Alteration Of Pyrrhotite Research Articles

An overview of chalcophile element contents of pyrrhotite, pentlandite, chalcopyrite, and pyrite from magmatic Ni-Cu-PGE sulfide deposits

We have compiled the trace element concentrations in pyrrhotite, pentlandite, chalcopyrite, and pyrite from magmatic Ni-Cu-PGE ore deposits with the aim of understanding their petrogenesis and whether these minerals can be used as indicator minerals. Among the samples, there are some of the most studied world-class Ni-Cu- (Aguablanca, Duluth, Jinchuan, Noril’sk-Talnakh-Kharaelakh, Sudbury, Voisey’s Bay, and others) and PGE-dominated (Bushveld, Lac des Iles, Stillwater, Great Dyke, and Penikat) deposits. Crustal assimilation may be constrained using As/Se and Sb/Se ratios in pentlandite. The degree of interaction between the silicate and sulfide liquids (R-factor) can be estimated by the content of highly chalcophile elements (Dsulf liq/sil liq above 1000) in sulfide minerals. The fractional crystallization of the sulfide liquid can be traced using Se/Te ratios of pentlandite. Pyrite formed by exsolution from MSS has higher Rh, Ru, Ir, and Os than co-existing pyrrhotite, whereas pyrite formed by hydrothermal alteration of pyrrhotite inherits the Rh, Ru, Ir, and Os contents of the pyrrhotite it replaced. Sulfide minerals are preserved in transported glacial cover and their trace element chemistry can be used to discriminate their source. Pentlandite from Ni-Cu deposits has much lower Rh and Pd concentrations than those from PGE-dominated deposits, pyrite from magmatic deposits has higher Co/Sb and Se/As ratios relative to pyrite from hydrothermal deposits, and chalcopyrite from magmatic deposits has much higher Ni and lower Cd concentrations than those from hydrothermal deposits.

Non-destructive, multi-method, internal analysis of multiple inclusions in a single diamond: First occurrence of mackinawite (Fe,Ni)1+xS

A single gem lithospheric diamond with five sulfide inclusions from the Udachnaya kimberlite (Siberia, Russia) has been analyzed non-destructively to track the growth conditions of the diamond. Sulfides are the most abundant mineral inclusions in many lithospheric diamond crystals and are the most favorable minerals to date diamond crystals by Re-Os isotope systematics. Our investigation used non-destructive, micro-techniques, combining X-ray tomography, X-ray fluorescence, X-ray powder diffraction, and Raman spectroscopy. This approach allowed us to determine the spatial distribution of the inclusions, their chemical and mineralogical composition on the microscale, and, finally, the paragenetic association, leaving the diamond host completely unaffected. The sample was also studied by X-ray diffraction topography to characterize the structural defects of the diamond and to obtain genetic information about its growth history. The X-ray topographic images show that the sample investigated exhibits plastic deformation. One set of {111} slip lamellae, corresponding to polysynthetic twinning, affects the entire sample. Chemical data on the inclusions still trapped within the diamond show they are monosulfide solid solutions of Fe, Ni and indicate a peridotitic paragenesis. Micro-X-ray diffraction reveals that the inclusions mainly consist of a polycrystalline aggregate of pentlandite and pyrrothite. A thorough analysis of the Raman data suggests the presence of a further Fe, Ni sulfide, never reported so far in diamonds: mackinawite. The total absence of any oxides in the sulfide assemblage clearly indicates that mackinawite is not simply a “late” alteration of pyrrhotite and pentlandite due to secondary oxidizing fluids entering diamond fractures after the diamond transport to the surface. Instead, it is likely formed as a low-temperature phase that grew in a closed system within the diamond host. It is possible that mackinawite is a more common phase in sulfide assemblages within diamond crystals than has previously been presumed, and that the percentage of mackinawite within a given sulfide assemblage could vary from diamond to diamond and from locality to locality.

