"Rock of the Month # 46, posted for April 2005" --- Sample 538.
Sample donated by Christian Ballhaus and Eugen Stumpfl, August 1985. Polished by George Taylor, photograph by Ontario Geological Survey. This sawn slab is 24.0x7.7x1.3 cm in size. The concentric graphite structures enclose other minerals such as quartz, carbonate and pyrrhotite. Collected from the centre of a pothole by C.G. Ballhaus, on 19 level, B14 incline, Brakspruit shaft area of Rustenberg Platinum Mines, in the southwest Bushveld complex, 25 km east of Rustenberg.
This page is dedicated to Eugen F. Stumpfl (1931-2004), an ore geologist and mineralogist of wide-ranging interests, who made extensive contributions to the understanding of platinum deposits, including those in the Bushveld complex. This site includes a "research bibliography" of Eugen, a partial selection of the prolific output of the professor, his students and colleagues. 61 items are included, 1958-2004.
Graphite, most often identified in the form of shiny black flakes, is an hexagonal mineral, soft (1-2 on the Mohs scale), of low specific gravity (circa 2.2), with a greasy feel. Being soft and sectile (easily cut) it readily separates to mark fingers and fabric. Crystallographic studies distinguish different structural forms or polymorphs, hexagonal graphite-2H and rhombohedral graphite-3R. It is opaque, and in reflected light the highly directional nature of its physical and optical properties (along or across the individual flakes, "anisotropy") make it a very distinctive mineral under the microscope. Like its more glamourous polymorph, diamond, it is a native element, carbon. The occurrences of graphite are diverse, and this unusual mineral is deserving of more than one appearance in this series!
Typical Occurrence of Graphite
Graphite as a mineral is probably unfamiliar to most people, in part because its most common modes of occurrence are in high- grade metamorphic terranes, commonly as black and shiny mica- like flakes disseminated within schists, gneisses and marbles of sedimentary origin. However, in processed form it is recognizable to all, in "pencil lead" and some lubricants. The distinctive sheet- like crystals of graphite can accommodate impurity elements in weakly-bonded spacings between adjacent hexagonally-packed planes. Highly pure synthetic graphite is of great interest in materials science and technology.
Both the occurrence and the diverse uses of graphite have been documented long since, two detailed reviews concerning graphite deposits in the Proterozoic Grenville province in Quebec, Ontario and New York (Cirkel, 1907; Alling, 1917). Historically, the purest graphite has been mined in the high-grade metamorphic terranes of Sri Lanka, where the mineral occur in veins of remarkable lustre and purity (Katz, 1987; Silva, 1987). Carbonaceous matter in parts of the Earth's crust subject to lower grades of metamorphism generally exhibits a lower degree of crystallinity, and some confusing terminology applies. Graphite may also occur, less commonly, in igneous rocks, a striking example of which is illustrated here. Most natural graphite can be divided in practical fashion as either flake or amorphous material. The economic classification becomes quite complex, with grain size of the flake graphite a key factor. Typical graphite deposits are quite small by modern mining standards (Sutphin and Bliss, 1990).
The Bushveld Graphite
The Bushveld complex of South Africa is the world's largest layered intrusion, home to the world's largest resources of metals such as platinum and chromium. Its unparalleled mineral endowment includes the Merensky Reef, a thin layer within the thick igneous stratigraphy containing sulphides and rare minerals enriched in the six platinum group elements (PGE) and gold. Other layers are rich in oxide minerals, such as chromite and magnetite, containing chromium, iron and vanadium resources.
Graphite is not a common mineral in typical igneous rocks, and seldom very abundant. However, the Bushveld contains some famous exceptions to the rule. Unusual olivine-rich (dunite) pipes in the eastern Bushveld contain graphite which occurs as globules, as intergranular films between olivine grains, and along fractures cutting chrome spinel grains (Stumpfl and Rucklidge, 1982). Graphite is also present in the peculiar "pothole" structures which cut across the layered igneous sequences which compose the bulk of the great volume of rocks within the Bushveld complex. The potholes cut both strongly- and weakly-mineralized units such as the Merensky Reef and the so-called Bastard Reef. Textures and mineral compositions are suggestive of high volatile activity, with the presence of hydrous phases such as amphiboles and micas, and enrichments in chlorine (Ballhaus and Stumpfl, 1985). The graphite displays remarkable layered and spherulitic textures, often associated with sulphides and other minerals. The host potholes may be concentrated above anticlines in the metasedimentary basement to the intrusion (Ballhaus, 1988).
Graphite also occurs in meteorites, in various classes of iron meteorites, ureilites and chondrites. Most notably, graphite is one of the principal mineral species that are identified as presolar, interstellar grains, which may be found at levels of a few parts per million in primitive meteorites. Such grains are generally minute, microns (0.001 mm) or less in diameter, and have isotopic characteristics consistent with origin as "stardust" formed in stellar atmospheres (Anders and Zinner, 1993). This exotic "space graphite" may form in astrophysical environments such as AGB stars, novae and Wolf-Rayet stars. Much larger (pea to fist-sized) nodules of graphite, often with associated sulphide or metal, are prominent in iron meteorites, and were documented a century before (e.g., Farrington, 1901).
Alling,HL (1917) The Adirondack graphite deposits. New York State Museum Bull. 199, 150pp.
Anders,E and Zinner,E (1993) Interstellar grains in primitive meteorites: diamond, silicon carbide, and graphite. Meteoritics 28, 490-514.
Ballhaus,CG (1988) Potholes of the Merensky Reef at Brakspruit Shaft, Rustenburg Platinum Mines: primary disturbances in the magmatic stratigraphy. Econ.Geol. 83, 1140-1158.
Ballhaus,CG and Stumpfl,EF (1985) Occurrence and petrological significance of graphite in the Upper Critical Zone, western Bushveld complex, South Africa. EPSL 74, 58-68.
Cirkel,F (1907) Graphite, its Properties, Occurrence, Refining and Uses. Department of Mines, Ottawa, 307pp. plus maps.
Farrington,OC (1901) The constituents of meteorites: I. J.Geol. 9, 393-408.
Katz,MB (1987) Graphite deposits of Sri Lanka: a consequence of granulite facies metamorphism. Mineralium Deposita 22, 18-25.
Silva,KKMW (1987) Mineralization and wall-rock alteration at the Bogala graphite deposit, Bulathkohupitiya, Sri Lanka. Econ.Geol. 82, 1710-1722.
Stumpfl,EF and Rucklidge,JC (1982) The platiniferous dunite pipes of the Eastern Bushveld. Econ.Geol. 77, 1419-1431.
Sutphin,DM and Bliss,JD (1990) Disseminated flake graphite and amorphous graphite deposit types: an analysis using grade and tonnage models. CIM Bull. 83 no.940, 85-89, August.
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