Structural changes during microstructural development in natural salt samples

Accepted Manuscript Structural changes during microstructural development in natural salt samples Caterina E. Tommaseo PII:

S0191-8141(14)00290-9

DOI:

10.1016/j.jsg.2014.12.004

Reference:

SG 3169

To appear in:

Journal of Structural Geology

Received Date: 17 September 2013 Revised Date:

30 November 2014

Accepted Date: 14 December 2014

Please cite this article as: Tommaseo, C.E., Structural changes during microstructural development in natural salt samples, Journal of Structural Geology (2015), doi: 10.1016/j.jsg.2014.12.004. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Structural changes during microstructural development in natural salt

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samples

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Caterina E. Tommaseo

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Technical University of Berlin, Department of Mineralogy, Ackerstraße 76, ACK9, 13355 Berlin, Germany, [email protected]

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Keywords

Natural salt samples; bulk texture; EXAFS analyses; active slip systems, texture components.

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ACCEPTED MANUSCRIPT Abstract

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This paper presents the microstructural and structural properties of two natural salt samples

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from Iran and Portugal. Their strength-ductility behavior is explored by uniaxial compression

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experiments that, demonstrate that the observed differences lie not only in the inhomogeneous

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composition of the samples and their different grain sizes but also in their internal

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microstructural inhomogeneities. Microstructural inhomogeneities and substructural changes

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were observed with the SEM method (Scanning Electron Microscopy), the thin section

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microscope investigations and the EXAFS (Extended X-ray Absorption Fine Structure)

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spectroscopy. EBSD (Electron backscattered diffraction) analyses show maximum in the

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[100] or [110] directions in the inverse pole figure. This demonstrates that recrystallization

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and grain growth processes took place together. A weak [111] texture sometimes observed

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nearby indicates that residual stress is still present in the recrystallized sample.

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Comparison of the textural and structural results show that local structural

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rearrangements take place during textural development; this is attributed to different

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orientations influencing the deformation and recrystallization of adjacent grains influencing

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the bulk final texture.

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1. Introduction

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Deformation and recrystallization processes induce structural changes in materials. Since

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NaCl has a simple face-centered cubic structure consisting of two atoms, with the same

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structural order, it would therefore be of great help to understand how complex structural and

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microstructural processes correlate in such a simple and well-known material. The direct

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relationship between the microstructural development and the order/disorder state in a

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structure is expected to be controlled by the activation of different slip systems during

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deformation and recrystallization and / or through the sample impurity content (Guillopé and

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Poirier, 1979).

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The goal of this work is to improve understanding of subject still open to debate: how Error! Unknown switch argument.

ACCEPTED MANUSCRIPT structural changes correlate with the micro textural changes that take place during complex

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deformation and recrystallization processes of natural salt samples. EXAFS (Extended X-ray

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Absorption Fine Structure) analyses are combined with microstructural analyses for the first

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time here to explore a complex field that is relevant to many aspects of the earth and material

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sciences. Insights into the relationships between recrystallization and deformation processes

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operating on microscopic scales are of great importance in salt bodies targeted for the storage

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of energy or dangerous (like radioactive) waste (Herrmann, 1979; Brewitz and Rothfuchs,

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2007).

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Studies by Schléder et al. (2007) of gamma-irradiated and etched thin sections of

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several rock salt specimens in reflected and transmitted light were able to distinguish dynamic

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from static recrystallization. The statically recrystallized sample still showed micro-cracks that

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had not been consumed by migrating boundaries.

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An older work (Bestmann et al., 2005) also showed some microstructures inherited

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from the consumed grain that survived in the swept area after grain migration between

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substructure grains. In fact, the driving force for static recrystallization arises from internal

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strain energy, which induces nucleation and subsequent grain boundary migration (Humphreys

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and Hatherly, 1996).

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Finally Schléder et al. (2007) confirmed the theory established from other salt bodies:

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that the deformation mechanism in their samples involved dislocation creep accompanied by

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fluid assisted grain boundary migration. Nonetheless they also point to additional processes,

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which probably influenced the microstructural development of halite in rocksalt. These

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processes involved local recrystallization and incomplete healing by grain boundary migration

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of micro-cracks in the recrystallized material.

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Furtheron Schléder and Urai (2007) and Desbois et al. (2010) showed that in extrusive

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salts, associated with relatively low stresses, mainly solution-precipitation (SP) creep coupled

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with grain-boundary sliding (GBS) takes place.

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The contribution of Zavada et al. (2012) is a continuation of the microstructure studies

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of the Werra rocksalt (Schléder et al., 2008) emphasizing the role of micro cracking for the

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enhancement of solution-precipitation (SP) creep coupled with grain boundary sliding (GBS). Error! Unknown switch argument.

ACCEPTED MANUSCRIPT More recently, Leitner et al. (2013) also observed deformation producing cm-sized

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euhedral halite cubes with rhombohedron, parallelepiped or tetragonal crystallography that

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depended on the direction of compression, as described earlier by Görgey (1912). The

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deformation observed in these larger grains can be also correlated with the deformation and

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recrystallization of µm sized grains in the polycrystalline matrix of the samples reported in

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this paper.

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Here, for the first time EXAFS spectroscopy, a sensitive method for the detection of

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ordered/disordered state, gives new insight in the near range order of the structure. It was

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possible to show which parts of the halite structure were affected by various recrystallization

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and deformation processes. In addition, the correlation between the structural parameters and

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such microstructural results as the preferred grain orientations affirm the presence of sub-

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structural changes even in recrystallized material. Subgrains, solid/fluid inclusions and

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microcracks observed in the SEM and thin sections give first hints about the deformation and

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recrystallization processes taking place and the microstructural inhomogeneities in the

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polycrystalline halite samples. The grain orientations and their grain orientations’

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neighbourhood in combination with the substructural data show that the locally observed

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phenomena are in good correlation to the bulk texture properties.

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A clear distinction was made between the crystal structure and the microstructure to

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describe the samples. The local structure of the halite was described in the atomic length scale

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with help of the EXAFS spectroscopy and the microstructure was described on the nm-cm

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scale with help of the EBSD method and the thin sections.

