Magnetic identification of selected natural iron oxides and sulphides

*Corresponding author. Tel.:#44 131 650 1000; fax:#44131 668 3184; e-mail: [email protected].

Journal of Magnetism and Magnetic Materials 183 (1998) 365—374

Magnetic identification of selected natural iron oxidesand sulphides

C. Peters*, R. ThompsonDepartment of Geology and Geophysics, University of Edinburgh, Grant Institute, West Mains Road, The King+s Bldg,

Edinburgh EH9 3JW, UK

Received 10 June 1997

Abstract

Measurements of initial susceptibility, remanent magnetisations and hysteresis loops have been carried out at roomtemperature on a range of characterised iron oxides and sulphides in order to attempt qualitative identification of theindividual minerals. The minerals studied were magnetite, titanomagnetite, haematite, pyrrhotite and greigite. It wasfound to be possible to qualitatively identify all the minerals (except titanomagnetites from magnetites) from each otherusing simple susceptibility and remanence ratios. Using discriminant analysis on both the remanence and hysteresis loopdata, it was found to be possible to also distinguish the titanomagnetites from the magnetites purely on the basis of theroom-temperature measurements. ( 1998 Elsevier Science B.V. All rights reserved.

Keywords: Greigite; Haematite; Magnetite; Pyrrhotite; Rock magnetism; Titanomagnetite

1. Introduction

Natural samples are complicated mixtures ofmany different types of magnetic crystals, includinga wide range of grain sizes. Some crystals are dis-persed, some in clumps. Often magnetic mineralsoccur as minor constituents in quite low concentra-tion of just a few parts per thousand. This researchexplores the question of which magnetic pa-rameters or combination of parameters are the most

useful in helping to characterise natural materialswhich contain mixtures of magnetic minerals.Natural magnetic minerals display a very widerange of magnetic properties. Many of the indi-vidual properties show overlapping characteristicsbetween minerals. However, it is found that certaincombinations of magnetic properties can be highlydiagnostic.

2. Samples

A range of 56 natural samples of iron oxidesand sulphides have been collected from various

0304-8853/98/$19.00 ( 1998 Elsevier Science B.V. All rights reserved.PII S 0 3 0 4 - 8 8 5 3 ( 9 7 ) 0 1 0 9 7 - 4

sediment, soil and rock types. The minerals selectedfor this study are the iron oxides magnetite (Fe

3O

4),

titanomagnetite (Fe3~x

TixO

4, 0(x(1), and hae-

matite (aFe2O

3), and the iron sulphides pyrrhotite

(Fe1~x

S, 0(x(0.13) and greigite (Fe3S4). The

samples were selected on the basis of (i) containingonly one magnetic mineral (i.e. no mixtures),and (ii) to span a wide a range of domain states and(iii) to cover a range of concentrations. The min-eralogies of the samples were determined by Curietemperature and thermomagnetic analyses. Curietemperatures between 575 and 600°C were ob-tained for magnetite. The titanomagnetite Curietemperatures ranged from 90 to 530°C and haema-tite from 675 to 695°C. The c-transition wasobserved in three pyrrhotites at temperaturesbetween 200 and 235°C, with subsequent Curietemperatures between 250 and 280°C. Monoclinicpyrrhotite exhibited Curie temperatures between300 and 325°C. The greigites displayed character-istic non-reversible drops in magnetisation between330 and 410°C.

3. Magnetic measurements

All the measurements carried out in this studyare based on inducing or growing magnetisationsin the laboratory. It is possible to induce and growlaboratory magnetisations in various ways.

3.1. Magnetic susceptibility

Magnetic susceptibility is the most commonlymeasured magnetic parameter due to ease andrapid nature of measurement. Magnetic susceptibil-ity is a measure of how easily a material can bemagnetised. It gives a rough indication of the mag-netic concentration. Initial magnetic susceptibilityis defined to be the ratio of the induced magnetisa-tion to the applied magnetic field. The volumesusceptibility, k, is dimensionless in SI units. Alter-natively on a specific basis, the mass susceptibility,s, is measured in lm3 kg~1. Specific susceptibilityis used throughout the present study. A Bartingtonsusceptibility bridge (sensor type MS1B) and aDigico susceptibility bridge were used to measureinitial magnetic susceptibility. The calibration of

the bridges were checked using a standard sampleof ammonium ferrous sulphate ((NH

4)2Fe(SO

4)2)

6H2O).

