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Thermal magnetic susceptibility data on natural iron sulfides of northeastern Russia
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Russian Geology and Geophysics 54 (2013) 464–474 www.elsevier.com/locate/rgg
Thermal magnetic susceptibility data on natural iron sulfides of northeastern Russia P.S. Minyuk a,*, E.E. Tyukova a, T.V. Subbotnikova a, A.Yu. Kazansky b, A.P. Fedotov c a
North-East Interdisciplinary Scientific Research Institute, named after N.A. Shilo, Far Eastern Branch of the Russian Academy of Sciences, ul. Portovaya 16, Magadan, 685000, Russia b A.A. Trofimuk Institute of Petroleum Geology and Geophysics, Siberian Branch of the Russian Academy of Sciences, pr. Akademika Koptyuga 3, Novosibirsk, 630090, Russia c Limnological Institute, Siberian Branch of the Russian Academy of Sciences, ul. Ulan-Batorskaya 3, Irkutsk, 664033, Russia Received 19 January 2011; received in revised form 30 May 2012; accepted 18 October 2012
Abstract Dependences of magnetic susceptibility (MS) on the temperature of natural iron sulfide samples (pyrite, marcasite, greigite, chalcopyrite, arsenopyrite, pyrrhotite) from the deposits of northeastern Russia were studied. The thermal MS curves for pyrite and marcasite are the same: On heating, MS increases at 420–450 ºC, and unstable magnetite (maghemite) and monoclinic pyrrhotite with a well-defined Hopkinson peak are produced. In oxygen-free media with carbon or nitrogen, magnetite formation is weak, whereas pyrrhotite generation is more significant. The heating curves for chalcopyrite are similar to those for pyrite. They show an increase in MS at the same temperatures (420–450 ºC). However, stable magnetite is produced, whereas monoclinic pyrrhotite is absent. In contrast to that in pyrite, marcasite, and chalcopyrite, magnetite formation in arsenopyrite begins at >500 ºC. Arsenopyrite cooling is accompanied by the formation of magnetite (S-rich arsenopyrite) or maghemite (As-rich arsenopyrite) with a dramatic increase in MS. Arsenopyrite with an increased S content is characterized by insignificant pyrrhotite formation. Greigite is marked by a decrease in MS on the heating curves at 360–420 ºC with the formation of unstable cation-deficient magnetite. Monoclinic pyrrhotite is characterized by a decrease in MS at ~320 ºC, and hexagonal pyrrhotite, by a transition to a ferrimagnetic state at 210–260 ºC. The addition of organic matter to monoclinic pyrrhotite stimulates the formation of hexagonal pyrrhotite, which transforms back into monoclinic pyrrhotite on repeated heating. The oxidation products of sulfides (greigite, chalcopyrite) show an increase in MS at 240–250 ºC owing to lepidocrocite. © 2013, V.S. Sobolev IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved. Keywords: magnetic susceptibility; temperature dependence of magnetic susceptibility; pyrite; marcasite; greigite; arsenopyrite; chalcopyrite; pyrrhotite
Introduction Thermal magnetic susceptibility measurements are a kind of thermomagnetic analysis in which magnetic susceptibility (MS) is studied under continued heating. It is a sensitive method for identifying magnetic and some Fe-containing minerals, including iron sulfides (pyrite, marcasite, chalcopyrite, arsenopyrite, pyrrhotite, greigite). These minerals are Cu, As, and H2SO4 ores and often accompany Au–Ag deposits. Some sulfide minerals contain Au and Ag admixtures and, therefore, are of interest as ores of these elements. Some iron sulfides (pyrite, marcasite, greigite) are widespread in sediments of different genesis (marine, lacustrine). They form
* Corresponding author. E-mail address: [email protected] (P.S. Minyuk)
mainly in reducing media, characterizing certain conditions of sediment formation and diagenesis, and are widely used to reconstruct paleodepositional environments. Thermal MS studies have become a common practice over the last few decades, especially with the appearance of equipment sets for this analysis (AGICO kappabridges and furnaces, Czech Republic). A combination of the technique for heating experiments with computer processing and presentation of results permits comparing data from laboratories worldwide. Published data indicate a wide spectrum of thermomagnetic curves, whose shape is influenced by the qualitative composition of ferrimagnetic materials, concentration, grain size, and the presence of Fe-containing paramagnetic minerals, which become ferrimagnetic upon heating. The results of heating experiments are greatly influenced by reducing admixtures (sulfur, nitrogen, carbon), which are often
1068-7971/$ - see front matter D 201 3, V.S. So bolev IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.rgg.2013.03.008 +
P.S. Minyuk et al. / Russian Geology and Geophysics 54 (2013) 464–474
present in sediments. Releasing sulfur upon heating, iron sulfides act as reducing agents themselves and influence temperature transformations of other minerals (e.g., iron oxides and hydroxides), thus complicating data interpretation. Although temperature dependences of the MS of sulfides have long been studied, the distinctive features of individual minerals and the influence of the enclosing substratum on the results of heating are poorly known. Methods We studied samples of natural iron sulfides, mainly from different Au–Ag deposits of northeastern Russia (less often, from sediments). High-temperature MS studies were conducted on an MFK1FA multifunctional kappabridge with a CS-3 high-temperature control unit (AGICO). The samples were heated to 700 °C and then cooled to room temperature (both at a rate of 12– 13 °C/min). For the quantitative description of the thermal curves, we used the MS variation indices proposed by F. Hrouda (2003): A40 = (k40 – K40)/K40 × 100) and Amax = (k – K)max/K40 × 100, where k40 (K40) is the MS value at 40 °C during cooling (heating) and (k – K)max is the maximum difference between MS values at equal cooling and heating temperatures. The samples were heated mainly in air; sometimes, reducing agents were added: carbon (sucrose), nitrogen (carbamide (NH2)2CO), and arsenic. The additives made up <5% of the mineral volume. The mixture was preagitated. According to special experiments, MS remains unchanged when the reducing agents themselves are heated to 700 °C and then cool to room temperature. On the other hand, metallic As is known to sublimate at 615 °C; carbamide turns into NH4NCO, NH3, CO2, etc., at T ≥ 150 °C (Knunyants, 1992); and sucrose melts and caramelizes at 186–200 °C, with decomposition at higher temperatures (Knunyants, 1983). The hysteresis parameters, including the remanent saturation magnetization (Jrs), saturation magnetization (Js), induced magnetization (Ji), coercivity (Hc), and remanent coercivity (Hcr), were measured on a J-meter automatic coercivity spectrometer (Kazan Federal University). The maximum field induction was 500 mT. The measurements were taken both for the original samples and upon heating with different additives. The element composition (Na, Mg, Al, Si, P, K, Ca, Ti, Fe, Mn, S) of individual mineral grains was determined on a Camebax microprobe before and after the heating. The detection limit was 1% for Na, 0.8% for Al and Si, and 0.5% for the other elements. Some minerals were studied on a QEMSCAN system (Australia), which included an EVO 50 SEM with a Quantax Esprit EDS (Bruker). Results and discussion Pyrite and marcasite. Pyrite FeS2 is unstable upon heating. When pyrite and marcasite are heated in air, a wide
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spectrum of magnetic minerals forms at different heating stages: hematite, magnetite, maghemite, and pyrrhotite (Kopp and Kerr, 1958); hematite, iron sulfates, and/or pyrrhotite (depending on the particle size) (Dunn et al., 1989a,b); hematite (Schorr and Everhart, 1969); hematite and magnetite/mag- hemite (Eneroth and Koch, 2003); magnetite, hematite, and pyrrhotite (Jørgensen and Moyle, 1982). Pyrite annealing at 610 °C during different periods of time produces pyrrhotite, iron monosulfide, α-Fe, magnetite, hematite, and maghemite (Prasad et al., 1985). Heating in carbon dioxide produces pyrrhotite, magnetite, maghemite, and hematite (Bhargava et al., 2009; Fegley et al., 1995). Pyrite heating with simultaneous studies of transformations by the X-ray (Bhargava et al., 2009) or Mössbauer method (Ferrow and Sjöberg, 2005) revealed quite a complicated set of intermediate mineral phases. For example, FeSO4, ε-Fe2O3, Fe2(SO4)3, β-Fe2O3, α-Fe2O3, and pyrrhotite were determined by Mössbauer spectroscopy. In situ X-ray structural analysis of pyrite showed that heating in a vacuum (T > 400 °C) produces hematite and magnetite; in air (T > 420 °C), hematite; in argon and nitrogen (T > 550 °C), pyrrhotite; in carbon dioxide, pyrrhotite (T > 500 °C), magnetite (T > 600 °C), and hematite (T > 700 °C) (Bhargava et al., 2009). An overview of literature on the mechanism and kinetics of pyrite transformations in different media is given in (Hu et al., 2006). When pyrite is heated, Js and/or MS increase after 420– 450 °C and decrease near the Curie point of magnetite; this is explained by the formation of this mineral on heating (Wang et al., 2008). Some researchers estimate the concentration of iron sulfides (pyrite) in sediments from the intensity of the peak on thermomagnetic curves (Emiroglu et al., 2004). However, the peak intensity depends on the grain size: Powdered pyrite does not yield such a susceptibility peak (Ferrow and Sjöberg, 2005). Also, pyrite content is determined from the magnetite newly formed during the heating (Grachev et al., 2008). The Js(T) cooling curves bend near the Curie temperature region of pyrrhotite. The k(T) cooling curves show a dramatic pyrrhotite peak at T ~ 320 °C (Wang et al., 2008). The before-heating MS of the pyrite samples studied was 11 × 10–9 m3/kg. The following pyrite composition was determined on the microprobe: 46.50 wt.% Fe, 53.55 wt.% S; n = 7. The pyrite susceptibility was studied during heating and cooling to different temperatures in air and with carbamide (nitrogen) and sucrose (carbon) additives. The initial pyrite was used for the next cycle. The chemical composition of the grains was determined on the microprobe after each cycle. The additives were used to trace their influence on the beginning and intensity of temperature transformations of pyrite. Curves typical of two full heating–cooling cycles in air (to 700 °C) are shown in Fig. 1a. During the heating, MS begins to increase at 430–450 °C and then decreases at the Curie temperature of magnetite. This newly formed mineral is temperature-unstable, as evidenced by irreversible heating– cooling curves at 700–400 °C (Fig. 1c). A distinct Hopkinson peak of pyrrhotite is observed on the cooling curves of cycle 1 at T ~ 320 °C. Judging by the ratios
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Fig. 1. Temperature dependences of magnetic susceptibility: a, pyrite; b, marcasite; c, fragment of the heating–cooling curves for cycle 1, pyrite. Numbers in italics indicate the heating.
