fabrication of synthetic calcite–muscovite rocks with variable texture — an analogue to...

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Tectonophysics 449 (2

Fabrication of synthetic calcite–muscovite rocks with variable texture — Ananalogue to cataclasite fabrics?

Volkmar Schmidt a,⁎,1, Luigi Burlini b, Ann M. Hirt a, Bernd Leiss c

a Institute for Geophysics, ETH Zurich, CH-8093 Zürich, Switzerlandb Institute for Geology, ETH Zurich, CH-8092 Zürich, Switzerland

c GZG, University of Göttingen, D-37077 Göttingen, Germany

Received 18 August 2006; received in revised form 24 November 2007; accepted 30 November 2007Available online 15 December 2007

Abstract

A series of large diameter calcite–muscovite aggregates has been prepared from calcite and muscovite powders, in order to gain a betterunderstanding of how texture develops in impure carbonate rocks. The development of the microstructure and the crystallographic preferredorientation (CPO, texture) during the preparation process is described. The synthetic rocks have been fabricated from powders of calcite andmuscovite by uniaxial cold-pressing at loads up to 400 MPa and subsequent hot isostatic pressing (HIPping) at pressures of 150 to 170 MPa and atemperature of 670 °C. The resulting textures and microstructures are homogeneous throughout the samples. The calcite CPO is generated by rigidbody rotation and twinning during cold-pressing and is not significantly altered by recrystallization during HIPping. Grain growth during HIPpingis observed in pure calcite samples, but is inhibited through high porosity and the presence of muscovite in the mixed aggregates. The preferredorientation of the calcite c-axes is found to increase with increasing uniaxial cold pressure, and to be independent of the muscovite content. Themagnetic bulk susceptibility of the starting material has been changed by the formation of ferromagnetic impurities during fabrication. Comparisonof the samples to natural calcite fabrics from fault zones show the potential of the experiments and fabric analyses presented to analyze and tobetter understand the deformation mechanisms of fault zones.© 2007 Elsevier B.V. All rights reserved.

Keywords: Crystallographic preferred orientation; Texture; Calcite; Muscovite; Compaction; Cataclasite

1. Introduction

Carbonate rocks can show pronounced anisotropic physicalproperties due to crystallographic preferred orientation (CPO),also called texture (cf., Owens and Bamford, 1976, Wenk et al.,1987). The influence of the CPO on the physical properties, suchas seismic and magnetic anisotropy (AMS) and thermaldilatation of carbonate rocks, is of great interest and has beeninvestigated in numerous studies (e.g., Owens and Rutter, 1978;Burlini et al., 1998; Khazanehdari et al., 1998; Burlini andKunze, 2000; Leiss andWeiss, 2000). A systematic study of rockproperties as a function of CPO requires a well-characterized

⁎ Corresponding author. Fax: +41 44 633 1065.E-mail address: [email protected] (V. Schmidt).

1 Now at: Kali-Umwelttechnik GmbH, D-99706 Sondershausen, Germany.

0040-1951/$ - see front matter © 2007 Elsevier B.V. All rights reserved.doi:10.1016/j.tecto.2007.11.055

starting material. For pure calcite rocks, Carrara marble andSolnhofen limestone are often used as natural standards. Fortwo-phase rocks, however, it is difficult to find such standards innature. Therefore, we present in this study a method to prepareuniform and easily reproducible calcite–muscovite rocks withCPOs of variable strength. The fabric characterization of thesesynthetic samples provide information on (1) deformationmechanisms and the development of compaction textures andmicrostructures related to cold-pressing and on (2) microstruc-tural recovery processes of cataclastically deformed calcite andcalcite/muscovite fabrics related to hot-pressing. Especially theresults of the latter processes help to better understand com-parable fabrics in nature.

A series of two-phase samples has been prepared from calciteandmuscovite powders in varying proportions. The samples werecompacted by cold uniaxial pressing to produce an initial CPO,and then annealed by hot isostatic pressing (HIP), which reduces

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106 V. Schmidt et al. / Tectonophysics 449 (2008) 105–119

the porosity and is expected not to alter the CPO (Barnhoorn et al.,2005). The large volume of the samples allows for severalspecimens to be obtained from each sample for subsequentmeasurements and testing. In this paper, the homogeneity of thesamples is verified, their magnetic properties are characterized,and the progressive change of microstructure (grain size, shapeanisotropy, porosity) and texture related to increasing compactionand muscovite content is described.

