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Monodomain behaviour in multiphase oxidized titanomagnetite
396
Earth and Planetary Science Letters, 34 (1977) 396-402 © Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands
I4]
MONODOMAIN BEHAVIOUR IN MULTIPHASE OXIDIZED TITANOMAGNETITE J.B. O'DONOVAN 1 and W. O'REILLY Department of Geophysics and Planetary Physics, School of Physics, The University of Newcastle upon Tyne, Newcastle upon Tyne, NE1 7R U [Great Britain)
Received November 27, 1976 Revised version received January 17, 1977
Large grains, of the order of tens of microns in size, of synthetic titanomagnetite have been oxidized to produce an intergrowth of phases, essentially similar to that produced by the natural process of deuteric oxidation. The scale of the intergrowth is at the limit of the optical range but the electron microscope reveals the characteristic lamella microstructure. The magnetic hysteresis properties of the most highly oxidized material, having probably about 10% of residual spinel phase, are typical of dispersed monodomain magnetite rods. The investigation therefore supports the model in which it is proposed that titanomagnetite grains in a slowly cooled basalt may carry a highly stable component of natural remanent magnetization.
1. Introduction The identification of remanence-carrying minerals with the high stability necessary to retain a faithful record of the palaeogeomagnetic field is a prime objective of rock magnetism research. It is believed that such stability requires the mineral grains to be in the monodomain state. The titanomagnetites and their derivatives are the principal magnetic minerals in basaltic rocks. Thus equidimensional titanomagnetite grains of typical composition possessing the necessary stability must be below about 1.5/amin size [1]. There is, as yet, no direct evidence for the existence of equidimensional magnetite grains in the stable monodomain state although calculations yield an upper limit of "-0.08 vm [2]. However, hysteresis loops [3] and low-temperature "remanence transition" characteristics [4] of elongated magnetic particles in the optical range indicate the monodomain state. "Hard" components of the natural remanent magnetization of basalts are therefore likely to reside in fine-grain titanomagnetite (or titanomaghemite) and magnetite rods with long dimension ~1 ~m. 1 Now at: NASA/Goddard Space Flight Center, Greenbelt Road, Greenbelt, Maryland 20771, U.S.A.
The mode of occurrence of the magnetic phases in a basalt depends essentially on the bulk composition, the environment of emplacement and the rate of cooling. In the case of a "basic" basalt, the composition of the first-formed near-stoichiometric titanomagnetite lies approximately in the range Fe2.sTi0.sO 4 to Fe2.4Tio.60 4. The titanium content is lower in more "acid" rocks. A fast cooling rate results in an essentially homogeneous titanomagnetite even in the oxidizing atmosphere of a basalt. The grain size distribution will extend below the optical limit and, in such cases, a magnetically significant fraction o f the material may be in the monodomain state [5,6]. Slow cooling allows the grains to approach equilibrium with the oxidizing environment which arises due to the buffering action of the basalt itself, cooling as a closed system [7], or due to contamination of the basalt by oxygen from air or water. The titanomagnetite becomes increasingly oxidized as temperature falls. At high temperature (>~400 ° C) the region in which the cation-deficient spinel structure is stable is strictly limited [8]. As deuteric oxidation proceeds beyond this limit the oxidation product is an intergrowth o f a near-stoichiometric spinel phase becoming progressively richer in iron together with a haemoilmenite phase rich in titanium. It has been proposed
397 that the magnetic regions of the subdivided grains become monodomains and that such grains may account for the highly stable component of natural remanent magnetization of many rocks [9]. This model has been criticized in that magnetostatic interactions between the magnetic regions will prevent them from acting as independent monodomain grains [10]. However, this effect will clearly become less important as the residual magnetic fraction becomes progressively smaller. A third source of monodomain remanence carriers are the non-magnetic silicates which, when subject to deuteric oxidation, produce magnetic inclusions [11 ] often with needle-like shapes. The purpose of the present work is to make an experimental test of the second of the three mechanisms, that in which deuteric oxidation o f large grains of titanomagnetite results in monodomain behaviour. The first requirement, homogeneous titanomagnetite of similar composition to that occurring in basalts in the form of large grains several tens of microns in size, is best met with well-characterized near-
stoichiometric synthetic samples. Although it is possible to approximately simulate the natural process of deuteric oxidation using suitable buffers, the requirement of the present test is that the end product of laboratory oxidation should be essentially similar to natural intergrowths, that is, regions of iron-rich spinel phase within a matrix of non-magnetic phases. Oxidation in air produces such material while at the same time being more convenient than oxidation within a buffered system and allowing continuous monitoring of the progress of oxidation.