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2. Experimental Results

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2.1. Natural samples

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Details about the local geology and sampling sites of the samples from Iran (Iran50, Iran167)

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and Portugal (LBport, L4port) can be found in Desbois et al. (2010) and references therein and

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from the thesis of Pedro Terrinha (1989). A brief summary is given here: The samples Iran167 Error! Unknown switch argument.

ACCEPTED MANUSCRIPT and Iran50 are both from the distal part of the fountain. Iran167, a pure coarse grained salt

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type typical for the southern glacier of the Kuh-e-Namak (Dashti) extrusive fountain,

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represents the younger part of the Infra-Cambrian Hormuz sedimentary sequence, that consists

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mostly of rock salt and underlies the overlying sediments of Paleozoic and Mesozoic age. The

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Iran50 sample represents the extruded older (Precambrian) part of Hormuz salt sequence. The

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older sequence characterizes salt deposited in shallow continental basin with fading volcanic

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activity and solid impurities are thus probably of volcanic origin (personal communication:

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Prokop Závada).

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The samples L4Port and LBport derive (originate) from the Loule salt rock diapir,

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which is part of major evaporite complex that discontinuously underlies the Mesozoic rocks of

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the Algarve Basin. The salt is of Hetangian age and lies on a Triassic sequence of continental

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detrital and volcano-sedimentary rocks (pre-rifting sequences) and uplifted/penetrated the

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overlying Mesozoic rocks mainly made up of limestones and marls (Thesis of Terrinha, 1989).

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The LBport and L4port samples come from an underground mine excavated in the

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Loule diapir. The LBport sample arises close to the entrance into the underground mine and

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represents a strongly deformed banded domain marked by mylonitic fabrics, finely alternating

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bands of rock salt and tight isoclinal folds and foliation. The L4port sample is characterized

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by magnetic foliation. The macroscopic rock salt shows porphyroclasts up to 10 mm

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embedded in recrystallized matrix (personal communication: Prokop Závada).

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2.2. Synthesized samples

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The polycrystalline NaCl used in this study was supplied by the Sigma Aldrich in Steinheim at

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Heidenheim. The purity of the polycrystalline salt is 99.9% with trace amounts of bromide,

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iodide, potassium hexacyanoferrate, nitrite, phosphate, sulphate and heavy metals (e.g. Pb).

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The salt was delivered as an analytical fine-grained powder with a grain size of around 50-150

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µm. Small portions of the powder were ground in an agate mortar with different grinding

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times to produce powders with inhomogeneous grain sizes. The polycrystalline material was

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separated by a set of sieves ranging from <45 µm to >70 µm. At this stage the synthesized Error! Unknown switch argument.

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sample with grain size around 70 µm was used in the experiments. The starting material was produced by cold pressing pure and SiO2 gel-doped

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polycrystalline halite of different grain sizes using a 100 ton uniaxial press. A steal container

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with small piston at the bottom was filled with about 60-70g powder and then closed with an

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11.4 cm long piston which was then used to press a pellet 2-3 cm high and of 5 cm diameter.

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The powder was cold pressed at 200 MPa for 20 seconds before being heated for about 7 days

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at 150°C after which a diamond drill was used to core a cylindrical sample 8 mm in diameter

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and 2-3 cm in height. The samples were drilled dry.

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The compression was performed during in situ heating and deformation experiments with an apparatus described in Wang et. al. (2007).

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The in situ heating and deformation experiments on polycrystalline halite were

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performed at the BW5 beamline at the German synchrotron in Hamburg (DESY). For the

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experiments a monochromatic X-ray beam (ë=0.124 Å in the first beamtime run and 0.127 Å

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in the second beamtime run) was focused through 500 x 500 µm slits on the cylindrical

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sample.

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The measurements were performed at beamline BW5 in 1° steps in the omega angle

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range of +/-20° and in a second beamtime run in 0.2° steps in the omega angle range of +/-

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40°, while the cylindrical samples with 8 mm diameter were heated and deformed under axial

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compression with the tensile- compression device of Kammrath and Weiss (Wang et al.,

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2007) and from TU Clausthal (5KN). Heating was applied by supplying direct current to self-

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made ceramic rings wound with isotan wire. The rings are mounted at opposite ends of the

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sample core. The temperature range chosen varied from room temperature until 300°C in

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100°C steps (with 10°C/second steps), which are relevant temperatures for engineering and

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natural halokinetic conditions (20-200°C) as the operational conditions for example in the

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case of radioactive waste and gas storage. A thermo couple was mounted on the sample and

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and near the ceramic rings to detect the temperature and to prove that there is no temperature

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gradient due to the very heat conductivity of salt. At a fixed temperature the samples were

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compressed axially at 4 N/sec steps until for example 500 N where the load was kept constant

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for the performed texture measurements (15-20 min.) and subsequently compressed with

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increasing loads until a maximum load of 700N, 1000N, 2000N, 2400N depending from the

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behaviour of the sample in dependence of temperature and additives to not overpass the

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breaking point.

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The plastically deformed pure and SiO2 gel-doped NaCl (of 70 µm grain size) samples from the in situ experiments were used for the EXAFS experiments.

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2.3. Thin sections

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The halite samples were ground down to 100 µm and after that polished with the finest sand

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paper (2400 grade). Finally the polished surface was etched with an iron chloride

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(FeCl3·6H2O) solution (0.5 wt.-%) prepared by adding 100 ml deionized water to the 500 ml

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halite saturated solution and adding iron chloride (FeCl3·6H2O). For better handling the 2.5

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gram iron chloride was dissolved in the 100 ml deionized water before adding to the halite

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saturated solution. The thin section was then suspended in the etching solution for about 60

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seconds and then rinsed with n-hexane. Immediately after rinsing the thin sections were dried

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with compressed air. Representative for all four sample one thin section each from Iran (Iran

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167) (Figure 1a-c) and Portugal (L4port) (Figure 1d) were prepared for the microstructural

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studies.

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2.4. XRD

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The four natural salt samples from Iran (Iran 50, Iran 167) and Portugal (LB port, L4 port)

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were pulverized and analyzed by XRD. X-ray diffraction analyses (XRD) were used to

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characterize the mineralogical composition of the samples collected from natural salt deposits.