2. Methods

2.1. Description of material

Calcite powder was prepared from: (a) pure reagent gradecalcite powder, and (b) by grinding a piece of Carrara marble.The chemically pure reagent grade calcite powder (Riedel-deHaën, Germany, No. 31208) was sieved to remove the fractionabove 100 µm. The material has been characterized byBarnhoorn et al. (2005). The Carrara marble was taken fromthe same block from the Lorano quarries described by Pieriet al. (2001) and consists of 98% calcite with occasional grainsof quartz, white mica, dolomite, epidote and pyrite. For themagnetic measurements, it was essential to avoid introducingadditional traces of iron during the preparation process. In thefirst step, the block was cut into plates of about 2 cm thicknesswith a stainless steel diamond saw blade. These plates werecrushed to gravel fraction with a hammer and a hydraulic press.During crushing, the marble was covered with two plates ofstainless steel to avoid contamination through abrasion of ironfrom the hammer or the press. The marble was then milled inan agate mill and sieved to remove the fraction N100 µm.The resulting powder shows a wide grain size distribution,which can favor a uniform grain-size distribution after HIPping(Zhang et al., 2000).

Muscovite was purchased in three batches from differentmineral dealers, referred to as Mu1, Mu2, and Mu3. Mu1 andMu2 were pegmatitic crystals of about 5 cm size; Mu3 consistedof small flakes of 1 mm size. The total iron content, whichinfluences the magnetic properties, was determined by laserablation ICPMS. The analysis showed no significant differencesbetween muscovite from the three batches, in which 11,000 ppm

Table 1Overview of synthesized samples of calcite (Cc) and calcite+muscovite (Mu) uniax

Hand pressed 20 MPa

Chemical calcite syn-5 syn-20100% Calcite (Cc) 0–5 0-20L

0-20Cc with 5% Mu1 5–5 5–20

Cc with 10% Mu1 – 10–20Cc with 30% Mu2 – 30–20Cc with 50% Mu2 – 50–20ACc with 50% Mu3 – 50–20BCc with 70% Mu3 – 70–20100% Mu3 – 100–20

Calcite powder is from Carrara marble, except for those in the first row (samples syn-xproduced twice, since a leak occurred on the first Hot Isostatic Pressing.

Fe was found for single crystals of Mu1 and Mu3, and about10,000 ppm for Mu2. There was a high variation, however, inFe-content for Mu3 with values between 8000 and 16,000 ppm.The muscovites Mu1 and Mu2 were cleaned in ethanol and withan ultrasonic cleaner to remove adhering particles. Large crystalswere cut down to a size of less than 1 cm andmilled with an agatemill for several minutes. Mu3 could be milled directly. Finally,the fraction N100 µm was removed by sieving.

Mixtures of calcite and muscovite with seven different vol-ume ratios between 0 and 100% (Table 1) were prepared.Therefore, the muscovite content in these samples is muchhigher than in the content in previous studies, e.g., Olgaard andEvans (1988). The samples with 50% calcite were preparedtwice using different batches of muscovite, which gave a total of40 samples.

2.2. Room-temperature uniaxial press and high-temperatureisostatic press

Before pressing, the powder was dried for at least 1 h in adrying oven at 120 °C. The reagent grade calcite powder wasdried for at least 24 h. The powder was cold-pressed in stainlesssteel jackets (Stainless 304, 18Cr 10Ni; W.-Nr. 1.4301) bystepwise filling and pressing of small portions at the same load toachieve a homogeneous compaction. The pressing was donewith an Enerpac-H-Frame 50-tons-press up to a maximumpressure of 400 MPa. The hand-pressed samples were pressedmanually only. The respective maximum load during the press-ing process was held for about one second. The jackets had twodifferent diameters: 5.12 cm for samples pressed up to 100 MPa,and 3.65 cm for samples pressed at 200 MPa or 400 MPa. Thelength of the cylinders was 20.30 cm, thus the total samplevolume was about 420 cm3 and 210 cm3, respectively. For puremuscovite samples, a cylinder with only the half-length wasused. The cold-pressed samples were stored for 12 h at 120 °Cuntil the jackets were welded (TIG-welding-process). Dueto leakage during HIPping, four samples (0–20L, 0–400L, 5–200L, and 5–400L) had to be prepared anew. Therefore, forsubsequent samples the quality of the sealing was tested in awater bath under low pressure. Finally, the stainless steel jacketswere sandblasted to clean them before the hot pressing. The

ially cold-pressed at different pressures

40 MPa 100 MPa 200 MPa 400 MPa

syn-40 syn-100 syn-200 syn-4000–40 0–100 0–200 0–400L

0–4005–40 5–100 5–200L 5–400L

5–200 5–400– 10–100 10–200 –– 30–100 30–200 –– 50–100A 50–200A –– 50–100B 50–200B –– 70–100 70–200 –– 100–100 100–200 –

), where artificial chemical powder was used. Note that some samples have been

Table 2Properties of specimens after cold-pressing (C-P) and after HIPing

Specimen Muscovitecontent (%)

Load atC-P (MPa)

Density afterC-P (g/cm3)

Porosity afterC-P (%)

Density afterHIP (g/cm3)

Porosity afterHIP (%)

Magnetic Susceptibilityafter HIP (10−9 m3/kg)

Average grain sizeof calcite (μm)

