b'Pyritethe firestoneFeatureTable 1.Pyrite: summary of properties.Synonyms Pyrite, iron pyrites, common pyrites, mundic, fools goldColour, lustre Brassy yellow, metallic splendent lustrepyrite has a very high light reflectanceChemical Formula Fes 2(46.6% Fe, 53.4% S): Fe(S 2 )Molecular Structure Face centred cubic: Fe atoms occupy the corners and centre faces of cube; S pairs (dumb bells) occupy mid points of cube edges and the cube centre [see Fig.4.7, Rickard (2015)]; covalent bonding of Fe and SStoichiometry Nonstoichiometric: mineral and trace element inclusions are common, and significantOxidation 2FeS 2 +7O 2 +2H 2 O2H 2 SO 4 +2FeSO 4 +260kJ/mole, in the presence of oxygen and moisture an exothermicFigure 1.Pyrite crystal cube (lacking striations) from Logrono Spain, left; reaction occursheat + sulphuric acid + ferrousone pyritohedron crystal from Arusha Tanzania, middle; aggregated striated sulphate which when hydrated is the mineralpyritohedra from Huallanca Peru, right.melanterite FeSO 4. 7H 2 O (density 1.85g/cc)Gottschalk and Buehler (1910) note other possible oxidations of pyrite producing sulphur, sulphursurely have contributed to the development of language. So, dioxide, and hydrogen sulphide intelligence flourished. Members of early human groups carried Density 5.02g/cc, can vary a compact strike-a-spark ignition kit comprising a nodule of pyrite (Mohs H=6) held in one hand, a rounded rod or piece of Mohs hardness 6-6, very hard but less than flint or quartz, 7 harder stone such as durable flint (H=7) held in the other hand Magnetic volume4 x 10-5SI, a weak paramagnetic (Figure 2), and tinder (dried vegetation)all could be placed in susceptibility a leather pouch (Voynick 2018a). Nodular pyrite (Figure 3) has a Electrical conductivity Nominally 1000 S/m but can vary considerablyradiant texture and an irregular surface conducive to optimum depending on texture and semiconductor typeshattering and sparking. A fist-sized, coarsely crystalline cube (p,n) or pyritohedron (or aggregates of them) are not suitable for fire P wave velocity 8.10 km/s, very high striking. If pyrite and flint were not available by collecting or S wave velocity 5.18 km/s trading then recourse was had to the frictional wood-on-wood technique.Poissons ratio 0.15, lowYoungs modulus 304.8 GPa, very high for a mineral and indicative ofA piece of pyrite has a passivated surface in its natural state i.e. stiffnesspyrite is brittle and not ductile it is shielded by the adsorption of oxygen, hydroxyls and the Thermal conductivity 20 W/m/oK [quite high, compare feldspar, quartzlike held on its surface by van der Waals forces and dangling 2,8 W/m/oK respectively] bonds on the ridges of its micro-rugosity. This results in layers, a few molecules deep, which impede or prevent pyrite oxidising. Note: Sulphur (S, 2.1g/cc) amounts very approximately to 0.1% of the earths crust; most of this occurs in iron sulphides, chiefly pyrite. Iron amounts to 5% of the earths crust andPyrite is brittle (Table 1) with a pronounced conchoidal fracture occurs widely in silicates, oxides and sulphides. producing sharp fragments. When struck, imparted kinetic Table 2.Pyrites (collective term for yellowish sulphides): selected pyrite and pyritic samples.Name Colour Formula Crystallinity HardnessDensityConductivity(shades of yellow) (Moh scale) (g/cc)Iron pyrites Bronze yellow toFes 2 Cubic 6 5.0 VariesPYRITE pale brass yellowWhite iron pyrites Pale bronze yellow Fes 2 Orthorhombic 6 4.9 LowMARCASITEArsenical pyrites Silver whitepaleFe As S Pseudo orth. 6 6.1 LowARSENOPYRITE copper tarnishCopper pyrites Brass yellow CuFeS 2 Tetragonal 4 4.2 HighCHALCOPYRITEMagnetic pyrites Reddish-brownishFe 1-n s Monoclinic (magnetic) &4 4.6 Very highPYRRHOTITE bronze yellow (n 0.2) hexagonal varietiesCapillary pyrites Pale brass yellow toNiS Hexagonal 3 5.5 Very highMILLERITE bronze yellowNotes:cited data from various sources, approximate onlymarcasite is a low temperature unstable diomorph of pyritepyrite and arsenopyrite commonly associated with gold deposits and these minerals can host blebs of gold in their own structureconductivity qualitative indications: low = 100s S/m or less, high = 1000s S/m, very high = 10,000s S/m or morefor pyrites conductivity ranges see Table 3 (limited data set)metallic lustre sulphides only in this Table e.g. orpiment, As 2 S 3 , a soft lemon yellow mineral is not includedpyrite is the sulphide with the highest exothermic oxidation and best sparking; marcasite also sparks, as does arsenopyrite (accompanied by a garlic odour)53 PREVIEW DECEMBER 2019'