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The BRUKER D2 phaser diffractometer at BW5 beamline at the German synchrotron in

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Hamburg was employed under the following conditions: CuKα radiation, graphite secondary

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monochromator, 40 kV, 30 mA, step scanning at 0.02°/5 sec in the range of 5-80° 2θ; the

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findIt diffraction database was used for peak identification (G: gypsum, A: anhydrite, E:

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Eugsterite, S: salt/halite, Q: quartz) (Figure 3a+b).

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2.5. Stress strain

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The uniaxial compression experiments at room temperature (Figure 4) were performed at the

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engineering laboratory of the geophysics department at Ernst-Reuter Platz (TU Berlin). The

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length and area of the surface of the cylindrical sample and the compression velocity (0.14

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mm/sec) were used as starting parameters to calculate the strength and ductility of the sample

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with the help of the acquired stress/strain curves (Figure 4). For the compression experiments

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all natural samples in exception of the coarse grained L4 port sample were used (Iran 167, Iran

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50, LB port). The natural samples were also compared to the synthesized samples (pure and

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SiO2 gel-doped NaCl) (Figure 4b). For each sample two measurements were performed.

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2.6. EXAFS measurements

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The natural polycrystalline salt samples (Iran 167, and L4port) and the synthesized samples

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(pure and SiO2 gel-doped NaCl) with the halite reference phase were measured at the Cl K

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edge (2820 eV) to examine the near range order in the recrystallized and deformed samples.

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Since sodium has the same near-range order as chlorine only two samples (L4port and the

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synthesized SiO2 gel-doped NaCl) were measured at the Na K-edge (1071 eV)

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reliability of the results.

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The experiments at the Cl K edge were conducted using the HIKE station at the KMC-

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1 (Schäfers et al., 2007; Gorgoi et al., 2009), soft X-ray double crystal monochromator,

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beamline of the BESSY II synchrotron facility at Helmholtz Zentrum Berlin in Germany. The

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experiments were performed under UHV condition with an overall energy resolution of 0.26

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eV with 2010 eV photon energy in the energy range between 2 and 12 KeV.

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The EXAFS measurements of Cl K edge were collected in 0.25 and 0.5 eV steps until

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1000 eV over the edge with 1 eV delay and 1 eV lifetime using a double crystal

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monochromator.

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The experiments at the Na K edge were performed at the SurIcat (Surface Investigation Error! Unknown switch argument.

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and Catalysis), a photoemission end station equipped with the high resolution electron energy

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analyzer Scienta SES 100 that allows angular resolution of 0.01° as well as integrated

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measurements. A Bruker fluorescence detector and the sample drain current signal can be

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used for X-ray absorption investigations.

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eV over the edge using a Plane Grating Monochromator PM4.

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The EXAFS measurements of Na K edge were collected in 0.25 eV steps until 1000

Data reduction and analysis were performed using the WINXAS software package (Ressler, 1998).

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2.7. SEM (Scanning Electron Microscopy)/EBSD (Electron Backscatter Diffraction)

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All images were acquired in the BSE (backscattered electrons) mode and shown at the same

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magnification of 50x (Figure 2a). The pattern quality is shown with the band contrast (BC)

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image (Figure 2c), as scalar value measured for each diffraction pattern collected. Essentially

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BC is related to the brightness level of diffraction bands above a normalized background, and

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is affected by the diffraction intensity for a phase, dislocation/crystallographic defect intensity

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and orientation. Grain boundaries are normally visible as low pattern quality (dark) linear

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features (Figure 2c). This is partly related to the observed high angle grain boundaries in the

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LAGB/HAGB distribution map (Figure 2d). Nonetheless in a few regions indicated with

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arrows the detected HAGB (Figure 2d) are not seen as dark linear features as expected in the

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BC map. Further on the texture spread of the texture components is low (sharp patterns).

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EBSD mapping of CPO (crystal preferred orientation) was performed on a Hitachi S-

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2700 scanning electron microscope equipped with a Nordlys II EBSD detector at ZELMI

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(Zentraleinrichtung Elektronenmikroskopie at TU Berlin). Pattern acquisition was carried out

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using acceleration voltage of 20 kV, beam current of 5-8 nA and 33 mm working distance,

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using step size of 7-12 µm. The EBSD patterns were indexed with HKL Channel 5 software.

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EBSD patterns were of sufficient quality to index more than 60-70% of measured points

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(Figure 2b).

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100 grains within 1x1 cm² area of each sample were recorded for statistically reliable

EBSD maps, depicting the crystallographic orientation of a minimum of about

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information about the texture. The CPO (crystallographic preferred orientation: take up)

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patterns are expressed in m.r.d. (multiples of a random distribution) (figure 12). The recorded map highlights the distribution of the LAGB (low angle grain

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boundaries) and HAGB (high angle grain boundaries) as dark and bright features (Figure 2d).

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3. Results and discussion

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3.1. Microstructural observations

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The inhomogeneous microstructure of the samples IRAN 167 (Figure 1a-c) and L4Port

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(Figure 1d) shown in the thin section images in transmitted light reflect the different

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deformation mechanisms. At the 120° triple junction usually present among recrystallized

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grains, channel-like and separated island like fluid inclusions are observed (Figure 1a). Also

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solid inclusions (gypsum grain) are detected. The observed fluid and solid inclusion at the

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grain boundaries could have caused grain boundary sliding (GBS). The presence of the second

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phase anhydrite and water in the grain boundaries (Figure 1a) enable solution precipitation (or

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pressure solution) phenomena. In fact, Poirier (1985) explained that the diffusion of matter

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along the grain boundaries during diffusion creep creates the driving force for grain boundary

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sliding (GBS) and vice versa. During diffusion creep defects are migrating due to vacancies in

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the crystal lattice. Both diffusion creep and GBS are therefore strongly coupled (Poirier,

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1985). Závada et al. (2012) suggested that also SP (solution precipitation) and GBS are

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strongly correlated mechanisms in the Werra salt. This is due to the fact that during solution

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precipitation creep diffusion takes place around grain boundaries, where the follow laws for

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diffusion creep and SP are similar (Blenkinsop, 2000). Furtheron the coupled activity of SP

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creep and GBS generates low degree of CPO (crystal preferred orientation) and abundant

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substructure-free grains (Schléder and Urai, 2007; Desbois et al., 2010). The development of a

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low degree of CPO for the samples presented in this paper will be shown later in the section

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3.5.

