Chemical elements
  Iron
    History of Iron
    Mineralogy
    Isotopes
    Energy
    Production
    Application
    Physical Properties
    Chemical Properties
      Iron Hydride
      Ferrous fluoride
      Aluminium pentafluoferrite
      Ferric fluoride
      Ammonium ferrifluoride
      Barium ferrifluoride
      Potassium ferrifluoride
      Sodium ferrifluoride
      Thallous ferrifluoride
      Ferrous diferrifluoride
      Ferrous monoferrifluoride
      Ferrous chloride
      Ammonium tetrachlorferrite
      Ferric chloride
      Tetrachlorferrates
      Pentachlorferrates
      Ferroso-ferric chloride
      Ferrous perchlorate
      Ferric perchlorate
      Ferrous chlorate
      Ferric chlorate
      Ferrous Oxychlorides
      Ferrous bromide
      Ferric bromide
      Ferric chloro-bromide
      Ferrous bromate
      Ferrous iodide
      Ferric iodide
      Ferric iodate
      Ferrous oxide
      Ferrous hydroxide
      Triferric tetroxide
      Ferric oxide
      Ferrous acid
      Calcium ferrite
      Cobalt ferrite
      Cupric ferrite
      Cuprous ferrite
      Magnesium ferrite
      Nickel ferrite
      Potassium ferrite
      Sodium ferrite
      Zinc ferrite
      Barium ferrate
      Strontium ferrate
      Barium perferrate
      Calcium perferrate
      Potassium perferrate
      Sodium perferrate
      Strontium perferrate
      Iron Subsulphides
      Ferrous sulphide
      Ferric sulphide
      Potassium ferric sulphide
      Sodium ferric sulphide
      Cuprous ferric sulphide
      Iron disulphide
      Ferrous sulphite
      Ferric sulphite
      Potassium ferri-tetrasulphite
      Potassium ferri-disulphite
      Potassium ferri-sulphite
      Ammonium ferri-sulphite
      Sodium ferri-disulphite
      Sodium hydrogen ferri-tetrasulphite
      Ferrous sulphate
      Ferrous copper sulphate Fe
      Ferrous ammonium sulphate
      Ferrous potassium sulphate
      Ferrous aluminium sulphate
      Basic ferrous sulphate
      Ferric sulphate
      Ammonium ferri-disulphate
      Trisodium ferri-trisulphate
      Ferric Alums
      Ferric ammonium alum
      Ferric potassium alum
      Ferric rubidium alum
      Ferroso-ferric sulphate
      Ferrous amido-sulphonate
      Ferric amido-sulphonate
      Ferrous thiosulphate
      Ferrous pyrosulphate
      Ferrous tetrathionate
      Ferric selenide
      Iron diselenide
      Iron Selenites
      Ferrous selenate
      Ferric rubidium selenium alum
      Ferric caesium selenium alum
      Ferric tellurite
      Ferrous chromite
      Ferrous chromate
      Iron nitride
      Nitro-Iron
      Ferrous nitrate
      Ferric nitrate
      Ferrous Nitroso Salts
      Potassium ferro-heptanitroso sulphide
      Sodium ferro-heptanitroso sulphide
      Ammonium ferro-heptanitroso sulphide
      Tetramethyl ammonium ferro-heptanitroso sulphide
      Ferro-dinitroso Sulphides
      Potassium ferro-dinitroso thiosulphate
      Triferro phosphide
      Diferro phosphide
      Iron monophosphide
      Iron sesqui-phosphide
      Ferrous hypophosphite