Notes

syn-5-0 0 b5 0.83 69.4 Friablesyn-20-0 0 20 1.44 46.9 – – −3.63 Friablesyn-40-0 0 40 1.57 42.1 – – −4.27 Friablesyn-100-0 0 100 1.73 36.2 – – −4.24 Friablesyn-200-0 0 200 1.98 26.9 – – −4.06 Friablesyn-400-0 0 400 2.19 19.3 – – −4.17 Friable0-5-0 0 b5 1.78 34.3 2.03 25.2 −3.98 2.40-20L-0 0 20 – – 2.07 23.6 100 Leak0-20-4 0 20 1.91 29.5 2.61 3.7 −3.47 4.80-40-0 0 40 1.99 26.6 2.63 3.0 −3.80 3.80-100-1 0 100 2.12 21.8 2.62 3.5 −3.40 4.20-200-0 0 200 2.24 17.3 2.61 3.7 −3.48 4.80-400L-0 0 400 – – 2.23 17.7 −1.32 Leak0-400-1 0 400 2.30 15.1 2.62 3.5 −3.35 7.85-5-0 5 b5 – – 2.33 14.1 −0.545-20-0 5 20 – – 2.56 5.6 −1.745-40-2 5 40 1.94 28.6 2.57 5.4 −0.865-100-4 5 100 – 24.9 2.60 4.4 −0.495-200L-3 5 200 – – 2.15 20.9 −0.90 Leak5-200-2 5 200 2.26 15.7 2.55 6.3 0.39 2.75-200-R 5 200 2.26 15.7 2.56 5.6 −1.065-400L-3 5 400 – – 2.22 18.2 −1.88 Leak5-400-2 5 400 2.32 10.1 2.58 5.0 −1.3610-20-3 10 20 – – 2.49 8.6 3.8010-100-2 10 100 2.12 22.1 2.52 7.5 3.0610-200-3 10 200 – – 2.49 8.4 4.00 2.110-200-R 10 200 – – 2.52 7.4 3.0330-20-2 30 20 1.67 39.1 2.33 15.0 17.530-100-2 30 100 1.96 28.5 2.37 13.5 16.630-200-2 30 200 2.10 23.4 2.41 12.2 16.9 2.750-20A-2 50 20 1.65 40.3 2.27 18.0 21.650-100A-2 50 100 1.93 30.2 2.30 16.7 21.050-200A-2 50 200 2.08 24.8 2.32 16.0 21.0 2.350-20B-S 50 20 1.74 37.1 2.14 22.6 39.650-20B-SR 50 20 1.74 37.1 2.14 22.7 38.850-100B-S 50 100 2.01 27.3 2.21 20.0 37.050-200B-S 50 200 2.10 24.1 2.24 19.0 36.9 2.970-20-0 70 20 1.75 37.2 2.10 24.5 35.170-100-0 70 100 2.12 23.9 2.19 21.5 35.770-100-R 70 100 2.12 23.9 2.17 22.0 36.270-200-0 70 200 2.23 20.0 2.24 19.7 34.0 2.870-200-R 70 200 2.23 20.0 2.28 18.1 34.3100-20-0 100 20 1.92 31.9 2.42 14.1 36.2100-100-0 100 100 2.23 20.9 2.48 12.1 36.2100-200-0 100 200 2.29 18.8 2.49 11.9 36.1

The muscovite content given does not consider the porosity. ‘Load’ is the pressure used during uniaxial cold pressing in the first stage of the preparation, which wasdone at room temperature. ‘leak’ indicates leakage in the jackets during HIPping. Properties after C-P refer to the whole sample.

107V. Schmidt et al. / Tectonophysics 449 (2008) 105–119

cylinders were hot isostatically pressed (HIPped) with a largevolume internally heated gas apparatus (Abra). They wereHIPped for 3 h at an isostatic pressure of 150 to 170MPa and at atemperature of 670 °C.

To distinguish the effects of the cold-pressing from those ofthe HIPping, four additional samples were prepared from thecalcite powder ground from Carrara marble. These sampleswere only uniaxially cold-pressed at hand-pressure, 40, 100 and400 MPa, respectively. Since the resulting samples were veryfragile, they were soaked with resin to allow for the preparationof thin-sections.

2.3. Sample description and density measurements

After HIPping or impregnating, respectively, the cylinderswere cut in half normal to the cylinder axis and cylindrical coresof 2.54 cm diameter and 7 to 10 cm length were drilled from themiddle part of the pressed material parallel to the compressionaxis. The material close to the jacket was not used and thuspossible fringe effects were avoided. These cores were used toprepare thin sections and specimens for magnetic and texturemeasurements. The samples made from reagent grade calcitepowder were very friable even after HIPping. They could not be

Fig. 1. (a) Dependence of the degree of compaction of the cold-pressed sampleson the axial load. (b) Dependence of the porosity of HIPped samples on themuscovite content for three different uniaxial cold pressings. Trend lines show apolynomial fit to the data.


used for density measurements and the fabrication of thinsections. For magnetic measurements, standard-size cylindersof 2.54 cm diameter and a length of 2.2 cm were drilled out. Thenames of the specimens are composed of the sample name and asuffix (Table 2). For the sake of simplicity, in the followingsections we refer to the specimens using their muscovite orcalcite content without considering the porosity.

