magnetic anisotropy of calcite at room-temperature

(2006) 63–73www.elsevier.com/locate/tecto

Tectonophysics 418

Magnetic anisotropy of calcite at room-temperature

Volkmar Schmidt a,⁎, Detlef Günther b, Ann M. Hirt a

a Institute of Geophysics, ETH-Hönggerberg, CH-8093 Zürich, Switzerlandb Laboratory for Inorganic Chemistry, ETH-Hönggerberg, CH-8093 Zürich, Switzerland

Received 12 May 2005; received in revised form 29 September 2005; accepted 5 December 2005Available online 3 March 2006

Abstract

The intrinsic room temperature magnetic properties of pure calcite were determined from a series of natural crystals, and theywere found to be highly dependent on the chemical composition. In general, dia-, para-, and ferromagnetic components contributeto the magnetic susceptibility and the anisotropy of magnetic susceptibility (AMS). With a combination of magnetic measurementsand chemical analysis these three contributions were determined and related to their mineralogical sources. The intrinsicdiamagnetic susceptibility of pure calcite is −4.46±0.16×10−9 m3/kg (−12.09±0.5×10−6 SI) and the susceptibility difference is4.06±0.03×10−10 m3/kg (1.10±0.01×10−6 SI). These diamagnetic properties are easily dominated by other components. Theparamagnetic contribution is due to paramagnetic ions in the crystal lattice that substitute for calcium; these are mainly iron andmanganese. The measured paramagnetic susceptibility agrees with the values calculated from the known concentration ofparamagnetic ions in the crystals according to the Curie law of paramagnetic susceptibility. Substituted iron leads to an increase inthe AMS. The paramagnetic susceptibility difference was found to correlate linearly with the iron content for concentrationsbetween 500 and 10,000 ppm. An empirical relation was determined: (k1−k3)para (kg/m3)=Fe-content (ppm)×(1±0.1)×10−12 (kg/m3/ppm). The maximum susceptibility difference (Δk=k1−k3) was found to be unaffected by iron contents below 100 ppm.Ferromagnetic contributions due to inclusions of ferromagnetic minerals can dominate the susceptibility. They were detected byacquisition of isothermal remanent magnetization (IRM) and their contribution to the AMS was separated by high-fieldmeasurements.© 2006 Elsevier B.V. All rights reserved.

Keywords: Calcite; Magnetic anisotropy; Anisotropy of magnetic susceptibility (AMS); Torsion magnetometer; LA-ICP-MS

1. Introduction

The anisotropy of magnetic susceptibility (AMS) is aphysical property of rocks that is very sensitive tomineral alignment. AMS measurements are a fast meth-od to characterize rock fabrics (Wood et al., 1976;Kligfield et al., 1977), and in numerous investigationsAMS has been used to clarify structural deformation(cf., Borradaile and Henry, 1997). Although carbonate

⁎ Corresponding author.E-mail address: [email protected] (V. Schmidt).

0040-1951/$ - see front matter © 2006 Elsevier B.V. All rights reserved.doi:10.1016/j.tecto.2005.12.019

rocks play an important role in natural and experimentaldeformation processes, their AMS has been seldomexamined due to the weak diamagnetic susceptibility ofcalcite. However, several investigations have shown thepotential of AMS as a strain indicator in calcite rocks.Besides the experimental deformation study of Owensand Rutter (1978), de Wall et al. (2000) measured AMSof a calcite marble of a shear zone in Thassos (Greece)and showed its use in characterizing rock fabrics. Evanset al. (2003) reviewed the deformation mechanisms ofcalcite and examined how they affect AMS fabrics inlimestone. To fully exploit the potential of using AMS

64 V. Schmidt et al. / Tectonophysics 418 (2006) 63–73

as an indicator of rock deformation in calcitic rocks, thefactors that affect the magnetic susceptibility in calcitemust be understood. Several studies have examined themagnetic anisotropy in calcite. However, little informa-tion has been provided on the composition of thesamples studied.

AMS is described by a second rank tensor with theeigenvalues k1≥k2≥k3. The mean susceptibility kM isthe arithmetic average of the eigenvalues (kM=(k1+k2+k3) / 3). The susceptibility tensor is represented geomet-rically by an ellipsoid, in which k1, k2 and k3 are thelengths of the principal axes. The anisotropy can bedescribed by the susceptibility difference Δk=k1−k3and theU-parameter (Jelinek, 1981). TheU-parameter isa shape factor, similar to the T-parameter, but based onsusceptibility differences, where U=(2×k2−k1−k3) /(k1−k3). Therefore, a perfectly oblate shape wouldresult in U=1 and a perfectly prolate shape in U=−1.These parameters can be determined from the torsionbalance measurements and they are, in contrast to manyother parameters, independent of the bulk susceptibility.