3.2. Anhysteretic remanent magnetisations

Anhysteretic remanent magnetisations, ARMs,are grown by superimposing a small direct fieldonto a larger smoothly decreasing alternating field.In this study an alternating field (a.f.) of 99 mT wassuperimposed on a direct field of 0.1 mT using anadapted Molyneux AC Demagnetiser. A Molspinfluxgate magnetometer (calibrated using a standardsample of magnetic tape of known magnetic mo-ment) was used to measure the remanent magnet-isations. The ARM gives an indication of themagnetic concentration. Further information canbe obtained by stepwise a.f. demagnetisation of theinitial ARM (SARM). In this study demagnetisa-tion was carried out in alternating fields of 5, 10, 20,40, 80 and 99 mT. The remanent magnetisationswere normalised with respect to SARM for plottingagainst the corresponding field values.

Fig. 1 shows a summary of the ARM demag-netisation curves obtained for the magnetites,titanomagnetites, pyrrhotites and greigites. All theextreme samples are plotted in order to illustratethe range each mineral covers. The pyrrhotite sam-ples show the greatest variation. All the other min-erals plot within the pyrrhotite range. There is nooverlap between the magnetite and greigite sam-ples, thus indicating the potential ARM measure-ments have for distinguishing different minerals.The titanomagnetites are slightly harder than themagnetites, i.e. the titanomagnetites resist demag-netisation more.

3.3. Isothermal remanent magnetisations

Isothermal remanent magnetisations, IRMs, aregrown by placing a sample in a direct field andsubsequently removing the field. A pulse magnet-iser was used to grow IRMs in fields up to 300 mTand electromagnets to grow IRMs in fields of 1 T.All IRMs were measured using a Molspin fluxgatemagnetometer. Single IRM measurements givea rough indication of magnetic concentration.Much more information can be obtained by studying

366 C. Peters, R. Thompson / Journal of Magnetism and Magnetic Materials 183 (1998) 365—374

Fig. 1. Selection of 13 ARM demagnetisation curves for magnetites, titanomagnetites, pyrrhotites and greigites to illustrate the greatrange of behaviour in natural samples.

the variation of IRMs with changing fieldvalues. Three common ways of carrying out suchstudies of coercivity spectra are as follows:

1. IRM acquisition curves are obtained by growingIRMs in successively higher fields. Fields of 20,40, 60, 80, 100, 200, 300, and 1000 mT were usedin this study. The IRM grown in 1 T is referredto as the saturation isothermal remanent mag-netisation (SIRM).

2. DC demagnetisation of SIRM. Fields of !20,!40, !60, !80, !100, !200, !300 and!1000 mT were used in this study. (B

0)#3

isdefined as the field required to reduce the mag-netisation to zero.

3. AF demagnetisation of SIRM. Fields of 5, 10,20, 40, 80 and 99 mT were used within thistechnique. B

1@2Iis defined as the alternating

field required to reduce the IRM to half of itsSIRM.

Fig. 2 shows 15 examples of IRM acquisitioncurves. Extreme results for all the types of ironoxides and sulphides studied have been plotted.The haematite samples are magnetically the

hardest, i.e. they resist magnetisation and are notsaturated in a field of 1 T. Greigite has very distinc-tive IRM acquisition curves compared to the otherminerals. The normalised greigite IRM data startswith very low ratios ()0.02) and then rapidly in-creases to high ratios (*0.9) in fields between 40and 200 mT. The other minerals do not show sucha rapid increase in any field range. As in the case ofARM demagnetisation, pyrrhotite shows a widerange of stabilities.

3.4. Hysteresis loops

Hysteresis loops can be measured in a variety ofways. In this study hysteresis loops were measuredusing a Molyneux vibrating sample magnetometer(VSM). The VSM was calibrated using knownmasses of the paramagnetic salts ammonium fer-rous sulphate ((NH

4)2Fe(SO

4)2) 6H

2O) and copper

sulphate (CuSO4) 5H

2O). Measurements were car-

ried out between fields of #1 and !1 T.The most commonly used hysteresis parameters

are indicated in Fig. 3 on a typical hysteresis loop.These are the saturation magnetisation, M

4, which

is the maximum magnetisation a sample can attain

C. Peters, R. Thompson / Journal of Magnetism and Magnetic Materials 183 (1998) 365—374 367

Fig. 2. Extreme IRM acquisition curves for 15 magnetites, titanomagnetites, haematites, pyrrhotites and greigites.