2. Experimental
Sample synthesis. Polycrystalline titanomagnetites, in which a fourth species of cation, Mg2+, was included to more closely simulate naturally occurring material, were prepared by a method of partial selfbuffering involving two fringes [12]. X-ray analysis, chemical analysis and Curie point determination showed that single-phase near-stoichiometric material
TABLE 1 Synthesis and analysis of multiphase oxidized titanomagnetites. The bulk composition, expressed in terms of the oxidation parameter z (the fraction of Fe 2+ in the starting material converted to Fe3+), was determined by gravimetry. The phases present (Sp = spinel, Hi = haemo-ilmenite, Psb = pseudobrookite) were determined using a Guinier-de Wolff X-ray powder camera Starting composition
Heat treatment in air
Bulk composition (z parameter)
X-ray analysis phases present
d311 (A)
Fe2.35Mgo.osTio.604
4 hr at 200°C + 3 hr, 20 min at 300°C 4 hr at 470°C 2 hr, 45 min at 400°C + 3 hr, 20 rain at 580°C
0 0.185 0.643 0.860
Sp Sp Sp + tti Sp + Hi+ Psb
2.56 2.55 2.55 2.53
Fe2.25Mgo.15Ti0.604
-
0
Sp
2.55
unknown time probably at 200°C 40 rain at 260°C 4 hr, 5 rain at 390°C
0.065 0.134 0.379
Sp Sp Sp + Hi
2.56 2.55 2.55
Fe2.15Mg0.2 sTio.604
2 hr, 10 rain at 320°C 80 min at 380°C + 2 hr at 480°C 2 hr, 15 rain at 460°C + 1 hr, 35 min at 670°C
0 0.231 0.642 0.884
Sp Sp Sp + Hi Sp + Hi + Psb
2.56 2.55 2.57 2.53
Fe2.0sMg0.35Ti0.604
1 hr, 30 rain at 430°C 1 hr, 40 rain at 510°C 3 hr, 20 rain at 390°C + 1 hr, 50 min at 690°C
0 0.357 0.540 0.929
Sp Sp + Hi Sp + Hi Sp + Hi + Psb
2.56 2.54 2.53 2.53
398
had been produced. The compositions were Fe2.asMgo.osTio.604, Fe2.2sMgo.l sTio.604, Fe2.1sMgo.2s" Tio.604 and Fe2.05Mgo.35Tio.604. The sintered pellets were hand-crushed to pass through a 44/am sieve. The samples were oxidized in air in a Stanton thermobalance at temperatures between 200 and 700°C to give a series of products with increasing degree of oxidation, the degree of oxidation being calculated from the observed weight increase.
X-ray analysis. The phase uniformity of the oxidation products was investigated using a Guinier-de Wolff Xray powder camera. A summary of the heat treatment, bulk chemical composition of the samples and the resuits of the X-ray analysis is contained in Table 1. The spinel phase persists up to the highest degrees of oxidation obtained, the spinel d311 appearing to be virtually unchanged ("-2.55-2.56 A) until a moderately high degree of oxidation has been reached (z,
1.0
the fraction of Fe'* converted to Fe j÷, "-0.5-0.6) after which it falls to ~2.53 A. The spinel lines become progressively broadened until a splitting into two components becomes clear (d311 ~ 2.57 and 2.53 A) at about z ~ 0.6. Samples with higher degrees of oxidation show only single lines with the lower dspacing. The first sign of a new phase (haemo-ilmenite) does not become apparent until z ~ 0.4. The haemo-ilmenite lines are much fainter than the spinel lines until the higher z values are reached, after which the spinel lines have the lowest intensities. Pseudobrookite lines with intensities comparable to the haemo-ilmenite lines were found for the most highly oxidized samples
Thermomagnetic analysis. Thermomagnetic curves of the samples in the lower range of oxidation (z < 0.6) are irreversible whether heated in evacuated capsules or in air up to 650°C, although the behaviour is quite
1.0-
to
om
c o
O °N m
0 N
.m
r-
E
0
E N
"o
O
N
E 8
E"
0
z
Z
0' 0
100 200 300 ~00 500 600 TemperQture *C
Fig. 1. Thermomagnetic curve o f encapsulated oxidized titanomagnetite (Fe2.35Mgo.osTio.604 oxidized to z = 0.185). The upper Curie temperature is about the same on both heating and cooling curves. The lower Curie temperature is lower on the cooling run than on the heating run.
0
0
I
I
I
I
I
100
200
300
~;00
500
Temperature
600
*C
Fig. 2. Thermomagnetic curve of encapsulated oxidized titanomagnetite (Fe2.35Mgo.osTi0.604 oxidized to z = 0.643).