The source of diamagnetism is not the aligning ofpreexisting magnetic moments, but an electronic re-sponse to a magnetic field. The movement of the orbitalelectrons according to Lenz' law induces a field in theopposite direction, which results in negative suscepti-bility. In calcite, the oxygen atoms of the carboxyl groupare arranged in the plane perpendicular to the c-axis andtheir π-bonds with the carbon atom facilitate movementof the electrons. When a field is applied parallel to thec-axis, the electron orbits can be larger in the planenormal to the c-axis and produce a stronger response,which is expressed in a stronger diamagnetism parallelto the c-axis (cf. Lonsdale, 1938; Pauling, 1979;O'Handley, 2000).

The magnetic properties of calcite have been of greatinterest since the 19th century. Michael Faraday deter-mined that calcite was diamagnetic, but due to his expe-rimental setup he was not able to detect any anisotropy(Tyndall, 1851). Furthermore, Tyndall (1851) investi-gated spheres of calcite with a torsion balance and founda directional dependence of the susceptibility. He foundthat the lowest susceptibility was sub-parallel to thecrystallographic c-axis and the ratio of the maximum tominimum susceptibility was 1.10, a value that has beenconfirmed in subsequent studies. König (1887) alsoinvestigated calcite spheres and obtained a value of1.43×10−6 (SI) for Δk. Using the values of Tyndall hedetermined the susceptibility values relative to air (k′)with k′1=−14.30×10−6 (SI) and k′3=−15.73×10−6

(SI). Voigt and Kinoshita (1907) carried out more mea-surements on calcite plates and constrained the maxi-

mum and minimum values of susceptibility to k′1=−12.4and k′3=−13.8×10−6 (SI). These values are most oftencited for calcite (Povarennykh, 1964; Owens andBamford, 1976; Owens and Rutter, 1978; Bleil andPetersen, 1982; Rochette, 1988).

Owens and Rutter (1978) investigated calcitecrystals with better constrained chemical composition(b65 wt. ppm for Mg, and b20 wt. ppm for all otherimpurity elements). They determined Δk=1.19×10−6

(SI). It should be noted that the shape of the AMSellipsoid was not perfectly oblate, which would beexpected for a perfect calcite crystal. The U-parameterwas −0.93 on average. Rochette (1988) demonstratedhow the magnetic susceptibility and anisotropy aredependent on the Fe-content in calcite and on ferro-magnetic inclusions.

Improvements in the sensitivity of measurementtechniques make it possible to re-examine the magneticsusceptibility of calcite and its anisotropy with greaterprecision and reliability. In this study the AMS of aseries of calcite crystals was measured in low and highfields. A high-field torque magnetometer was used toseparate any ferromagnetic contribution to the AMS.The chemical composition of the crystals was analyzedusing laser ablation-inductively coupled plasma massspectrometry (Jackson et al., 1992; Durrant, 1999) toassess the influence of paramagnetic ions in the calcitestructure.

2. Methods/measurement procedure

2.1. Sample preparation

Eighteen well-grown calcite single crystals fromdifferent localities and one sample of synthetic calcitepowder were analyzed (Table 1). The crystals representa variety of colours and their original size ranged from 4to 130 g. Visible impurities or intergrowths in thecrystals were sliced away. Crystals larger than themaximum measurable sample size of about 30 g weresplit into individual parts and were measured separately.This provided information on the homogeneity of themagnetic properties of the crystal. Some samples couldbe drilled along the crystallographic c-axis, afterorienting the sample with a specially constructedmount, which was made in the ETH Physics workshop.Before the measurements, the samples were cleanedwith alcohol and an ultrasonic cleaner to remove dustand possible impurities from the cutting and drilling.Anisotropy in dia- and paramagnetic materials is due tocrystalline anisotropy only; therefore the shape of thesamples is unimportant. In the majority of cases the

Table 1Calcite crystals

Sample Location Mass[g]

Colour

C1A Gonzen, Switzerland 6.41 Clear,transparent

C1B Gonzen, Switzerland 6.65 Grey, semi-transparent

C2 ? 20.49 Greenish, semi-transparent

C4 ? 4.34 Dark brown,opaque

C5 Mexico 28.00 Clear,transparent

C6 ? 24.45 Clear,transparent

C7 Iceland 10.59 Clear,transparent

C8 Chihuahua, Mexico 4.61 Clear,transparent

C10 Mexico 6.01 Pinkish,transparent

C11 Mexico 20.26 Greenish,transparent

C12 Chihuahua, Mexico 26.43 Red banded,opaque

C13 Chihuahua, Mexico 26.19 Greenish, semi-transparent

C14 Pachapaqui, Peru 4.54 White to pinkish,opaque

C15 Durango, Mexico 26.99 Pinkish,transparent

C16 Synthetic powder,Riedel-de Haen

3.77 White

AG, GermanyC17 Italy 24.43 White, opaqueC18 Santa Eulalia,

Chihuahua, Mexico5.73 Clear,

transparentU1A ? 4.59 White, opaqueU1B ? 5.49 White, opaque

Mass of individual specimens.