Jrs/Js and Hcr/Hc, the particles are single-domain ones. The MS variation indices for cycle 1 are A560 = –41, A306 = 275.3, and A40 = 99.3. The cycle 1 curves are also irreversible. Monoclinic pyrrhotite and temperature-unstable magnetite are determined from them. The MS variation indices for cycle 2 are A568 = –1.5, A306 = 4.8, and A40 = 1.3. According to these data, pyrite continues to transform into unstable magnetite and pyrrhotite during this cycle. Heating–cooling curves for different temperatures were obtained to determine the pyrrhotite formation temperature. It was found that the curves of the heating cycle are reversible up to 400 °C; after 450 °C, the cooling curves are above those of heating, reflecting magnetite formation. Also, such curves were obtained for the 500 °C cycle. A slight pyrrhotite peak appears on the cooling curves only after 600 °C and becomes prominent after the 700 °C cycle. Pyrite heating with sucrose showed that MS begins to increase at 420 °C, peaks at ~490 °C, and decreases at ~600 °C.
The cooling curve is irreversible, with a prominent Hopkinson peak of pyrrhotite, and without the magnetite phase. The MS variation indices for cycle 1 are A494 = –2.9, A318 = 393, and A40 = 81.5. The cycle 2 curves are complicated. The heating curves have peaks at 315 (monoclinic pyrrhotite) and 365 °C (?), and the cooling ones have them at 320 and 350 °C, respectively. Pyrite heating with carbamide indicates that the MS curves are almost reversible before 450 °C (Fig. 2a–d); after 500 °C, the cooling curves have a slight peak at 300 °C, which is due to the monoclinic pyrrhotite produced by the heating (Fig. 2e). Monoclinic pyrrhotite is more distinct on the cooling curves after heating to ≥550 °C: It is reflected in increasing MS near the Curie point (Fig. 2f–h). So, heating in air revealed a considerable increase in MS at ~430–450 °C, and that with sucrose, at ~425 °C. This reflects the oxidation of pyrite decomposition products with the formation of magnetite, judging by MS(T). On the curves
Fig. 2. k(T) curves for cycles of pyrite heating with carbamide (nitrogen). a–h, See explanation in the text. Inset shows the heating curve.
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for the heating with sucrose, the magnetite “hump” is gentle and shows a maximum at lower temperatures (490 °C), whereas during the heating with carbamide, the transformations begin at higher temperatures (~500 °C). The characteristic magnetite “hump” on the heating curves is considerably weaker than that for the samples heated without additives. However, pyrrhotite forms at lower temperatures (500 °C) and in larger amounts; this significantly affects the MS values. The MS of a large pyrite sample (0.1–2.0 mm) increased by 15–16 times upon heating in air and by 76 times after the addition of sucrose and carbamide. The susceptibility of the same sample in powdered form (0.02–0.05 mm) increased by 550 times with respect to the initial value. Framboidal pyrite, which occurs widely in sediments of different ages and is characterized by microscopic size (Wilkin and Barnes, 1997), is expected to undergo more intense temperature transformations than coarse-grained pyrite. Additives favor the formation of pyrrhotite rather than magnetite (apparently, because of oxygen deficiency). Remarkably, the pyrite composition, determined on the microprobe upon heating even to 700 °C, remains unchanged: 45.72 wt.% Fe, 52.36 wt.% S, and n = 5, though TMA reveals the presence of pyrrhotite. After 20-minute thermal treatment of the sample at 700 °C, two phases of pyrite (47.86 wt.% Fe, 49.76 wt.% S, n = 18) and pyrrhotite (61.25 wt.% Fe, 37.44 wt.% S, n = 16) compositions appear in the grains (Fig. 3). The variations in the MS of pyrite at 2–700 A/m reach 70% of the initial value; note that the maximum variation is observed at low fields (2–40 A/m) and might be due to measurement errors at low fields (Hrouda et al., 2006). The heating is followed by a directed increase in MS with the field value, as is typical of pyrrhotite (Hrouda et al., 2006; Worm et al., 1993). At 700 A/m, MS increased by 20% for large (0.1–2 mm, sample PRS-700) grains and by 7% for small ones (0.02–0.05 mm, sample PRS-P700) (Fig. 4c). Marcasite FeS2 is a rhombic mineral. The before-heating MS of marcasite (sample MZ) was 62 × 10–9 m3/kg. The marcasite studied contains 46.00 norm. wt.% Fe and 54.00 norm. wt.% S; n = 6. The thermal MS data are analogous to those on pyrite (Fig. 1b). The MS variation indices for heating in air are A40 = 21.3, A307 = 57, and A566 = –14. The MS value increased by 22 times after the heating. The heating produces mainly high-coercivity monoclinic pyrrhotite. Hc = 57.3, and Hcr = 76.2 (Table 1). As is the case with pyrite, sucrose addition increases MS upon heating. Magnetic susceptibility on the heating curves begins to increase at higher temperature (500 °C); the magnetite “hump” is gentle; and the pyrrhotite peak is more prominent on the cooling curves. The MS variation indices for this cycle are A40 = 37.7, A313 = 129.4, and A516 = 3.4. The after-heating MS value was ~40 times higher. No increase in the MS of magnetite was observed on the heating or cooling curves during the heating with arsenic. At
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Fig. 3. Fragment of a pyrite grain heated to 700 °C for 20 min with two phases. Po, Pyrrhotite; Py, pyrite; G, galena.
the same time, there is a distinct pyrrhotite peak on the cooling curves (not shown). The MS variation indices are A40 = 3124 and A314 = 8360. Additives, as is the case with pyrite, favor the formation of monoclinic pyrrhotite rather than magnetite. Greigite Fe3S4 is a ferrimagnetic cubic mineral, a sulfide analog of magnetite (Skinner et al., 1964). It is known as melnikovite in Russian publications (Polushkina and Sidorenko, 1963). The magnetic properties of greigite are described in detail in the review by Roberts et al. (2011). Temperature transformations hamper exact location of the Curie point for greigite: 333 °C (Spender et al., 1972), >322 °C (Roberts, 1995), >350 °C (Chang et al., 2008), or >463 °C (Bol’shakov and Dolotov, 2011). As greigite was heated at 300–400 °C, X-ray diffractometry revealed pyrite, marcasite, and pyrrhotite; above 400 °C, structural varieties of Fe2O3 were observed (Krs et al., 1992). Temperature transformations of greigite begin between 238 and 282 °C (Skinner et al., 1964). Note that the 148-h-long thermal treatment of the samples at 282 °C yielded only 5% of pyrrhotite. Of the many cited thermomagnetic curves for greigite, the cooling curves show newly formed monoclinic pyrrhotite only in (Chang et al., 2008; Roberts et al., 2011). Thermomagnetic analysis Js(T) and thermal MS measurements k(T) were used to study synthetic and natural greigite and greigite-bearing sediments. The curves Js(T) and k(T) are irreversible during the heating and cooling. Sometimes, the cooling curves are above the heating ones (type 1) (Brachfeld et al., 2009; Dekkers et al., 2000; Roberts, 1995); other times, below them (type 2) (Chang et al., 2008; Roberts et al., 2011; Vasiliev et al., 2007). On all the heating curves, magnetization (Js) and/or MS decrease to 350–420 °C (less often, 450 °C). This is followed by the formation of a temperature-stable (type 1) or temperature-unstable (type 2) magnetic mineral (presumably magnetite). Greigite from the sediments of Lakes Hövsgöl (Mongolia) (Kazansky et al., 2005; Nourgaliev et al., 2005) and El’gygytgyn (Chukchi Peninsula) was studied. It occurs as loose black grain accumulations. It is highly oxidized in some samples. Greigite nodules and grains for the studies were selected under
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Fig. 4. Dependence of the MS of pyrrhotite (a), greigite (b), and pyrite (c) on the magnetic field.
