b'Pyritethe firestoneFeaturePyrite and the environment; geohazards Arid zone pyritePyrite often occurs in coal as disseminations, coatings, andIn the field, pyrite in veins or in massive replacement bodies lenses. Such pyrite, when exposed, may be inert or reactive.weathers and forms a limonitic capping i.e. a gossan (Blanchard If reactive it can rapidly oxidise in moist mine conditions.1968). Limonite is a general field term for a mix of haematite Reactivity depends on grain size and shape providing increased(Fe 2 O 3 ), goethite FeO(OH), and lepidocrocite FeO(OH). surface area of the pyrite and porosity for air to reach reactiveAlteration products of pyrite are frequently seen in arid sites on the pyrite (Beamish 2017). Under moist mine conditionsareas in the oxidation zones of massive sulphides as cellular, such pyrite can oxidise rapidly producing iron sulphates suchspongy, boxwork structures which are developed through as greenish melanterite Fe2+SO 4. 7H 2 O (also known as copperas,limonitic gossans which are ore indicators in economically green vitriol, iron vitriol). Fine acicular crystals of low densitymineralised pyritic sulphides. Pyrite often forms tough limonite melanterite (1.9g/cc) readily mix with coal dust and, if inhaled,pseudomorphs with the outline of the parent pyrite fully contribute to coal workers lung disease (pneumoconiosis). preserved (Figure 8). The replacement may be partial (a coating Potential acid sulphate soils occur in about 3 000 000 haon the pyrite crystal) or it may be complete.of pyritic Australian Holocene coastal floodplains. Pyrite is safely inert in the reducing conditions of its deposition, butPyritephysical propertiesoxidises on exposure to oxygen when the soil is disturbedPyrite, FeS 2 , has an intriguing set of physical properties for a by excavations involved in engineering infrastructure andubiquitous mineral (Table 1). Compared to most minerals, it is agriculture (e.g. drains). The oxidation equations can bequite dense, it manifests a very weak para-magnetism (unusual complex but are broadly summarised by the entry in Table 1.for an Fe compound), it carries a very fast compressional wave, Oxidation of pyrite generates sulphuric acid; for each moleits Youngs Modulus (a proxy for low ductility or stiffness) is of pyrite that is oxidised four moles of acid are producedextremely high, as is its thermal conductivity. All these make (Indraratna, Blunden, and Nethery 1999). Needless to say, thisfor a salient combination of characteristics that are, more can have catastrophic environmental effects and requiresor less, consistent for various pyrite occurrences. However careful assessment and management in affected areas. another important property, perhaps of most importance Disseminated pyrite can cause problems in the constructionto a geophysicist, is resistivity, (or its inverse: conductivity), industries using building stone, dimension stone, and rockand this is certainly not consistent. Thousands of resistivity fill (Ray 1988; McNally 1988; Smith 1999). Small amounts ofmeasurements have been made on pyrite, perhaps more than pyrite can ruin an otherwise attractive stone. In building andany other sulphide. Most of these measurements have been dimension stone, such as granites and slates, pyrite oxidationon single crystals. Pyrites resistivity, generally, is low, but it generates rusty blemishes along fissures and on surfaces. In rockis quite variable and difficult to predict or anticipate in field fill used for dams and embankments the presence of pyrite leadswork even if important factors such as mode of conduction, to pollution from mobile metals and acidic drainage. Alterationcrystallinity, alteration, and texture are known. Information on can be biochemical (Irdi and Booher 1994). Bacteria do occur inpyrite resistivities can be found in Harvey (1928), Telkes (1950), rocks. The bacterium Thiobacillus ferro-oxidaus converts pyrite toParasnis (1956), Hill and Green (1962), Parkhomenko (1967), ferric ions and sulphuric acid. When the ferric ions further reactShuey (1975), and Olhoeft et al. (1981).with the pyrite a self-sustaining reaction can ensue. If calcitePyrite is a semiconductor. Conduction can be n type occurs with the pyrite, as in some roofing slates, gypsum is(electrons) or p type (holes, actually electrons hopping into formed from the sulphuric acid with flaking and spalling. lattice holes and leaving holes in their wake). Trace or minor Pyrite and technologyPyrite has found applications in modern technology. Rechargeable lithium batteries have aluminium cathodes containing disseminated pyrite grains. Solar energy projects may find a use for semiconducting crystalline pyrite as a cheap photovoltaic absorber of radiant energy when sprayed in thin layers on exposed panels (Voynick 2018b).Pyrite and early life on EarthIn sediments, a very common type of pyrite occurs as clustered microscopic aggregates. This is known as framboid pyrite. The accumulation of tiny pyrite grains has the appearance of a raspberry, and its formation is thought to be linked to anaerobic bacterial processes, such as the reduction of sulphates to sulphides, which have gone on for billions of years. The iron-sulphur world is a supposition of geochemists and biologists and is based on the premise that the origin of life required the active involvement of iron sulphides. These were ubiquitous evenFigure 8.Pseudomorphs of limonite, FeOH.nH 2 O, after pyrite, result from before atmospheric oxygen appeared, and acted as catalysts andextreme alterations in arid zones. The external features of the original cube (left) conductors in biological reactions requiring electron transfer.and pyritohedron (right) have been preserved. The cube is from Nilinghou South Where would we be without pyrite? Rickard (2015) provides a fullAustralia, the pyritohedron is from Mkushi Zambia. Density for each is 3.7 g/cc, and lucid discussion of this fascinating topic. and mag k 110 x 10-5 SI. Both samples have very high resistivity, > 100 000 ohms.DECEMBER 2019 PREVIEW 58'