65V. Schmidt et al. / Tectonophysics 418 (2006) 63–73

shape of the crystals allowed the determination of thecrystallographic orientation of the sample.

2.2. Determination of chemical composition

Due to the interest in the elemental composition ofthe calcite material, laser ablation inductively coupledplasma mass spectrometry (LA-ICP-MS) was used toanalyze all 18 crystals used for further AMS studies. Adetailed description of the technique and the workingprinciple can be found elsewhere (Günther andHeinrich, 1999). A 193 nm ArF laser ablation system(GeoLas M, MicroLas, Göttingen, Germany) wascoupled to an ICP-mass spectrometer (ICP-MS DRC,Perkin Elmer/Sciex, Norwalk, USA) for direct solid

analysis. Thematerial was ablated at a fluence of 17 J cm2

at a repetition rate of 10 Hz. The spatial resolution(crater size) was adjusted to 80 μm and each sample wasanalyzed three times at different locations. The replicateanalysis provided information about the homogeneity ofthe element distribution within the crystals. An elementmenu of 33 isotopes was measured to get the major,minor and trace element composition of the samples,which included all elements with possible paramagneticions. The data acquisition was 90 s, where 30 s gasbackground was measured followed by a 60 s ablationand ICP-MS signal detection. Further details about thedata acquisition protocol can be found in Longerich etal. (1996). For quantitative analysis, NIST 610 glass wasused as the external calibration standard. The Castoichiometry at a concentration of 40.4 wt.% wasused for internal standardization. Afterwards the valueswere further corrected for stoichiometry assuming thatdivalent Na, Mg, Mn, Fe and Sr substitute for Ca.

2.3. Isothermal remanent magnetization (IRM)acquisition

The acquisition of IRM was used to identify theferromagnetic inclusions in the crystals. These inclu-sions can increase the susceptibility of the samples andaffect their magnetic anisotropy. The results of IRMacquisition were useful to prove the purity of the crystalsstudied, and to explain anomalous magnetic propertiesof impure crystals.

The samples were magnetized with a pulse magne-tizer (ASC Scientific, IM-10-30) first in one direction ina field of 2.5 T, and then in the opposite direction inprogressively higher fields from 10 mT up to 2.5 T. Thisprocedure allows the coercivity of remanence Hcr to bedetermined. The IRM acquisition of three samples wasmade before and after cleaning the crystals to determinethe presence of ferromagnetic contaminants.

2.4. Measurement of magnetic anisotropy

Magnetic susceptibility and low-field AMS weremeasured at room temperature with a KLY-4S suscep-tibility meter (AGICO, Brno) in an alternating field of300 A/m and 875 Hz. The mean susceptibility wascalculated from the full susceptibility tensor. Thesensitivity of the instrument for standard size samplesof 10 cm3 is 3×10−8 (SI) for the bulk susceptibility and2×10−8 (SI) for the AMS. Measurements of the low-field AMS on an AGICO KLY-2 susceptibility meter atthe Laboratory for Natural Magnetism (LNM) at theETH Zurich gave results that were not significantly

Table 2Chemical composition from LA-ICP-MS

Sample Na Mg Ca Ti V Cr Mn Fe Ni Cu Sr Tb Dy Ho Er Pb

C 1A Average 5.2 2090 389355 0.5 1.2 1.2 840 8310 0.11 b0.06 826 0.04 0.23 0.04 0.09 0.99St dev. 8.8 29.2 0.9 0.0 0.2 37 107 0.19 68 0.01 0.02 0.01 0.02 0.12

C 1B Average 1.8 2196 388651 4.6 1.3 2.2 940 8770 b0.9 b0.1 970 0.04 0.29 0.05 0.08 1.41St dev. 2.7 104 8.0 0.2 0.2 30 285 85 0.02 0.05 0.00 0.07 0.31

C 2 Average 11.7 315 397424 b0.3 0.0 0.8 120 886 b0.3 2.9 2210 4 1.41 7.93 1.62 4.24 26.96St dev. 0.6 3.5 0.0 0.1 1.1 18 5.0 12 0.04 0.31 0.04 0.12 0.84