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The strain free grains near the solid inclusion anhydrite (Figure 1c) show apparently Error! Unknown switch argument.

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dragging microstructure that seems to indicate GBM mechanism compatible with decreasing

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density of dislocation. In fact, the absence of slip lines (bands) or subgrains suggest that the

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principal deformation mechanism was solution precipitation creep accompanied by grain

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boundary migration and grain boundary sliding (Urai et al., 2008). Further deformation mechanisms influencing the microstructural development in both

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samples are the presence of healed cracks reaching the grain boundary and fluid inclusions

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(Figure 1c). The observed microcracks at the grain boundary triple junctions (Figure 1b-c) or

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grain boundary ledge (Figure 1a) form due to stress accumulations by grain boundary sliding

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(Kranz, 1983).

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Furtheron subgrain-boundaries and slip bands (Figure 1b-c) are observed as in the studies of Urai et al. (2008b), which are an indication for dislocation creep.

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Obviously the deformation mechanisms operating simultaneously to grain boundary

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sliding is diffusional creep and dislocation creep. The last two mentioned deformation

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mechanisms prevent overlaps and voids between sliding grains.

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3.2. XRD

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The diffraction patterns (Figures 3a+b) confirm halite (80-85%) as the major constituent with

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smaller amounts of gypsum (10-5%) and eugsterite (10-5%). Anhydrite (10-5%) was also

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detected as an additional component in samples Iran 167 and LB port followed by quartz (10-

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5%) in the samples Iran 50 and LB port.

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3.3. Stress/strain

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The stress/strain curves acquired by loading the natural halite samples by monotonic uniaxial

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compression until rupture at room temperature are shown in Figure 4a. Comparison with the

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synthesized samples (Figure 4b) should clarify the differences between the natural samples.

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The small discrepancies between the first and second compression experiments with the

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natural samples (Figure 4a + b) are probably due to small heterogeneities between samples Error! Unknown switch argument.

ACCEPTED MANUSCRIPT due to different mineral grains and their inner-structure, responding to the load in different

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ways. The yield strength increases as the averaged grain size decreases (Figure 4a + b). This

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correlates well with the fact that the finer the grains, the larger the area of grain boundaries to

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hinder dislocation motion. Smaller grain sizes usually increase the strength. By contrast, the

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rupture strength showed no obvious correlation with grain size. One reason for this could be

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that the macroscopically and microscopically inhomogeneous natural samples began

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deformation from different starting points, as verified by the EXAFS and EBSD results.

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Comparison with the synthesized samples (Figure 4b) emphasizes the complexity of

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the stress-strain behavior of the natural samples. The rupture strength tends to decrease in

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samples doped with SiO2 gel while the strength and stiffness increase at the same time.

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Studies of Renard et al. (2001) show that clay particles trapped along salt grain

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contacts enhance mechanically pressure solution by sustaining open grain contacts. They

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favour the diffuse transport of Na+ and Cl- to the pore space and inhibit grain boundary

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formation. The presence of clay particles thus has important consequences for the ductile

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behaviour of the crust both in diagenetic conditions and in the gouges of active faults. For

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intergranular pressure solution, the two models of water film diffusion and undercutting are

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possible. In both cases the removal of material at grain-to-grain contacts causes the grains to

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move together (i.e. converge) without requiring internal deformation of the grains. This

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process leads to macroscopic densification and/or shear creep of a rock, fault gouge or

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sediment. Similar results were obtained from Zubtsov et al. (2004).

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Hickman and Evans (1995) studied deformation at the contact between single-crystal

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halite and fused silica lenses that were immersed in brine. When a convex halite lens was

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pressed against a flat halite lens, no convergence occurred. However, the contact area between

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the lenses grew as halite dissolved from the free surfaces of the lenses, diffused through the

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pore fluid and precipitated at the perimeter of the contact spot. This process, called neck

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growth is analogous to crack healing and is driven by gradients in surface curvature (Hickman

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and Evans, 1992). In similar experiments with a halite crystal pressed against a flat silica

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surface, convergence by intergranular pressure solution did occur. Pressure solution, neck

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growth and crack healing can affect fault zone rheology in markedly different ways. The three Error! Unknown switch argument.

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processes can lead to the time-dependent strengthening of faults between earthquakes or allow

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the formation of pressure seals along faults which prevent overpressured pore fluids from

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escaping. Mineralogic observations on gouges from the San Andreas and other mature fault

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zones at shallow depths indicate that these gouges are mineralogically diverse; contain large

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percentages of illites, smectites and other clays; and are very fine-grained. In the experiments

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of Hickman and Evans (1992), no neck growth and a dramatic acceleration of pressure

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solution rates were observed at the interfaces between dissimilar materials (halite and fused

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silica) and in the presence of intergranular clay. These results suggest that intergranular clays

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and the juxtaposition of different minerals within crustal fault zones can poison neck growth

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and, by enhancing interphase boundary diffusivities, promote intergranular pressure solution.

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During plastic deformation one of the main mechanisms is the slip of dislocations with

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crystallographically controlled geometries (Poirier, 1985). The slip directions vary from

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crystal to crystal, and some movements will be hindered by less favorably oriented neighbors.

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Dislocations interact among themselves. As a result, the likelihood of plastic deformation

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depends on the ability of dislocations to move. It is probable that some grains deform much

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more than others (Kocks et al., 1998). This would result in large variation in such

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microstructures as dislocation density and subgrain geometry. Silica gel additives in the halite

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matrix induce lower strain accumulations. This is expressed as samples strengthening (i.e.

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higher Young’s moduli), which correlates well with a decrease in plasticity. Even in the

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simple NaCl system it becomes clear that reproducible trends during the deformation

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processes depend on temperature and additional phases and are surprisingly complex.

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3.4. EXAFS

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Good fits above the second coordination shell were not possible due to the signals in the

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EXAFS spectrum above 7 Å-1 being weak and noisy. Nonetheless, as shown in table 1,

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reliable results were obtained by using the short k range below 7 Å-1. For each of the Na-Cl

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and Na-Na and Cl-Na and Cl-Cl subshells in the halite samples, separate phase and amplitude Error! Unknown switch argument.