a binocular microscope. Both oxidized and nonoxidized greigite was studied. The MS of the samples varies from 11 × 10–6 to 70 × 10–6 m3/kg. It remains almost unchanged at different fields except low ones, with great measurement errors (Fig. 4b). Jrs/Js = 0.4–0.6, and Hcr/Hc = 1.3–1.9 (Table 1). Several heating–cooling cycles were performed. Cycle 1. The k(T) curves are irreversible. On the heating curves, MS decreases from 270–300 to 360–420 °C, increases, and then decreases at 585–590 °C (Fig. 5a). The decrease in MS is due to greigite decomposition, as evidenced by the irreversible heating–cooling curves for samples heated to 320 and 450 °C (Fig. 5b, c). The increase in MS is due to magnetite formation. The cooling curves are usually above the heating ones, and susceptibility increases dramatically at 590 °C (samples C15, C132, C62, C18) or ~640 °C (sample E688). The cooling curves are only seldom below the heating ones. Cycle 2. The heating curves for all the samples are characterized by a dramatic decrease in MS at 650 °C; the cooling ones, by an increase at 600–610 or 630 °C (sample E668) (Fig. 5d, f). All the cooling curves are below the heating ones. The newly formed minerals are partly oxidized to hematite. Cycle 3. The curves are almost reversible, with a decrease in MS at ~620 °C (Fig. 5d, f). They are gently convex with a maximum at ~450 °C. No Hopkinson effects are observed. Very similar MS curves were obtained upon heating goethite with sucrose. A slight MS increase begins at ~240 °C on all the curves for the samples studied (Fig. 5a). It might be due to the appearance of λ-pyrrhotite or transformations of iron hydrox-
ides into oxide forms (probably, magnetite (maghemite)). The iron hydroxides were produced by greigite oxidation (weathering). The samples with removed oxidized nodules (as a rule, ocherous) are characterized by a slight, if any, increase in MS at these temperatures. Therefore, the increase in MS is attributed to the dissociation of iron hydroxides (presumably lepidocrocite) (Burov and Yasonov, 1979). At the same temperatures, the MS of greigite from the marine sections of Italy (Sagnotti and Winkler, 1999), Chukchi Sea sediments (Brachfeld et al., 2009), flyschoid sediments of Iran (Aubourg and Robion, 2002), and lacustrine sediments of Tibet (Hu et al., 2002). Also, an increase in magnetization was observed during the heating of greigite from the lacustrine sediments of England (Snowball and Thompson, 1990). Chalcopyrite (CuFeS2). Chalcopyrite annealing is accompanied by complicated reactions with the formation of iron oxides, iron and copper sulfates, etc. An overview of this question can be found in (Prasad and Pandey, 1998). Note that the MS of chalcopyrite increases considerably upon heating above 400 °C (measurements were taken with cold samples). Hematite is the only newly formed magnetic mineral determined here by X-ray diffractometry (Sahyoun et al., 2003). Oxidized and nonoxidized chalcopyrite from the ore deposits of northeastern Russia was studied. Its MS was ~30 × 10−9 m3/kg. Oxidized chalcopyrite in caverns and microcracks is covered with an iron hydroxide crust. Chalcopyrite contains 35.10 norm. wt.% Cu, 30.94 norm. wt.% Fe, and 33.96 norm. wt.% S; n = 5. The oxidized areas are poor in S:
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P.S. Minyuk et al. / Russian Geology and Geophysics 54 (2013) 464–474 Table 1. Hysteresis characteristics of iron sulfides Mineral, sample
Heating
Hc
Hcr
Js
Jrs
Jrs/Js
Hcr/Hc
2
mT
Am /kg
Pyrite, PRS
1st
45.4(43.7)
59.2
1.9(2.4)
1.1
0.58(0.46)
1.30(1.35)
Pyrite, PRS + N
1st
21.9(21.7)
27.5
4.6(5.3)
2.2
0.48(0.41)
1.26(1.27)
Pyrite, PRS + C
1st
20.4(20.3)
24.7
4.2(4.9)
2.34
0.56(0.47)
1.2(1.2)
Marcasite, MZ
1st
57.3(55.9)
76.2
3.4(3.9)
2.0
0.59(0.51)
1.3(1.3)
Greigite, C15
Before heating
61.3(60.4)
80.4
2.4(2.6)
1.4
0.61(0.55)
1.3(1.3)
Greigite, C15
1st
24.2(23.8)
47.6
5.7(6.3)
1.7
0.30(0.27)
1.9(1.9)
Greigite, C25-102
Before heating
35.5(35.1)
67.8
5.8(6.2)
2.3
0.39(0.37)
1.9(1.9)
Greigite, C25-110
Before heating
13.4(13.2)
61.3
8.8(9.2)
1.31
0.14(0.14)
4.6(4.6)
Greigite, C25-110
1st
16.5(16.3)
29.1
10.7(11.6)
2.48
0.23(0.21)
1.8(1.8)
Greigite, C15-132
Before heating
45.5(44.8)
76.8
8.2(8.9)
3.29
0.40(0.37)
1.7(1.7)
Greigite, C15-132
Heating to 450 °C
7.4
36.5
0.78
0.06
0.08
4.9
Greigite, C15-132
1st
13.9(13.8)
31.7
19.1(20.3)
3.56
0.19(0.18)
2.3(2.3)
Greigite, C15-132
2nd
9.1(9.0)
22.9
3.9(4.2)
0.77
0.19(0.18)
2.53(2.55)
Greigite, E668
Before heating
50.7(48.9)
80.1
1.3 (1.6)
0.66
0.51(0.42)
1.6(1.6)
Greigite, E668
1st
30.0(29.1)
56.8
0.9(1.2)
0.36
0.39(0.3)
1.9(1.9)
Greigite, E668
2nd
35.2(29.3)
77.3
0.5(0.7)
0.18
0.36(0.26)
2.2(2.6)
Chalcopyrite, KD
Before heating
20.2
54.3
0.034
0.0031
0.09
2.6
Chalcopyrite, KD
1st
37.4(36.7)
53.9
1.9(2.1)
0.75
0.4(0.35)
1.4(1.4)
Chalcopyrite, HP + C
2nd
42.2 (41.5)
60.9
3.1(3.4)
1.3
0.41(0.37)
1.44(1.47)
Chalcopyrite, HP + C
3rd
34.8(34.2)
51.8
6.1(6.8)
2.3
0.38(0.34)
1.49(1.51)
Arsenopyrite, 19/V-84
2nd
16.6(16.5)
28.8
4.9(5.1)
1.2
0.24(0.24)
1.73(1.74)
Arsenopyrite, 11251
2nd
15.0(14.9)
25.8
1.9(2.1)
0.6
0.32(0.29)
1.72(1.74)
Arsenopyrite, 182/Sh-84
2nd
15.3(15.4)
26.7
0.8(0.8)
0.2
0.21(0.21)
1.75(1.74)
Arsenopyrite, 333/V-84
2nd
19.5(19.5)
32.3
2.3(2.3)
0.6
0.26(0.26)
1.66(1.66)
Pyrrhotite, PMZ
Before heating
4.8(4.8)
9.6
8.4(11.4)
1.08
0.13(0.09)
1.9(2.0)
Pyrrhotite, PMZ
1st
9.1(8.9)
13.9
13.3(16.0)
2.99
0.22(0.18)
1.53(1.55)
Pyrrhotite, PMZ + C
1st
12.3 (12.1)
18.2
5.5(6.9)
1.9
0.34(0.27)
1.5(1.5)
Pyrrhotite, PMZ + C
2nd
10.5(10.4)
18.4
11.7(12.8)
2.7
0.23(0.21)
1.7(1.7)
Fe(SO)4
1st
50.1
396.4
0.17
0.01
0.1
7.9
Note. The parameters before correction for the paramagnetic component are given in parentheses; N, heating with nitrogen (carbamide); C, heating with carbon (sucrose).
64.33 norm. wt.% Fe, 34.25 norm. wt.% O, and 1.42 norm. wt.% S. An oxidized grain is shown in Fig. 6. During heating in air, MS begins to increase at ~400 °C (decomposition and oxidation of the decomposition products), peaks at 530 °C, and decreases at the Curie temperature of magnetite (Fig. 7a). On the cooling curves, MS increases at 595 °C. The after-heating MS value is 257 times higher. Single-domain magnetite is produced. Hcr/Hc = 0.4, and Jrs/Js = 1.4 (Table 1). The curves of the second heating and cooling are irreversible but typical of magnetite, with prominent Hopkinson peaks. The cooling curve is above the heating one, indicating continued magnetite formation in this cycle. The heating curves for a 1:1 chalcopyrite–hematite mixture (not shown) are identical to those for pure chalcopyrite, with
a slight Hopkinson peak of hematite. However, the considerably higher position of the cooling curve indicates a larger amount of newly formed magnetite. Judging by the intensity of the Hopkinson peaks on the cooling curves, the hematite component makes up only 3% of the magnetite one. Complicated k(T) curves were observed when chalcopyrite was heated with sucrose (Fig. 7b). The first-heating curves are the same as those for pure chalcopyrite. The cooling curves are located above the heating ones and reflect an increase in MS at 610 and ~590 °C. The second-heating curves show two distinct phases with Hopkinson peaks at ~570 and ~610 °C (MS decreases at 635 °C). The same phases are observed on the cooling curves. The cycle 3 curves are typical of magnetite, with Hopkinson effects at 575–585 °C. Apparently, sucrose stimulated the formation of unstable cation-deficient magnet-
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Fig. 5. k(T) curves (a–d, f) and hysteresis parameters (e, g) of greigite. a, Curves for heating to 700 C; b, c, k(T) curves for 320- and 450-degree cycles; d, f, k(T) curves for 700-degree cycles. Numbers in italics indicate heating and the after-heating hysteresis parameters. Gr, Greigite; M, magnetite; Mg, maghemite; L, lepidocrocite.
Fig. 6. Grain fragment of oxidized chalcopyrite. Ch, Chalcopyrite; G, galena; bright white spots indicate a mineral phase with Ag, Sb, Cu, S, and Fe.
ite. This is evidenced by the hysteresis parameters Hc and Hcr (Table 1), whose values are lower than those for hematite but slightly higher than those for magnetite (Peters and Dekkers, 2003). After the third heating, these parameters are close to those of once-heated chalcopyrite without additives. When chalcopyrite was heated with arsenic, no magnetite was observed on the heating curves (Fig. 7c, III), though there was a distinct increase in the MS of magnetite at 600 °C on the cooling curves. Magnetite in this experiment formed above the Curie temperature. On the heating curves for oxidized chalcopyrite, MS shows a slight increase at ~240–250 °C (Fig. 7c, II, III). As is the case with greigite, this increase might be due to iron hydroxides (lepidocrocite).
