Structure of grain boundaries in wet, synthetic … of grain boundaries in wet, synthetic...

Structure of grain boundaries in wet, syntheticpolycrystalline, statically recrystallizing halite – evidencefrom cryo-SEM observations

O. SCHENK1, J . L. URAI1 AND S. PIAZOLO2 ,*1Geologie – Endogene Dynamik, RWTH Aachen University, Germany; 2Department of Earth Science, Liverpool University,

Liverpool, UK

ABSTRACT

It is well known from nature and experiments that the presence of brine strongly affects the microstructural evo-

lution and the mechanical and transport properties of halite. Existing interpretations of the grain boundary struc-

ture in deformed, wet, salt samples annealed statically at room temperature are based on indirect evidence from

reflected light microscopy and conventional scanning electron microscopy. This paper presents direct observations

of fluid-filled grain boundaries using the cryogenic-scanning electron microscope (cryo-SEM) in which the grain

boundary fluids were frozen before breaking the samples. The rapid cooling transforms the brine into two phases,

i.e. ice and hydrohalite, which are easily recognized from characteristic segregation patterns. We studied samples

of wet, synthetic, polycrystalline halite annealed under static conditions at room temperature. In coarse-grained

samples, fine-scale segregation patterns were observed at the boundaries of the primary recrystallizing grains.

These points indicate the existence of fluid films with a thickness in the range of 30 nm, but the finer scale struc-

ture of the fluid remains unknown. In fine-grained samples, the distribution and reorganization of fluids with

annealing time is recorded by the combination of contact healing and successive accumulation of fluids in triple

junction tubes. The contact healing is attributed to the small initial grain size, such that the fluid film necks down

by accumulating the fluids into previously existing triple junctions via neck growth. Electron backscatter diffraction

measurements of both primary and secondary recrystallized grains indicate that they are euhedral, i.e. the grain

growth morphology is controlled by the anisotropy of the grain boundary energy of the growing grain, which

results in planar growth faces.

Key words: cryo-SEM, fluids, grain boundary, halite, morphology, recrystallization.

Received 7 December 2004; accepted 16 September 2005

Corresponding author: Oliver Schenk, Geologie-Endogene Dynamik, RWTH Aachen University, Lochnerstr. 4-20,

52056 Aachen, Germany

E-mail: [email protected]. Tel.: +49 241 80 95416. Fax: +49 241 80 92358.

Geofluids (2006) 6, 93–104

INTRODUCTION

The presence of fluids has major effects on the dynamics

of geological processes in the Earth’s mantle and crust

(e.g. Fyfe et al. 1978). In minerals, such as quartz (Gri-

ggs 1974; Hirth & Tullis 1992; Jaoul et al. 1984; Kro-

nenberg & Tullis 1984; Post & Tullis 1998; Tullis &

Yund 1982), feldspar (Dimanov et al. 1999; Tullis et al.

1996), olivine (Mei & Kohlstedt 2000a,b), bischofite

(Urai 1983), carnallite (Urai 1985) and halite (e.g.

Spiers et al. 1990; Urai et al. 1986a), water-rich fluids

play a significant role in recrystallization, grain growth

and development of texture (Evans et al. 2001; Urai

et al. 1986a).

Halite has been investigated in detail because the inter-

play of fluid morphology and deformation can be studied

at relatively low temperatures and pressures (e.g. Urai et al.

1986b), allowing systematic laboratory experiments of

practical duration (Watanabe & Peach 2002). The results

are also relevant to other fluid-bearing minerals, including

partially molten rocks (Spiers et al. 1988).*Present address: Department of Geology and Geochemistry,

Stockholm University, 10691 Stockholm, Sweden.

Geofluids (2006) 6, 93–104

� 2006 The Authors, Journal compilation � 2006 Blackwell Publishing Ltd

The effect of fluids on grain boundary mass transfer in

halite has been demonstrated by numerous examples from

nature, and also by experiments during both fluid-phase

diffusional creep (pressure solution) (de Meer et al. 2002;

Hickman & Evans 1991; Martin et al. 1999; Peach 1991;

Schutjens 1991; Spiers & Schutjens 1990; Spiers et al.

1990) and fluid-assisted grain boundary migration (Peach

et al. 2001; Schenk & Urai 2004; Urai et al. 1986b;

Watanabe & Peach 2002). However, the structure of the

halite grain boundaries that contain water is still a matter

of debate. Firstly, for pressure solution, three different,

nonexclusive models have been proposed (den Brok et al.