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functions were calculated using the FEFF7 program (Zabinsky et al., 1995). Matsuo et al.

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(2005) confirmed that using this short k-range gives reliable results. As listed in Table 1, the distances between the first two coordination shells are in good

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agreement with those in the pure NaCl reference phase. Deviations are probably caused by

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defects developing during structural rearrangements occurring during such complex

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deformation and recrystallization processes. Nonetheless, the F-test, with values ranging

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between 0 and 0.3 and a variation of residuals within 95%, indicate that all fitting parameters

364

are independent. The fit residual of the F-test demonstrates the significance of additional

365

fitting parameters.

SC

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366

Furthermore, one has to take into account that a short k-range implies a spread of

367

distances. The practical limit for resolving Är with k-range 3-13 Å is around 0.05 Å. That

368

means that, in the present case, the k-range up to 7 Å-1 with a spread of around 0.1 Å in the r

369

space (in comparison to the theoretical values) has to be expected, which correlates well with

370

the results listed here. The higher error value (≥0.1 Å) observed in some experiments is

371

probably due to a mix of different bond lengths in the samples. In fact, the existence of

372

homogenous bond lengths in the pure NaCl phase (reference) phase are the low error value of

373

0.005-0.008 Å (Table 1) and the fitted Fourier transform of the EXAFS spectrum measured at

374

the Cl K edge. The hot in situ deformed synthetic samples and the natural samples show

375

higher error values of up to 0.2 Å (Table 1). These inhomogenous bond lengths are attributed

376

to deformation and recrystallization of impure samples inducing extra distortions,

377

dislocations, and defects etc., goodness of the fits of the Fourier transforms to the EXAFS

378

spectra of the synthesized and natural sample is shown in Figures 5, 6 and 7.

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-1

In addition, the Debye-Waller factor of the Cl backscatter is higher than the Na

380

backscatter. By contrast, the results obtained from the measurements performed at the Na K-

381

edge show the opposite trend. Thus using the Debye-Waller factor as a measure of the relative

382

motion between the central atom and the back-scattering atoms is increased if the shell

383

contains unresolved contributions from back-scattering at slightly different distances. The lack

384

of good quantitative measurements of the atomic distances necessitated a qualitative approach

385

to include shells of higher coordination. The “filtered” EXAFS spectra were used to shed light Error! Unknown switch argument.

ACCEPTED MANUSCRIPT 386

on the structural order. Fourier filtering the r-range between 1.3 Å and 4.5 Å and between 1.3

387

Å and 5.7 Å of the measurements performed at the Cl K edge (Figure 8) and at the Na K-edge

388

(Figure 9) exposes noticeable differences. These differences evidently lie in disturbance of the

389

bond lengths between the third and fourth coordination shells. In addition the synthesized pure and SiO2-gel doped halite samples were compared

391

(Figure 8c). The resulting differences in EXAFS signals affirm the quantitative results from

392

the fit of the first two coordination shells (Table 1 and Figure 10 + 11). Nonetheless the high

393

Debye-Waller factor for the Cl-Cl and Cl-Na bond lengths in the SiO2-gel doped halite sample

394

are similar to those in the reference phase in which the grains are obviously less deformed

395

than those in the pure sample.

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One explanation for this behavior could be that deformation takes place preferentially

397

in slip direction [0-11] on the slip plane (011). This will be also shown in the next section 3.5.

398

and it was just demonstrated with the presence of slip lines in the thin section. The preferred

399

shifting parallel to the (011) plane and to the [0-11] and [100] directions is due to the missing

400

repulsive forces in contrary to the shifting along the (100) plane, where equal ions face each

401

other. Therefore the Na-Cl and Cl-Cl bond lengths at 5.02 Å and 5.79 Å are preferentially

402

affected by such shifting (Figure 12).

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3.5. Texture development

405

The motivation for the EBSD mapping was not only to characterize the bulk textures of the

406

four naturally deformed samples from Iran (Iran 167, Iran 50) and Portugal (LB port, L4 port),

407

but also to gain information about the orientation of grains around a specific grain orientation.

408

Since the sample orientation could not be backtracked, a sample coordination system was

409

defined by measuring the samples in both cross-sections and longitudinal cuts. The texture

410

analyses show a weak texture of around 2 m.r.d. Further on a few grains show a low texture

411

spread of specific texture components from the original grain orientation and therefore a

412

sharpering of the texture, which is indicative for substructure-free (dislocation free) grains. In

413

contrary grain orientations with a high texture spread (deviation from a standard texture

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ACCEPTED MANUSCRIPT component) within a grain show a bad quality (low sharpness) of the diffractions patterns in

415

the EBSD maps. It is known that, when a crystal is deformed the dimensions of its lattice are

416

distorted. This non-uniformity in the lattice leads to a greater distribution of the angles at

417

which a crystallographic plane diffracts. An indication for the bad quality is that certain

418

regions with high dislocation density appear as darker shades on an EBSD map (non-indexed

419

areas shaded black). The comparison of the Euler angle map with the low and high angle

420

boundaries distribution map shows that deformed regions will have a higher concentration of

421

low angle grain boundaries (e.g. 2-10° misorientation) (Figure 2d dark features).

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Recrystallization taking place due to grain boundary migration (motion through the

423

material) develop a low-energy grain configuration with triple junctions approaching 120°

424

angles (Stipp et al., 2002). This was also observed in the thin sections of the sample (IRAN

425

167) (Figure 1a). Yet another indication is the presence of subgrains (Figure 1a). Furtheron

426

the regions marked with an arrow (Figure 2d bright features) contains high angle boundaries

427

that are not visible as boundaries in the BC map (Figure 2c). Here probably nucleation could

428

have been taken place at pre-existing high angle boundaries. In this case new grains are

429

formed due to accumulation of dislocations leading to the formation of subgrains, whose

430

misfit angles increase with strain (Azuma, 1986). Alternatively grain boundaries become

431

mobile, without the appearance of new boundaries, due to the increased driving force for grain

432

boundary migration as a result of dislocation accumulation (Bailey and Hirsch, 1962). Also

433

subgrain rotation recrystallization is possible, which occurs at large strain.

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Since all these samples show approximately the same bulk texture, two samples (Iran

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435

167 and LBport) were chosen as representatives of all samples: the consequent results are

436

plotted in Figures 13 to 16.