The densities of the samples were determined from theirdimensions and weights. The density of the HIPped samples wasdetermined for each specimen, the density of the cold-pressedsample for the sample only. Porosities were calculated by com-parison of these densities to the theoretical single crystal densities

Fig. 2. a. Micrographs of impregnated pure calcite samples uniaxially cold-pressedsections with crossed polarizer. Compression direction is vertical; the scale bar is 10pressures taken from ultra-thin sections with crossed polarizer. Compression directiHIPped samples with 5% muscovite at different cold uniaxial pressures taken fromscale bar is 100 μm. Annealing time: 3 h. d. Micrographs of HIPped samples with 30taken from ultra-thin sections with crossed polarizer. Compression direction is verticaluniaxially cold pressed at 200 MPa with 50% and 100% muscovite content taken fromscale bar is 100 μm. Annealing time: 3 h.

of calcite (2710 kg/m3) and muscovite (2820 kg/m3). The degreeof compaction has been calculated from the densities, whereas avalue of 100% corresponds to total compaction, i.e., no porosity.

2.4. Microstructural analysis

Ultra-thin sections were made from all HIPped samples formicrostructural analysis. Sections were cut in the middle of thecylinders and parallel to the cylinder axis, which corresponds tothe compression direction during the initial cold uniaxialpressing. Both sides of the thin sections were polished to avery fine polishing grade (0.05 mm alumina suspension) in orderto obtain ultra-thin sections (1–5 μm thick). Two-dimensionalgrain size (diameter of a circle with equivalent area) histogramswere calculated from drawings of grain boundary outlines,traced from light micrographs. On average, 1100 grains persection were analyzed. No corrections for sectioning effects ofthe three-dimensional grains were performed in order to allowdirect comparison with previous work. The shape-preferredorientation (SPO) of the grains was determined by fitting ellipsesto the digitized grains using the image analysis program ImageJ1.34 s (Rasband, 1997–2006). Because of the uncertainties intracing grain boundaries of very small grains, only grains with anequivalent diameter ≥1 μm were considered. The orientationdistributions of the long axes of the grains were plotted in rosediagrams, where the sector is proportional to the number ofgrains with its long axis in the respective direction. The volumeand area fractions histograms have been calculated using theprogram StripStar by Heilbronner and Bruhn (1998).

2.5. X-ray texture analysis

X-ray texture measurements were carried out on a PANalyticalX-ray diffractometer (X'pert PRO MRD, PW3040) especiallyconfigured for the texture analysis of rock samples (Leiss, 2005;Leiss and Ullemeyer, 2006). A primary, optically parallel X-raybeamwith a diameter up to 7 mm and high intensity generated bya polycapillary allows relative large sample volumes to bemeasured within a short time. The CPOs were measured on a 5°by 5° grid and plotted as pole figures. A short exposure time of 1 sper grid point is accomplished by using 40 kVand 40 mA. On thesecondary beam side, a parallel plate collimator with a divergenceof 0.27° and a proportional detector were installed. To evaluatetexture homogeneities, three cylindrical specimens used for mag-netic measurements were cut normal to the cylinder axis into twodisks. Four pole figures for each specimen were obtained bymeasuring the textures at each side of both disks. Since c-axismaxima are located in the centre of the pole-figure, i.e. parallel to

by hand, at 40 MPa, 100 MPa and 200 MPa, respectively, taken from ultra-thin0 μm. b. Micrographs of HIPped pure calcite samples at different cold uniaxialon is vertical; the scale bar is 100 μm. Annealing time: 3 h. c. Micrographs ofultra-thin sections with crossed polarizer. Compression direction is vertical; the% muscovite (left) and 70% muscovite (right) at different uniaxial cold pressures; the scale bar is 100 μm. Annealing time: 3 h. e. Micrographs of HIPped samplesultra-thin sections with crossed polarizer. Compression direction is vertical; the

Fig. 2 (continued ).


the cylinder axis of the sample, defocusing corrections can beneglected, which allows for a direct comparison of the resultsbetween samples.

3. Results

3.1. Porosity and compaction

The densities and porosities after cold-pressing and afterHIPping are shown in Table 2. The porosities after cold-pressingare between 10 and 40%, and they decrease with increasing load.The degree of compaction of the cold-pressed samples is shownin Fig. 1a. The compaction obtained during cold-pressing at aspecific load differs considerably depending on the calcite–muscovite ratio. Samples containing 0%, 5%, and 100% mus-covite are easiest to compact. In contrast, samples with 30% or50% muscovite show about 10% less compaction. Samplesmade from the synthetic calcite powder are even more difficultto compact. Moreover, these samples did not solidify duringHIPping and could not be used for further measurements.