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Fig. 7. k(T) curves for chalcopyrite: a, Oxidized chalcopyrite; b, nonoxidized chalcopyrite with carbon; c, fragment of the heating–cooling curves for nonoxidized chalcopyrite (I), oxidized chalcopyrite (II), and oxidized chalcopyrite with arsenic (III). Numbers in italics indicate the heating.
Fig. 8. k(T) curves for arsenopyrite: a, First heating; b, c, curves typical of two heating cycles for As- (b) and S-rich (c) arsenopyrite. Pm, Monoclinic pyrrhotite.
Arsenopyrite (FeAsS). Natural arsenopyrite from different areas of the Pionerskii ore cluster (Magadan Region) was studied. The samples differ in chemical composition (Table 2). The first-heating curves are almost the same for all the samples studied (Fig. 8a). An increase in MS on the heating curves is observed at ~500 °C, and this agrees with literature data (Burov and Yasonov, 1979). The cooling curves are above the heating ones. For different samples, they are characterized by a dramatic increase in MS at 650–600 °C (formation of cationdeficient magnetite). There is a slight bend on some curves at ~320 °C, indicating the formation of monoclinic pyrrhotite. These samples have an increased S content (Fig. 8c, Table 2).
The cycle 2 curves for As-rich arsenopyrite are identical: MS plummets at 663–664 °C and increases dramatically at the same temperature on the cooling curves (Fig. 8b, Table 2). The irreversible character of the curves in this cycle indicates the instability of the mineral (apparently, cation-deficient magnetite). Pyrrhotite is a series of minerals with the chemical formula Fe1-xS, where x varies from 0 (troilite) to 0.125. Some data on the magnetic properties of pyrrhotite are given in (Brodskaya, 1980; Dekkers, 1988, 1989; Li and Franzen, 1996; Peters and Dekkers, 2003; Novikov et al., 1988; Worm et al., 1993; etc.).
Table 2. Chemical composition of arsenopyrite from the Pionerskii ore cluster Sample no.
Locality
Fe
As
S
47.25
18.83
at.% 19/V-84
Klin deposit, leucocratic granites
33.57
333/V-84
Igumenovskoe deposit, sulfide-quartz vein
33.90
46.70
19.20
182/Sh-84
Igumenovskoe deposit, aplite
34.70
42.78
21.75
11251
Igumenovskoe deposit, breccia-like veined body
35.04
42.83
21.98
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Fig. 9. k(T) curves for pyrrhotite: a, Monoclinic; b, hexagonal and monoclinic; c, monoclinic, heated with carbon to 200, 250, and 350 °C; d–f, monoclinic, heated with carbon to 700 °C. Pm (Ph), Monoclinic (hexagonal) pyrrhotite.
Monoclinic pyrrhotite and a monoclinic-hexagonal pyrrhotite mixture from different ore deposits of northeastern Russia were studied. The thermal MS curves do not differ much from the previous ones (Kontny et al., 2000). The heating–cooling curves typical of monoclinic pyrrhotite are shown in Fig. 9a. They show a distinct Hopkinson peak and a slight magnetite “hump.” Such curves were obtained for pyrrhotite from the Zolotistoe (sample 1/V-03, 47.25 at.% Fe) and Shturmovskoe (sample 26/V-03, 46.96 at.% Fe; sample 3210/V-88, 46.96 at.% Fe). When the monoclinic-hexagonal pyrrhotite mixture was heated, MS decreased at T = 320 °C, indicating monoclinic pyrrhotite (Fig. 9b), and increased at 225–230 °C owing to the transition of hexagonal pyrrhotite to a ferrimagnetic state (λ-transition). The decrease in MS at T = 265–280 °C is associated with the Curie point of this phase. In cycle 1, there is a well-defined magnetite peak, close to a pyrrhotite one in intensity. On cycle 2 curves, it is more intense than a pyrrhotite one, indicating continued magnetite formation. Hexagonal pyrrhotite is not observed on the reheating curves. Heating with organic matter. The cycle 1 curves are complicated: Several phases are distinguished on them (Fig. 9d). On the heating curve, MS begins to increase at 210 °C and then decreases to 260 °C. This part of the curve shows the formation of hexagonal pyrrhotite and its transition to a ferrimagnetic state. Monoclinic pyrrhotite is preserved with a Hopkinson peak at T = 310 °C. Along with pyrrhotite,
the curves feature phases with transformation at 470, 520, and 630 °C. The decrease in MS at T = 630 °C is due to unstable cation-deficient magnetite, whose Curie point shifts to that of magnetite as it cools. Mineral phases with MS decreasing at T = 470 and 520 °C are also shown on the cooling curve. Monoclinic and hexagonal pyrrhotite are insignificant. In general, after-heating MS was half as high (apparently, mainly owing to the monoclinic-hexagonal transition). Cycle 2 is also characterized by the formation of hexagonal pyrrhotite, but there is a distinct decrease in MS at 295 °C. The curve is marked by a dramatic decrease in MS at the Curie temperature of monoclinic pyrrhotite (325 °C) and a slight magnetite “hump.” All the phases are also reflected on the cooling curve, and hexagonal pyrrhotite is the most prominent. Magnetic susceptibility increases after the heating to the initial value. Cycle 3 is marked by dramatic decreases in MS at ~300 and 325 °C, which might indicate the presence of hexagonal and monoclinic pyrrhotite. Magnetite is more distinct than that in cycle 2. Magnetite and monoclinic pyrrhotite are observed on the cooling curves. Hexagonal pyrrhotite is manifested in a slight bend at T ~ 290 °C. Heating cycles to 200, 250, and 350 °C showed that the curves are reversible up to 200 °C (Fig. 9c). Note that these experiments involved heating the same sample and reheating it to the next temperature after cooling. The curves for the next cycle (250 °C) clearly show the formation of hexagonal pyrrhotite and its transition to a ferrimagnetic state. Newly
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Fig. 10. k(T) curves for the first heating and cooling of iron sulfate.
formed pyrrhotite is temperature-unstable and absent from the cooling curve; note that MS decreases. Hexagonal pyrrhotite was not observed during heating to 350 °C, but the monoclinic variety is well-defined (Fig. 9c). As pointed out above, the heating of some sulfide minerals produces iron sulfates as intermediate phases. Thermal MS studies were conducted for a standard reagent, iron sulfate Fe(SO4). On the first-heating curves, MS increases at 640 °C and plummets at the Curie temperature of hematite, indicating the formation of this mineral. Judging by the shape of this curve, MS contains >90% paramagnetic component. The Hopkinson peak for hematite is more intense on the cooling curve (Fig. 10). The after-heating hysteresis parameters are Hc = 50.1 mT; Hcr = 396.4 mT; and Jrs/Js = 0.1. The total MS decreased after the heating owing to the paramagnetic component. The heating–cooling curves for cycle 2 are almost reversible and typical of hematite.
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by heating–cooling curves, marcasite does not transform into magnetite when heated with arsenic. The MS(T) curves for chalcopyrite are similar to those for pyrite. However, stable magnetite is produced, whereas monoclinic pyrrhotite is absent. Magnetite formation also takes place during the second heating cycle, leading to an even greater increase in MS. If arsenic is added, magnetite forms above its Curie point. On the curves for heating with carbon (sucrose), magnetite and temperature-unstable cation-deficient magnetite with a Curie point at ~635 ºC are more distinct in cycle 2. This magnetite disappears during the subsequent heating cycles. In contrast to that in pyrite, marcasite, and chalcopyrite, magnetite formation in arsenopyrite begins at T >500 ºC. Arsenopyrite cooling is accompanied by the formation of magnetite (S-rich arsenopyrite) or cation-deficient magnetite (As-rich arsenopyrite) with a dramatic increase in MS. Arsenopyrite with an increased S content is characterized by insignificant pyrrhotite formation, but this should be confirmed with samples from other deposits. Greigite is marked by a decrease in MS on the heating curves at 360–420 ºC with the formation of unstable cationdeficient magnetite. The thermal MS curves do not differ from the literature data. Monoclinic pyrrhotite is characterized by a decrease in MS at ~320 ºC, and hexagonal pyrrhotite, by a transition to a ferrimagnetic state at 210–260 ºC. The addition of organic matter to monoclinic pyrrhotite stimulates the formation of hexagonal pyrrhotite, which transforms back into monoclinic pyrrhotite on reheating. The oxidation products of greigite and chalcopyrite show an increase in MS at 240–250 ºC owing to lepidocrocite. The study was supported by the Russian Foundation for Basic Research and the Far Eastern Branch of the Russian Academy of Sciences (grants no. 12-05-00286, 12-III-A-08191).