Geology and Geochronology of the Xietongmen (Xiongcun) Cu-Au Porphyry District, Southern Tibet, China

Geology and Geochronology of the Xietongmen (Xiongcun) Cu-Au Porphyry District, Southern Tibet, China

Sulfide oxidation and distribution of metals near abandoned copper mines in coastal environments, Prince William Sound, Alaska, USA

Abstract The oxidation of sulfide-rich rocks, mostly leftover debris from Cu mining in the early 20th century, is contributing to metal contamination of local coastal environments in Prince William Sound, Alaska. Analyses of sulfide, water, sediment, precipitate and biological samples from the Beatson, Ellamar, and Threeman mine sites show that acidic surface waters generated from sulfide weathering are pathways for redistribution of environmentally important elements into and beyond the intertidal zone at each site. Volcanogenic massive sulfide deposits composed of pyrrhotite and (or) pyrite + chalcopyrite + sphalerite with subordinate galena, arsenopyrite, and cobaltite represent potent sources of Cu, Zn, Pb, As, Co, Cd, and Hg. The resistance to oxidation among the major sulfides increases in the order pyrrhotite ≪ sphalerite < chalcopyrite ⩽ pyrite; thus, pyrrhotite-rich rocks are typically more oxidized than those dominated by pyrite. The pervasive alteration of pyrrhotite begins with rim replacement by marcasite followed by replacement of the core by sulfur, Fe sulfate, and Fe–Al sulfate. The oxi dation of chalcopyrite and pyrite involves an encroachment by colloform Fe oxyhydroxides at grain margins and along crosscutting cracks that gradually consumes the entire grain. The complete oxidation of sulfide-rich samples results in a porous aggregate of goethite, lepidocrocite and amorphous Fe-oxyhydroxide enclosing hydrothermal and sedimentary silicates. An inverse correlation between pH and metal concentrations is evident in water data from all three sites. Among all waters sampled, pore waters from Ellamar beach gravels have the lowest pH (∼3) and highest concentrations of base metals (to ∼25,000 μg/L), which result from oxidation of abundant sulfide-rich debris in the sediment. High levels of dissolved Hg (to 4100 ng/L) in the pore waters probably result from oxidation of sphalerite-rich rocks. The low-pH and high concentrations of dissolved Fe, Al, and SO4 are conducive to precipitation of interstitial jarosite in the intertidal gravels. Although pore waters from the intertidal zone at the Threeman mine site have circumneutral pH values, small amounts of dissolved Fe2+ in the pore waters are oxidized during mixing with seawater, resulting in precipitation of Fe-oxyhydroxide flocs along the beach–seawater interface. At the Beatson site, surface waters funneled through the underground mine workings and discharged across the waste dumps have near-neutral pH (6.7–7.3) and a relatively small base-metal load; however, these streams probably play a role in the physical transport of metalliferous particulates into intertidal and offshore areas during storm events. Somewhat more acidic fluids, to pH 5.3, occur in stagnant seeps and small streams emerging from the Beatson waste dumps. Amorphous Fe precipitates in stagnant waters at Beatson have high Cu (5.2 wt%) and Zn (2.3 wt%) concentrations that probably reflect adsorption onto the extremely high surface area of colloidal particles. Conversely, crystalline precipitates composed of ferrihydrite and schwertmannite that formed in the active flow of small streams have lower metal contents, which are attributed to their smaller surface area and, therefore, fewer reactive sorption sites. Seeps containing precipitates with high metal contents may contribute contaminants to the marine environment during storm-induced periods of high runoff. Preliminary chemical data for mussels (Mytilus edulis) collected from Beatson, Ellamar, and Threeman indicate that bioaccumulation of base metals is occurring in the marine environment at all three sites.

Fluid inclusions and quantitative model for gold precipitation in the Tiouit deposit (Anti-Atlas, Morocco)

In the Tiouit gold deposit (Anti-Atlas, Morocco), a paragenetic study has revealed the existence mainly of two generations of gold, the earliest one was cogenetic with ferro-arseniferous stage I; the second generation (most important) is associated with the minerals of a cupro-zinciferous sulphide stage II. A microthermometric study of fluid inclusions demonstrates that the main part of the gold was introduced by an aqueous solution of relatively low salinity (1.7 m Nacl) and deposited at about 400 °C and 500 bars in microfractures crosscutting early sulphides. A calculated quantitative model for the fluid-mineral interaction demonstrates that a significant amount of gold (3 mg deposited from 1 kg of solution, i.e. 43% of total gold), transported as bisulphide complexes ( HAu ( HS ) 2 0 , AuHS 0 and Au ( HS ) 2 - ), can be deposited as a result of a decrease in both H 2S activity and oxygen fugacity produced by the alteration of pyrrhotite into pyrite.