      Ferric hypophosphite
      Ferrous phosphite
      Ferric phosphite
      Ferrous orthophosphate
      Ferrous hydrogen orthophosphate
      Ferrous dihydrogen orthophosphate
      Ferric orthophosphate
      Sodium ferri-diorthophosphate
      Ammonium ferri-diorthophosphate
      Sodium ferri-triorthophosphate
      Ferric dihydrogen orthophosphate
      Acid ferric orthophosphate
      Ferrous metaphosphate
      Ferric metaphosphate
      Ferrous pyrophosphate
      Ferric pyrophosphate
      Hydrogen ferri-pyrophosphate
      Sodium ferro-pyrophosphate
      Ferrous thio-orthophosphite
      Ferrous thio-orthophosphate
      Ferrous thio-pyrophosphite
      Ferrous thio-pyrophosphate
      Iron sub-arsenide
      Iron mon-arsenide
      Iron sesqui-arsenide
      Iron di-arsenide
      Iron thio-arsenide
      Ferrous met-arsenite
      Ferric arsenite
      Ferrous ortho-arsenate
      Ferric ortho-arsenate
      Ferro mono-antimonide
      The di-antimonide
      Ferrous thio-antimonite
      Ferric ortho-antimonate
      Triferro carbide
      Diferro carbide
      Iron dicarbide
      Iron pentacarbonyl
      Diferro nonacarbonyl
      Iron tetracarbonyl
      Ferrous carbonate
      Ferrous bicarbonate
      Ferrous potassium carbonate
      Complex Iron Carbonates
      Ferrous thiocarbonate
      Ferrous thiocarbonate hexammoniate
      Ferrous cyanide
      Ferro-cyanic acid
      Aluminium ferrocyanide
      Aluminium ammonium ferrocyanide
      Ammonium ferrocyanide
      Barium ferrocyanide
      Calcium ferrocyanide
      Calcium ammonium ferrocyanide
      Cobalt ferrocyanide
      Copper ferrocyanide
      Ammonium cuproferrocyanide
      Barium cuproferrocyanide
      Lithium cuproferrocyanide
      Magnesium cuproferrocyanide
      Potassium cuproferrocyanide
      Sodium cuproferrocyanide
      Ammonium cupriferrocyanide
      Potassium cupriferrocyanide
      Potassium ferrous cupriferrocyanide
      Sodium cupriferrocyanide
      Strontium cupriferrocyanide
      Lithium ferrocyanide
      Magnesium ferrocyanide
      Magnesium ammonium ferrocyanide
      Manganese ferrocyanide
      Nickel ferrocyanide
      Potassium ferrocyanide
      Potassium aluminium ferrocyanide
      Potassium barium ferrocyanide
      Potassium calcium ferrocyanide
      Potassium cerium ferrocyanide
      Potassium magnesium ferrocyanide
      Potassium mercuric ferrocyanide
      Silver ferrocyanide
      Sodium ferrocyanide
      Sodium cerium ferrocyanide
      Strontium ferrocyanide
      Thallium ferrocyanide
      Zinc potassium ferrocyanide
      Ferricyanic acid
      Ammonium ferricyanide
      Barium ferricyanide
      Barium potassium ferricyanide
      Calcium ferricyanide
      Calcium potassium ferricyanide
      Cobalt ferricyanide
      Copper ferricyanide
      Lead ferricyanide
      Magnesium ferricyanide
      Mercuric ferricyanide
      Mercurous ferricyanide
      Potassium ferricyanide
      Sodium ferricyanide
      Strontium ferricyanide
      Zinc ferricyanide