Fig. 3. Typical hysteresis loop: main features and parameters.

in the presence of a magnetic field; the remanencemagnetisation, M

34, which is the magnetisation

remaining after the applied field has been reducedto zero following saturation of the sample; and theco ercivity, (B

0)#, which is the reverse field required

to reduce the magnetisation to zero. Additionally,

high field, s)&, and low field, s

-&, susceptibilities can

be calculated from the hysteresis loops. The high-field susceptibility reflects the paramagnetism/diamagnetism within a sample. The ferrimagneticcomponent of the saturation magnetisation, M

4&,

can be determined from the relationshipM

4&"M

4!s

)&]H

4.

4. Variation of magnetic properties with grain size

There are of course many parameters that can bemeasured and used to characterise magnetic mate-rials. s and (B

0)#3

have been measured most fre-quently by previous workers. The variation of (B

0)#3

with grain size is summarised in Figs. 4 and 5 forpreviously published data of pyrrhotite and haema-tite, respectively. The magnetic properties of pyr-rhotite are complicated by the existence ofmonoclinic and hexagonal forms and their co-exist-ence in natural samples. However, (B

0)#3

shows analmost linear relationship with grain size for pyr-rhotite. Only a few coarse grained samples fromRef. [1] fall out with this trend. Dekkers [1] at-tributes the discrepencies in his samples as due tothe presence of silicate intergrowths which reduce


Fig. 4. Variation of remanent coercivity with grain size (lm) forpyrrhotite. The data are from Refs. [9,10,1].

Fig. 5. Variation of remanent coercivity with grain size (lm) for haematite. The data are taken from Refs. [11—13,2].

the effective grain size. In comparison, we observea large variation in (B

0)#3

for any given grain size ofhaematite. The lack of an observed trend in thevariation of magnetic properties with grain sizehighlights how little is currently known about hae-matite, despite several detailed studies.

Ref. [2] gives good summaries of the change ofmagnetic properties with grain size for magnetite

and titanomagnetite. A peak at &1 lm is ob-served in the variation of (B

0)#3

with grain sizefor both magnetite and titanomagnetite. However,the remanent coercivities of titanomagnetiteare higher than those for magnetite, indicatingthat titanomagnetite is magnetically harderthan magnetite. The variation of magnetic proper-ties with grain size for greigite remains to bestudied.

5. Qualitative identification

Of the five common magnetic minerals studied,haematite is the only one which can be separatedfrom the others using a single magnetic parametersuch as (B

0)#or (B

0)#3. Magnetite, titanomagnetite,

pyrrhotite and greigite have similar magnetic prop-erties, therefore no single room-temperature mag-netic parameter was found which can conclusivelyidentify any of these four minerals. However, cer-tain parameters can indicate the likelihood ofa mineral. For example, SIRM/s is a very usefulparameter for indicating the likely presence of pyr-rhotite. Similar to the results found in this study,Ref. [1] found very high values of SIRM/s forpyrrhotite. Ref. [3] also reports high values of


Fig. 6. SIRM/s versus (B0)#3

showing a clear distinction of hae-matite from the other four minerals, and a reasonable distinctionfor most pyrrhotites.

Fig. 7. SIRM/s versus ARM(40 .T)

/SARM showing cleardistinction of pyrrhotite from magnetite, titanomagnetite andgreigite.

SIRM/s for greigite. This study, however, indicatesthat the SIRM/s values for greigite are less thanthose for pyrrhotite and similar to those of hardmagnetite/titanomagnetite. Additionally, very lowvalues of SIRM/s ((4 kA m~1) indicate very softmagnetite/titanomagnetite. It is much more diffi-cult to find individual magnetic properties whichindicate the presence of hard magnetite. Ref. [4] listcertain criteria which are useful for distinguishingbetween pyrrhotite and magnetite. These are, firstfor pyrrhotite, M

34/M

4'0.2, (B

0)#3/(B

0)#(1.5 and

M34/k'(B

0)#3, and secondly for magnetite, M

34/

M4(0.2, (B

0)#3/(B

0)#'2 and M

34/k((B

0)#3

(un-less the grains are single domain and strongly an-isotropic). The results from the present study andother previously published data indicate that thegeneralisations of Ref. [4] of the M

34/M

4and

(B0)#3/(B

0)#

ratios are true for a large number ofnatural samples.