GLADIUSITE, Fe3+2(Fe2+,Mg)4(PO4)(OH)11(H2O), A NEW HYDROTHERMAL MINERAL SPECIES FROM THE PHOSCORITE CARBONATITE UNIT, KOVDOR COMPLEX, KOLA PENINSULA, RUSSIA

Gladiusite, ideally Fe 3+ 2 (Fe 2+ ,Mg) 4 (PO 4 )(OH) 11 (H 2 O), monoclinic, a 16.959(2), b 11.650(3), c 6.266(6) A, β 90.00(5)°, V 1238(1) A 3 , space group P 2 1 / n , Z = 4, is a new mineral species from hydrothermal assemblages associated with the phoscorite–carbonatite unit of the Kovdor alkaline-ultramafic complex, Kola Peninsula, Russia. It occurs in vugs within veins of mineralized dolomite carbonatite. Associated minerals are dolomite, pyrochlore, several generations of magnetite, pyrite, rutile, a ternovite-like phase, catapleiite, bobierrite, rimkorolgite, juonniite, strontiowhitlockite, collinsite (including a Sr-rich variety) and chlorite. Gladiusite occurs as acicular aggregates and as free-standing radial clusters ( 3 , respectively. Gladiusite is biaxial negative and strongly pleochroic, α 1.722(2), β 1.730(2), γ 1.737(2), 2 V calc. = 78.3°. The strongest reflections in the X-ray powder-diffraction pattern [ d in A( I )( hkl )] are: 9.61(53)(110), 6.87(77)(210), 5.83(89)(020), 4.805(100)(220), 3.787(62)(130), 3.533(84)(230), and 2.868(66)(140). Electron-microprobe analysis and Mossbauer spectroscopy gave (wt.%) FeO 25.00, Fe 2 O 3 29.90, MgO 11.16, MnO 0.78, P 2 O 5 12.46, TiO 2 0.04, H 2 O(calc.) = 20.18, sum = 99.51 wt.%; the H 2 O content, obtained by the Penfield method, is 19.7 wt.%. The unit formula is Fe 3+ 2.00 [Fe 2+ 2.02 Mg 1.61 Fe 3+ 0.17 Mn 2+ 0.06 ] ∑3.86 (P 1.02 O 4 )(OH) 11 (H 2 O) on the basis of 16 O atoms. The simplified formula is Fe 3+ 2 (Fe 2+ ,Mg) 4 (PO 4 )(OH) 11 (H 2 O). Crystal-structure study showed gladiusite to be extensively twinned on [001]. Gladiusite is considered to be a product of hydrothermal alteration of pyrrhotite and fluorapatite, the primary Fe and P minerals in the dolomite carbonatite. The mineral is named in accordance with the morphology of the crystals, which resemble double-edge swords ( gladius , in Latin) in SEM images.

Mineralogical study of base metal tailings with various sulfide contents, oxidized in laboratory columns and field lysimeters

Oxidation of a flotation-derived, low-sulfide tailings containing approximately 0.4 wt.% S was compared with simultaneously oxidized tailings containing 1.0 wt.% S and 2.5 wt.% S to assess their acid generating characteristics. Each tailings type was exposed to oxidation for three years in laboratory columns and in lysimeter pits in the field. In these tailings the sulfide mineral of principal concern with respect to acid generation is pyrrhotite (Fe 1-x S). In past studies the alteration of pyrrhotite has been characterized by initial replacement with marcasite (FeS2) and ferric iron sulfates, which are followed by development of ferric oxyhydroxides such as goethite and lepidocrocite. Macroscopic characterization of the tailings shows varying and progressive degrees of oxidation correlative with the three different sulfur contents. As expected, the tailings with the lowest sulfur content are the least oxidized, and those with the highest sulfur content have reacted the most. The column tests, which represent accelerated reaction conditions relative to those for the lysimeter pits, show much higher degrees of oxidation, and a markedly more distinct boundary between the oxidized and unoxidized zones; as well, differences among the three tailings types are more pronounced.