399 different in the two cases. In the case of vacuumheating two Curie temperatures are found on both heating and cooling runs. The higher Curie temperature is about the same on both runs but the lower Curie temperature is reduced in value after heating. Examples are shown in Figs. 1 and 2. The air-heated runs give complex heating curves as the sample oxidizes further during the run. The cooling curves are simple and show a single Curie point. Samples in the higher range of oxidation state show virtually reversible thermomagnetic curves. Air-heating results in a cooling curve having the same shape as the heating curve (Fig. 3) but reduced magnetization ( ~ 6 0 - 8 0 % of the heating curve). The general trend is to find lower Curie temperatures in the samples with higher magnesium content.
Electron microscope investigation. Samples in the higher oxidation range were selected for electron microscope investigation. Polished sections were , etched for 10 seconds in 40% hydrofluoric acid and
1.0
tO
carbon replicas prepared of the etched surface. A typical electron micrograph of a replica is shown in Fig. 4. This shows a well-developed microstructure similar to that found in basaltic titanomagnetites which hav.e been subject to deuteric or laboratory oxidation. The etched grains were also inspected under the optical microscope but the microstructure is at the limit of the range of the instrument and no inhomogeneity was observable even using a Nomarski interference contrast device.
Constituents of the multiphase oxidation product. The evolution of the samples in the lower oxidation range is complex. The irreversibility of the thermomagnetic curves presumably indicates the presence o f a nonstoichiometric spinel (titanomaghemite) which inverts during the heating run. The phase with the higher Curie point could conceivably have been produced by inversion of the titanomaghemite during the heating run itself but the broadening of the X-ray lines would suggest that it was already present after oxidation in the thermobalance. The identification of the spinel phases and interpretation of the thermomagnetic curves requires further experiment. Although models could be proposed at this point involving (say) two
\
O N t¢I
E °
N ~
J
O
E O Z
0
100 200 300 /~)0 500 600 Temperature "C
Fig. 3. Air run thermomagnetic curve of highly oxidized titanomagnetite (Fe2.osMg0.35Tio.604 oxidized to z = 0.929).
Fig. 4. Electron micrograph of a carbon replica of etched oxidized titanomagnetite grain (Fe2.osMgo.35Tio.604 oxidized to z = 0.929) showing well-developed microstructure.
400
~
32
/,00
2,~
3oo ,~
"~ 8
0
0
I 0
I I 0.2 0.& 0.6 Oxidation parameter ( B u l k composition
~Mrs = 0.8 1.0 Z )
tion of magnesium in the spinel constituent, further experiments would be needed before such models could be confidently defended. In any event the samples of primary interest in the present study lie in the higher range of oxidation. X-ray and thermomagnetic analysis suggests that samples in the higher oxidation range contain virtually a single spinel phase, although the thermomagnetic curves may have a high-temperature tail (Fig. 3). Xray analysis also reveals a predominance of non-magnetic phases which may be effective in sub-dividing the grains into regions containing a near-stoichiometric iron-rich spinel. Magnetic hysteretie properties. A vibrating sample
Fig. 5. Variation of magnetic hysteresis parameters with degree of oxidation for titanomagnetite grains of the order of tens of microns in size. The oxidation parameter z describes the bulk composition of the multiphase intergrowths. spinel phases with different levels of iron concentration and degrees of non-stoichiometry inverting (possibly by a two-stage process) with preferential loca-
magnetometer was used to obtain magnetic hysteresis loops at room temperature. The samples were saturated in the maximum available field (13 kOe). The values of saturation magnetization (Ms), saturation isothermal remanence (Mrs) and coercive force (He) so obtained are summarized in Fig. 5 as zones within which the experimental values lie. In the lower oxidation range the coercive force is ~ 5 0 - 1 0 0 Oe and the ratio Mrs]M s ~ 0 . 1 5 - 0 . 3 . In the higher oxidation range Hc reaches values in excess of 400 Oe and the ratio M r s / M s rises to 0 . 4 - 0 . 6 (see Table 2).
TABLE 2 Magnetic hysteresis parameters of multiphase oxidized titanomagnetites. H c and Mrs determined for the first-listed specimen are considered to be anomalously high for a coarse-grain sintered titanomagnetite. This may possibly be due to adhesion of fine particles to grains contained in the specimen used in the vibration magnetometer measurement of this particular sample Starting composition
z Parameter
H e (Oe)
Hrc/H c
MDF/Hc
Fe2.3sMgo.osTi0.604
0 0.185 0.643 0.860
(120) 145 100 455
(1.07) 2.5 2.1 1.3
(0.61) 1.5 1.3 1.0
(0.29) 0.27 0.19 0.53
Fe2.2 sMgo. t sTio.604
0 0.065 0.134 0.374
40 130 145 70
2.5 3.7 2.6 2.6
1.5 2.5 1.6 1.7
0.15 0.27 0.29 0.19
Fe2.15Mgo.2sTio.604
0 0.231 0.624 0.884
40 80 50 455
2.4 2.8 3.1 1.0
1.3 1.8 1.9 0.8
0.15 0.20 0.14 0.42
Fe2.05Mgo.35Tio.604
0 0.375 0.540 0.929
35 60 45 385
2.6 2.1 2.7 1.1
1.4 1.4 1.6 0.9
0.15 0.17 0.14 0.59
Mrs/Ms
401
The direct and alternating field (AF) demagnetization of Mrs was also studied. The ratio of coercive force of remanence (Hrc) so obtained to coercive force (He) lies in the rangc 2- 4 for all specimens except the most highly oxidized for which Hrc/H c 1--1.3. The values of median destructive field (MDF) obtained from the AF demagnetization curves are generally intermediate between the values of Hc and Hrc except for the most highly oxidized samples in some of which MDF < 11c (Table 2).
