"Rock of the Month #172, posted for October 2015" ---

A "pyrite sun"

from a coal mine in the Midwestern state of Illinois. These discoidal bodies differ dramatically from the typical cubic or modified cubic forms of pyrite as it is found most commonly in nature: see, for example, the varied but clearly cubic habits in three other Rocks of the Month:

The pyrite suns are said to occur in a narrow black shale layer, situated on top of a coal seam 100 feet (91 metres) below surface. This is not unusual. In Britain, where most coal resources are of Carboniferous age, "shales are the dominant rock of the Coal Measures, generally forming the roof of coal seams" (Raistrick and Marshall, 1939, p.21).

Pyrite, marcasite and sphalerite are sulphides that occur in a range of sedimentary rocks. Carboniferous coals of the Midwestern states such as Illinois, Iowa, Missouri and Kansas contain minerals such as kaolinite, pyrite, sphalerite and calcite (Whelan et al., 1988). Pyrite is common in reduced black shales rich in organic matter, as in the Devonian of Illinois and Indiana (Anderson et al., 1987); the Carboniferous of Indiana, Illinois, Missouri and Kansas (Coveney et al., 1987; Desborough et al., 1990; Schultz and Maynard, 1990); and Cretaceous shales of South Dakota (Leventhal and Taylor, 1990).

The pyrite disc occurs in a shale layer associated with coal seams. It also contains traces of plant fossils, ascribed to the fern-like genus Alethopteris. The age is Carboniferous (359-299 Ma), and probably Pennsylvanian (upper Carboniferous, 323-299 Ma). These plant fossils are found widely, e.g., in the central USA (White in Taff, 1899); the British Isles (Anon, 1969; Bowden et al., 2005); Spain; and elsewhere in Europe (Turek et al., 1988).

There are at least three possible modes of origin, explaining the peculiar form of the sulphide. These include:

• 1. Fossil replaced by pyrite? Fossils may be preserved in detailed pyritic casts generated by molecular replacement (e.g., Briggs and Edgecombe, 1992). As described by Elizabeth Kolbert (2014), fossils such as trilobites can be replaced by pyrite. The same applies for gastropods in Illinois (Eckert, 1987). Microfossils may also be pyritized and thus preserved, as in Cretaceous strata in the Cariboo-Chilcotin region of south-central British Columbia (Haggart et al., 2011). However, the pyrite suns are not any obvious type of fossil, despite superficial similarities in morphology to echinoderms such as sand dollars.
• 2. Deformation of pre-existing crystals? Sulphides, quartz and kaolinite may occur as epigenetic cleat (joint) -filling minerals within coal (Finkelman, 1992). Black shales tend to be organic carbon-rich, highly reducing, and good traps for a variety of trace metals. Yet the shale host rock does not seem to be appreciably deformed, nor metamorphosed, and similar radiating pyrite bodies are not observed in shear zones within Precambrian carbonaceous metasediments.
• 3. A concretion? The sulphide bodies are distinct from metre-scale coal balls, carbonate-rich permineralized peat found generally in the coal seam itself, in areas where the roof rock is marine sediment (De Maris, 2000). The upper Carboniferous Pittsburgh coal bed at Pursglove, West Virginia, USA, contains both pyrite- and dolomite-dominant concretions. Pyrite tends to form at higher pH than marcasite (not found at Pursglove). Some of the sulphidic bodies weathered and disintegrated fast in the laboratory. The Pittsburgh coal is generally succeeded by several feet of shale, but limestone is locally present, and could promote an alkaline environment (Foster and Feicht, 1946). Looking at Figures 1-2, it seems possible that the pyrite nucleated on organic matter or other nuclei, and grew outwards during diagenesis, within the as-yet-uncompacted clay-rich sediment that would soon consolidate into shale.

A sulphidic concretion is the preferred explanation. A significant early contribution was the work of Van Horn and Van Horn (1933). Their study of Fe sulphide concretions in Devonian- lower Carboniferous shales and sandstones near Cleveland, Ohio investigated the mineralogy of nodular masses which can be either massive or radiating. The acicular masses were commonly assumed to be orthorhombic marcasite rather than cubic pyrite. Such "marcasite suns" were known in the bituminous shales at Sparta in Illinois, the location of this Rock of the Month. The structures were found by x-ray diffraction to be pyrite (and thus are "pyrite suns"). The two polymorphs may also form intergrowths, as noted at the Mount Carroll, Mulford and Irene quarries in northern Illinois (Richards et al., 1995), though it should be noted that these occurrences are in vugs in Ordovician limestones, the sulphides are microscopic to mm-sized, and furthermore feature growth of marcasite blades and chains of small pyrite cubes on nuclei of acicular marcasite. If a polished thin section is made, the two polymorphs are easily distinguished in reflected light, since pyrite is isotropic, and marcasite markedly anisotropic.