      Ferrous hydrogen ferrocyanide
      Ferrous potassium ferrocyanide
      Prussian Blues
      Ferrous ferrocyanide
      Ferric ammonium ferrocyanide
      Nitroprussic acid
      Sodium nitroprusside
      Ammonium nitroprusside
      Barium nitroprusside
      Cobalt nitroprusside
      Nickel nitroprusside
      Potassium nitroprusside
      Carbonyl Penta-Ferrocyanides
      Carbonyl ferrocyanic acid
      Barium carbonyl ferrocyanide
      Copper carbonyl ferrocyanide
      Ferric carbonyl ferrocyanide
      Potassium carbonyl ferrocyanide
      Silver carbonyl ferrocyanide
      Sodium carbonyl ferrocyanide
      Strontium carbonyl ferrocyanide
      Uranyl carbonyl ferrocyanide
      Sodium ammonio ferrocyanide
      Potassium aquo ferrocyanide
      Potassium aquo ferricyanide
      Sodium aquo penta-ferricyanide
      Potassium sulphito ferrocyanide
      Ferrous thiocyanate
      Ferric thiocyanate
      Sodium ferrothiocyanate
      Sodium ferrithiocyanate
      Potassium ferrithiocyanate
      Iron subsilicide
      Iron monosilicide
      Iron disilicide
      Triferro disilicide
      Ferrous orthosilicate
      Ferrous magnesium orthosilicate
      Ferrous metasilicate
      Ferric silicate
      Diferro boride
      Iron monoboride
      Iron diboride
      Ferrous chlorborate
      Ferrous bromborate
    Corrosion
    Iron Salts
    PDB 101m-1aeb
    PDB 1aed-1awd
    PDB 1awp-1beq
    PDB 1bes-1c53
    PDB 1c6o-1ci6
    PDB 1cie-1cry
    PDB 1csu-1dfx
    PDB 1dgb-1dry
    PDB 1ds1-1e08
    PDB 1e0z-1ehj
    PDB 1ehk-1f5o
    PDB 1f5p-1fnp
    PDB 1fnq-1fzi
    PDB 1g08-1gnl
    PDB 1gnt-1h43
    PDB 1h44-1hdb
    PDB 1hds-1i5u
    PDB 1i6d-1iwh
    PDB 1iwi-1jgx
    PDB 1jgy-1k2o
    PDB 1k2r-1kw6
    PDB 1kw8-1lj0
    PDB 1lj1-1m2m
    PDB 1m34-1mko
    PDB 1mkq-1mun
    PDB 1muy-1n9x
    PDB 1naz-1nx4
    PDB 1nx7-1ofe
    PDB 1off-1p3t
    PDB 1p3u-1pmb
    PDB 1po3-1qmq
    PDB 1qn0-1ra0
    PDB 1ra5-1rxg
    PDB 1ry5-1smi
    PDB 1smj-1t71
    PDB 1t85-1u8v
    PDB 1u9m-1uyu
    PDB 1uzr-1vxf
    PDB 1vxg-1wri
    PDB 1wtf-1xlq
    PDB 1xm8-1y4r
    PDB 1y4t-1ygd
    PDB 1yge-1z01
    PDB 1z02-2a9e
    PDB 2aa1-2azq
    PDB 2b0z-2boz
    PDB 2bpb-2ca3
    PDB 2ca4-2cz7
    PDB 2czs-2dyr
    PDB 2dys-2ewk
    PDB 2ewu-2fwl
    PDB 2fwt-2gl3
    PDB 2gln-2hhb
    PDB 2hhd-2ibn
    PDB 2ibz-2jb8
    PDB 2jbl-2mgh
    PDB 2mgi-2o01
    PDB 2o08-2ozy
    PDB 2p0b-2q0i
    PDB 2q0j-2r1h
    PDB 2r1k-2spm
    PDB 2spn-2vbd
    PDB 2vbp-2vzb
    PDB 2vzm-2wiv
    PDB 2wiy-2xj5
    PDB 2xj6-2ylj
    PDB 2yrs-2zon
    PDB 2zoo-3a17
    PDB 3a18-3aes
    PDB 3aet-3bnd
    PDB 3bne-3cir
    PDB 3ciu-3dax
    PDB 3dbg-3e1p
    PDB 3e1q-3eh4
    PDB 3eh5-3fll
    PDB 3fm1-3gas
    PDB 3gb4-3h57
    PDB 3h58-3hrw
    PDB 3hsn-3ir6
    PDB 3ir7-3k9y
    PDB 3k9z-3l4p
    PDB 3l61-3lxi
    PDB 3lyq-3mm8
    PDB 3mm9-3n62
    PDB 3n63-3nlo
    PDB 3nlp-3o0f
    PDB 3o0r-3p6o
    PDB 3p6p-3prq
    PDB 3prr-3sel
    PDB 3sik-3una
    PDB 3unc-4blc
    PDB 4cat-4erg
    PDB 4erm-4nse
    PDB 4pah-8cat
    PDB 8cpp-9nse