C 4 Average 18.0 279 399836 6.0 1.3 1.3 0.8 190 0.47 0.16 40 b0.02 0.04 0.01 b0.02 0.12St dev. 7.7 121 2.7 0.5 1.1 0.4 70 0.81 0.14 1.8 0.06 0.01 0.03

C 5 Average 20.0 566 399449 0.2 1.6 100 21 100 0.03 0.24 0.06 0.19 0.10St dev. 1.8 32 0.0 0.4 2.6 0.4 5.2 0.00 0.03 0.01 0.02 0.01

C 6 Average 0.2 1650 397621 0.1 0.5 0.8 180 53 480 0.04 0.26 0.06 0.15 0.51St dev. 0.1 26 0.2 0.0 0.2 0.6 0.9 6.0 0.00 0.02 0.00 0.01 0.24

C 7 Average 47 400114 1.6 210 24 0.05 82 0.01 0.14St dev 4.2 0.6 8.3 0.6 0.08 5.8 0.03 0.05

C 8 Average 0.2 1510 398013 0.2 1.5 170 26 222 0.02 0.17 0.03 0.10 0.03St dev. 0.3 81 0.3 0.4 14 2.0 58 0.04 0.25 0.06 0.18 0.05

C 10 Average 35.7 727 399139 0.2 0.6 93 44 0.04 170 0.04 0.27 0.07 0.20 0.53St dev. 1.3 2.9 0.0 0.1 1.0 3.0 0.04 2.1 0.00 0.01 0.00 0.03 0.19

C 11 Average 31.8 760 399136 0.2 1.6 96 25 144 0.04 0.30 0.09 0.25 0.17St dev. 0.6 7.8 0.0 0.3 1.8 1.4 2.0 0.01 0.03 0.01 0.05 0.15

C 12 Average 1.9 290 399774 2.6 0.1 0.6 4.1 105 0.02 228 0.17 0.89 0.19 0.43 2.31St dev. 0.2 43 2.9 0.1 0.4 5.3 46 0.05 70 0.09 0.44 0.11 0.21 0.97

C 13 Average 26.9 1150 397142 108.1 0.8 1180 670 0.16 0.06 102 0.70 4.37 1.02 2.72 0.33St dev. 1.7 81 38.2 0.2 160 880 0.27 0.06 3.7 0.64 3.45 0.74 1.89 0.09

C 14 Average 89.8 2390 338534 0.2 1.9 69500 150 66 0.47 2.64 0.56 1.34 0.43St dev. 45.6 300 0.2 0.6 1860 2.5 5.1 0.08 0.25 0.09 0.23 0.08

C 15 Average 45.6 950 398765 0.3 0.9 120 28 238 0.06 3.45St dev. 1.4 20 0.0 0.9 0.6 2.6 4.0 0.06 2.00

C 16 b500 N399960 b10 b5 b1000C 17 Average 200 5108 392840 2.1 2.7 25 62 0.4 0.10 195 0.04 0.3 0.07 0.18 0.4

St dev. 60.4 161 0.5 0.5 0.2 3.5 0.7 0.08 2.5 0.00 0.03 0.01 0.02 0.02C 18 Average 33.6 37 400266 0.1 0.8 1.0 68 2.6 14 0.04 0.04 0.05 0.01 1.4

St dev. 7.8 2.4 0.1 0.2 0.5 4 2.5 0.6 0.02 0.00 0.07 0.02 1.02U 1A Average 159 276 381461 0.6 1.2 11880 9860 0.02 127 3.2 26.8 6.7 22 1.3

St dev. 23.1 4.5 0.2 0.1 145 650 0.04 19 1.4 12.5 3.2 10.8 0.5U 1B Average 184 273 380156 0.7 0.9 12370 10792 0.15 160 3.8 32.1 7.9 26 0.9

St dev. 101.6 14.6 0.2 0.1 198 240 0.20 6.2 0.23 2.2 0.8 3.43 0.26

Averages and standard deviations of the 3 or 4 analyses are given in ppm. Composition of C16 (powder) was given from manufacturer.

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63–73

Ba

1.720.19na

83.70.83na

na

0.050.04na

na

0.010.02na

1.441.280.110.03na

na

b501.90.30.140.093.61.36092

Fig. 1. IRM acquisition curves. (a) Samples with low-coercivity component only, (b) samples with high-coercivity component.


different from the KLY-4S measurements. Because themeasured susceptibilities were usually negative, theprincipal susceptibilities had to be reordered because ofthe automatic ranking by their absolute values (Hrouda,2004). It should be noted that the mean susceptibilitymeasurements with the KLY susceptibility bridge aremade in air. To obtain correct results for samples withvery weak magnetic susceptibility, the values have to becorrected by the susceptibility of air. Air is paramagneticand its susceptibility was determined to be 0.38×10−6