netic hardness (indicated by remanence demagnetization linear and rotational hysteresis characteristics of the bulk rock) observed after laboratory heating of some basalts in air [5,14,15]. The increased hardness in bulk magnetic properties is clearly due to the multiphase oxidation product of the titanomagnetite rather than the result of some other process, e.g., oxidation of silicates.
4. Conclusion 3. Discussion Although highly oxidized, titanomagnetite grains produced in the present experiments would be categorized as being within Class I of the optically based oxidation scale developed to describe the Fe-Ti oxides in basalts [13]. Class I grains appear to be structurally homogeneous and are expected to be in the multidomain state. The Curie temperature of an unaltered titanomagnetite of typical composition is about 200°C. The association of a higher Curie tern> o perature (~500 C, say) with apparently homogeneous grains in a basalt is therefore taken to indicate that such grains are not truly Class I. However, the absence of a Curie temperature characteristic of an iron-rich spinel (~500°C) may not in itself be a sufficient condition from which to infer the absence of alteration. This is because during exsolution or inversion magnesium appears to seek the spinel phase with a consequent reduction in the Curie temperature. From considerations of the bulk chemical composition (assigning residual Fe 2+ to the spinel phase), X-ray diffraction line intensities and assuming a spinel saturation magnetization in the range 2 0 - 5 0 emu/g it seems that the spinel phase constitutes about 10-20% of the most highly oxidized samples (z "~ 0.9). Magnetostatic interaction between the magnetic zones is therefore likely to be very weak. This is borne out by the magnetic hysteresis parameters, as the values of Hc are typical of those obtained for samples of dispersed magnetite rods and the Mrs/Ms and Hrc/Hc ratios are consistent with a randomly orientated assemblage of non-interacting monodomain particles with uniaxial anisotropy. The results of the present study support the generally accepted interpretation of the increased mag-
The present study supports the model in which it is proposed that deuteric oxidation of large grains of titanomagnetite may result in effectively isolated regions of residual spinel phase with monodomain characteristics and corresponding high remanence stability.
Acknowledgements This work forms part of a NERC-sponsored research programme "Thermoremanence in titanomagnetites". One of the authors (J.B.O'D.) has been in receipt ofa NERC studentship. We thank Dr. Z. Hauptman for invaluable assistance in the preparation of materials and the staff of the Electron Optical Unit, University of Newcastle upon Tyne.
References 1 H.C. Soffel, The single domain-multidomain transition in natural intermediate titanomagnetites, Z. Geophys. 37 (1971)451. 2 R.F. Butler and S.K. Banerjee, Theoretical single-domain grain size range in magnetite and titanomagnetite, J. Geophys. Res. 80 (1975) 4049. 3 A.H. Morrish and S.P. Yu, Magnetic measurements on individual microscopic ferrite particles near the singledomain size, Phys. Rev. 102 (1956) 670. 4 T. Nagata, K. Kobayashi and M.D. Fuller, Identification of magnetite and haematite in rocks by magnetic observation at low temperature, J. Geophys. Res. 69 (1964) 2111. 5 E. Larson, M. Ozima, M. Ozima, T. Nagata and D. Strangway, Stability of remanent magnetization of igneous rocks, Geophys. J. R. Astron. Soc. 17 (1969) 263. 6 R.B. Hargraves and N. Petersen, Notes on the correlation between petrology and magnetic properties o f basaltic rocks, Z. Geophys. 37 (1971) 367.