Iron disulphide, FeS2






Iron disulphide, FeS2, occurs in nature as the mineral mundic or iron pyrites belonging to the cubic system. When broken it exhibits a conchoida fracture. It is pale brass-yellow in colour, and of hardness 6 5; by both of these features it is readily distinguishable from copper pyrites, which is darker in colour and softer (hardness 3.5-4).

The word pyrites was used by the ancients to include a number of minerals which yield sparks when struck with a hammer. Iron pyrites is frequently found of botryoidal, spherical, or stalactitic form, having been deposited from solutions containing iron sulphate through the agency of organic matter. For this reason pyrites is frequently found in coal, and is known to miners as brass or fools' gold. Most probably its presence assists the spontaneous inflammability of coal, although it is not such an important factor as was at one time believed.

Frequently fossils are found, particularly in London Clay, consisting of pyrites, the decaying organism having presumably reduced sulphates of iron present in the infiltrating waters.

When pure the density of iron pyrites is 5.027 at 25° C. Nickel and cobalt are sometimes present, probably as isomorphous intermixtures of their corresponding sulphides; copper may also be present, perhaps as chalcopyrite. Thallium, silver, and even gold have been found in pyrites, the last-named in sufficient quantity to render the mineral a profitable source of that precious metal, as, for example, in British Columbia, where auriferous pyrites is largely worked.

From electric conductivity measurements the solubility of pyrites in water has been calculated as follows: -

Gram-molecules per Litre
Pyrites from Freiberg48.9×10-6
Artificial pyrites40.8×10-6


Aerated waters charged with calcium carbonate appear to decompose pyrites very slowly at the ordinarv temperature, yielding limonite. Thus: -

4FeS2 + 15O2 + 3H2O + 8CaCO3 = 2Fe2O3.3H2O + 8CaSO4 + 8CO2;

Distilled water, in the presence of air, slowly oxidises pyrites to ferrous sulphate and sulphuric acid.

Sulphur monochloride has no action on pyrites in the cold, but effects its complete decomposition at 140° C.

Another form of iron disulphide occurs in nature as the mineral marcasite, which possesses a radiated structure and is frequently found as irregular balls on chalky downs. When broken open the fracture exhibits a fibrous crystalline structure radiating from the centre - whence the name radiated pyrites. The fresh fracture is almost white in colour, and if quite pure the marcasite would probably be quite tin-white in appearance. When pure, its density at 25° C. is 4.887. Marcasite has in general a density of 4.68 to 4.85. It oxidises more readily than pyrites, becoming covered, upon exposure to air, with white fungus-like growths of ferrous sulphate.

When heated to 450° C. and upwards, marcasite changes slowly into pyrites, but this change does not appear to be reversible. It is not accelerated by pressures even of 10,000 atmospheres, but the change increases in rapidity with rise of temperature, being accompanied by an evolution of heat and a decrease in the electrical resistance of the mineral. A sample of marcasite on being heated to 610° C. increased in density from 4.887 to 4.911, changed in colour, and gave the chemical reactions characteristic of pyrites. The density (4.911) is too low for ordinary pyrites (5.02), but apparently that is due to the porosity of the product obtained under the experimental conditions.

Sulphur monochloride has no action on marcasite in the cold, but decomposes it completely at 140° C.