Table 3Susceptibility parameters from high- and low-field measurements and percferromagnetic component (ferro) to the high-field AMS

Sample Low-field measurements High-field measu

kM [m3/kg] k1−k3 [m3/kg] U k1−k3 [m3/kg]

Group 1 C16 −4.37E−09C17 −4.36E−09 3.84E−10 0.79 3.71E−10C12 −4.24E−09 4.09E−10 0.97 3.99E−10C10 −4.14E−09 4.18E−10 0.77 4.05E−10C11 −4.13E−09 4.26E−10 0.90 4.04E−10C5 −4.12E− 09 4.03E−10 0.98 4.09E−10

C18 −4.06E−09C15 −4.04E−09 4.08E−10 0.96 4.10E−10C6 −3.86E−09 4.33E−10 0.89 4.06E−10

C8 −3.84E−09 5.12E−10 0.74 4.05E−10

C7 −3.32E−09 4.58E−10 0.99 4.04E−10

Group 2 C2 −1.10E−10 3.91E−10 −0.22 4.07E−10C13 2.93E−09 1.08E−09 −0.90 9.21E−10C4 1.21E−08 5.52E−11 −0.75 3.94E−10C1B 2.16E−08 8.45E−09 −0.89 8.66E−09C1A 2.21E−08 8.44E−09 −0.98 8.47E−09U1A 6.96E−08 9.22E−09 −0.87 9.39E−09U1B 7.00E−08 9.27E−09 −0.90 9.57E−09C14 2.04E−07 4.62E−10 0.41

Mr is the isothermal remanent magnetization at 2.5 T, and Hcr the remanent

(SI), in agreement with values cited in the literature(Nakagawa et al., 1999). All values given in this studyare corrected for air. This may not be the case in earlierstudies.

The high-field AMS was determined at room tem-perature with a torsion magnetometer at the LNM at theETH Zurich (Bergmüller et al., 1994). This devicemeasures susceptibility differences in a plane, thereforeonly the deviatoric part of the susceptibility tensor isobtained. The sensitivity of the instrument is 5×10−8 Nm,

entage of contribution of the dia/paramagnetic component (para) and

rements IRM acquisition

U Para [%] Ferro [%] Mr at 2.5 T [A/m] Hcr [mT]

0.86 99±6 1±4 4.2E−04 550.98 99±2 1±2 3.4E−02 3500.98 98±6 2±4 1.8E−04 350.99 99±4 1±3 3.5E−04 501.00 98±10 2±9 7.3E−04 50

before cleaning: 1.6E−033.6E−03 35

0.98 99±5 1±3 4.1E−04 550.98 99±13 1±10 4.4E−04 50

before cleaning: 1.5E−030.92 96±6 4±4 1.6E−04 40

before cleaning: 7.2E−040.98 97±13 3±7 9.1E−05 35

−0.96 94±5 6±3 4.0E−02 60−0.93 100±7 0±3 1.1E−02 6500.96 97±8 3±5 1.0E−01 15

−0.82 100±2 0±1 4.9E−04 50−0.99 99±3 1±2 6.6E−04 90−0.88 99±6 1±3 5.0E−02 75−0.94 99±8 1±4 1.6E−02 70

1.0E−03 45

coercivity. Samples are ordered by increasing susceptibility.

Fig. 2. Mean susceptibility plotted against iron and manganese content.Inset: enlarged view for 0–2000 ppm.


which allows for measurements of susceptibility dif-ferences of 5×10−9 (SI) for a sample size of 10 cm3.Because of the weak anisotropy of the samples it wasimportant to measure the background signal of thedevice and subtract it from the torque curves.

Four to seven fields between 800 and 1700 mT wereused to measure each crystal. This allows for the sep-aration of any contribution from ferrimagnetic inclu-sions, since their magnetization will be saturated(Martin-Hernandez and Hirt, 2001). The remainingsignal is due to diamagnetic, paramagnetic and an-tiferromagnetic contributions to the AMS. Plotting thetorque amplitude as a function of applied field isindicative of the presence of unsaturated antiferromag-netic minerals, e.g., hematite. Their contribution to theAMS can be separated, but measurement in more fieldsis necessary (Martin-Hernandez and Hirt, 2004).

Susceptibilities were normalized with the mass of thesamples. The given volume susceptibility values weredetermined based on the density of pure calcite, which is2710 kg/m3 (Gaines et al., 1997).