402 7 A.F. Buddington and D.tl. Lindsley, Iron-titanium oxide minerals and synthetic equivalents, J. Petrol. 5 (1964) 310. 8 Z. Hauptman, High-temperature oxidation, range of nonstoichiometry and Curie point variation of cation deficient Fe2.4Ti0.604+.r, Geophys. J. R. Astron. Soc. 38 (1974) 29. 9 D.W. Strangway, E.E. Larson and M. Goldstein, A possible cause of high magnetic stability in volcanic rocks, J. Geophys. Res. 73 (1968) 3787. 10 F.D. Stacey and S.K. Banerjee, The Physical Principles of Rock Magnetism (Elsevier, Amsterdam, 1974) 6 3 - 6 5 . 11 M.E. Evans, M.W. McElhinny and A.C. Gifford, Single domain magnetite and high coercivities in a gabbroic in-
trusion, Earth Planet. Sci. Left. 4 (1968) 142. 12 J.B. O'Donovan, Studies on synthetic analogies of carriers of the palaeomagnetic record, Ph.D. Thesis, Univ. of Newcastle upon Tyne (1975). 13 R.L. Wilson and N.D. Watkins, Correlation of petrology and natural magnetic polarity in Columbia River basalts, Geophys. J. R. Astron. Soc. 12 (1967) 405. 14 P.M. Davis and M.E. Evans, Interacting single-domain properties of magnetite intergrowths, J. Geophys. Res. 81 (1976) 989. 15 A.J. Manson and W. O'Reilly, Submicroscopic texture in titanomagnetite grains in basalt studied using the torque magnetometer and electron microscope, Phys. Earth Planet. Inter. 11 (1976) 173.
Earth and Planetary Science Letters, 34 (1977) 396-402 © Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands
I4]
MONODOMAIN BEHAVIOUR IN MULTIPHASE OXIDIZED TITANOMAGNETITE J.B. O'DONOVAN 1 and W. O'REILLY Department of Geophysics and Planetary Physics, School of Physics, The University of Newcastle upon Tyne, Newcastle upon Tyne, NE1 7R U [Great Britain)
Received November 27, 1976 Revised version received January 17, 1977
Large grains, of the order of tens of microns in size, of synthetic titanomagnetite have been oxidized to produce an intergrowth of phases, essentially similar to that produced by the natural process of deuteric oxidation. The scale of the intergrowth is at the limit of the optical range but the electron microscope reveals the characteristic lamella microstructure. The magnetic hysteresis properties of the most highly oxidized material, having probably about 10% of residual spinel phase, are typical of dispersed monodomain magnetite rods. The investigation therefore supports the model in which it is proposed that titanomagnetite grains in a slowly cooled basalt may carry a highly stable component of natural remanent magnetization.
1. Introduction The identification of remanence-carrying minerals with the high stability necessary to retain a faithful record of the palaeogeomagnetic field is a prime objective of rock magnetism research. It is believed that such stability requires the mineral grains to be in the monodomain state. The titanomagnetites and their derivatives are the principal magnetic minerals in basaltic rocks. Thus equidimensional titanomagnetite grains of typical composition possessing the necessary stability must be below about 1.5/amin size [1]. There is, as yet, no direct evidence for the existence of equidimensional magnetite grains in the stable monodomain state although calculations yield an upper limit of "-0.08 vm [2]. However, hysteresis loops [3] and low-temperature "remanence transition" characteristics [4] of elongated magnetic particles in the optical range indicate the monodomain state. "Hard" components of the natural remanent magnetization of basalts are therefore likely to reside in fine-grain titanomagnetite (or titanomaghemite) and magnetite rods with long dimension ~1 ~m. 1 Now at: NASA/Goddard Space Flight Center, Greenbelt Road, Greenbelt, Maryland 20771, U.S.A.
The mode of occurrence of the magnetic phases in a basalt depends essentially on the bulk composition, the environment of emplacement and the rate of cooling. In the case of a "basic" basalt, the composition of the first-formed near-stoichiometric titanomagnetite lies approximately in the range Fe2.sTi0.sO 4 to Fe2.4Tio.60 4. The titanium content is lower in more "acid" rocks. A fast cooling rate results in an essentially homogeneous titanomagnetite even in the oxidizing atmosphere of a basalt. The grain size distribution will extend below the optical limit and, in such cases, a magnetically significant fraction o f the material may be in the monodomain state [5,6]. Slow cooling allows the grains to approach equilibrium with the oxidizing environment which arises due to the buffering action of the basalt itself, cooling as a closed system [7], or due to contamination of the basalt by oxygen from air or water. The titanomagnetite becomes increasingly oxidized as temperature falls. At high temperature (>~400 ° C) the region in which the cation-deficient spinel structure is stable is strictly limited [8]. As deuteric oxidation proceeds beyond this limit the oxidation product is an intergrowth o f a near-stoichiometric spinel phase becoming progressively richer in iron together with a haemoilmenite phase rich in titanium. It has been proposed
397 that the magnetic regions of the subdivided grains become monodomains and that such grains may account for the highly stable component of natural remanent magnetization of many rocks [9]. This model has been criticized in that magnetostatic interactions between the magnetic regions will prevent them from acting as independent monodomain grains [10]. However, this effect will clearly become less important as the residual magnetic fraction becomes progressively smaller. A third source of monodomain remanence carriers are the non-magnetic silicates which, when subject to deuteric oxidation, produce magnetic inclusions [11 ] often with needle-like shapes. The purpose of the present work is to make an experimental test of the second of the three mechanisms, that in which deuteric oxidation o f large grains of titanomagnetite results in monodomain behaviour. The first requirement, homogeneous titanomagnetite of similar composition to that occurring in basalts in the form of large grains several tens of microns in size, is best met with well-characterized near-
stoichiometric synthetic samples. Although it is possible to approximately simulate the natural process of deuteric oxidation using suitable buffers, the requirement of the present test is that the end product of laboratory oxidation should be essentially similar to natural intergrowths, that is, regions of iron-rich spinel phase within a matrix of non-magnetic phases. Oxidation in air produces such material while at the same time being more convenient than oxidation within a buffered system and allowing continuous monitoring of the progress of oxidation.