437

The pole figures (Figure 13 and 14), reveal the orientations of particular

438

crystallographic planes rather than of individual crystals. The calculated (100), (110) and

439

(111) pole figures, smoothed with a 10° Gauss filter to reduce stochastic defects, show visible

440

orthorhombic sample symmetry. The symmetry of an orthorhombic sample was imposed on

441

the pole figures (Figure 13 and 14) to show the presence of texture components with the slip

442

plane (011) as for example in the observed grain orientations (011)[0-11] and (011)[100] Error! Unknown switch argument.

ACCEPTED MANUSCRIPT 443

(Figure 13 and 14). For further information about the alignment of crystallographic poles with a particular

445

sample direction, the inverse pole figures are shown in Figure 15 with maxima at <110> and

446

between <100> and <111>. Finally, with the crystallite orientation distribution function

447

defined by three Euler angles (obtained directly from the EBSD mapping), the particular grain

448

orientations could be identified. The main texture component here is the (011)[100]. The

449

EBSD mapping allowed identification of the single grain orientations and therefore also the

450

orientations of the neighboring grains. The mostly detected preferred orientation (011)[100] in

451

all samples is preferentially surrounded by orientations ranging from (011)[1-33] until

452

(011)[0-11] by a rotation around the ϕ1 angle (Figure 16). In addition, diffuse (broadened)

453

texture components are also detected as neighbors. This indicates that the boundaries have a

454

high mean misorientation. In fact as mentioned above the grain boundary migration results in

455

lattice preferred orientation, if grains with particulate orientations grow at the expense of the

456

others. The driving force for grain boundary migration is the difference in free energy between

457

grains on both sides of a grain boundary, resulting in dislocation creep as main deformation

458

mechanism. The relation between dislocation density and crystal orientation is closely related

459

to the stress distribution in a deforming polycrystal. These results correlate well with the

460

EXAFS analysis presented in the preceding section and the observations in the thin sections

461

(slip bands in Figure 1a).

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4. Conclusions

464

The various structural and microstructural results presented in this paper are remarkably

465

consistent. The inhomogeneities in the texture development are reflected in the halite

466

structure. The microstructural phenomena, e.g. the slip bands observed in the thin sections,

467

could be defined as a specific slip system showing dislocation creep as main deformation

468

mechanism with the aid of the EXAFS spectroscopy, a sensitive method for detecting the near

469

range order in the structure. It was possible to show that the third and fourth coordination

470

shells in the halite structure are mostly affected during structural development. This is in good Error! Unknown switch argument.

ACCEPTED MANUSCRIPT agreement with the microstructural development, in which (011)[0-11] and (011)[100] are the

472

most active slip systems in the halite structure as shown using the EBSD method. The

473

different resulting bond lengths in the third and fourth coordination shells, shown qualitatively

474

by different frequencies of the overlapping signals in the Fourier filtered EXAFS spectra, are

475

evidently in good correlation with different preferred grain orientations developing in adjacent

476

grains. Local changes in the structure obviously control the microstructural development of

477

halite and rock salt. The mutual agreement between structural and microstructural

478

development provides information about the complex recrystallization and deformation

479

processes taking place.

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Deformation processes took place by several coupled activity of solution precipitation

481

and grain boundary sliding but obviously also by healed cracks, subgrains and dislocation

482

creep.

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The presence of the second phase anhydrite and water in the grain boundaries enable

484

solution precipitation (or pressure solution) phenomena. The diffusion of matter along the

485

grain boundaries during diffusion creep creates the driving force for grain boundary sliding

486

(GBS) and vice versa. Obviously the deformation mechanisms operating simultaneously to

487

grain boundary sliding is diffusional creep and dislocation creep. The last two mentioned

488

deformation mechanisms prevent overlaps and voids between sliding grains.

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The driving force for recrystallization is the strain energy stored in individual grains.

490

Micro strains are the result of small changes in the local lattice parameters resulting from

491

defects, imperfections and variations in the crystal structure as demonstrated by the structural

492

parameters (atomic distances, coordination numbers) obtained from the EXAFS analyses.

493

Therefore is was possible to prove the observed slip bands by defining the slip system in the

494

halite structure activated by dislocation creep.

495 496

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The locally observed substructural and microstructural phenomena are in good correlation to the bulk texture properties.

497 498

Error! Unknown switch argument.

ACCEPTED MANUSCRIPT Acknowledgements

500

Many thanks to BESSY (Dr. A. Vollmer) for beamtime at KMC1 (HIKE station) and SurIcat.

501

Many thanks to Dr. M. Gorgoi and Dr. Ovsyannikov for their great technical support during

502

the measurements of the EXAFS spectra. Many thanks for HASYLAB for beamtime at BW5,

503

and R. Nowak, J. Bednarcik, M. von Zimmermann for support and counseling. Many thanks

504

also to H.G. Brokmeier, W. Reimers and G. Franz supporting my work. Many thanks to

505

Prokop Zavada for fruitful discussions. Furthermore I want to thank technicians M. Hill and

506

C. Zecha of the TU Berlin (Department of Mineralogy and Materials sciences) for the

507

preparation of the samples used for the EBSD measurements. Many thanks to the ZELMI

508

(Zentraleinrichtung Elektronenmikroskopie in TU Berlin) for beamtime and J. Nissen for the

509

great technical support in the performance of the EBSD measurements. Many thanks also to

510

G. Koch and T. Ressler for the EXAFS Program of T. Ressler placed at my disposal and the

511

interesting exchanges and discussion in the EXAFS analyses.

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References

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Bailey, J.E., Hirsch, P.B., 1962. The recrystallization process in some polycrystalline metals.

517

Proceedings of the royal society of London A 267, 11-30.

518

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516

519

Bestmann, M., Prior, D.J., 2003. Intragranular dynamic recrystallization in naturally deformed

520

calcite marble: diffusion accommodated grain boundary sliding as a result of subgrain rotation

521

recrystallization. Journal of Structural Geology 25, 1597-1613.

522

Error! Unknown switch argument.

ACCEPTED MANUSCRIPT Brewitz, W., Rothfuchs, T., 2007. Concepts and technologies for radioactive waste disposal in

524

rock salt. Acta Montanistica Slovaca 12: 67-74.