Conclusions The thermal MS studies indicate that the heating rate and treatment at certain temperatures are important in sulfide transformations. Heating at a rate of 12–13 ºC/min does not lead to complete transformations; for example, pyrite also transforms into pyrrhotite during the subsequent heating cycles. Temperature transformations of minerals are influenced by the particle size and the heating medium. Every mineral studied has its distinctive features. Pyrite and marcasite are characterized by the same thermal MS curves. Unstable cation-deficient magnetite and monoclinic pyrrhotite with a well-defined Hopkinson peak are produced during the heating. In oxygen-free media with carbon or nitrogen, magnetite formation is weak, whereas pyrrhotite generation is more significant. The application of additives to find out whether the formation of ferrimagnetic minerals can be suppressed ended in failure. Carbamide (nitrogen) addition leads to insignificant magnetite formation at higher temperatures (>500 ºC) and, at the same time, to earlier generation of monoclinic pyrrhotite (500 ºC). Judging
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Thermal magnetic susceptibility data on natural iron sulfides of northeastern Russia P.S. Minyuk a,*, E.E. Tyukova a, T.V. Subbotnikova a, A.Yu. Kazansky b, A.P. Fedotov c a
North-East Interdisciplinary Scientific Research Institute, named after N.A. Shilo, Far Eastern Branch of the Russian Academy of Sciences, ul. Portovaya 16, Magadan, 685000, Russia b A.A. Trofimuk Institute of Petroleum Geology and Geophysics, Siberian Branch of the Russian Academy of Sciences, pr. Akademika Koptyuga 3, Novosibirsk, 630090, Russia c Limnological Institute, Siberian Branch of the Russian Academy of Sciences, ul. Ulan-Batorskaya 3, Irkutsk, 664033, Russia Received 19 January 2011; received in revised form 30 May 2012; accepted 18 October 2012
Abstract Dependences of magnetic susceptibility (MS) on the temperature of natural iron sulfide samples (pyrite, marcasite, greigite, chalcopyrite, arsenopyrite, pyrrhotite) from the deposits of northeastern Russia were studied. The thermal MS curves for pyrite and marcasite are the same: On heating, MS increases at 420–450 ºC, and unstable magnetite (maghemite) and monoclinic pyrrhotite with a well-defined Hopkinson peak are produced. In oxygen-free media with carbon or nitrogen, magnetite formation is weak, whereas pyrrhotite generation is more significant. The heating curves for chalcopyrite are similar to those for pyrite. They show an increase in MS at the same temperatures (420–450 ºC). However, stable magnetite is produced, whereas monoclinic pyrrhotite is absent. In contrast to that in pyrite, marcasite, and chalcopyrite, magnetite formation in arsenopyrite begins at >500 ºC. Arsenopyrite cooling is accompanied by the formation of magnetite (S-rich arsenopyrite) or maghemite (As-rich arsenopyrite) with a dramatic increase in MS. Arsenopyrite with an increased S content is characterized by insignificant pyrrhotite formation. Greigite is marked by a decrease in MS on the heating curves at 360–420 ºC with the formation of unstable cation-deficient magnetite. Monoclinic pyrrhotite is characterized by a decrease in MS at ~320 ºC, and hexagonal pyrrhotite, by a transition to a ferrimagnetic state at 210–260 ºC. The addition of organic matter to monoclinic pyrrhotite stimulates the formation of hexagonal pyrrhotite, which transforms back into monoclinic pyrrhotite on repeated heating. The oxidation products of sulfides (greigite, chalcopyrite) show an increase in MS at 240–250 ºC owing to lepidocrocite. © 2013, V.S. Sobolev IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved. Keywords: magnetic susceptibility; temperature dependence of magnetic susceptibility; pyrite; marcasite; greigite; arsenopyrite; chalcopyrite; pyrrhotite
Introduction Thermal magnetic susceptibility measurements are a kind of thermomagnetic analysis in which magnetic susceptibility (MS) is studied under continued heating. It is a sensitive method for identifying magnetic and some Fe-containing minerals, including iron sulfides (pyrite, marcasite, chalcopyrite, arsenopyrite, pyrrhotite, greigite). These minerals are Cu, As, and H2SO4 ores and often accompany Au–Ag deposits. Some sulfide minerals contain Au and Ag admixtures and, therefore, are of interest as ores of these elements. Some iron sulfides (pyrite, marcasite, greigite) are widespread in sediments of different genesis (marine, lacustrine). They form
* Corresponding author. E-mail address: [email protected] (P.S. Minyuk)
mainly in reducing media, characterizing certain conditions of sediment formation and diagenesis, and are widely used to reconstruct paleodepositional environments. Thermal MS studies have become a common practice over the last few decades, especially with the appearance of equipment sets for this analysis (AGICO kappabridges and furnaces, Czech Republic). A combination of the technique for heating experiments with computer processing and presentation of results permits comparing data from laboratories worldwide. Published data indicate a wide spectrum of thermomagnetic curves, whose shape is influenced by the qualitative composition of ferrimagnetic materials, concentration, grain size, and the presence of Fe-containing paramagnetic minerals, which become ferrimagnetic upon heating. The results of heating experiments are greatly influenced by reducing admixtures (sulfur, nitrogen, carbon), which are often
1068-7971/$ - see front matter D 201 3, V.S. So bolev IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.rgg.2013.03.008 +
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present in sediments. Releasing sulfur upon heating, iron sulfides act as reducing agents themselves and influence temperature transformations of other minerals (e.g., iron oxides and hydroxides), thus complicating data interpretation. Although temperature dependences of the MS of sulfides have long been studied, the distinctive features of individual minerals and the influence of the enclosing substratum on the results of heating are poorly known. Methods We studied samples of natural iron sulfides, mainly from different Au–Ag deposits of northeastern Russia (less often, from sediments). High-temperature MS studies were conducted on an MFK1FA multifunctional kappabridge with a CS-3 high-temperature control unit (AGICO). The samples were heated to 700 °C and then cooled to room temperature (both at a rate of 12– 13 °C/min). For the quantitative description of the thermal curves, we used the MS variation indices proposed by F. Hrouda (2003): A40 = (k40 – K40)/K40 × 100) and Amax = (k – K)max/K40 × 100, where k40 (K40) is the MS value at 40 °C during cooling (heating) and (k – K)max is the maximum difference between MS values at equal cooling and heating temperatures. The samples were heated mainly in air; sometimes, reducing agents were added: carbon (sucrose), nitrogen (carbamide (NH2)2CO), and arsenic. The additives made up <5% of the mineral volume. The mixture was preagitated. According to special experiments, MS remains unchanged when the reducing agents themselves are heated to 700 °C and then cool to room temperature. On the other hand, metallic As is known to sublimate at 615 °C; carbamide turns into NH4NCO, NH3, CO2, etc., at T ≥ 150 °C (Knunyants, 1992); and sucrose melts and caramelizes at 186–200 °C, with decomposition at higher temperatures (Knunyants, 1983). The hysteresis parameters, including the remanent saturation magnetization (Jrs), saturation magnetization (Js), induced magnetization (Ji), coercivity (Hc), and remanent coercivity (Hcr), were measured on a J-meter automatic coercivity spectrometer (Kazan Federal University). The maximum field induction was 500 mT. The measurements were taken both for the original samples and upon heating with different additives. The element composition (Na, Mg, Al, Si, P, K, Ca, Ti, Fe, Mn, S) of individual mineral grains was determined on a Camebax microprobe before and after the heating. The detection limit was 1% for Na, 0.8% for Al and Si, and 0.5% for the other elements. Some minerals were studied on a QEMSCAN system (Australia), which included an EVO 50 SEM with a Quantax Esprit EDS (Bruker). Results and discussion Pyrite and marcasite. Pyrite FeS2 is unstable upon heating. When pyrite and marcasite are heated in air, a wide
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spectrum of magnetic minerals forms at different heating stages: hematite, magnetite, maghemite, and pyrrhotite (Kopp and Kerr, 1958); hematite, iron sulfates, and/or pyrrhotite (depending on the particle size) (Dunn et al., 1989a,b); hematite (Schorr and Everhart, 1969); hematite and magnetite/mag- hemite (Eneroth and Koch, 2003); magnetite, hematite, and pyrrhotite (Jørgensen and Moyle, 1982). Pyrite annealing at 610 °C during different periods of time produces pyrrhotite, iron monosulfide, α-Fe, magnetite, hematite, and maghemite (Prasad et al., 1985). Heating in carbon dioxide produces pyrrhotite, magnetite, maghemite, and hematite (Bhargava et al., 2009; Fegley et al., 1995). Pyrite heating with simultaneous studies of transformations by the X-ray (Bhargava et al., 2009) or Mössbauer method (Ferrow and Sjöberg, 2005) revealed quite a complicated set of intermediate mineral phases. For example, FeSO4, ε-Fe2O3, Fe2(SO4)3, β-Fe2O3, α-Fe2O3, and pyrrhotite were determined by Mössbauer spectroscopy. In situ X-ray structural analysis of pyrite showed that heating in a vacuum (T > 400 °C) produces hematite and magnetite; in air (T > 420 °C), hematite; in argon and nitrogen (T > 550 °C), pyrrhotite; in carbon dioxide, pyrrhotite (T > 500 °C), magnetite (T > 600 °C), and hematite (T > 700 °C) (Bhargava et al., 2009). An overview of literature on the mechanism and kinetics of pyrite transformations in different media is given in (Hu et al., 2006). When pyrite is heated, Js and/or MS increase after 420– 450 °C and decrease near the Curie point of magnetite; this is explained by the formation of this mineral on heating (Wang et al., 2008). Some researchers estimate the concentration of iron sulfides (pyrite) in sediments from the intensity of the peak on thermomagnetic curves (Emiroglu et al., 2004). However, the peak intensity depends on the grain size: Powdered pyrite does not yield such a susceptibility peak (Ferrow and Sjöberg, 2005). Also, pyrite content is determined from the magnetite newly formed during the heating (Grachev et al., 2008). The Js(T) cooling curves bend near the Curie temperature region of pyrrhotite. The k(T) cooling curves show a dramatic pyrrhotite peak at T ~ 320 °C (Wang et al., 2008). The before-heating MS of the pyrite samples studied was 11 × 10–9 m3/kg. The following pyrite composition was determined on the microprobe: 46.50 wt.% Fe, 53.55 wt.% S; n = 7. The pyrite susceptibility was studied during heating and cooling to different temperatures in air and with carbamide (nitrogen) and sucrose (carbon) additives. The initial pyrite was used for the next cycle. The chemical composition of the grains was determined on the microprobe after each cycle. The additives were used to trace their influence on the beginning and intensity of temperature transformations of pyrite. Curves typical of two full heating–cooling cycles in air (to 700 °C) are shown in Fig. 1a. During the heating, MS begins to increase at 430–450 °C and then decreases at the Curie temperature of magnetite. This newly formed mineral is temperature-unstable, as evidenced by irreversible heating– cooling curves at 700–400 °C (Fig. 1c). A distinct Hopkinson peak of pyrrhotite is observed on the cooling curves of cycle 1 at T ~ 320 °C. Judging by the ratios
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Fig. 1. Temperature dependences of magnetic susceptibility: a, pyrite; b, marcasite; c, fragment of the heating–cooling curves for cycle 1, pyrite. Numbers in italics indicate the heating.