HIPping reduces the porosities to 3–25%. For calcite-richsamples, the porosity after HIPping is independent of the loadduring cold-press for samples pressed at ≥20 MPa. An in-creasing amount of muscovite causes higher porosities also atlarger pressures. Abnormally high porosities in samples 0–20L,0–400L, 5–200L and 5–400L are the result of leakage in thesteel jackets during HIPping. The dependence of the porosity onthe muscovite content is shown in Fig. 1b. Note that for the end-member samples the porosity is low and has a weak dependenceon the uniaxial load, while for the 50:50 mixtures the variation inporosity with load reaches a maximum.

Samples containing 50% muscovite were produced twice bythe same operator using different batches of muscovite. Thedensity difference between the samples containing 50% mus-covite that were produced twice was only 0.1 g/cm3, whichdemonstrates the excellent reproducibility achieved by the sam-ple preparation procedure. The difference between specimensfrom the same sample was 0.02 g/cm3 on average, based onfive samples, which additionally demonstrates the very goodhomogeneity.



3.2. Microstructure

3.2.1. Cold-pressed samplesThe microstructure of the pure calcite samples after cold-

pressing is homogeneous at the mm-scale and shows larger grainsevenly distributed in a matrix of smaller grains. The large grainsize fraction of the hand-pressed sample and the sample pressed at40 MPa shows extensive fracturing along the rhombohedral

cleaveage planes forming new cleavage rhombs (Fig. 2a). It canbe implied that the fracturing develops during pressing and notduring grinding; otherwise the large fractured crystals would bealready disintegrated duringmilling. At 40MPa some twinning isalready observable but twinning intensity increases significantlyat pressures of 100 and 400MPa. At 400MPa, twins occasionallyare bent and some of the large grains show serrated grain bound-aries as observed in the HIPped samples, which could be due to

Fig. 3. Rose diagrams of grain long axis orientations (a) of the HIPped pure calcite samples at different uniaxial cold pressures, and (b) of the muscovite fraction ofHIPped samples uniaxially cold-pressed at 200 MPa. Only grains with area greater than 1 μm2 have been included in the analysis, compaction direction is 90°.


gliding on the twin planes. The grain in the middle of themicrograph of the sample pressed at 400 MPa still shows thefracture network, which has been developed in the first stages ofthe cold-pressing (Fig. 2a). The most striking difference betweencold-pressed and the HIPped samples is that the fine grainedmatrix of the cold-pressed samples does not show any graingrowth. A significant decrease of the number and size of largegrains with progressive load due to fracturing is not observed.

3.2.2. HIPped pure calcite samplesThe microstructure of the pure calcite samples after HIPping

is homogeneous throughout the sample volume at the mm-scalewith larger grains evenly distributed in a matrix of small grains(Fig. 2b). The grain boundaries of the larger grains are straightto weakly lobate at low pressures. At higher pressures, theyshow serrated grain boundaries due to an intergrowth of the re-crystallized matrix grains at the expense of the large grains. Atthe same time, the intensity of twinning and the thickness of thetwin lamellae increase with cold pressure within the largergrains. Sample 0–400 shows a foam-structure; the re-crystal-lized matrix grains have straight boundaries and make up about70% of the area. A shape preferred orientation (SPO) of thecalcite grains is barely visible by eye; however, in the rosediagrams there is a weak SPO parallel and perpendicular to thepressure direction in sample 0–5 (Fig. 3a).

The number-weighted grain-size histograms show a widegrain size spectrumwith a broadmaximum between 1 and 25 µm(Fig. 4a). When the area or volume fractions are considered, the

histograms show a bimodal grain size distribution (Fig. 4c). Inall samples, the large grain size fraction shows grain sizes up to60 µm and the grains with a diameter ≤30 µm make up morethan 50% of the sample's volume (Fig. 4c). It is striking that themean grain size of the smaller fraction increases from 2 µm in thehand-pressed sample to 8 µm in the sample cold-pressed at400 MPa, while sample 0–20 has an abnormally high averagegrain size (Fig. 4a). Plotting the grain size histograms versus thecold-pressure shows the increase in grain size with cold pressuremore clearly (Fig. 4e). This trend is caused by grain growthduring HIPping. The grain size distribution of sample 0–400shows a dominant peak due to re-crystallized grains with a meandiameter of about 10 µm.