2002): (i) the thin film model, (ii) the island-channel

model and (iii) the island-crack model.

In the thin film boundary model (Hickman & Evans

1991; Renard & Ortoleva 1997; Rutter 1976) the grains

are separated by a thin, structured water film with a thick-

ness of a few nanometres. This film is proposed to transmit

the contact stress, and dissolution followed by diffusion is

the process of transport of material. The island-channel

boundary model (Lehner 1990; Spiers & Schutjens 1990)

is based on the assumption that, during pressure solution,

the fluids residing in thin films are squeezed out between

the grains, resulting in solid–solid contact (islands)

through which the contact stresses are transmitted,

together with water-filled channels through which the

material transport takes place by diffusion. This microscop-

ically rough island-channel structure is dynamically stable.

The island-crack boundary model (den Brok 1998; Dysthe

et al. 2003; Gratz 1991) proposes static islands that are

separated by microfracture-controlled fluid channels. In

contrast to the solid–solid contact of the island-channel

model, the islands in this model contain thin films compar-

able to the earlier proposed thin film boundary model.

However, compared to the thin film model, the total diffu-

sivity in the island-crack model is increased by the presence

of the microcracks.

Secondly, experiments on wet polycrystalline halite

deformed at temperatures between room temperature and

150�C in the nondilatant field indicate that halite recrystal-

lizes readily both during and after deformation (Drury &

Urai 1990; Peach et al. 2001; Spiers et al. 1990; Urai

et al. 1986a,b; Watanabe & Peach 2002). The grain

boundaries are interpreted to contain thin fluid films. A

method to show the presence of such brine films in halite

samples is the application of the ether test (Spiers et al.

1986): cooling during evaporation of the ether disrupts

the fluid film into isolated droplets. Urai et al. (1986b)

demonstrated the presence of fluid films from scanning

electron microscope (SEM) observations on deformed,

water-bearing halite samples: 1 month after cessation of

deformation grain boundaries were smooth, whereas sam-

ples annealed for 1 year contained grain boundaries with

isolated cavities marking former fluid inclusions. The

authors interpreted these results as evidence of grain

boundary brine films that shrink into isolated fluid inclu-

sions after grain boundary migration stops. Similar obser-

vations were shown by in situ experiments conducted on

wet bischofite, where water-filled grain boundaries neck

down after cessation of grain boundary migration (Urai

1987).

The fluid-filled grain boundaries are interpreted to

migrate by (i) dissolution of the deformed grains, (ii) diffu-

sion through the brine film and (iii) precipitation on the

low-index facets of the recrystallized grains forming

smooth grain surfaces (e.g. Spiers et al. 1990; Urai et al.

1986a), comparable to the step model of Gleiter (1969).

However, so far the nature of such fluid films in migrating

boundaries has been inferred only indirectly.

A significant problem is that the observations were

made after removal of the stress, and this could have led

to a redistribution of the fluid by viscous flow of brine

(Hickman & Evans 1991; Peach et al. 2001; Watanabe

& Peach 2002). This problem was avoided in a recent

study in which the microstructural evolution of wet,

compacted, statically recrystallizing halite samples with

different initial grain sizes was presented (Schenk & Urai

2004). The microstructural evolution in Schenk & Urai

(2004) is briefly summarized in Fig. 1: in the coarse-

grained samples, primary recrystallization resulted in the

nucleation and growth of euhedral grains, which replaced

the original, deformed grains (Fig. 1A). Inside the fine-

grained samples, primary recrystallization was followed by

normal grain growth, but stopped after a few hours

because of contact healing (Fig. 1B). Exaggerated grain

growth (secondary recrystallization) is initiated at this

stage (Fig. 1C).

Schenk & Urai (2004) interpreted the growth of the

euhedral primary and secondary recrystallized grains to be

due to the presence of brine films on the grain boundaries,

as dry grain boundaries only migrate at temperatures above

approximately 400�C (Franssen 1993; Guillope & Poirier

1979). However, the details of the fluid distribution and

its influence on grain boundary migration were only partly

resolved, because fluid was removed during sample pre-

paration.