3. Results and interpretation

3.1. Chemical composition

The results of the chemical analysis are summarizedin Table 2. Standard deviations of the determinedelement concentrations are low, which demonstratesthat the element distribution in the crystals is homoge-neous. The total content of impurity elements rangesfrom approximately 180 to 72,300 ppm. The concentra-tion of paramagnetic ions Fe and Mn is between 70 and70,000 ppm. Here it is important to note that for elevencrystals this concentration was very low (b250 ppm).The concentration of Mg (C17), Sr (C2) or Mn (C14)can be high in individual crystals. The concentrations ofsodium, vanadium, yttrium and cerium were general-lyb100 ppm. Concentrations of all other analyzedelements were determined to be below 100 ppm. Themineral/chondrite patterns of the rare earth elementswere typical for calcite, which indicates that theanalysed material was ablated from the calcite crystalsitself and not from possibly intergrown mineralinclusions.

3.2. IRM acquisition

IRM acquisition curves of almost all samples show arapid increase in magnetization at low fields andsaturation is reached below 500 mT, mainly below 300mT (Fig. 1). The shape of the curve indicates a low

coercivity ferromagnetic component, possibly due tomagnetite and/or maghemite. Three samples (C14, U1A,U1B) show in addition to this low coercivity componenta high coercivity component. The IRM of another threesamples (C1A, C12, C13) is dominated by a highcoercivity component as seen from a high Hcr (Table 3),which does not saturate below 2000 mT. However, inevery case the strength of the IRM indicating the amountof the ferromagnetic phases was very low and did notaffect the high-field torsion measurements. The mea-sured torque in high fields was linear as a function of thesquare of the applied field (B2) for every sample.Nevertheless, no samples with an evident high-coerciv-ity phase were considered in the determination of theintrinsic magnetic properties of calcite.

The IRM at 2500 mT is very low and indicates therelative amount of magnetic phases present in thesamples (Table 3). The majority of the samples show anIRM less than 0.005 A/m. Three samples of this groupwere also measured before cleaning. The cleaningprocedure reduced the magnetization drastically, butdid not change the shape of the IRM acquisition curve.Therefore, we assume that these very low magnetiza-tions can be attributed to adhesive particles and do notindicate crystalline inclusions in the samples. Sampleswith stronger IRM probably contain ferromagneticinclusions and were therefore not used for thedetermination of the magnetic properties of calcite.Sample C4 provides the strongest magnetization with avery low coercivity (Hcr=15 mT), which probablyindicates fine grained magnetite. These sizes of inclu-sions can hardly be detected by Laser Ablation ICP-MS.

3.3. Bulk susceptibility with theoretical calculation

The bulk susceptibilities of the samples range from−4.37 to 204×10−9 m3/kg (Table 3). Twelve of the

Fig. 3. Measured mean susceptibility (full symbols) and calculatedparamagnetic susceptibility (open symbols). Samples are ordered byincreasing susceptibility (see Table 2), every second labeled. Inset: firstten samples with stretched susceptibility axis.


nineteen samples are diamagnetic, and eight samples havesusceptibilities between −4.5 and −4.0×10−9 m3/kg.This wide range of susceptibilities shows that, apartfrom the diamagnetic nature of pure calcite, para- andferromagnetic impurities must contribute to the suscep-tibility. Divalent paramagnetic ions, mainly Fe2+ andMn2+ (Table 2), substitute for Ca2+ and increase thesusceptibility. This is illustrated by the good linearcorrelation between susceptibility and the contents ofiron and manganese (Fig. 2).

Knowing the chemical composition of the crystals,the theoretical paramagnetic susceptibility can becalculated according to the Curie law of paramagneticsusceptibility (Bleil and Petersen, 1982). For thecalculation the concentrations of Fe, Mn, V, Ti, Cr, Ni,Cu, and rare earth elements were considered. Thecalculation assumes that all ions substitute for divalentcalcium and hence are divalent and, furthermore, that

Fig. 4. Directions of minimum andmaximum susceptibilities relative to the c-ax

there is no effective interaction between the magneticions.

The calculated and measured susceptibilities agreewell as shown in Fig. 3. Note that the calculated valuesdo not include the diamagnetic susceptibility of calcite,and thus must be higher than the measured values by thisamount. The good agreement between measured andtheoretical values justifies the assumption that the para-magnetic ions are divalent. Fe and Mn ions contributemore than 95% to the paramagnetic susceptibility foreach crystal, and therefore have the greatest influence.The increase of susceptibility is 2.3×10−10 m3/kg per100 ppm Fe and 3.4×10−10 m3/kg per 100 ppm Mn.The paramagnetic susceptibility of almost all samplescan be quantitatively related to their content of para-magnetic ions. For samples C2, C13, C4, U1A, and U1Bthe measured susceptibility is higher than calculated,because an additional ferromagnetic contribution to thesusceptibility is present as shown by IRM acquisition.The successful calculation of the susceptibility from thechemical composition for natural crystals of calcitemakes it generally possible to deduce the chemicalcomposition from magnetic susceptibility, when noferromagnetic contributions exist.

