals. In their habit, these bear a striking morphological resemblance to the spheres of rhodochrosite that came from the same locality, such as the famed “Snail” specimen. These poldervaartites are amber to peach-pink coloured and some spheres are up to 3-4 cm diameter, some on matrix. Some are deep-orange coloured. The myriad of crystal terminations on these particular species emit a sparkle of light, and coupled with the attractive colour, are truly beautiful examples of this rare type-locality species. Then there are specimens that appear in intermediate form. These consist of clusters of parallel intergrown crystals resembling sheaves of wheat. They too have very high lustre and some are translucent to transparent. The cream-coloured poldervaartites fluoresce deep-red under short-wave ultraviolet light. Associated species are few. Acicular pale-blue celestine crystals up to 5 mm occur on some specimens, but not many. Microcrystalline hematite, minor andradite, bultfonteinite and rare oyelite constiute some of the minerals in the and on the oxidised matrix to the poldervaartite.

The “rarity” value of these poldervaartites now comes into question. As a few thousand specimens have been collected, it can be said that it is now a “common” mineral from the region. But it remains a type-locality species and unless many more specimens are again discovered, it is likely to remain rare. This is typical for the pockets from the high-grade manganese ore. Pockets of high-grade Wessels-type ore are not rare, but most contain minerals that are once-off discoveries. The ettringite-sturmanite of the early 1980’s is a classic case where thousands of crystals were recovered and many collectors virtually lost interest in the seemingly endless flood of specimens. However, they have never been as plentiful since and most were quickly absorbed into the mineral-hungry collector market.

The zone that produced these minerals has now been mined out and no more specimens are forthcoming.

References.

Cairncross, B., Beukes, N.J. and Gutzmer, J. (1997). The Manganese Adventure. Associated Ore & Metal Corporation, Johannesburg. ISBN 0-620-21235-7, 250 pages.

Dai, Y., Harlow, G.E. and McGhie, A.R. (1993). Poldervaartite Ca(Ca_0.5Mn_0.5)(SiO₃OH)(OH), a new acid nesosilicate from the Kalahari manganese field, South Africa: crystal structure and description. American Mineralogist, 78, 1082-1087.