In this paper, we therefore set out to investigate directly,

and in detail, the nature of the fluid in grain boundaries

during different stages of recrystallization within com-

pacted, polycrystalline halite samples. To do this we stu-

died samples using the cryo-SEM. Small rock chips are

shock-frozen to a very low temperature (approximately

)190�C) and can be used for chemical characterizations

(Samson & Walker, 2000; Timofeeff et al. 2001) and for

visualization of the distribution of fluids in rocks, in partic-

ular to investigate qualitatively fluid–mineral interfaces

(Durand & Rosenberg 1998; Mann et al. 1994; Monma

et al. 1997). In order to characterize the crystallographic

94 O. SCHENK et al.

� 2006 The Authors, Journal compilation � 2006 Blackwell Publishing Ltd, Geofluids, 6, 93–104

orientations of grain boundaries we used electron backscat-

ter diffraction (EBSD) analysis.

METHODS

Experimental methods

Preparation of compacted samples.

The sample preparation technique was that used by Schenk

& Urai (2004). Here we summarize the procedure briefly.

Wet sodium chloride powder of analytical grade (Roth,

Art. 9265.3; NaCl content >99.9%) of two different grain

size classes (<10 and 200–355 lm) was compacted (cold-

pressed, T ¼ approximately 25�C) uniaxially with a pres-

sure of 150 MPa for 5 min. The resulting aggregates have

connected brine-filled porosities of less than 2%, together

with volumetrically very minor, localized occurrences of

air-filled pores. The samples were annealed at room tem-

perature (24 ± 1�C) over periods of up to 9 months in

small, air-tight containers with small amounts of saturated

salt solution. This solution was not in contact with the

samples, ensuring that the vapour pressure of H2O around

the samples was buffered at the equilibrium value. Only

samples 139 and 141 were stored in a wet salt mush (see

Table 1 for a detailed description of the compacted sam-

ples).

Analytical methods

Preparation for cryo-SEM observations.

For direct observation of the brine-filled grain boundary

sections of synthetic halite samples, we used a field emis-

sion scanning electron microscope (FESEM JSM-6300F,

JEOL) equipped with a dedicated cryo-preparation cham-

ber (CT 1500 HF, Oxford Instruments) at the Depart-

ment of Plant Cell Biology, Wageningen University, The

Netherlands. Where necessary, the samples were ground to

the required thickness of approximately 1.6 mm very care-

fully to minimize any damage, at a temperature of approxi-

mately 25�C. The sample was then placed into the slot of

the cryo-SEM holder and fixed with carbon conductive

cement (Leit-C, Neubauer chemicals). It was secured addi-

tionally by tightening carefully the screw of the holder.

Subsequently, the whole assembly (sample with holder)

was immersed in liquid nitrogen ()196�C). Once frozen,

the whole unit was transferred into the cryo-preparation

chamber at a temperature of )90�C and a pressure of

1.3 · 10)3 Pa (high vacuum conditions), in which the

sample was fractured using a cold knife ()196�C) at ade-

quate distance from the tightened holder (see sketch in

Fig. 2A). After approximately 5 min (sublimation stage),

the sample was sputter coated with 8 nm platinum and

subsequently transferred into the SEM on the sample

Fig. 1. Schematic illustration of the microstruc-

tural evolution of wet compacted, statically

recrystallizing halite samples. The processes and

grain size evolution of the samples described in

the present paper are identical to that described

in Schenk & Urai (2004). Note that normal grain

growth is inhibited, whereas growth of primary

recrystallized and exaggerated grains continue.

Table 1. Overview of samples described in this

paper. Sample Type of salt Starting grain size (lm) Annealing time (days) Observations

1-XXs Brine: Roth – 0.50 Segregation pattern

085a Roth 200–355 252.23 Primary recrystallization

111a Roth <10 180.97 Normal grain growth

138a Roth <10 29.93 Exaggerated grain growth

139* Roth <10 6.70 Normal grain growth

141* Roth <10 1.10 Normal grain growth

146 Roth 200–355 139.98 Primary recrystallization

152 Roth 200–355 0.50 Primary recrystallization

*Stored in salt mush.

Structure of wet grain boundaries in halite 95

� 2006 The Authors, Journal compilation � 2006 Blackwell Publishing Ltd, Geofluids, , 93–104

holder, at a temperature between )170 and )190�C. Ima-

ges were recorded digitally.

Bicrystal ‘calibration’ tests.

To interpret the frozen structure of brine inside the com-

pacted samples, we compared them with the structure of a

thin film of frozen, saturated sodium chloride solution

between halite plates. For this an industrially grown halite

single crystal was cleaved along {100} cleavage facets into

two thin wafers, which were bonded with a droplet of sat-

urated sodium chloride solution (Roth, Art. 9265.3; NaCl

content >99.9%) for 12 h, relying on capillary action to

avoid applying any external stress. Subsequently, this

assembly was inserted into a cryo-SEM holder and frozen

at a temperature of )90�C for 5 min, before it was frac-

tured inside the cryo-preparation chamber.