Figure 2. A close-up of the pyrite aggregate, composed of radiating acicular to platy domains of sulphide.

References

Anderson,TF, Krugler,J and Raiswell,R (1987) C-S-Fe relationships and the isotopic composition of pyrite in the New Albany Shale of the Illinois Basin, U.S.A. Geochim.Cosmochim.Acta 51, 2795-2805.

Anon (1969) British Palaeozoic Fossils. British Museum (Natural History), London, 3rd edition, 208pp.

Bowden,AJ, Burek,CV and Wilding,R (editors) (2005) History of Palaeobotany: Selected Essays. Geol.Soc. Spec.Publ. 241, 312pp.

Briggs,DEG and Edgecombe,GD (1992) The gold bugs. Natural History 101 no.11, 36-43.

Coveney,RM, Leventhal,JS, Glascock,MD and Hatch,JR (1987) Origins of metals and organic matter in the Mecca Quarry Shale Member and stratigraphically equivalent beds across the Midwest. Econ.Geol. 82, 915-933.

DeMaris,PJ (2000) Formation and distribution of coal balls in the Herrin Coal (Pennsylvanian), Franklin county, Illinois basin, USA. J.Geol.Soc.London 157, 221-228.

Desborough,GA, Hatch,JR and Leventhal,JS (1990) Geochemical and mineralogical comparison of the Upper Pennsylvania Stark Shale Member of the Dennis Limestone, east-central Kansas, with the Middle Pennsylvanian Mecca Quarry Shale Member of the Carbondale Formation in Illinois and of the Linton Formation in Indiana. In `Metalliferous Black Shales and Related Ore Deposits - Proceedings, 1989 United States Working Group Meeting, International Geological Correlation Program Project 254' (Grauch,RI and Huyck,HLO editors), USGS Circ. 1058, 85pp., 12-30.

Eckert,AW (1987) Earth Treasures Volume 1, the Northeastern Quadrant. Harper and Row Perennial Library, 476pp.

Finkelman,RB (1992) Controls on epigenetic cleat-filling mineralization in bituminous coal samples. Program and Abstracts, V.M.Goldschmidt Conference, Reston, Virginia, 134pp., 35.

Foster,WD and Feicht,FL (1946) Mineralogy of concretions from Pittsburgh coal seam, with special reference to analcite. Amer.Mineral. 31, 357-364.

Haggart,JW, Mahoney,JB, Forgette,M, Carter,ES, Schroder-Adams,CJ, MacLaurin,CI and Sweet,AR (2011) Paleoenvironmental and chronological constraints on the Mount Tatlow succession, British Columbia: first recognition of radiolarian and foraminiferal faunas in the intermontane Cretaceous back-arc basins of western Canada. Can.J. Earth Sci. 48, 952-972.

Kolbert,E (2014) Bug bed. New Yorker 90 no.24, 25 August, 20.

Leventhal,J and Taylor,C (1990) Comparison of methods to determine degree of pyritization. Geochim.Cosmochim.Acta 54, 2621-2625.

Raistrick,A and Marshall,CE (1939) The Nature and Origin of Coal and Coal Seams. English Universities Press Ltd, London / Jarrold and Sons Ltd, Norwich, 282pp.

Richards,RP, Clopton,EL and Jaszczak,JA (1995) Pyrite and marcasite intergrowths from northern Illinois. Mineral.Record 26 no.2, 129-138.

Schultz,RB and Maynard,JB (1990) Midcontinent Virgilian (Upper Pennsylvanian) black shales in eastern Kansas. In `Metalliferous Black Shales and Related Ore Deposits - Proceedings, 1989 United States Working Group Meeting, International Geological Correlation Program Project 254' (Grauch,RI and Huyck,HLO editors), USGS Circ. 1058, 85pp., 68-78.

Taff,JA (1899) Geology of the McAlester-Lehigh coal field, Indian territory. USGS 19nd Annual Report, Part III - Economic Geology, 423-601.

Turek,V, Marek,J and Benes,J (1988) Fossils of the World: a Comprehensive Practical Guide to Collecting and Studying Fossils. Arch Cape Press, New York, 1990 edition, 495pp. (1988)

Van Horn,FR and Van Horn,KR (1933) X-ray study of pyrite or marcasite concretions in the rocks of the Cleveland, Ohio, quadrangles. Amer.Mineral. 18, 288-294.

Whelan,JF, Cobb,JC and Rye,RO (1988) Stable isotope geochemistry of sphalerite and other mineral matter in coal beds of the Illinois and Forest City basins. Econ.Geol. 83, 990-1007.

Figure 3. Two pyrite suns in host shale, as presented in their "flats" at the Denver Mineral Show, 2015. Featured specimen at right. At lower left is a perfect cube of pyrite from Navajun in Spain, the "textbook" cubic crystal!

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Graham Wilson, 01-04 October 2015, addition on 13 November 2015

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