3.2.3. HIPped calcite–muscovite samplesIn the two-phase aggregates, the calcite and muscovite are

well mixed and homogeneously distributed at the mm-scale(Fig. 2c–e). The thin sections show some large, intensivelytwinned calcite grains in a matrix of smaller grains. In samplescontaining ≤10% muscovite, the boundaries of larger grainsbecome serrated in strongly compacted samples, whereas in sam-ples with higher muscovite content they remain straight. Oftenmuscovite flakes are oriented parallel to the faces of the calcitecrystals. Grain growth is weak in samples with 5%muscovite andvirtually absent in samples with higher muscovite content. Thiscan be seen in the calcite grain-size distributions of the two-phasesamples uniaxially pressed at 200 MPa (Fig. 4b, d, e). The shapeand the maximum of these distributions are similar to that of the

Fig. 4. a. Grain size histograms of the pure calcite samples. The lognormal distributions of the two-dimensional equivalent diameters are shown. Themean of the lognormaldistributions (d) and its standard deviation are indicated. b. Grain size histograms for the calcite fraction of calcite–muscovite aggregates uniaxially cold pressed at 200MPa.Explanations as in a. c. Grain size histograms of the HIPped pure calcite samples. Black and white bars show the volume and area fractions, respectively, per grain sizeinterval. d. Grain size histograms of the HIPped samples cold pressed at 200 MPa. Black and white bars show the volume and area fractions, respectively, per grain sizeinterval. e. Combined view of the grain size histograms for theHIPped samples showing the volume fractions. Only grainswith equivalent diameter≤24 μmare considered.Above: Histogram of muscovite-bearing samples cold-pressed at 200 MPa. Below: Grain size histogram of pure calcite samples cold-pressed at different pressures.



Fig. 5. IRM acquisition curves for selected samples. The magnetization isnormalized by the saturation magnetization of remanence.


hand-pressed pure calcite sample (Fig. 4a). Smaller grains of about2 μm equivalent diameter dominate the grain size distributions,independent of the muscovite content. Grains with a diameterof 5–10 μm – thought to be due to grain growth – are under-represented in the two-phase samples compared to the pure calcitesample pressed at 200MPa (0–200), where this grain-size fractionis the largest volume fraction (Fig. 4c).

The muscovite fraction shows a wide grain size distributionwith maximum grain sizes of 100 µm. The small grains tendto cluster or agglomerate around the larger muscovite grains(Fig. 2c–e). Some small grains may have been scraped off thelarger grains, because there is an increase in the number of smallgrains with increasing pressure. The size and outline of the smallgrains could not be properly determined in the micrographs.Therefore this fraction could not be regarded for the micro-structural analysis. The larger muscovite grains are bent andkinked. In general, the basal planes of the muscovite crystals areoriented perpendicular to the compression axis (Fig. 3b); how-ever, calcite grains can act as rigid particles and hamper theorientation in this direction. An almost perfect orientation isfound only for the pure muscovite samples.

3.3. Magnetic properties

If samples are being used for a study to compare magneticanisotropy (AMS) with CPO, it is important that magnetomi-neralogical changes and formation of ferromagnetic mineralsare minimized during the sample preparation. Traces of fer-romagnetic minerals such as magnetite and hematite can formfrom iron impurities that could be introduced during the prep-aration, such as abrasion from tools, diffusion of iron from thejackets or release of iron from mica. The magnetic suscept-ibility and acquisition of isothermal remanent magnetization(IRM) were measured to monitor changes in the magneticproperties. IRM acquisition, which is a very sensitive tool todetect traces of ferromagnetic minerals in the ppm-range, canbe used to distinguish between ferromagnetic phases based ontheir coercivity (magnetic hardness) and their saturation of theremanence.

The susceptibility of the Carrara marble before grinding andof the powder afterwards was the same with kM=−4.4×10−9 m3/kg (−12.0×10−6 SI). Hence, the crushing of therocks did not introduce magnetic particles in the powder. kM ofthe muscovites before grinding was 37.2×10−9, 31.2×10−9,and 36.5×10−9 m3/kg (105×10−6, 88×10−6, and 103×10−6 SI)forMu1,Mu2, andMu3, respectively; that of the ground powderswas insignificantly higher. The susceptibility of the HIPpedsamples was in the range of the values expected from the startingmaterial, but slightly higher in general (cf., Table 2). The increasewas typically about 3×10−6 SI for the pure calcite samples and upto 20×10−6 SI for the samples containing muscovite. However,the samples containing 100% muscovite had a lower suscept-ibility than the powder. The susceptibility differed by about 26%on average between specimens of the same sample (Table 2).

The IRM measurements showed virtually no ferromag-netic material in the marble before grinding. The remanentmagnetization Mrs induced by a field of 2.5 T was below5×10−7 A m2/kg. In the muscovite (Mu3) before milling, sat-uration was not reached at 2.5 T and Mrs was 4×10

−5 A m2/kg.Large differences in the IRM results are observed amongstthe pressed samples. Samples with the same muscovitecontent, however, show similar IRM parameters, independentof the load during cold pressing. Samples with 30 to 70%muscovite have the highest remanent magnetization of about5×10−4 A m2/kg and are saturated by 300 mT (Fig. 5), in-dicating magnetite or maghemite as the ferromagnetic phasethat was formed. The samples with 10% muscovite had Mrs of1.6×10−4 A m2/kg and saturated below 1 T. Samples with 0%,5%, or 100% muscovite had a weaker magnetization, b1.1×10−4 A m2/kg, and did not reach saturation by 2 T, whichindicates a high-coercivity phase, such as hematite. Thesamples from the cylinders that leaked during HIPping, showthe largest high-coercivity signal, which indicates that theleakage promoted the oxidation of iron phases to hematite orgoethite. All numbers indicate amounts of ferromagnetic par-ticles in the ppm-range.