EBSD.

Detailed EBSD analysis was conducted on selected samples

to investigate the nature of grain boundaries of primary

and secondary recrystallized grains in the annealed samples.

To obtain a high-resolution EBSD pattern, the samples

(thick, compacted, unfrozen sections) were mechanically

polished using 1200-, 2400- and 4000-grade carborundum

paper, removing dust at regular intervals with a blast of dry

compressed air. To remove surface damage, the samples

were chemically polished in pure analytical grade methanol

for 10 sec, and then immediately and vigorously rinsed in

a jet of diethyl ether. Finally, samples were carbon coated

to reduce charging during EBSD analysis.

Samples were analysed in a field-emission gun (FEG)

CamScan X500 SEM at the University of Liverpool. Full

crystallographic orientation data were obtained from EBSD

patterns using a 20 kV acceleration voltage and a beam

current of 7 nA. EBSD patterns were auto-indexed using

the CHANNEL 5.03 software of HKL Technology. The cen-

tre of five to six Kikuchi bands was detected automatically,

whereby the solid angles calculated from the patterns were

compared with the calculated halite patterns originating

from 47 reflectors. Data were obtained by moving the

beam at a fixed step size of 2 lm. The average percentage

of EBSD patterns that could not be indexed ranged

between 30% and 35%; most of the unindexed analyses

were at high-angle grain boundaries. The maps were proc-

essed to remove erroneous data in order to provide a more

complete reconstruction of the microstructure (Prior et al.

2002). The accuracy of individual EBSD orientation meas-

urements is better than 1�. The misorientation angle

between grains was calculated by selecting the minimum

misorientation angle and its corresponding axis from all

possible symmetric variants (cf. Wheeler et al. 2001). We

present data obtained in a combination of displays: (i)

maps showing the spatial distribution of grains and their

crystallography in different grey scales and (ii) three-

dimensional representation of the crystallographic orienta-

tion of individual grains of special interest.

Observations

The microstructural evolution of the samples is identical

to that observed by Schenk & Urai (2004). Both the

Fig. 2. Cryo-SEM micrographs of the single crystal – brine setup: (A) two

thin wafers of halite single crystals (XX-1 and XX-2) were attached to each

other parallel to the {100} facets with a droplet of saturated brine and bro-

ken with a cold knife inside the cryo-chamber; (B and C) details of the seg-

regation pattern of the frozen brine film with the two phases, hydrohalite

and ice, the latter as negative imprint because of evaporation during subli-

mation.

96 O. SCHENK et al.


fine- and coarse-grained samples are characterized by a

porosity connected through triple junction tubes (to pre-

vent confusion we use the terms triple junction and triple

junction tubes solely in a geometric sense, without relat-

ing them to textural equilibrium as those conditions were

not attained in our experiments). In the present study,

grain boundary morphology during recrystallization was

studied on broken surfaces of samples annealed for differ-

ent periods using the cryo-FESEM. An overview of the

samples described in this paper and the main observations

are given in Table 1.

Segregation patterns of frozen brine

Setup with halite single crystals and brine.

In order to correctly identify the earlier presence of brine

at grain boundaries, we ‘calibrated’ our observations

against the segregation pattern on faces of a sample in

which brine was enclosed between the cleavage faces of

two single crystal wafers of halite (Fig. 2A). Because of the

rapid cooling, the saturated brine shock-freezes and is

transformed into the two phases, i.e. ice and hydrohalite

(NaCl · 2H2O) (Bodnar 1993; Roedder 1984). During

the sublimation period at the sample stage (5 min at

)90�C), the ice crystals evaporate leaving behind voids in a

hydrohalite foam (Fig. 2B,C) (we call the resultant texture

a ‘segregation pattern’). Accordingly, if such pores and

‘foam’ are observed in our samples, we suggest that they

are indicative of the earlier presence of brine.

General comparison of segregation patterns observed

in experimental samples.

Brine inclusions are common in halite, as illustrated in a

coarse-grained sample inside an original, deformed grain

(Fig. 3). This frozen fluid inclusion shows the characteristic

segregation pattern described above, indicating that it rep-

resents frozen salt solution, i.e. with the two phases hydro-

halite and (evaporated) ice. Inside both the coarse- and

fine-grained samples, similar patterns were observed in

pores, triple junction tubes and on grain boundaries, dem-

onstrating that they are also brine in a frozen state. Thus,

these patterns provide direct evidence for the distribution

of brine inside the compacted samples.