2. Experimental
Sample synthesis. Polycrystalline titanomagnetites, in which a fourth species of cation, Mg2+, was included to more closely simulate naturally occurring material, were prepared by a method of partial selfbuffering involving two fringes [12]. X-ray analysis, chemical analysis and Curie point determination showed that single-phase near-stoichiometric material
TABLE 1 Synthesis and analysis of multiphase oxidized titanomagnetites. The bulk composition, expressed in terms of the oxidation parameter z (the fraction of Fe 2+ in the starting material converted to Fe3+), was determined by gravimetry. The phases present (Sp = spinel, Hi = haemo-ilmenite, Psb = pseudobrookite) were determined using a Guinier-de Wolff X-ray powder camera Starting composition
Heat treatment in air
Bulk composition (z parameter)
X-ray analysis phases present
d311 (A)
Fe2.35Mgo.osTio.604
4 hr at 200°C + 3 hr, 20 min at 300°C 4 hr at 470°C 2 hr, 45 min at 400°C + 3 hr, 20 rain at 580°C
0 0.185 0.643 0.860
Sp Sp Sp + tti Sp + Hi+ Psb
2.56 2.55 2.55 2.53
Fe2.25Mgo.15Ti0.604
-
0
Sp
2.55
unknown time probably at 200°C 40 rain at 260°C 4 hr, 5 rain at 390°C
0.065 0.134 0.379
Sp Sp Sp + Hi
2.56 2.55 2.55
Fe2.15Mg0.2 sTio.604
2 hr, 10 rain at 320°C 80 min at 380°C + 2 hr at 480°C 2 hr, 15 rain at 460°C + 1 hr, 35 min at 670°C
0 0.231 0.642 0.884
Sp Sp Sp + Hi Sp + Hi + Psb
2.56 2.55 2.57 2.53
Fe2.0sMg0.35Ti0.604
1 hr, 30 rain at 430°C 1 hr, 40 rain at 510°C 3 hr, 20 rain at 390°C + 1 hr, 50 min at 690°C
0 0.357 0.540 0.929
Sp Sp + Hi Sp + Hi Sp + Hi + Psb
2.56 2.54 2.53 2.53
398
had been produced. The compositions were Fe2.asMgo.osTio.604, Fe2.2sMgo.l sTio.604, Fe2.1sMgo.2s" Tio.604 and Fe2.05Mgo.35Tio.604. The sintered pellets were hand-crushed to pass through a 44/am sieve. The samples were oxidized in air in a Stanton thermobalance at temperatures between 200 and 700°C to give a series of products with increasing degree of oxidation, the degree of oxidation being calculated from the observed weight increase.
X-ray analysis. The phase uniformity of the oxidation products was investigated using a Guinier-de Wolff Xray powder camera. A summary of the heat treatment, bulk chemical composition of the samples and the resuits of the X-ray analysis is contained in Table 1. The spinel phase persists up to the highest degrees of oxidation obtained, the spinel d311 appearing to be virtually unchanged ("-2.55-2.56 A) until a moderately high degree of oxidation has been reached (z,
1.0
the fraction of Fe'* converted to Fe j÷, "-0.5-0.6) after which it falls to ~2.53 A. The spinel lines become progressively broadened until a splitting into two components becomes clear (d311 ~ 2.57 and 2.53 A) at about z ~ 0.6. Samples with higher degrees of oxidation show only single lines with the lower dspacing. The first sign of a new phase (haemo-ilmenite) does not become apparent until z ~ 0.4. The haemo-ilmenite lines are much fainter than the spinel lines until the higher z values are reached, after which the spinel lines have the lowest intensities. Pseudobrookite lines with intensities comparable to the haemo-ilmenite lines were found for the most highly oxidized samples
Thermomagnetic analysis. Thermomagnetic curves of the samples in the lower range of oxidation (z < 0.6) are irreversible whether heated in evacuated capsules or in air up to 650°C, although the behaviour is quite
1.0-
to
om
c o
O °N m
0 N
.m
r-
E
0
E N
"o
O
N
E 8
E"
0
z
Z
0' 0
100 200 300 ~00 500 600 TemperQture *C
Fig. 1. Thermomagnetic curve o f encapsulated oxidized titanomagnetite (Fe2.35Mgo.osTio.604 oxidized to z = 0.185). The upper Curie temperature is about the same on both heating and cooling curves. The lower Curie temperature is lower on the cooling run than on the heating run.