Jrs/Js and Hcr/Hc, the particles are single-domain ones. The MS variation indices for cycle 1 are A560 = –41, A306 = 275.3, and A40 = 99.3. The cycle 1 curves are also irreversible. Monoclinic pyrrhotite and temperature-unstable magnetite are determined from them. The MS variation indices for cycle 2 are A568 = –1.5, A306 = 4.8, and A40 = 1.3. According to these data, pyrite continues to transform into unstable magnetite and pyrrhotite during this cycle. Heating–cooling curves for different temperatures were obtained to determine the pyrrhotite formation temperature. It was found that the curves of the heating cycle are reversible up to 400 °C; after 450 °C, the cooling curves are above those of heating, reflecting magnetite formation. Also, such curves were obtained for the 500 °C cycle. A slight pyrrhotite peak appears on the cooling curves only after 600 °C and becomes prominent after the 700 °C cycle. Pyrite heating with sucrose showed that MS begins to increase at 420 °C, peaks at ~490 °C, and decreases at ~600 °C.
The cooling curve is irreversible, with a prominent Hopkinson peak of pyrrhotite, and without the magnetite phase. The MS variation indices for cycle 1 are A494 = –2.9, A318 = 393, and A40 = 81.5. The cycle 2 curves are complicated. The heating curves have peaks at 315 (monoclinic pyrrhotite) and 365 °C (?), and the cooling ones have them at 320 and 350 °C, respectively. Pyrite heating with carbamide indicates that the MS curves are almost reversible before 450 °C (Fig. 2a–d); after 500 °C, the cooling curves have a slight peak at 300 °C, which is due to the monoclinic pyrrhotite produced by the heating (Fig. 2e). Monoclinic pyrrhotite is more distinct on the cooling curves after heating to ≥550 °C: It is reflected in increasing MS near the Curie point (Fig. 2f–h). So, heating in air revealed a considerable increase in MS at ~430–450 °C, and that with sucrose, at ~425 °C. This reflects the oxidation of pyrite decomposition products with the formation of magnetite, judging by MS(T). On the curves
Fig. 2. k(T) curves for cycles of pyrite heating with carbamide (nitrogen). a–h, See explanation in the text. Inset shows the heating curve.
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for the heating with sucrose, the magnetite “hump” is gentle and shows a maximum at lower temperatures (490 °C), whereas during the heating with carbamide, the transformations begin at higher temperatures (~500 °C). The characteristic magnetite “hump” on the heating curves is considerably weaker than that for the samples heated without additives. However, pyrrhotite forms at lower temperatures (500 °C) and in larger amounts; this significantly affects the MS values. The MS of a large pyrite sample (0.1–2.0 mm) increased by 15–16 times upon heating in air and by 76 times after the addition of sucrose and carbamide. The susceptibility of the same sample in powdered form (0.02–0.05 mm) increased by 550 times with respect to the initial value. Framboidal pyrite, which occurs widely in sediments of different ages and is characterized by microscopic size (Wilkin and Barnes, 1997), is expected to undergo more intense temperature transformations than coarse-grained pyrite. Additives favor the formation of pyrrhotite rather than magnetite (apparently, because of oxygen deficiency). Remarkably, the pyrite composition, determined on the microprobe upon heating even to 700 °C, remains unchanged: 45.72 wt.% Fe, 52.36 wt.% S, and n = 5, though TMA reveals the presence of pyrrhotite. After 20-minute thermal treatment of the sample at 700 °C, two phases of pyrite (47.86 wt.% Fe, 49.76 wt.% S, n = 18) and pyrrhotite (61.25 wt.% Fe, 37.44 wt.% S, n = 16) compositions appear in the grains (Fig. 3). The variations in the MS of pyrite at 2–700 A/m reach 70% of the initial value; note that the maximum variation is observed at low fields (2–40 A/m) and might be due to measurement errors at low fields (Hrouda et al., 2006). The heating is followed by a directed increase in MS with the field value, as is typical of pyrrhotite (Hrouda et al., 2006; Worm et al., 1993). At 700 A/m, MS increased by 20% for large (0.1–2 mm, sample PRS-700) grains and by 7% for small ones (0.02–0.05 mm, sample PRS-P700) (Fig. 4c). Marcasite FeS2 is a rhombic mineral. The before-heating MS of marcasite (sample MZ) was 62 × 10–9 m3/kg. The marcasite studied contains 46.00 norm. wt.% Fe and 54.00 norm. wt.% S; n = 6. The thermal MS data are analogous to those on pyrite (Fig. 1b). The MS variation indices for heating in air are A40 = 21.3, A307 = 57, and A566 = –14. The MS value increased by 22 times after the heating. The heating produces mainly high-coercivity monoclinic pyrrhotite. Hc = 57.3, and Hcr = 76.2 (Table 1). As is the case with pyrite, sucrose addition increases MS upon heating. Magnetic susceptibility on the heating curves begins to increase at higher temperature (500 °C); the magnetite “hump” is gentle; and the pyrrhotite peak is more prominent on the cooling curves. The MS variation indices for this cycle are A40 = 37.7, A313 = 129.4, and A516 = 3.4. The after-heating MS value was ~40 times higher. No increase in the MS of magnetite was observed on the heating or cooling curves during the heating with arsenic. At
467
Fig. 3. Fragment of a pyrite grain heated to 700 °C for 20 min with two phases. Po, Pyrrhotite; Py, pyrite; G, galena.
the same time, there is a distinct pyrrhotite peak on the cooling curves (not shown). The MS variation indices are A40 = 3124 and A314 = 8360. Additives, as is the case with pyrite, favor the formation of monoclinic pyrrhotite rather than magnetite. Greigite Fe3S4 is a ferrimagnetic cubic mineral, a sulfide analog of magnetite (Skinner et al., 1964). It is known as melnikovite in Russian publications (Polushkina and Sidorenko, 1963). The magnetic properties of greigite are described in detail in the review by Roberts et al. (2011). Temperature transformations hamper exact location of the Curie point for greigite: 333 °C (Spender et al., 1972), >322 °C (Roberts, 1995), >350 °C (Chang et al., 2008), or >463 °C (Bol’shakov and Dolotov, 2011). As greigite was heated at 300–400 °C, X-ray diffractometry revealed pyrite, marcasite, and pyrrhotite; above 400 °C, structural varieties of Fe2O3 were observed (Krs et al., 1992). Temperature transformations of greigite begin between 238 and 282 °C (Skinner et al., 1964). Note that the 148-h-long thermal treatment of the samples at 282 °C yielded only 5% of pyrrhotite. Of the many cited thermomagnetic curves for greigite, the cooling curves show newly formed monoclinic pyrrhotite only in (Chang et al., 2008; Roberts et al., 2011). Thermomagnetic analysis Js(T) and thermal MS measurements k(T) were used to study synthetic and natural greigite and greigite-bearing sediments. The curves Js(T) and k(T) are irreversible during the heating and cooling. Sometimes, the cooling curves are above the heating ones (type 1) (Brachfeld et al., 2009; Dekkers et al., 2000; Roberts, 1995); other times, below them (type 2) (Chang et al., 2008; Roberts et al., 2011; Vasiliev et al., 2007). On all the heating curves, magnetization (Js) and/or MS decrease to 350–420 °C (less often, 450 °C). This is followed by the formation of a temperature-stable (type 1) or temperature-unstable (type 2) magnetic mineral (presumably magnetite). Greigite from the sediments of Lakes Hövsgöl (Mongolia) (Kazansky et al., 2005; Nourgaliev et al., 2005) and El’gygytgyn (Chukchi Peninsula) was studied. It occurs as loose black grain accumulations. It is highly oxidized in some samples. Greigite nodules and grains for the studies were selected under
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Fig. 4. Dependence of the MS of pyrrhotite (a), greigite (b), and pyrite (c) on the magnetic field.