In order to facilitate a comparative study of the two sulphides, their more important chemical and physical properties are listed in the accompanying table: -

Pyrites.Marcasite.
ColourBrass-yellowWhite.
AppearanceStriated cubes or pentagonal dodecahedraFibrous, radiating.
Crystal systemCubicRhombic.
StreakGreenish to brownish blackGreyish to brownish black.
Density range4.8 to 5.24.7 to 4.8.
Density at 25° C. when pure.5.0274.887.
FractureConchoidalUneven, radiating.
Hardness (Mohs' scale)6 to 6.56 to 6.5. Frequently a trifle softer than pyrites.
Hardness relative to topaz (1000)182 to 199134 to 140
Specific heat0.13060.1332
Heat of combustion1550 calories1550 calories.
Effect of heating to 450° C.StableConverted into pyrites.
Action of airMore stable than marcasiteReadily oxidises.
Action of caustic soda solutionLess resistant than marcasiteFairly resistant.
Action of nitric acid, Density 1.4Dissolves completelySulphur deposited.


When pyrites and mareasite are not distinctly crystallised it is frequently difficult to distinguish between them, as the foregoing characteristics are not clearly discernible. A convenient chemical method has been devised, however, which enables a discrimination to be made with certainty. It consists in boiling the mineral with iron alum, containing 1 gram of ferric iron and 16 c.c. of 25 per cent, sulphuric acid per litre. The proportion of sulphur oxidised in the case of pyrites is 60.4 per cent, of the total sulphur contained in the mineral; in the case of mareasite it is only 18 per cent. The reaction may be considered as taking place in two stages, namely: -

FeS2 + Fe2(SO4)3 = 3FeSO4 + 2S,
6Fe2(SO4)3 + 2S + 8H2O = 12FeSO4 + 8H2SO4.

A labile phase of iron disulphide occurs in nature under the name melnikovite. It is a black, finely divided substance which impregnates certain miocene clays in Russia, and differs in many respects from the black hydrated sulphide of iron usually found in black muds of lakes. In composition it corresponds to the formula FeS2. It is magnetic, and its true density is probably 4.2 to 4.3. Cold, dilute hydrochloric acid readily attacks it, evolving hydrogen sulphide. It has probably been derived from a colloidal deposit of an iron sulphide.


Laboratory Preparation of Iron Disulphide

In the laboratory iron disulphide may be prepared by several wet methods. Thus, it is formed when ferrous sulphide is boiled with flowers of sulphur; and when sodium trisulphide is added slowly to a boiling solution of ferrous sulphate, provided excess of the trisulphide is avoided. Sulphur is simultaneously precipitated. It is obtained also by boiling the freshly precipitated monosulphide suspended in water with sulphur in the absence of alkalies; by the action of sodium thiosulphate solution upon ferrous sulphate in sealed tubes at temperatures even below 100° C.: -

4Na2S2O3 + FeSO4 = FeS2 + 3S + 4Na2SO4;

by the action of hydrogen sulphide upon ferrous thiosulphate, or on ferric or ferrous sulphate; by heating metallic iron with a solution of sulphur dioxide at about 200° C., or phosphorus penta-sulphide and ferric chloride: -

6FeCl3 + 2P2S5 = 3FeCl2 + 3FeS2 + 4PSCl3.

As obtained by these methods, the iron disulphide is usually either amorphous or consists of a mixture of minute crystals of pyrites and mareasite. By prolonging the reactions crystals can be obtained of sufficient size to render possible an examination under the microscope. By heating an intimate mixture of ferric oxide, sulphur, and ammonium chloride, Wohler obtained small brass-yellow crystals, probably of pyrites.

Constitution of Pyrites and Marcasite

When marcasite is heated to 200° C. in sealed tubes with copper sulphate solution, the product contains iron in the ferrous state only. Pyrites, under similar treatment, yields about 20 per cent, of its iron in the ferrous form, the remainder being ferric. It has therefore been suggested that marcasite is represented by the formula



or a polymeride, whilst pyrites is 4Fe•••S2.Fe••S2.

On the other hand, both marcasite and pyrites, when heated with excess of bismuth chloride in an atmosphere of dry carbon dioxide, yield only ferrous iron.