Sulfide Oxidation and Production of Gossans, Ashanti Mine, Ghana

At the Justice mine, in the Ashanti goldfields of southwestern Ghana, chemical weathering of gold- bearing sulfide-rich lodes has produced a series of characteristic mineralogical and geochemical features that are diagnostic. In this type of gold mineralization, the most abundant sulfides are arsenopyrite, pyrite, pyrrhotite, and chalcopyrite with minor bornite and sphalerite. Gold occurs predominantly as native gold, spatially associated and chemically bound with arsenopyrite. Elsewhere gold-silver tellurides are present in quartz veins. During sulfide oxidation, arsenopyrite is replaced by amorphous and crystalline Fe-Mn arsenates, goethite, hematite, and arsenolite in box- and ladderwork textures. In the extremely weathered gossans exposed at surface or in exploration pits, goethite, hematite, and scorodite are present as pseudomorphs of oxidized arsenopyrite, which can be used as a visual pathfinder for gold-arsenic mineralization. As with arsenopyrite, pyrite and pyrrhotite alteration produces boxwor...

Magnetic monitoring of pyrrhotite alteration during thermal demagnetization

Pyirhotite progressively oxidizes via magnetite to hematite during thermal demagnetization of porous artificial samples containing well‐defined natural pyrrhotite. Rockmagnetic and X‐ray diffraction determinations show that the intermediate alteration stages consist of pyrrhotite‐magnetite‐pyrite intergrowths. Alteration is quite differently expressed in two ctaiically virtually identical pyrrhotites.

Magnetic properties of natural pyrrhotite. II. High- and low-temperature behaviour of Jrs and TRM as function of grain size

Twelve grain-size fractions, ranging from 250 νm down to < 5 νm, of four natural pyrrhotite samples were homogeneously dispersed in a non-magnetic matrix to study their magnetic behaviour during thermal demagnetization and low-temperature treatment. Normalized decay curves of the isothermal saturation remanence ( J rs) obtained by continuous thermal demagnetization are identical for all samples studied. They are characterized by a continuous decay up to the maximum blocking temperature of 325°C. Small grain-size fractions have a blocking temperature spectrum slightly shifted to lower temperatures. Decay curves for J rs obtained by stepwise thermal demagnetization yield a different behaviour for coarse- and fine-grained fractions. The former show a monotonous decay up to the maximum blocking temperature, whereas the latter show an enhanced decay at 210–245°C, followed by a small remanence increase at 260–300 ° C. The enhanced decay is tentatively related to the formation of non-magnetic anomalous pyrrhotite; the increase with its upper stability temperature. The impact of pyrrhotite alteration and metastable preservation of high-temperature superstructures upon rapid cooling is discussed. The TRM intensity decreases with grain size, possibly owing to the absence of closure domains in pyrrhotite. Normalized TRM decay curves are virtually identical for all fractions. Stepwise thermal demagnetization indicates narrow blocking temperature spectra for coarse grain-size fractions, slightly broadening with decreasing grain size. Low-temperature cycling of J rs shows a gradually increasing reversible remanence behaviour with decreasing grain size. This concurs with a gradual approach of the SD state in the smallest grain-size fractions (< 5 νm). TRM shows a reversible low-temperature behaviour throughout the investigated grain-size range.

Thermochronology of economic mineral deposits; dating the stages of mineralization at Panasqueira, Portugal, by high-precision 40 / 39 Ar age spectrum techniques on muscovite

40 Ar/ 39 Ar age spectrum dates for 13 muscovites have been used to reconstruct the thermal history (thermochronology) of the Panasqueira, Portugal, tin-tungsten deposit, a deposit spatially associated with a belt of Hercynian plutons. Muscovite samples with an age difference as small as 2.2 m.y. (0.7% of the age) are statistically distinct. Statistics are even better for comparison of multiple samples from separate events; that is, a difference of 0.9 m.y. (0.3%) can be resolved in this approximately 300-m.y.-old deposit. The major tin and tungsten ore-forming stages, which are the oxide-silicate stage, the main sulfide stage, and greisenization, occurred between 296.3 + or - 0.8 (1 Sigma ) and 291.6 + or - 0.8 m.y. (1 Sigma ). The first substage of the oxide-silicate stage was a short-lived thermal pulse at 296.3 + or - 0.6 m.y.; the fluids responsible may have emanated from the known granite cupola. The main sulfide stage was active at 294.5 + or - 0.9 m.y. as a slightly longer lived pulse with oldest evidence for this stage (295.8 + or - 0.6 m.y.) coming from areas farthest away from the known cupola and youngest evidence (293.5 + or - 0.8 m.y.) closest to the cupola. A second substage of the oxide-silicate stage occurred as a short-lived thermal pulse at 292.9 + or - 0.7 m.y., synchronous with greisenization of the cupola and alteration of the silica cap at 292.1 + or - 0.4 m.y. The duration of activity of the oxide-silicate stage, the main sulfide stage, greisenization, and alteration of the silica cap based on the ages of all 13 muscovites was greater than 4.2 + or - 0.5 m.y. (1 Sigma ). Minor argon loss from all dated muscovites occurred during later reheating, probably during the longer lived pyrrhotite alteration stage. A single center, the known cupola, had a prolonged role and was the source for main sulfide stage, oxide-silicate stage II, greisenization, and alteration of the silica cap and possibly oxide-silicate stage I and the pyrrhotite alteration stage; however, a separate source for these latter two stages cannot be ruled out.This study is an example of a new and powerful application of 40 Ar/ 39 Ar age spectrum dating of muscovite. Because of the high precision demonstrated in this study, it is now possible to establish time constraints necessary for solving some of the long-standing problems in economic geology. Beyond this, the unique geologic situation of Panasqueira has allowed us to quantify the thermal characteristics of muscovite. Published fluid inclusion data have been used to estimate a muscovite argon closure temperature of approximately 325 degrees C during rapid cooling or short reheating and a temperature of approximately 270 degrees C during slow cooling or extended reheating. Argon-loss patterns displayed by all dated muscovites resulted from reheating after original closure; the mechanism for this argon loss appears to have been argon transport by volume diffusion. Thus, 40 Ar/ 39 Ar age spectrum dating of muscovite can be used to evaluate thermal conditions controlling argon diffusion as well as age, duration, and number of episodes of mineralization.