3.4. Low- and high-field AMS

The AMS results can be subdivided into two groups(Table 3). The first consists of pure crystals with lowsusceptibility that have a virtually perfect oblate AMSshape with k3 subparallel to the crystallographic c-axis,as expected for pure calcite (Fig. 4a). The second groupis composed of impure crystals with higher susceptibil-ity, which— except for C4— show an almost perfectlyprolate AMS shape with k1 subparallel to the c-axis(Fig. 4b).

is (inclination 90°), samples of (a) group 1 and (b) group 2 (see Table 2).

Fig. 5. Sample C15, (a) torque per unit volume as a function of measurement angle, (b) amplitude of the 2θ-term as a function of B2. Planes 1 and 2 aresubparallel, plane 3 normal to the crystallographic c-axis.


In the first group the deviation of the k3 directionfrom the c-axis is within 2.0° on average for high-fieldAMS and 3.4° for low-field AMS, which is within theuncertainty range of the measurement. In high-fieldmeasurements the torque is dominated by a 2θ-signal(Fig. 5a) and is linear to B2 with zero intercept (Fig. 5b);therefore there was no significant ferromagnetic contri-bution (Table 3). Hence, the susceptibility differencesmeasured in low and high fields are of comparablemagnitude (Fig. 6), although on close inspection thelow-field AMS tends to be higher. The U factors fromthe low-field measurements cannot be regarded asreliable, because the anisotropies were close to themeasurement limits of the instrument. In the high-fieldmeasurements the U-parameter was very close to 1.00(Fig. 7). The U-parameter of sample C17 is relativelylow and the only reasonable explanation is that C17 isnot a single crystal. Therefore, this sample was excludedfrom further consideration. For the rest of the group,U is0.98 on average. Lower absolute values of theU-parameters can arise from slight misorientationbetween planes. This can also lead to an overestimationof the degree of anisotropy. For this reason the U factorwas used as a measure for the quality of the

Fig. 6. Susceptibility differencesΔk of diamagnetic samples measuredin low and high fields.

measurement. Values close to +1 or −1 indicate aprecise measurement.

In the second group the deviation of the k1-directionfrom the c-axis is only 2.2° on average. In high-fieldmeasurements the torque was linear to B2 and nosignificant ferromagnetic contribution to the AMS wasfound except for C2. The susceptibility differencesmeasured were similar for low- and high-field measure-ments. The AMS shape was nearly perfectly prolate,particularly in high-field measurements. This shape canbe explained by the substitution of Ca by Fe ions in thecrystal lattice. Thereby the composition of calcitechanges slightly towards that of siderite, which has astrong crystalline anisotropy of prolate shape with thehighest susceptibility along the c-axis (Jacobs, 1963).Iron-bearing calcite can thus be regarded as calcite witha small amount of siderite with subparallel c-axes. The

Fig. 7. Modified Jelinek plot for low-field AMS (closed symbols) anddia/paramagnetic part of the high-field AMS (open symbols). Insetshows values of the purest crystals.

Fig. 8. High-field susceptibility difference versus iron content. Inset:Enlarged view of samples with low Fe-content.


prolate AMS of siderite is about 1500 times strongerthan the oblate AMS of calcite, which is why even smallamounts of iron can control the AMS of the calcite. TheAMS of sample C2 (890 ppm Fe) is already dominatedby iron. Its shape is prolate (U=−0.96), but it still showsa negative susceptibility.

Sample C4 has an oblate AMS shape and a suscep-tibility difference close to that of the pure calcite. Its highsusceptibility results from the ferromagnetic inclusionsfound by IRM acquisition. However, these inclusions donot have a significant anisotropy, which is indicated by theinsignificant ferromagnetic part of the AMS (Table 3).SampleC14 contains a high amount ofMnwhich increasesthe bulk susceptibility but does not change the anisotropysignificantly. Because the sample showed skewed torquecurves the high-field AMS was not determined.

These examples show that the AMS is independentfrom the mean susceptibility. AMS is thus more appro-priately described with parameters that are not dependenton the susceptibility, e.g., susceptibility difference andU.

4. Discussion

4.1. Intrinsic magnetic properties of pure calcite

With the above results, the intrinsic susceptibility ofpure calcite was determined by subtracting the calcu-lated contribution due to paramagnetic ions from themeasured susceptibility. The intrinsic susceptibility ofthe ten purest samples with less than 250 ppm Fe andMn and negative bulk susceptibility was calculatedusing this procedure. The average of these values can beregarded as the intrinsic susceptibility of pure calcite. Itsvalue is −4.46±0.16×10−9 m3/kg, which is equivalentto −12.09±0.5×10−6 (SI). Without correction for airthe value was determined to be −12.47±0.5×10−6 (SI),which is in good agreement with the value provided byVoigt and Kinoshita (1907) of −12.87×10−6 (SI).