5.1. Biplots

Biplots are a useful qualitative graphical tech-nique for combining magnetic parameters in orderto emphasise the properties of certain minerals anddomain states. Figs. 6—8 show three examples ofsuch biplots. All three biplots involve magneticratios, as ratios largely eliminate the effects of mag-netic concentration.

We first consider the well-known biplot ofSIRM/s versus (B

0)#3, Fig. 6 (cf. Ref. [5]). Here

haematite can clearly be identified from the otherminerals due to its high remanent coercivities.Other interesting features of this graph are thedistinction (with the exception of one sample) ofpyrrhotite with high SIRM/s ratios from magnet-ite, titanomagnetite and greigite. The pyrrhotiteSIRM/s ratios of around 500 kA m~1 are compa-rable to those of haematite SIRM/s ratios. A differ-ence is also observed in Fig. 6 between the softmagnetite and titanomagnetite samples. The titano-magnetite samples have higher SIRM/s ratios ofup to 20 kA m~1. With the exception of onetitanomagnetite sample (which contains a mixtureof SPM and SD grains) the titanomagnetites followan almost linear trend on the SIRM/s versus (B

0)#3

graph. The greigite samples all plot together on theSIRM/s versus (B

0)#3

graph and have similar

SIRM/s ratios to the hard magnetite andtitanomagnetite samples of about 100 kA m~1.One problem with using the parameter SIRM/s isthat the s values are increased by the presence ofparamagnetic and superparamagnetic minerals.Thus, for very weak samples the susceptibilityvalues could be significantly increased by a para-magnetic contribution, which in turn would signifi-cantly lower their SIRM/s ratio.


Fig. 8. IRM(~100 .T)

/SIRM versus ARM(40 .T)

/SARM. On thisbiplot greigite is distinguished from magnetite, titanomagnetiteand pyrrhotite.

Fig. 9. Discriminant analysis biplot showing the differences be-tween magnetite and titanomagnetite using a linear combina-tion of ten magnetic parameters.

Although the SIRM/s versus (B0)#3

biplot ofFig. 6 shows a reasonable distinction of pyrrhotitefrom the other minerals, Fig. 7 yields a completedistinction of pyrrhotite from magnetite, titano-magnetite and greigite. This separation is achievedby combining the ratio SIRM/s with the ARMdemagnetisation ratio, ARM

40 .T/SARM. In addi-

tion, in Fig. 7 we find that for magnetite,titanomagnetite and pyrrhotite the samples withthe lowest ratios of SIRM/s and ARM

40 .T/SARM

are MD and plot towards the lower left-hand cor-ner. The samples with the highest ratios are SD andplot towards the upper right-hand corner.

We now consider a third biplot in which theARM demagnetisation ratio proves to be useful inidentifying greigite. The biplot of IRM

~100 .T/

SIRM versus ARM40 .T

/SARM is shown in Fig. 8.It generates a reasonable distinction betweengreigite and the other three low stability minerals.A trend from SD to PSD greigites is found on thegraph. The SD grains have high IRM

~100 .T/

SIRM and ARM40 .T

/SARM ratios and plot in themiddle right, while the PSD grains plot further tothe lower middle.

In summary, for the five minerals studied (i)haematite is relatively easy to distinguish with itshigh stabilities, (ii) pyrrhotite can be detected byhigh SIRM/s in comparison to (B

0)#3, and espe-

cially by high SIRM/s in comparison to ARM40 .T

/

SARM, (iii) greigite has high ARM stability incomparison to its IRM stability while (iv) magnet-ite and titanomagnetite are not distinguishable us-ing single magnetic parameters or biplots based onroom-temperature measurements. We turn now tomore formalized approaches to discrimination andtheir application to magnetic characterisation, inparticular to the problem pair of magnetite plustitanomagnetite.

6. Discriminant analysis

Discriminant analysis is a multivariate statisticalprocedure which compares variables from a num-ber of groups and then combines them linearly toproduce discriminant functions which show thegreatest separation and least dispersion betweenthe groups [6,7]. Discriminant analysis was carriedout on 16 room-temperature magnetic measure-ments to see if any combination could distinguishbetween the presence of magnetite as opposed totitanomagnetite. Discriminant analysis actuallymanages to find a linear combination of themeasurements which achieves a complete sepa-ration. In order to show the discrimination graphi-cally the discriminant variable is plotted in Fig. 9against ARM

40 .T/SARM. Of the 16 parameters

used in the analysis, 10 were selected by thealgorithm for inclusion in the discriminating


Fig. 10. Summary of magnetic measurements and identificationof different minerals.

function. The two main parameters which contrib-ute are found to be (B

0)#3

and B1@2I

.