0
0
I
I
I
I
I
100
200
300
~;00
500
Temperature
600
*C
Fig. 2. Thermomagnetic curve of encapsulated oxidized titanomagnetite (Fe2.35Mgo.osTi0.604 oxidized to z = 0.643).
399 different in the two cases. In the case of vacuumheating two Curie temperatures are found on both heating and cooling runs. The higher Curie temperature is about the same on both runs but the lower Curie temperature is reduced in value after heating. Examples are shown in Figs. 1 and 2. The air-heated runs give complex heating curves as the sample oxidizes further during the run. The cooling curves are simple and show a single Curie point. Samples in the higher range of oxidation state show virtually reversible thermomagnetic curves. Air-heating results in a cooling curve having the same shape as the heating curve (Fig. 3) but reduced magnetization ( ~ 6 0 - 8 0 % of the heating curve). The general trend is to find lower Curie temperatures in the samples with higher magnesium content.
Electron microscope investigation. Samples in the higher oxidation range were selected for electron microscope investigation. Polished sections were , etched for 10 seconds in 40% hydrofluoric acid and
1.0
tO
carbon replicas prepared of the etched surface. A typical electron micrograph of a replica is shown in Fig. 4. This shows a well-developed microstructure similar to that found in basaltic titanomagnetites which hav.e been subject to deuteric or laboratory oxidation. The etched grains were also inspected under the optical microscope but the microstructure is at the limit of the range of the instrument and no inhomogeneity was observable even using a Nomarski interference contrast device.
Constituents of the multiphase oxidation product. The evolution of the samples in the lower oxidation range is complex. The irreversibility of the thermomagnetic curves presumably indicates the presence o f a nonstoichiometric spinel (titanomaghemite) which inverts during the heating run. The phase with the higher Curie point could conceivably have been produced by inversion of the titanomaghemite during the heating run itself but the broadening of the X-ray lines would suggest that it was already present after oxidation in the thermobalance. The identification of the spinel phases and interpretation of the thermomagnetic curves requires further experiment. Although models could be proposed at this point involving (say) two
\
O N t¢I
E °
N ~
J
O
E O Z
0
100 200 300 /~)0 500 600 Temperature "C
Fig. 3. Air run thermomagnetic curve of highly oxidized titanomagnetite (Fe2.osMg0.35Tio.604 oxidized to z = 0.929).
Fig. 4. Electron micrograph of a carbon replica of etched oxidized titanomagnetite grain (Fe2.osMgo.35Tio.604 oxidized to z = 0.929) showing well-developed microstructure.
400
~
32
/,00
2,~
3oo ,~
"~ 8
0
0
I 0
I I 0.2 0.& 0.6 Oxidation parameter ( B u l k composition
~Mrs = 0.8 1.0 Z )
tion of magnesium in the spinel constituent, further experiments would be needed before such models could be confidently defended. In any event the samples of primary interest in the present study lie in the higher range of oxidation. X-ray and thermomagnetic analysis suggests that samples in the higher oxidation range contain virtually a single spinel phase, although the thermomagnetic curves may have a high-temperature tail (Fig. 3). Xray analysis also reveals a predominance of non-magnetic phases which may be effective in sub-dividing the grains into regions containing a near-stoichiometric iron-rich spinel. Magnetic hysteretie properties. A vibrating sample
Fig. 5. Variation of magnetic hysteresis parameters with degree of oxidation for titanomagnetite grains of the order of tens of microns in size. The oxidation parameter z describes the bulk composition of the multiphase intergrowths. spinel phases with different levels of iron concentration and degrees of non-stoichiometry inverting (possibly by a two-stage process) with preferential loca-
magnetometer was used to obtain magnetic hysteresis loops at room temperature. The samples were saturated in the maximum available field (13 kOe). The values of saturation magnetization (Ms), saturation isothermal remanence (Mrs) and coercive force (He) so obtained are summarized in Fig. 5 as zones within which the experimental values lie. In the lower oxidation range the coercive force is ~ 5 0 - 1 0 0 Oe and the ratio Mrs]M s ~ 0 . 1 5 - 0 . 3 . In the higher oxidation range Hc reaches values in excess of 400 Oe and the ratio M r s / M s rises to 0 . 4 - 0 . 6 (see Table 2).