Further, when heated to a red heat in an atmosphere of carbon dioxide, pyrites loses half its sulphur, and the residue consists of ferrous sulphide. A similar result is obtained when pyrites is heated at 300° to 400° C. in steam.

From these experiments it would appear that the iron is present in both minerals in the ferrous state. The evidence is thus seen to be conflicting and unreliable. The heat of combustion of the two minerals is the same, namely 1550 calories, which suggests a similar state of valency of the iron in both, and thus lends support to the latter view. The heat of formation is likewise the same, assuming the iron to be in the ferrous state. Some difference in the molecular structure of the minerals is to be anticipated, and Arbeiter suggests the following graphical formulae: -

or
PyritesMarcasite

Formation of Pyrites and Marcasite in Nature

As has already been mentioned, iron disulphide is obtained in both crystalline varieties - pyrites and marcasite - by the action of hydrogen sulphide upon a solution of ferric sulphate at the ordinary temperature. The reaction begins with the reduction of ferric sulphate to the ferrous salt with the deposition of sulphur

Fe2(SO4)3 + H2S = 2FeSO4 + H2SO4 + S.

This is followed by the formation of the disulphides, if the reaction is carried out in a closed vessel where the hydrogen sulphide is prevented from oxidation or escape.

FeSO4 + H2S + S = FeS2 + H2SO4.

At the ordinary temperature this reaction is very slow, but at 200° C. it is fairly rapid. The dark deposit is microcrystalline. On carrying out the reaction in a sealed tube at 100° C. with a solution containing 1 per cent, of free sulphuric acid, marcasite is the only product. Higher temperatures and reduction of acidity favour the production of pyrites, distinct crystals being produced at 200° C. Iron pyrites is formed in neutral or alkaline solutions, as, for example, by the action of sodium polysulphide on a ferrous salt. Marcasite is not produced under these conditions. The foregoing results are in harmony with the observation that whilst iron pyrites in nature is usually formed in deep veins from hot alkaline solutions, marcasite is produced near the surface from acid solutions.

Thus a recent formation of pyrites has been observed at Karlsbad, in the well-known springs, which have a temperature of about 55° C. The waters are faintly alkaline and contain dissolved sulphates and a trace of hydrogen sulphide. Similarly the Tuscan lagoons are gradually depositing pyrites, whilst the hot vapours of the Icelandic fumaroles are slowly converting the ferrous silicate of the rocks into pyrites.

Certain micro-organisms may indirectly facilitate the formation of natural pyrites and marcasite by evolving hydrogen sulphide as a product of reduction of sulphates. A considerable number of bacteria, algse, flagellata, and infusoria exhibit this kind of activity.

It is not probable, however, that they are directly responsible for any large quantity of pyrite or marcasite formation for several reasons. Thus, for example, pyrites is usually formed at considerable depths, but micro-organisms are relatively superficial inhabitants of the soil. Again, pyrites is not infrequently associated with copper pyrites and analogous minerals, which indicates the presence of copper in the original solutions. Now, copper is exceedingly poisonous to most micro-organisms. Finally, marcasite could hardly be produced direct by micro-organisms, as the presence of free acid, which is a condition of its formation, is fatal to organisms.

It seems fairly well agreed amongst geologists that pyrites has also been formed in certain cases by direct crystallisation from rock magmas. This could never happen with marcasite, however, owing to its instability at temperatures above 450° C.

Artificial crystals of marcasite closely resemble the natural mineral in colour, lustre, and axial ratios. Thus: -

MarcasiteAxial Ratio. a:b: c.
Natural0.7662: 1: 1.2342
do0.7623: 1: 1.2167
do0.7580: 1: 1.2122
Artificial0.7646: 1: 1.2176.


The striations likewise agree with rhombic rather than with cubic symmetry.

