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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: -
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: -
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.
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 Freiberg | 48.9×10-6 |
| Artificial pyrites | 40.8×10-6 |
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. | |
| Colour | Brass-yellow | White. |
| Appearance | Striated cubes or pentagonal dodecahedra | Fibrous, radiating. |
| Crystal system | Cubic | Rhombic. |
| Streak | Greenish to brownish black | Greyish to brownish black. |
| Density range | 4.8 to 5.2 | 4.7 to 4.8. |
| Density at 25° C. when pure. | 5.027 | 4.887. |
| Fracture | Conchoidal | Uneven, radiating. |
| Hardness (Mohs' scale) | 6 to 6.5 | 6 to 6.5. Frequently a trifle softer than pyrites. |
| Hardness relative to topaz (1000) | 182 to 199 | 134 to 140 |
| Specific heat | 0.1306 | 0.1332 |
| Heat of combustion | 1550 calories | 1550 calories. |
| Effect of heating to 450° C. | Stable | Converted into pyrites. |
| Action of air | More stable than marcasite | Readily oxidises. |
| Action of caustic soda solution | Less resistant than marcasite | Fairly resistant. |
| Action of nitric acid, Density 1.4 | Dissolves completely | Sulphur 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.
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 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  |
| Pyrites | Marcasite |
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: -
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: -
The change from pyrite to pyrrhotite when heated in an atmosphere of hydrogen sulphide is represented by the reversible reaction: -
FeS2 ⇔ FeS(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.
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: -
| Marcasite | Axial Ratio. a:b: c. |
| Natural | 0.7662: 1: 1.2342 |
| do | 0.7623: 1: 1.2167 |
| do | 0.7580: 1: 1.2122 |
| Artificial | 0.7646: 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. |
| 1 | 36.72 | 99.59 | 0.41 | 4.755 |
| 3 | 37.71 | 98.04 | 1.96 | 4.677 |
| 5 | 38.54 | 96.73 | 3.27 | 4.632 |
| 7 | 38.84 | 96.26 | 3.74 | 4.619 |
| 9 | 39.49 | 95.23 | 4.77 | 4.585 |
| 10 | 40.30 | 93.96 | 6.04 | 4.520 |
The change from pyrite to pyrrhotite when heated in an atmosphere of hydrogen sulphide is represented by the reversible reaction: -
FeS2 ⇔ FeS(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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