Chemical Composition and Stability of Violarite

Violarites of the general composition (Fe, Ni) 3 S 4 from a variety of natural Fe-Ni sulfide assemblages have been studied by electron microprobe analysis. The violarite solid solution extends from the Ni-rich end member polydymite to at least 17 at. percent Ni, compositions more Fe-rich than FeNi 2 S 4 being characteristic of pyrrhotite-bearing assemblages and compositions more Ni-rich than FeNi 2 S 4 of Ni-rich assemblages that do not contain pyrrhotite. Most of the violarites have compositions in the range of 56 to 57 at. percent S and thus are slightly S-deficient relative to the ideal M 3 S 4 composition.The predominant mode of formation of violarite is by alteration of pentlandite. Violarite also forms from millerite, but alteration of pyrrhotite to violarite has not been confirmed in the present study. A single crystal X-ray diffraction study of a partially violaritized pentlandite grain has shown that violarite has the same crystallographic orientation as the host pentlandite and the alteration can be effected by removal of excess metal atoms from pentlandite and redistribution of the remainder. It is suggested that violarite forms readily from pentlandite because of the ease with which the pentlandite structure can be converted to that of violarite.The stability field of violarite in the Fe-Ni-S system must enlarge to more Fe-rich compositions, toward the Fe-S join, as the temperature decreases. Likely phase relations at temperatures sufficiently low to include smythite and the extension of the violarite field to 17 at. percent Ni have been schematically presented.

Untersuchung des Magnetkies-Zwischenproduktes von Lindenfels, Odenwald, mit der Mikrosonde und im Infrarot

Untersuchung des Magnetkies-Zwischenproduktes von Lindenfels, Odenwald, mit der Mikrosonde und im Infrarot

Mineralogy, paragenesis and geothermometry of the tin and tungsten deposits of the eastern Andes, Bolivia