The chief commercial use of pyrites (including mareasite) is in the manufacture of sulphuric acid. The pyrites, on being roasted in air, yields sulphur dioxide and a residue of ferric oxide. Thus: -

4FeS2 + 11O2 = 2Fe2O3 + 8SO2

A small quantity of sulphur trioxide is also formed, its proportion ranging from 0.1 to about 8.5 per cent.

Magnetic pyrites or pyrrhotite is a mineral of somewhat variable composition ranging from 5FeS.Fe2S3 to 9FeS.Fe2S3, and is thus intermediate between ferrous sulphide (troilite) and iron pyrites. Some of the purest specimens yield an analysis closely approximating to that of ferrous sulphide. This is particularly the case with specimens derived from meteorites, the excess of sulphur having perhaps been lost during the heating of the meteorite.

Pyrrhotite has been prepared artificially by heating ferrous chloride and sodium carbonate with water in a rifle-barrel at 200° C. for 16 days in an atmosphere of hydrogen sulphide and carbon dioxide. The reaction proceeds easily between 80° and 225° C. at the lower temperature, yielding hexagonal crystals, and at the higher temperature rhombic ones.

In view of these experiments it seems probable that the natural mineral has been produced at a relatively low temperature from ferrous salts dissolved in water by the action of hydrogen sulphide on slightly acid solutions of a ferrous salt.

Artificial pyrrhotite has also been obtained by passing hydrogen sulphide over heated ferrous chloride, air having been previously expelled by passage of carbon dioxide; and by decomposing mareasite or iron pyrites with hydrogen sulphide at temperatures above 575° C. The reaction may be first detected at about this temperature and proceeds fairly rapidly at 665° C. The mineral is not a definite compound, but in all probability a solid solution of sulphur in iron sulphide.

The following data are interesting as illustrating the fall in density with rise of dissolved sulphur: -

No.Total Sulphur. Per cent.Calculated FeS. Per cent.Calculated Dissolved Sulphur. Per cent.Density at 4° C.
136.7299.590.414.755
337.7198.041.964.677
538.5496.733.274.632
738.8496.263.744.619
939.4995.234.774.585
1040.3093.966.044.520


The change from pyrite to pyrrhotite when heated in an atmosphere of hydrogen sulphide is represented by the reversible reaction: -

FeS2FeS(S)x + (1-x)S,

which is endothermic in the direction of left to right, a marked absorption of heat being evident at about 665° C., at which temperature pyrites is transformed into pyrrhotite. At 565° C. pyrrhotite is stable in an atmosphere of hydrogen sulphide, whilst at 550° C. it gradually passes over into pyrites. The transition temperature is thus approximately 565° C. Pyrrhotite melts at 1183° C. in a current of hydrogen sulphide.

There has been considerable discussion as to the crystal system to which pyrrhotite belongs. It is generally regarded as hexagonal, but several investigators have concluded that it is really rhombic, and the suggestion has even been made that the mineral is monoclinic. It seems highly probable that the mineral occurs in two varieties - namely a pyrrhotite, which is rhombic; and β pyrrhotite, which is hexagonal; the a variety being produced from solution at a higher temperature (circa 225° C.) than the β (circa 80° C.).

The axial ratios for the rhombic variety vary between the limits

a: b: с

0.5793: 1: 0.9267

and

0.5793: 1: 0.9927,

whilst for the hexagonal variety the value for с lies between 0.8632 and 0.8742.

Pyrrhotite is readily distinguished from ordinary pyrites by its softness, 3.5 to 4.5 (pyrites, 6 to 6.5), and by its crystal form and magnetic properties. Its specific heat is 0.1539.

The solubility of pyrrhotite in water, as calculated from electric conductivity measurements, is 53.6×10-6 gram-molecules per litre.

Oxysulphides of iron

Oxysulphides of iron are not known in nature; but a substance, to which the formula Fe2O3.3Fe2S3 is given, has been obtained by the action of hydrogen sulphide on ferric oxide at temperatures below red heat.
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