TABLE 2 Magnetic hysteresis parameters of multiphase oxidized titanomagnetites. H c and Mrs determined for the first-listed specimen are considered to be anomalously high for a coarse-grain sintered titanomagnetite. This may possibly be due to adhesion of fine particles to grains contained in the specimen used in the vibration magnetometer measurement of this particular sample Starting composition
z Parameter
H e (Oe)
Hrc/H c
MDF/Hc
Fe2.3sMgo.osTi0.604
0 0.185 0.643 0.860
(120) 145 100 455
(1.07) 2.5 2.1 1.3
(0.61) 1.5 1.3 1.0
(0.29) 0.27 0.19 0.53
Fe2.2 sMgo. t sTio.604
0 0.065 0.134 0.374
40 130 145 70
2.5 3.7 2.6 2.6
1.5 2.5 1.6 1.7
0.15 0.27 0.29 0.19
Fe2.15Mgo.2sTio.604
0 0.231 0.624 0.884
40 80 50 455
2.4 2.8 3.1 1.0
1.3 1.8 1.9 0.8
0.15 0.20 0.14 0.42
Fe2.05Mgo.35Tio.604
0 0.375 0.540 0.929
35 60 45 385
2.6 2.1 2.7 1.1
1.4 1.4 1.6 0.9
0.15 0.17 0.14 0.59
Mrs/Ms
401
The direct and alternating field (AF) demagnetization of Mrs was also studied. The ratio of coercive force of remanence (Hrc) so obtained to coercive force (He) lies in the rangc 2- 4 for all specimens except the most highly oxidized for which Hrc/H c 1--1.3. The values of median destructive field (MDF) obtained from the AF demagnetization curves are generally intermediate between the values of Hc and Hrc except for the most highly oxidized samples in some of which MDF < 11c (Table 2).
netic hardness (indicated by remanence demagnetization linear and rotational hysteresis characteristics of the bulk rock) observed after laboratory heating of some basalts in air [5,14,15]. The increased hardness in bulk magnetic properties is clearly due to the multiphase oxidation product of the titanomagnetite rather than the result of some other process, e.g., oxidation of silicates.
4. Conclusion 3. Discussion Although highly oxidized, titanomagnetite grains produced in the present experiments would be categorized as being within Class I of the optically based oxidation scale developed to describe the Fe-Ti oxides in basalts [13]. Class I grains appear to be structurally homogeneous and are expected to be in the multidomain state. The Curie temperature of an unaltered titanomagnetite of typical composition is about 200°C. The association of a higher Curie tern> o perature (~500 C, say) with apparently homogeneous grains in a basalt is therefore taken to indicate that such grains are not truly Class I. However, the absence of a Curie temperature characteristic of an iron-rich spinel (~500°C) may not in itself be a sufficient condition from which to infer the absence of alteration. This is because during exsolution or inversion magnesium appears to seek the spinel phase with a consequent reduction in the Curie temperature. From considerations of the bulk chemical composition (assigning residual Fe 2+ to the spinel phase), X-ray diffraction line intensities and assuming a spinel saturation magnetization in the range 2 0 - 5 0 emu/g it seems that the spinel phase constitutes about 10-20% of the most highly oxidized samples (z "~ 0.9). Magnetostatic interaction between the magnetic zones is therefore likely to be very weak. This is borne out by the magnetic hysteresis parameters, as the values of Hc are typical of those obtained for samples of dispersed magnetite rods and the Mrs/Ms and Hrc/Hc ratios are consistent with a randomly orientated assemblage of non-interacting monodomain particles with uniaxial anisotropy. The results of the present study support the generally accepted interpretation of the increased mag-
The present study supports the model in which it is proposed that deuteric oxidation of large grains of titanomagnetite may result in effectively isolated regions of residual spinel phase with monodomain characteristics and corresponding high remanence stability.
Acknowledgements This work forms part of a NERC-sponsored research programme "Thermoremanence in titanomagnetites". One of the authors (J.B.O'D.) has been in receipt ofa NERC studentship. We thank Dr. Z. Hauptman for invaluable assistance in the preparation of materials and the staff of the Electron Optical Unit, University of Newcastle upon Tyne.
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trusion, Earth Planet. Sci. Left. 4 (1968) 142. 12 J.B. O'Donovan, Studies on synthetic analogies of carriers of the palaeomagnetic record, Ph.D. Thesis, Univ. of Newcastle upon Tyne (1975). 13 R.L. Wilson and N.D. Watkins, Correlation of petrology and natural magnetic polarity in Columbia River basalts, Geophys. J. R. Astron. Soc. 12 (1967) 405. 14 P.M. Davis and M.E. Evans, Interacting single-domain properties of magnetite intergrowths, J. Geophys. Res. 81 (1976) 989. 15 A.J. Manson and W. O'Reilly, Submicroscopic texture in titanomagnetite grains in basalt studied using the torque magnetometer and electron microscope, Phys. Earth Planet. Inter. 11 (1976) 173.