a binocular microscope. Both oxidized and nonoxidized greigite was studied. The MS of the samples varies from 11 × 10–6 to 70 × 10–6 m3/kg. It remains almost unchanged at different fields except low ones, with great measurement errors (Fig. 4b). Jrs/Js = 0.4–0.6, and Hcr/Hc = 1.3–1.9 (Table 1). Several heating–cooling cycles were performed. Cycle 1. The k(T) curves are irreversible. On the heating curves, MS decreases from 270–300 to 360–420 °C, increases, and then decreases at 585–590 °C (Fig. 5a). The decrease in MS is due to greigite decomposition, as evidenced by the irreversible heating–cooling curves for samples heated to 320 and 450 °C (Fig. 5b, c). The increase in MS is due to magnetite formation. The cooling curves are usually above the heating ones, and susceptibility increases dramatically at 590 °C (samples C15, C132, C62, C18) or ~640 °C (sample E688). The cooling curves are only seldom below the heating ones. Cycle 2. The heating curves for all the samples are characterized by a dramatic decrease in MS at 650 °C; the cooling ones, by an increase at 600–610 or 630 °C (sample E668) (Fig. 5d, f). All the cooling curves are below the heating ones. The newly formed minerals are partly oxidized to hematite. Cycle 3. The curves are almost reversible, with a decrease in MS at ~620 °C (Fig. 5d, f). They are gently convex with a maximum at ~450 °C. No Hopkinson effects are observed. Very similar MS curves were obtained upon heating goethite with sucrose. A slight MS increase begins at ~240 °C on all the curves for the samples studied (Fig. 5a). It might be due to the appearance of λ-pyrrhotite or transformations of iron hydrox-
ides into oxide forms (probably, magnetite (maghemite)). The iron hydroxides were produced by greigite oxidation (weathering). The samples with removed oxidized nodules (as a rule, ocherous) are characterized by a slight, if any, increase in MS at these temperatures. Therefore, the increase in MS is attributed to the dissociation of iron hydroxides (presumably lepidocrocite) (Burov and Yasonov, 1979). At the same temperatures, the MS of greigite from the marine sections of Italy (Sagnotti and Winkler, 1999), Chukchi Sea sediments (Brachfeld et al., 2009), flyschoid sediments of Iran (Aubourg and Robion, 2002), and lacustrine sediments of Tibet (Hu et al., 2002). Also, an increase in magnetization was observed during the heating of greigite from the lacustrine sediments of England (Snowball and Thompson, 1990). Chalcopyrite (CuFeS2). Chalcopyrite annealing is accompanied by complicated reactions with the formation of iron oxides, iron and copper sulfates, etc. An overview of this question can be found in (Prasad and Pandey, 1998). Note that the MS of chalcopyrite increases considerably upon heating above 400 °C (measurements were taken with cold samples). Hematite is the only newly formed magnetic mineral determined here by X-ray diffractometry (Sahyoun et al., 2003). Oxidized and nonoxidized chalcopyrite from the ore deposits of northeastern Russia was studied. Its MS was ~30 × 10−9 m3/kg. Oxidized chalcopyrite in caverns and microcracks is covered with an iron hydroxide crust. Chalcopyrite contains 35.10 norm. wt.% Cu, 30.94 norm. wt.% Fe, and 33.96 norm. wt.% S; n = 5. The oxidized areas are poor in S:
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P.S. Minyuk et al. / Russian Geology and Geophysics 54 (2013) 464–474 Table 1. Hysteresis characteristics of iron sulfides Mineral, sample
Heating
Hc
Hcr
Js
Jrs
Jrs/Js
Hcr/Hc
2
mT
Am /kg
Pyrite, PRS
1st
45.4(43.7)
59.2
1.9(2.4)
1.1
0.58(0.46)
1.30(1.35)
Pyrite, PRS + N
1st
21.9(21.7)
27.5
4.6(5.3)
2.2
0.48(0.41)
1.26(1.27)
Pyrite, PRS + C
1st
20.4(20.3)
24.7
4.2(4.9)
2.34
0.56(0.47)
1.2(1.2)
Marcasite, MZ
1st
57.3(55.9)
76.2
3.4(3.9)
2.0
0.59(0.51)
1.3(1.3)
Greigite, C15
Before heating
61.3(60.4)
80.4
2.4(2.6)
1.4
0.61(0.55)
1.3(1.3)
Greigite, C15
1st
24.2(23.8)
47.6
5.7(6.3)
1.7
0.30(0.27)
1.9(1.9)
Greigite, C25-102
Before heating
35.5(35.1)
67.8
5.8(6.2)
2.3
0.39(0.37)
1.9(1.9)
Greigite, C25-110
Before heating
13.4(13.2)
61.3
8.8(9.2)
1.31
0.14(0.14)
4.6(4.6)
Greigite, C25-110
1st
16.5(16.3)
29.1
10.7(11.6)
2.48
0.23(0.21)
1.8(1.8)
Greigite, C15-132
Before heating
45.5(44.8)
76.8
8.2(8.9)
3.29
0.40(0.37)
1.7(1.7)
Greigite, C15-132
Heating to 450 °C
7.4
36.5
0.78
0.06
0.08
4.9
Greigite, C15-132
1st
13.9(13.8)
31.7
19.1(20.3)
3.56
0.19(0.18)
2.3(2.3)
Greigite, C15-132
2nd
9.1(9.0)
22.9
3.9(4.2)
0.77
0.19(0.18)
2.53(2.55)
Greigite, E668
Before heating
50.7(48.9)
80.1
1.3 (1.6)
0.66
0.51(0.42)
1.6(1.6)
Greigite, E668
1st
30.0(29.1)
56.8
0.9(1.2)
0.36
0.39(0.3)
1.9(1.9)
Greigite, E668
2nd
35.2(29.3)
77.3
0.5(0.7)
0.18
0.36(0.26)
2.2(2.6)
Chalcopyrite, KD
Before heating
20.2
54.3
0.034
0.0031
0.09
2.6
Chalcopyrite, KD
1st
37.4(36.7)
53.9
1.9(2.1)
0.75
0.4(0.35)
1.4(1.4)
Chalcopyrite, HP + C
2nd
42.2 (41.5)
60.9
3.1(3.4)
1.3
0.41(0.37)
1.44(1.47)
Chalcopyrite, HP + C
3rd
34.8(34.2)
51.8
6.1(6.8)
2.3
0.38(0.34)
1.49(1.51)
Arsenopyrite, 19/V-84
2nd
16.6(16.5)
28.8
4.9(5.1)
1.2
0.24(0.24)
1.73(1.74)
Arsenopyrite, 11251
2nd
15.0(14.9)
25.8
1.9(2.1)
0.6
0.32(0.29)
1.72(1.74)
Arsenopyrite, 182/Sh-84
2nd
15.3(15.4)
26.7
0.8(0.8)
0.2
0.21(0.21)
1.75(1.74)
Arsenopyrite, 333/V-84
2nd
19.5(19.5)
32.3
2.3(2.3)
0.6
0.26(0.26)
1.66(1.66)
Pyrrhotite, PMZ
Before heating
4.8(4.8)
9.6
8.4(11.4)
1.08
0.13(0.09)
1.9(2.0)
Pyrrhotite, PMZ
1st
9.1(8.9)
13.9
13.3(16.0)
2.99
0.22(0.18)
1.53(1.55)
Pyrrhotite, PMZ + C
1st
12.3 (12.1)
18.2
5.5(6.9)
1.9
0.34(0.27)
1.5(1.5)
Pyrrhotite, PMZ + C
2nd
10.5(10.4)
18.4
11.7(12.8)
2.7
0.23(0.21)
1.7(1.7)
Fe(SO)4
1st
50.1
396.4
0.17
0.01
0.1
7.9
Note. The parameters before correction for the paramagnetic component are given in parentheses; N, heating with nitrogen (carbamide); C, heating with carbon (sucrose).
64.33 norm. wt.% Fe, 34.25 norm. wt.% O, and 1.42 norm. wt.% S. An oxidized grain is shown in Fig. 6. During heating in air, MS begins to increase at ~400 °C (decomposition and oxidation of the decomposition products), peaks at 530 °C, and decreases at the Curie temperature of magnetite (Fig. 7a). On the cooling curves, MS increases at 595 °C. The after-heating MS value is 257 times higher. Single-domain magnetite is produced. Hcr/Hc = 0.4, and Jrs/Js = 1.4 (Table 1). The curves of the second heating and cooling are irreversible but typical of magnetite, with prominent Hopkinson peaks. The cooling curve is above the heating one, indicating continued magnetite formation in this cycle. The heating curves for a 1:1 chalcopyrite–hematite mixture (not shown) are identical to those for pure chalcopyrite, with
a slight Hopkinson peak of hematite. However, the considerably higher position of the cooling curve indicates a larger amount of newly formed magnetite. Judging by the intensity of the Hopkinson peaks on the cooling curves, the hematite component makes up only 3% of the magnetite one. Complicated k(T) curves were observed when chalcopyrite was heated with sucrose (Fig. 7b). The first-heating curves are the same as those for pure chalcopyrite. The cooling curves are located above the heating ones and reflect an increase in MS at 610 and ~590 °C. The second-heating curves show two distinct phases with Hopkinson peaks at ~570 and ~610 °C (MS decreases at 635 °C). The same phases are observed on the cooling curves. The cycle 3 curves are typical of magnetite, with Hopkinson effects at 575–585 °C. Apparently, sucrose stimulated the formation of unstable cation-deficient magnet-
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Fig. 5. k(T) curves (a–d, f) and hysteresis parameters (e, g) of greigite. a, Curves for heating to 700 C; b, c, k(T) curves for 320- and 450-degree cycles; d, f, k(T) curves for 700-degree cycles. Numbers in italics indicate heating and the after-heating hysteresis parameters. Gr, Greigite; M, magnetite; Mg, maghemite; L, lepidocrocite.
Fig. 6. Grain fragment of oxidized chalcopyrite. Ch, Chalcopyrite; G, galena; bright white spots indicate a mineral phase with Ag, Sb, Cu, S, and Fe.
ite. This is evidenced by the hysteresis parameters Hc and Hcr (Table 1), whose values are lower than those for hematite but slightly higher than those for magnetite (Peters and Dekkers, 2003). After the third heating, these parameters are close to those of once-heated chalcopyrite without additives. When chalcopyrite was heated with arsenic, no magnetite was observed on the heating curves (Fig. 7c, III), though there was a distinct increase in the MS of magnetite at 600 °C on the cooling curves. Magnetite in this experiment formed above the Curie temperature. On the heating curves for oxidized chalcopyrite, MS shows a slight increase at ~240–250 °C (Fig. 7c, II, III). As is the case with greigite, this increase might be due to iron hydroxides (lepidocrocite).