7. Discussion

We find that many natural magnetic mineralscan be satisfactorily distinguished from oneanother using a small number of laboratory-in-duced magnetisations or remanences produced atroom temperature. In particular, (i) haematite hasdistinctively high coercivities with (B

0)#3'100 mT;

(ii) pyrrhotite has an unusually high SIRM/s ratioin comparison to its remanent coercivity; (iii)greigite also has a high SIRM/s ratio but is distin-guishable from pyrrhotite by its higher ARM orIRM stability in comparison to its SIRM/s ratio;and (iv) fossil bacterial magnetosomes occur asinteracting chains; these give rise to unusuallystrong ARMs, with a SIRM/ARM ratio of less thanfive and an ARM mdf of over 40 mT. The remain-ing natural magnetic minerals are the iron-oxidespinels which occur in nature in a wide variety ofdomain and oxidation states. Unless they are inmixed assemblages, their probable domain state(single-, psuedo- or multi-domain) can often berapidly determined from a simple measure of co-ercivity (e.g. IRM

60 .T/SIRM) or from their M

3/M

4ratios. A major exception being that super-paramagnetic (spm) single-domain particles havesimilar low-stability properties and M

3/M

4ratios

to multi-domain particles (M3/M

4(0.05). Super-

paramagnetic particles can, however, be recog-nised, as they display a distinctively more rapid riseto saturation in their room-temperature hysteresisloops along with very low SIRM/s ratios. Curietemperatures are traditionally used to aid discrim-ination between pure magnetite, pure maghaemiteand the titanomagnetites. The overlapping remanentand hysteresis properties of this group of mineralsprevent us from distinguishing between them usingroom-temperature magnetic properties alone.

8. Summary of qualitative techniques

Fig. 10 summarises all the magnetic measure-ments used in this study and indicates their useful-

ness in identifying the different minerals and do-main states. For example, (i) ARM demagnetisationdata is used to identify pyrrhotite, greigite, magnet-ite and titanomagnetite, (ii) the hysteresis loopparameters give an indication of domain state, (iii)alternating field demagnetisation of SIRM is usefulfor looking at grain interactions and (iv) all themagnetic data is required to discriminate magnetiteand also titanomagnetite from the other minerals.

Fig. 11 summarises graphically the ways inwhich the different minerals can be identified usingsimple combinations of the room-temperaturemagnetic data. In Fig. 11a the overall mineralogyand domain state trends are sketched. In addition,goethite data from Ref. [8] was included to allowcomparison between the two magnetically hardminerals, haematite and goethite. Fig. 11a showsthat although haematite can be distinguished fromthe other minerals measured in this study, usingthis particular biplot there is a small overlap be-tween its magnetic properties and those of goethite.Pyrrhotite can be identified by combining the s,ARM demagnetisation and IRM acquisition dataas shown in Fig. 11b, and greigite can be identifiedby combining the ARM demagnetisation and IRMacquisition data as shown in Fig. 11c.

Distinguishing magnetite, and similarly titano-magnetite, from all the other minerals requireda more complicated combination of the room-temperature magnetic data. Discriminant analysis


Fig. 11. Summary of biplots used for qualitative identification of magnetic minerals.

shows that discrimination is achievable withroom-temperature magnetic data.

Acknowledgements

The work was supported by a NERC student-ship to CP.

References

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[6] W.C. Isphording, G.C. Flowers, J. Sediment Petrol. 50 (1)(1980) 31.

[7] P.B. de Menocal, E.P. Laine, P.F. Ciesielski, Phys. EarthPlanet. Inter. 51 (1988) 326.

[8] M.J. Dekkers, Ph.D. Thesis, University of Utrecht,1988.

[9] A. Menyeh, W. O’Reilly, Geophys. J. Int. 104 (1991)387.

[10] D.A. Clark, Geophys. Res. Lett. 11 (3) (1984) 173.[11] M.J. Dekkers, J.H. Linssen, Geophys. J. Int. 99 (1989) 1.[12] D.J. Dunlop, Ann. Geophys. 27 (3) (1971) 269.[13] R.L. Hartstra, Ph.D. Thesis, University of Utrecht, 1982.


Documents

Magnetic identification of selected natural iron oxides and sulphides