3.4. Texture

Pole figures of the calcite and themuscovite c-axes fromX-raytexture analysis show a typical compaction texture, which can be


described as c-axis fibre-type (Leiss and Ullemeyer, 1999).Axially symmetric single maxima are oriented parallel to thecompression axis at the cold-pressing for all samples (Fig. 6). Thetextures of the specimens 0-200-0, 0-400-1, and 30-200-2 havebeenmeasured on four different sections inside the specimen. Thefour resulting pole figures of each sample are virtually identical inshape and intensity, which proves texture homogeneity of thesamples (Fig. 6a, b). The strength of the calcite CPO generallyincreases with increasing cold-pressure, and is in the same rangefor samples cold-pressed at the same load (Fig. 6c). Somevariation in the CPO maxima occurs for the samples pressed at200 MPa; however, the texture intensity is well distinguishedfrom samples pressed at higher or lower pressures.

4. Discussion and conclusions

4.1. Sample characterization

Synthetic samples are often prepared by uniaxial cold-pressing and HIPping using cylinder jackets with a diameter

Fig. 6. Examples of calcite and muscovite textures represented by experimental X-raylowest contour line is equal to 1.0 multiples of random distribution (m.r.d.), contourright side of the pole figure. Compaction is in the z-direction). (a) Pole figures, measufigures, measured at four different positions in specimens 0-200-0 and 30-200-2. (c

b10 mm. In this study we have shown the fabrication of large-diameter samples (36 to 51 mm) of calcite–muscovite rocks thatdisplay different texture intensities depending on the cold-pressure. Analysis of thin sections illustrates the homogeneousmicrostructure that has developed for the different specimensfrom a sample, as well as for the different samples. Within thesamples almost no density variations were found, and we con-clude that the samples compacted homogenously. The texturethroughout the specimens has been demonstrated to be ho-mogeneous in shape and intensity by X-ray pole figures. Thehomogeneity of the samples makes them suitable for furthermeasurements of the texture and physical properties by methodsthat investigate the material on different scales. The broadergrain size spectrum of the powder ground from Carrara marblewas more suitable for the fabrication of the samples thansynthetic powder, because samples made from synthetic powderdid not solidify during the annealing.

The magnetic properties of the material were found to bechanged during the preparation. The susceptibility data of theCarrara marble before and after grinding indicate that no iron

pole-figures of the basal planes (equal-area projection, lower hemisphere). Thelevel interval is 0.25 m.r.d., and relative maxima for m.r.d. is given on the lowerred at four different positions in specimen 0-400-1. (b) Average of the four pole) Further examples of pole figures for a selection of specimens.


impurities were introduced in the powder by grinding. A changein the magnetic properties, however, occurred during HIPping.Since it was noted that the color of the aggregate was slightlygreyer immediately adjacent to the jacket, it is presumedthat traces of iron diffused from the jackets during HIPping.Depending on the redox conditions, either magnetite/maghe-mite or hematite was formed. Larger amounts of hematite wereformed in samples with higher porosity due to cold pressing at≤20 MPa or leaking, and therefore higher oxygen content.Formation of magnetic minerals could be reduced by using non-iron bearing jackets or spacers to prevent the influx of iron.Such small concentrations in the ppm-range can influence themagnetic anisotropy. In principle, there is also an influence ofnano-sized particles on the texture development of calcite(Herwegh and Kunze, 2002); however, in the described samplesthis effect is expected to be negligible, since the content of thesecond phases is extremely low.

4.2. Deformation mechanisms, grain growth and texturedevelopment

4.2.1. CalciteDuring cold-press, the calcite fraction has been deformed

during densification by means of intergranular sliding, fracturing,passive rotation and twinning. At low pressures, where twinningis not active, the weak c-axis CPO of calcite is thought to begenerated by rigid rotation of the generally rhombohedral grains.For idiomorphic grains, the long axes of the grains are parallel tothe basal plane and perpendicular to the c-axis. Since the longaxes statistically tend to align in the compaction plane, the c-axesparallel the compression direction. The idiomorphic shape of thecrystals can be often observed in the thin sections (Fig. 2a) and inalso in SEM pictures of the powders (Fig. 7). It is therefore hardlypossible to produce totally isotropic samples with this technique.For this purpose, a cold isostatic press should be used beforeHIPping. At higher pressures, the development of the c-axis fibre-type texture is further enhanced by twinning, which additionallyreorients the c-axis towards the maximum shortening direction(Evans et al., 2003). Healing of fractures in large grains occursduring cold-pressing at high pressures already.