Numerous geothermometers were applied to ores of the Eastern Andes of Bolivia in order to evaluate temperature as one factor in zoning and paragenesis of the deposits, and also to test the thermometers themselves where results could be checked against available field evidence. Some 225 specimens were analyzed representing 53 deposits and several ore types, chiefly the tin and tungsten ores.The stratigraphic record suggests that ores of the subvolcanic setting of central and southern Bolivia formed at depths on the order of 350 to 2,000 m and at total pressures of approximately 30 to 500 bars. Comparable estimates for the somewhat deeper deposits of plutonic association in the Cordillera Real are depths of 2,000 to 4,000 m and pressures of about 190 to 1,000 bars.A similar paragenesis is indicated for the tin and tungsten deposits and consists of (1) an early vein stage in which most tin and tungsten was deposited, (2) a base-metal sulfide stage, (3) hypogene alteration of pyrrhotite to pyrite, marcasite, and siderite and (4) late crusts and veinlets of siderite, fluorite, and a variety of hydrous phosphate minerals some of which are supergene.The Bolivian ores contain most of the sulfide minerals proposed, at one time or another, as geothermometers, but present attempts to use them have been largely unsuccessful. Pyrrhotite compositions were modified during replacement by pyrite, marcasite and siderite and most do not reflect initial depositional temperatures. Arsenopyrite gave only rough indications of temperature and was essentially useless as a geobarometer. Sphalerite displays complex polytypism in the Bolivian ores, but this could not be correlated with variables of geologic interest. The present iron content of sphalerite reflects initial equilibration with pyrrhotite or pyrite, but its significance in terms of temperature is debatable. Miscellaneous thermometers such as sulfide invariant points, the high-low quartz inversion, melting of native bismuth and sulfide exsolution temperatures are considered and some yield useful temperature limits. Fluid inclusion studies reveal highly systematic trends of salinity and depositional temperature during formation of the tin and tungsten ores. Tin-bearing fluids of the early vein stage were complex, NaCl-rich brines of low CO 2 content. During precipitation of cassiterite and early quartz, salinities reached values as high as 46 weight percent, but the later ore fluids were more dilute and gradually approached fresh water in closing stages of mineralization. Depositional temperatures first increased from about 300 degrees to 530 degrees C during the early vein stage and then declined steadily to less than 70 degrees C in the later stages. Minerals of the main sulfide stage formed as temperatures declined from about 400 degrees C to 260 degrees C. Fluid inclusions in siderite indicate temperatures of 260 degrees C to 200 degrees C for the hydrothermal alteration of pyrrhotite. Late fluorite of the Colquiri lode was precipitated at temperatures between 200 degrees C and 133 degrees C. The late hydrous phosphate minerals formed below 70 degrees C. Many events of the hypogene sequence could be attributed chiefly to cooling of the vein fluids and are so interpreted, but little is known of the wall rock alteration and its influence on paragenesis. Active boiling of the early vein fluids is indicated by the fluid inclusions in quartz and cassiterite. This boiling upgraded salt contents of the early vein fluids while driving CO 2 into the associated vapor phase. This vapor transported some mineral matter, as evidenced by daughter minerals in gas-rich inclusions, but the identity and quantitative significance of this material are unknown. The early boiling favored precipitation of quartz and cassiterite and may explain the restricted vertical distribution of very high-grade tin ore in several deposits of shallow origin in central and southern Bolivia. Evidence of boiling in several major deposits of the Cordillera Real suggests that these ores, though formed in the plutonic setting, were still deposited at depths of less than 3,000 m. This limit is compatible with that based on stratigraphic reconstructions of rock cover at the time of mineralization. All data gathered in this study are consistent with a single, prolonged event of mineralization for the individual tin and tungsten deposits throughout Bolivia. The deposits could differ in age, but there is no convincing evidence that they are hybrid products of hydrothermal regenerative processes. A magmatic source is preferred for the mineralizing NaCl brines and their contained metals, but the progressive cooling and dilution of these brines may reflect gradual influx of meteoric water to the magmatic hydrothermal system.

Paragenesis of iron sulphides in a Black Hills deposit

A small prospect mine near Deadwood, South Dakota, shows striking selective replacement of dolomite beds by quartz, pyrite, sphalerite, arsenopyrite, galena, chalcopyrite, and pyrrhotite. The occurrence of arsenopyrite, pyrrhotite, and gold values is significant in view of the occurrence of these minerals in the Homestake Mine, three miles distant.The remarkable feature of the ores is the abundant occurrence of four types of iron sulphide aside from small amounts of pyrrhotite. These varieties are: aggregates of anhedral grains of pyrite; euhedral pyrite crystals; stippled, brownish, fine-grained pyrite, and marcasite. The stippled brownish variety of pyrite is believed to have formed by alteration of pyrrhotite at a late stage in the mineralization apparently after the partial replacement of pyrrhotite by marcasite.

Note on the colour and alteration of Pyrrhotite

The colour of the well-known iron sulphide, pyrrhotite or pyrrhotine, is given in most treatises on mineralogy as 'bronze-yellow'. Dana calls it ' between bronze-yellow and copper-red ', and also notes that it is 'subject to speedy tarnish'. The accuracy of the last statement will not be doubted by any one who has had much acquaintance with mining operations in districts where pyrrhotite is a common lode mineral, but it does not appear to have been put on record in any scientific publication that the characteristic colour, not very accurately designated 'bronze-yellow', does not seem to be the original one of the unaltered mineral.