To determine a value for the intrinsic susceptibilitydifference Δk the values of the crystals with less than250 ppm iron and manganese and U-parameterbetween 0.98 and 1.00 were averaged. Only thesamples C5, C6, C7, C10, C11, and C15 fulfilled theseconditions. Their average susceptibility difference is4.06±0.03×10−10 m3/kg, which is equivalent to 1.10±0.01×10−6 (SI). This leads to values for k1=k2=−4.32×10−9 and k3=−4.73×10−9 m3/kg.

The average susceptibility difference is slightly lowerthan 1.19×10−6 (SI) reported by Owens and Rutter(1978). The reason for their higher value could be animperfect orientation of the samples, which is indicated bythe lower U-parameter of 0.93. Together with the above-

determinedmean susceptibility, the susceptibility ratio k1 /k3 is 1.097, which agreeswith the value of Tyndall (1851).

4.2. Chemical composition and AMS

An interesting question is whether there is aquantitative relationship between AMS and chemicalcomposition. A good correlation of susceptibility differ-ence and iron content was found (Fig. 8). For calcites withFe concentration between 500 and 10,000 ppm it appearsto be possible to calculate the paramagnetic contributionto Δk from the Fe content. The empirical relationship is

Dkpara ðm3=kgÞ ¼ Fe� content ðppmÞ � ð1F0:1Þ� 10−12ðm3=kg=ppmÞ:

For crystals with less than 100 ppm iron the AMSshows no dependence on the iron content (Fig. 8, inset).It is possible that the Fe2+ ions are too dispersed withinthe lattice and do not systematically interact to producean AMS. Further work would be needed to test thishypothesis. However, the absence of a correlationbetween AMS and Fe-content for these pure crystalsjustifies our method for determining the intrinsicsusceptibility difference of calcite in Section 4.1.

5. Conclusions

Room-temperature susceptibility and anisotropy ofnatural calcite crystals from different localities werecharacterized with a combination of measurementtechniques. The magnetic properties of natural calciteare shown to be highly dependent on chemicalcomposition, and are the result of dia-, para- andferromagnetic contributions. The intrinsic diamagneticsusceptibility and susceptibility difference of purecalcite were determined to be −4.46±0.16×10−9 m3/kg(−12.09±0.5×10−6 SI) and 4.06±0.03×10−10 m3/kg


(1.10±0.01×10−6 SI), respectively. The diamagneticproperties are very small and are therefore easilydominated by other components. Paramagnetic ions inthe crystal lattice increase the susceptibility andanisotropy. The paramagnetic susceptibility was corre-lated to the concentration of paramagnetic ions deter-mined by LA-ICP-MS. Iron and manganese increasethe susceptibility, whereas the paramagnetic anisotropycorrelates with the iron content. On the other hand,contributions due to inclusions of ferromagnetic min-erals can dominate the susceptibility, as also shown inHrouda et al. (2000). The ferromagnetic contribution tothe anisotropy can be separated by high-field methods.

The identified correlations between susceptibilityand chemical composition, as well as between ironcontent and AMS, give helpful constraints for theinterpretation of AMS results in rocks, in which calciteis responsible for the observed magnetic anisotropy.

Parameters, which are dependent on the bulksusceptibility, are affected by compositional changesof the minerals. For this reason the maximum sus-ceptibility difference Δk and the U-parameter are ap-propriate parameters to characterize the diamagneticAMS of carbonate rocks. For the characterization of theAMS of carbonate rocks, the measurement in high fieldsis recommended. When the calcite is sufficiently pure,the ferromagnetic contribution can be eliminated andparamagnetic impurities below 100 ppm appear to haveno effect on susceptibility differences. In general, wepropose that good AMS results can be obtained fromcalcitic rocks when their bulk susceptibility is negative.

Acknowledgements

This work was supported by the Swiss NationalScience Foundation, Project No. 200020-100224. Wethank Dr Peter Brack from the Institute of Mineralogyand Petrology, ETH Zurich, for providing us withseveral carbonate crystals, Kathrin Hametner from theInstitute of Inorganic Chemistry, ETH Hönggerberg, forthe LA-ICP-MS measurements, and Dr Agnes Kontnyfrom the Institute of Geology and Palaeontology,University of Heidelberg, for the use of the KLY-4Ssusceptibility bridge. We also thank two anonymousreviewers for their constructive comments. Contributionno. 1427, Institute of Geophysics, ETH Zurich.

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