525

Desbois, D., Zavada, P., Schleder, Z., Urai, J.L., 2010. Deformation and recrystallization

526

mechanisms in actively extruding salt fountain: Microstructural evidence for a switch in

527

deformation mechanisms with increased availability of meteoritic water and decreased grain

528

size (Qum Kuh, central Iran). Journal of Structural Geology 32, 580-594.

RI PT

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529

Görgey, R., 1912. Zur Kenntnis der Kalisalzlager von Wittelsheim im Ober-Elsaß,

531

Tschermaks. Mineral Petrogr Mitt 31: 339-468.

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532 533

Guillopé, M., Poirier, J.P., 1979. Dynamic recrystallization during creep of single crystalline

534

halite: an experimental study. J. Geophys. Res., 84, pp. 5557-5567.

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Gorgoi, M., Svensson, S., Schäfers, F., Öhrwall, G., Mertin, M., Bressler, P., Karis, O.,

537

Siegbahn, H., Sandell, A., Rensmo, H., Doherty, W., Jung, C., Braun, W., Eberhardt, W.

538

2009. High Kinetic Energy Photoelectron Spectroscopy Facility at BESSY: Progress and First

539

Results, Nuclear Instruments and Methods in Physics Research A 601, pp. 48-53

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541

Herrmann, G., 1979. Geowissenschaftliche Probleme bei der Endlagerung radioaktiver

542

Substanzen in Salzdiapiren Norddeutschlands. Band 68, Heft 3: 1076-1106.

543 544

Hickman, S.H., Evans, B., 1995. Kinetics of pressure solution at halite-silica interfaces and

545

intergranular clay films. Journal of Geophysical Research 100: 113-132.

546

Error! Unknown switch argument.

ACCEPTED MANUSCRIPT Humphreys, F.J., Hatherly, M., 1996. Recrystallization and related annealing phenomena. Vol

548

Pergamon pp. 497.

549

Kocks, U.F., Tomé, C.N., Wenk, H.-R., 1998. Texture and anisotropy: Preferred Orientations

550

in polycrystals and their effect on materials properties. Cambridge University Press.

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551 552

Kranz, R., 1983. Microcracks in rocks - a review. Tectonophysics 100 (1-3), 449-480.

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Leitner, C., Neubauer, F., Marschallingeer, R., Genser, J., Bernroider, M., 2013. Origin of

555

defromed halite hopper crystals, pseudomorphic anhydrite cubes and polyhalite in Alpine

556

evaporites (Austria, Germany). Int. J Earth Sci Geol Rundsch 102, 813-829.

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Matsuo, S., Nachimuthu, P., Lindle, D.W., Perera, R.C.C., Wakita H., 2005. Physica Scripta

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Vol. T115, 966-969.

562

Poirier, J.P., 1985. Creep of crystals. Cambridge University Press, Cambridge, UK, 260 pp.

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Renard, F., Dysthe, D., Feder, J., Biorlykke, K., Jamtveit, B., 2001. Enhanced pressure

564

solution creep rates induced by clay particles: Experimental evidence in salt aggregates.

565

Geophysical Research Letters, Vol. 28, No. 7, 1295-1298.

566 567

Ressler, T., 1998. J. Synch Rad., 5, 118.

Error! Unknown switch argument.

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F. Schäfers, M. Mertin, M. Gorgoi, KMC-1: a High Resolution and High Flux Soft x-Ray

570

Beamline at BESSY, Review of Scientific Instruments 78 (2007) 123102

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Schléder, Z., Burliga, St., Urai, J.L., 2007. Dynamic and static recrystallization-related

573

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574

gamma-irradiation. Neues Jahrbuch für Mineralogie. Journal of Mineralogy and geochemistry.

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Schléder, Z., Urai, J.L., Nollet, Sl, Hilgers, C., 2008. Solution-precipitation creep and fluid

577

flow in halite: a case study of Zechstein (z1) rocksalt from Neuhof salt mine (Germany).

578

International Journal of Earth Sciences 97, 5, 1045-1056.

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Stipp, M., Stünitz, H., Heilbronnen, R., Schmid, S.M., 2002. The eastern Tonale fault zone: a

581

natural "laboratory" for crystal plastic deformation of quartz over a temperature range from

582

250°C to 700°C. Journal of structural geology 24, 1861-1884.

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Terrinha, P., 1989. A study of the internal structure of the Loule diapir (Algarve Basin - S

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586

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Urai, J. L., Schléder, Z., Spiers, C. J., Kukla, P. A., 2008b. 5.2: Flow and Transport Properties

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of Salt Rocks. In: Littke, R., Bayer, U., Gajewski, D., Brink, H. J. & Winter, I. (Eds.),

589

Dynamics of complex intracontinental basins: The Central European Basin System.)

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Wang, X.D., Bednarcik, J., Saksl, K., Franz, H., Cao, Q.P., Jiang, J.Z., 2007. Tensile behavior

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of bulk metallic glasses by in situ x-ray diffraction. Applied Physics Letters, 91: 081913.

594

Zabinsky, S.I., Rehr, J.J., Ankudinov, A., Albers, R.C., Eller, M.J., 1995. Phys. Rev. B., 52,

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2995.

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Závada, P., Desbois, G., Schwedt, A., Lexa, O., Urai, J., 2012. Journal of Structural Geology,

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37, 89-104.

600

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Zubtsov, S., Renard, F., Gratier, J.P., Guiguet, R., Dysthe, D.K., Traskine, V., 2004.

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Experimental

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Tectonophysics 385, 45-57.

604

solution

compaction

of

synthetic

halite/calcite

aggregates.

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Figure captions

606

Fig. 1 Thin section images of a the sample Iran 167 showing interconnected fluid inclusions at

607

the grain boundaries (triple point: 120° angle), solid inclusion (gypsum grain) and healed

608

cracks originating at the grain boundary; b grain growth: curved grain boundary of grain 1.

609

Subgrain-free grain 1 grows at expense of grain 2, in which healed cracks and slip bands are

610

observed; c solid inclusion at the triple point junctions between strain free grains; d the

611

sample L4port from Portugal showing subgrains, fluid and solid inclusions.