During HIPping, the porosity is reduced and healing offractures as well as grain growth is observed. These processes

Fig. 7. Secondary electron SEM images of calcite grains (a) from a powder of ground C5 μm).

are not expected to change the texture type. Porosity re-duction can be accompanied by further twinning. Normalgrain growth is restricted by the short HIPping time andporosity of up to 20% (Olgaard and Evans, 1988). In the purecalcite samples, grain growth is mainly governed by theporosity after cold-pressing (Table 2). The more intensivegrain growth in sample 0–400 compared to sample 0–200,however, may be attributed to a higher dislocation densityin sample 0–400 due to stronger cold-pressing. Since itsporosity after both cold-pressing and HIPping is roughly thesame as in sample 0–200 and we do not believe that there isanother controlling parameter. In two-phase samples, graingrowth is effectively suppressed by the presence of muscoviteand the resulting pinning of grain boundaries as noted inother studies (Olgaard and Evans, 1986; Berger and Herwegh,2004).

The texture strength appears to be mainly dependent on thecold-press load and less on muscovite content and degree ofcompaction. For instance, the calcite CPO of specimens 0-200-0and 30-200-2 is of the same intensity, although the degreeof compaction after cold-pressing differs about 11% and theporosity after HIPping about 8%.

4.2.2. MuscoviteThe muscovite fraction is deformed by passive rotation,

elastic deformation (bending), kinking and (001) slip. In sam-ples with high calcite content, the muscovite is bent and kinkedbetween the calcite grains. Larger grains are not intensivelysheared; smaller grains are barely visible and may be fracturedthrough abrasion with the calcite matrix. In samples with≥30%muscovite, thicker muscovite grains are intensely sheared intostacks of smaller grains and kinking is reduced. In the puremuscovite samples thicker grains are present, which indicatesless (001) slip. We did not observe any significant grain growthin muscovite suggesting that grain boundary mobility was verylow at the HIPping temperature.

The CPO of muscovite with the c-axis normal to thecompaction direction results from passive rotation during cold-pressing, since the c-axis coincides with the short axis of thecrystals. The muscovite concentration influences the intensityof the shape preferred orientation of the grains (Fig. 3), andtherefore also the intensity of the CPO.

arrara marble (scale bar is 20 μm), and (b) from the chemical calcite (scale bar is


4.3. Significance to the understanding of natural cataclasitefabrics

The comparison of the fabrics from the cold press ex-periment at 400 MPa (Fig. 2a) with the fabric from additionalHIPping (Fig. 2b) impressively demonstrates grain growthof the calcite matrix by static recrystallisation. As describedabove, besides other parameters such as porosity, dislocationdensity is a controlling parameter for the intensity of graingrowth in the calcite matrix, which obviously achieves a crit-ical value between 200 and 400MPa where grain growth starts.The result is an equilibrated matrix fabric with large relicgrains. The partly serrated grain boundaries already developedduring cold-pressing and are only modified by HIPping in thesense that grain boundaries from the matrix grains migratedinto the relic grains. If one were to only observe the finalmicrostructure and texture, one could interpret the deformationmechanisms to be ductile deformation by dynamic recrystalli-zation of a former coarse-grained marble, since the indices forthe cataclastic deformation event are completely extinguished.Twinning could be interpreted as developing during a sub-sequent event.

Natural calcite fabrics from fault zones can be very similarto the fabrics produced in this study (e.g., Fredrich et al., 1989;Liu et al., 1999; Walter, 2004; Schubnel et al., 2006, Ebertet al., 2007). Interpretation of such fabric features in terms ofductile and brittle deformation mechanisms and their chron-ological order is generally difficult since both features can befound next to one another. This can be interpreted as a mutualoverprinting which originates in a succession of seismic andaseismic events. In this study we show that a fabric with ductiledeformation features can be also produced by cataclasis andsubsequent thermal equilibration. Our results demonstrate thatthe importance of a ductile component for the development ofmicrostructures in a fault zone can be easily overestimated.Especially in this view, more detailed experiments and fabricanalyses as presented here have the potential to contributesignificantly to a greater understanding of the coexisting in-teraction of brittle and ductile deformation mechanisms in faultzones.

Acknowledgments

R. Hoffmann is thanked for the preparation and setup ofthe cold press apparatus, M. Metzler for the assistance on theHot Isostatic Press, F. Pirovino for the preparation of the thinsections and Claudio delle Piane for providing a SEM image.P. Piguet and J. Sturzenegger are especially thanked for theireffort in the sample fabrication. The X-ray texture goniometerat the Geoscience Centre at the University of Göttingenwas financed by the VolkswagenStiftung. Helpful discussionswith Axel Vollbrecht and Jens Walter are greatly acknowl-edged. We also thank two anonymous reviewers for theirconstructive comments. The project was supported by theSwiss National Science Foundation, Project No. 200020–100224. Contribution no. 1525, Institute of Geophysics, ETHZurich.

Appendix A. Supplementary data

Supplementary data associated with this article can be found,in the online version, at doi:10.1016/j.tecto.2007.11.055.

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