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Fig. 7. k(T) curves for chalcopyrite: a, Oxidized chalcopyrite; b, nonoxidized chalcopyrite with carbon; c, fragment of the heating–cooling curves for nonoxidized chalcopyrite (I), oxidized chalcopyrite (II), and oxidized chalcopyrite with arsenic (III). Numbers in italics indicate the heating.
Fig. 8. k(T) curves for arsenopyrite: a, First heating; b, c, curves typical of two heating cycles for As- (b) and S-rich (c) arsenopyrite. Pm, Monoclinic pyrrhotite.
Arsenopyrite (FeAsS). Natural arsenopyrite from different areas of the Pionerskii ore cluster (Magadan Region) was studied. The samples differ in chemical composition (Table 2). The first-heating curves are almost the same for all the samples studied (Fig. 8a). An increase in MS on the heating curves is observed at ~500 °C, and this agrees with literature data (Burov and Yasonov, 1979). The cooling curves are above the heating ones. For different samples, they are characterized by a dramatic increase in MS at 650–600 °C (formation of cationdeficient magnetite). There is a slight bend on some curves at ~320 °C, indicating the formation of monoclinic pyrrhotite. These samples have an increased S content (Fig. 8c, Table 2).
The cycle 2 curves for As-rich arsenopyrite are identical: MS plummets at 663–664 °C and increases dramatically at the same temperature on the cooling curves (Fig. 8b, Table 2). The irreversible character of the curves in this cycle indicates the instability of the mineral (apparently, cation-deficient magnetite). Pyrrhotite is a series of minerals with the chemical formula Fe1-xS, where x varies from 0 (troilite) to 0.125. Some data on the magnetic properties of pyrrhotite are given in (Brodskaya, 1980; Dekkers, 1988, 1989; Li and Franzen, 1996; Peters and Dekkers, 2003; Novikov et al., 1988; Worm et al., 1993; etc.).
Table 2. Chemical composition of arsenopyrite from the Pionerskii ore cluster Sample no.
Locality
Fe
As
S
47.25
18.83
at.% 19/V-84
Klin deposit, leucocratic granites
33.57
333/V-84
Igumenovskoe deposit, sulfide-quartz vein
33.90
46.70
19.20
182/Sh-84
Igumenovskoe deposit, aplite
34.70
42.78
21.75
11251
Igumenovskoe deposit, breccia-like veined body
35.04
42.83
21.98
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Fig. 9. k(T) curves for pyrrhotite: a, Monoclinic; b, hexagonal and monoclinic; c, monoclinic, heated with carbon to 200, 250, and 350 °C; d–f, monoclinic, heated with carbon to 700 °C. Pm (Ph), Monoclinic (hexagonal) pyrrhotite.
Monoclinic pyrrhotite and a monoclinic-hexagonal pyrrhotite mixture from different ore deposits of northeastern Russia were studied. The thermal MS curves do not differ much from the previous ones (Kontny et al., 2000). The heating–cooling curves typical of monoclinic pyrrhotite are shown in Fig. 9a. They show a distinct Hopkinson peak and a slight magnetite “hump.” Such curves were obtained for pyrrhotite from the Zolotistoe (sample 1/V-03, 47.25 at.% Fe) and Shturmovskoe (sample 26/V-03, 46.96 at.% Fe; sample 3210/V-88, 46.96 at.% Fe). When the monoclinic-hexagonal pyrrhotite mixture was heated, MS decreased at T = 320 °C, indicating monoclinic pyrrhotite (Fig. 9b), and increased at 225–230 °C owing to the transition of hexagonal pyrrhotite to a ferrimagnetic state (λ-transition). The decrease in MS at T = 265–280 °C is associated with the Curie point of this phase. In cycle 1, there is a well-defined magnetite peak, close to a pyrrhotite one in intensity. On cycle 2 curves, it is more intense than a pyrrhotite one, indicating continued magnetite formation. Hexagonal pyrrhotite is not observed on the reheating curves. Heating with organic matter. The cycle 1 curves are complicated: Several phases are distinguished on them (Fig. 9d). On the heating curve, MS begins to increase at 210 °C and then decreases to 260 °C. This part of the curve shows the formation of hexagonal pyrrhotite and its transition to a ferrimagnetic state. Monoclinic pyrrhotite is preserved with a Hopkinson peak at T = 310 °C. Along with pyrrhotite,
the curves feature phases with transformation at 470, 520, and 630 °C. The decrease in MS at T = 630 °C is due to unstable cation-deficient magnetite, whose Curie point shifts to that of magnetite as it cools. Mineral phases with MS decreasing at T = 470 and 520 °C are also shown on the cooling curve. Monoclinic and hexagonal pyrrhotite are insignificant. In general, after-heating MS was half as high (apparently, mainly owing to the monoclinic-hexagonal transition). Cycle 2 is also characterized by the formation of hexagonal pyrrhotite, but there is a distinct decrease in MS at 295 °C. The curve is marked by a dramatic decrease in MS at the Curie temperature of monoclinic pyrrhotite (325 °C) and a slight magnetite “hump.” All the phases are also reflected on the cooling curve, and hexagonal pyrrhotite is the most prominent. Magnetic susceptibility increases after the heating to the initial value. Cycle 3 is marked by dramatic decreases in MS at ~300 and 325 °C, which might indicate the presence of hexagonal and monoclinic pyrrhotite. Magnetite is more distinct than that in cycle 2. Magnetite and monoclinic pyrrhotite are observed on the cooling curves. Hexagonal pyrrhotite is manifested in a slight bend at T ~ 290 °C. Heating cycles to 200, 250, and 350 °C showed that the curves are reversible up to 200 °C (Fig. 9c). Note that these experiments involved heating the same sample and reheating it to the next temperature after cooling. The curves for the next cycle (250 °C) clearly show the formation of hexagonal pyrrhotite and its transition to a ferrimagnetic state. Newly
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Fig. 10. k(T) curves for the first heating and cooling of iron sulfate.
formed pyrrhotite is temperature-unstable and absent from the cooling curve; note that MS decreases. Hexagonal pyrrhotite was not observed during heating to 350 °C, but the monoclinic variety is well-defined (Fig. 9c). As pointed out above, the heating of some sulfide minerals produces iron sulfates as intermediate phases. Thermal MS studies were conducted for a standard reagent, iron sulfate Fe(SO4). On the first-heating curves, MS increases at 640 °C and plummets at the Curie temperature of hematite, indicating the formation of this mineral. Judging by the shape of this curve, MS contains >90% paramagnetic component. The Hopkinson peak for hematite is more intense on the cooling curve (Fig. 10). The after-heating hysteresis parameters are Hc = 50.1 mT; Hcr = 396.4 mT; and Jrs/Js = 0.1. The total MS decreased after the heating owing to the paramagnetic component. The heating–cooling curves for cycle 2 are almost reversible and typical of hematite.
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by heating–cooling curves, marcasite does not transform into magnetite when heated with arsenic. The MS(T) curves for chalcopyrite are similar to those for pyrite. However, stable magnetite is produced, whereas monoclinic pyrrhotite is absent. Magnetite formation also takes place during the second heating cycle, leading to an even greater increase in MS. If arsenic is added, magnetite forms above its Curie point. On the curves for heating with carbon (sucrose), magnetite and temperature-unstable cation-deficient magnetite with a Curie point at ~635 ºC are more distinct in cycle 2. This magnetite disappears during the subsequent heating cycles. In contrast to that in pyrite, marcasite, and chalcopyrite, magnetite formation in arsenopyrite begins at T >500 ºC. Arsenopyrite cooling is accompanied by the formation of magnetite (S-rich arsenopyrite) or cation-deficient magnetite (As-rich arsenopyrite) with a dramatic increase in MS. Arsenopyrite with an increased S content is characterized by insignificant pyrrhotite formation, but this should be confirmed with samples from other deposits. Greigite is marked by a decrease in MS on the heating curves at 360–420 ºC with the formation of unstable cationdeficient magnetite. The thermal MS curves do not differ from the literature data. Monoclinic pyrrhotite is characterized by a decrease in MS at ~320 ºC, and hexagonal pyrrhotite, by a transition to a ferrimagnetic state at 210–260 ºC. The addition of organic matter to monoclinic pyrrhotite stimulates the formation of hexagonal pyrrhotite, which transforms back into monoclinic pyrrhotite on reheating. The oxidation products of greigite and chalcopyrite show an increase in MS at 240–250 ºC owing to lepidocrocite. The study was supported by the Russian Foundation for Basic Research and the Far Eastern Branch of the Russian Academy of Sciences (grants no. 12-05-00286, 12-III-A-08191).
Conclusions The thermal MS studies indicate that the heating rate and treatment at certain temperatures are important in sulfide transformations. Heating at a rate of 12–13 ºC/min does not lead to complete transformations; for example, pyrite also transforms into pyrrhotite during the subsequent heating cycles. Temperature transformations of minerals are influenced by the particle size and the heating medium. Every mineral studied has its distinctive features. Pyrite and marcasite are characterized by the same thermal MS curves. Unstable cation-deficient magnetite and monoclinic pyrrhotite with a well-defined Hopkinson peak are produced during the heating. In oxygen-free media with carbon or nitrogen, magnetite formation is weak, whereas pyrrhotite generation is more significant. The application of additives to find out whether the formation of ferrimagnetic minerals can be suppressed ended in failure. Carbamide (nitrogen) addition leads to insignificant magnetite formation at higher temperatures (>500 ºC) and, at the same time, to earlier generation of monoclinic pyrrhotite (500 ºC). Judging
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Editorial responsibility: A.D. Duchkov