612

Fig. 2 a SEM (scanning electron microscopy) image shows subgrains separated by low

613

angular (subgrain) boundary, in which the misorientation is typically less than 5°-15°; b the

614

subgrain region indicated by an arrow appears as darker shades in the EBSD map (Euler angle

615

map) (non-indexed areas shaded black) due probably to rough surface; c band contrast (BC)

616

image related to the brightness level of diffraction bands. The arrows indicate the regions in

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ACCEPTED MANUSCRIPT which twinning is takes place; d HAGB/LAGB map shows deformed regions with higher

618

concentration of low angle boundaries (e.g. 2°-10° misorientation) in grey and HAGB regions

619

with white features

620

Fig. 3 Diffractograms of the natural samples from a Portugal (LBport, L4port) and b Iran (Iran

621

167, Iran 50) with phase components (A: anhydrite, S: Halite, G: Gypsum, E: Eugsterite, Q:

622

quartz).

623

Fig. 4 Stress-strain curves recorded at room temperature of a natural samples and b compared

624

with synthesized pure and SiO2 gel doped halite

625

Fig. 5 Fourier transform of the EXAFS spectrum calculated in the range k=2.6-6.9 Å-1 with a

626

k1 weight. Continuous lines are the experimental data and the broken lines are the best fits in

627

R space in the range 1.3-4.2 Å of the pure NaCl reference powder measured at the Cl K edge.

628

Fig. 6 Fourier transform of the EXAFS spectrum calculated in the range k=2.6-6.9 Å-1 with a

629

k1 weight. Continuous lines are the experimental data, the broken lines are the best fits in R

630

space in the range 1.3-4.2 Å of the SiO2 gel doped NaCl reference phase with 70 µm grain

631

size (in situ deformed and heated) measured at the Cl K edge.

632

Fig. 7 Fourier transform of the EXAFS spectrum calculated in the range k=2.6-6.9 Å-1 with a

633

k1 weight. Continuous lines are the experimental data and the broken lines are the best fits in

634

R space in the range 1.3-4.2 Å of the natural sample from Portugal (L4port) measured at the

635

Cl K edge.

636

Fig. 8 Fourier filtering (FF) by Fourier transforming the k1χ(k) data of the measurements at

637

the Cl K edge (k range: k=2.6-6.9 Å-1) into the R space and back transforming the data of the

638

selected distance range a the first and second coordination shell and b the third and fourth

639

coordination shells into the k space. c zoomed k-range between 5 and 6.9 Å-1 in b.

640

Fig. 9 Fourier filtering (FF) by Fourier transforming the k1χ(k) data of the measurements at

641

the Na K edge (k range: k=2.6-6.9 Å-1) into the R space and back transforming the data of the

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643

coordination shells into the k space.

644

Fig. 10 Fourier transform of the EXAFS spectrum calculated in the range k=2.6-6.9 Å-1 with a

645

k1 weight. Continuous lines are the experimental data; the broken lines are the best fits in R

646

space in the range 1.4-4.13 Å of the natural sample from Portugal (L4port) measured at the Na

647

K edge.

648

Fig. 11 Fourier transform of the EXAFS spectrum calculated in the range k=2.6-6.9 Å-1 with a

649

k1 weight. Continuous lines are the experimental data, the broken lines are the best fits in R

650

space in the range 1.4-4.13 Å of the SiO2 gel doped NaCl reference phase with 70 µm grain

651

size (in situ deformed and heated) measured at the Na K edge.

652

Fig. 12 Structure model of halite (NaCl)

653

Fig. 13 Pole figures of a the longitudinal cut b and cross-section (Iran 167 sample) calculated

654

from around 100 single grain orientations.

655

Fig. 14 Pole figures of a the longitudinal cut b and cross-section (LBport sample) calculated

656

from around 100 single grain orientations.

657

Fig. 15 Inverse pole figures of a the longitudinal cut and b cross-section (Iran 167 sample)

658

calculated from around 100 single grain orientations with imposed orthorhombic sample

659

symmetry.

660

Fig. 16 ODF (Orientation distribution function) of a the longitudinal cut and b cross-section

661

(Iran 167 sample) calculated from around 100 single grain orientations with imposed

662

orthorhombic sample symmetry. The ODF show the preferred grain orientations (011)[100].

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663 664 665

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Table 1: Results of the quantitative analysis of the first and second coordination shells (Cl K

667

edge measurements); S0²=0.9 (fixed)

NaCl (70 µm) NaCl (70 µm) + SiO2 gel L4port IRAN 167

R [Å] 2.887 (+/- 0.005) 3.975 (+/- 0.008) 2.866 (+/- 0.132) 3.983 (+/- 0.167) 2.886 (+/- 0.132) 3.999 (+/- 0.167) 2.873 (+/- 0.132) 3.977 (+/- 0.167) 2.874 (+/- 0.007) 3.976 (+/- 0.009)

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668 669

σ² [Ų] 0.042 (+/- 0.005) 0.056 (+/- 0.005) 0.045 (+/- 0.008) 0.056 (+/- 0.014) 0.044 (+/- 0.008) 0.053 (+/- 0.014) 0.039 (+/- 0.008) 0.055 (+/- 0.014) 0.043 (+/- 0.009) 0.055 (+/- 0.009)

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N (fixed) NNa 6 NCl 12 NNa 6 NCl 12 NNa 6 NCl 12 NNa 6 NCl 12 NNa 6 NCl 12

SC

Sample NaCl reference phase

670

Table 2: Results of the quantitative analysis of the first and second coordination shells (Na K

671

edge measurements); S0²=0.9 (fixed)

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R [Å] 2.749 (+/- 0.091) 3.872 (+/- 0.171) 2.775 (+/- 0.052) 3.941 (+/- 0.149)

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N (fixed) NCl 6 NNa 12 NCl 6 NNa 12

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σ² [Ų] 0.036 (+/- 0.005) 0.041 (+/- 0.005) 0.024 (+/- 0.003) 0.045 (+/- 0.013)

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ACCEPTED MANUSCRIPT -microstructural analyses of natural salt samples from Iran and Portugal -structural rearrangements take place during textural development -different deformation and recrystallization behavior of adjacent grain orientations

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-different strength-ductility behavior due to microstructural inhomogeneities