The volume increase of approximately 15% that is associ-

ated with the transformation of saturated brine into the

two phases hydrohalite and ice might result in misinterpre-

tations of the microstructure as the previously brine-filled

grain boundaries or triple junctions expand. However,

Fig. 3B shows that the shock-freezing did not result in

fracturing of the surrounding grain. Furthermore, this

photomicrograph illustrates that the size of the pores left

by the evaporated ice is five times smaller than that in the

single crystal brine setup (Fig. 2). As the cooling rate was

the same for all samples, it is suggested that the dimen-

sions of the segregated components are related to the vol-

ume of the fluid.

Observations on coarse-grained samples during primary

recrystallization

Inside the coarse-grained aggregates (initial grain size:

200–355 lm), primary recrystallization dominated textural

evolution. Nucleation of primary recrystallized grains

occurred in high-strain zones close to the contact regions

of original, deformed grains. These new grains are charac-

terized by a cubic shape, and replace the surrounding

deformed material. This microstructural evolution is docu-

mented by the three samples that were annealed for

0.5 days, 4.5 months and 8.5 months (samples 152, 146

and 085a, respectively; see Table 1). Segregation patterns

of the frozen fluids indicate that brine was present in all

samples. If three or more recrystallized grains (i.e. strain-

free grains smaller than approximately 20 lm) are in con-

tact, the frozen brine is only present in triple junctions

(however not in the sense of textural equilibrium) (Fig. 4).

Fig. 3. Cryo-SEM micrographs of a coarse, deformed grain in sample 152

with a frozen fluid inclusion. The segregation pattern is similar to that of

the saturated brine in the setup shown in Fig. 2. See Table 1 for a detailed

description of the sample (also relevant to all sample micrographs shown in

this paper).



Grain boundary migration occurs if a strain-free grain is

in contact with a deformed grain. Inside sample 146

(annealed for 4.5 months), some of these grains are larger

than 100 lm (Fig. 5). This contact zone between the new

grain (recrystallized) and the old grain (deformed) is char-

acterized by a very thin segregation pattern indicating a

fluid film with a thickness of less than 30 nm (Fig. 5C).

Additionally, tiny pores visible on the lower crystal face of

the deformed crystals suggest that they are related to fro-

zen fluids (see arrow in Fig. 5C). This is supported by the

fact that no air-filled pores were observed inside new

grains. Backscatter analysis indicates that there are no

measurable differences in chemistry between the original

deformed grain, the recrystallized grain and the solid phase

of the segregation pattern (cf. Heard & Ryerson 1986),

suggesting that second phases other than brine did not

influence this contact region. Inside the same sample, a dif-

ferent but rare situation is displayed (Fig. 6). Here, the

porosity is predominantly air-filled, as shown by the lack of

segregation pattern. However, remnants of a frozen fluid

phase are observed at the contact zone between a recrystal-

lized grain and a deformed grain. The thickness of this

fluid film is approximately 200 nm (Fig. 6B,C) with a seg-

regation pattern similar to that shown in the grain bound-

ary region of Fig. 5C.

Observations on fine-grained samples during primary

recrystallization, normal grain growth and exaggerated

grain growth

Microstructural evolution of the fine-grained samples (ini-

tial grain size <10 lm) starts with primary recrystallization,

followed by a reduction of irregularities in grain boundary

morphology and rearrangement of porosity. These proces-

ses occur only in the first hours (see Schenk & Urai 2004).

There is no sign of significant normal grain growth; how-

ever, at this stage exaggerated grain growth begins in some

samples.

Fluids are present in all the fine-grained samples, whe-

ther they were stored in brine saturated environment or

salt mush, regardless of the annealing time ranging from

1 day to 6 months (samples 141, 139, 138a and 111a)

(see Table 1 for the detailed sample description). During

Fig. 4. Cryo-SEM micrographs of sample 152. At the contact of the two

original, deformed coarse grains new, recrystallized, cube-shaped grains

start to grow. Segregation pattern in a triple junction points to the presence

of fluids.

Fig. 5. Cryo-SEM micrographs of sample 146 showing thin segregation

pattern between the primary recrystallized and the deformed grain. This

pattern [arrow in (C)] is interpreted to represent a frozen fluid film with a

thickness of less than 30 nm. Note that the deformed grain is characterized

by a complex cleavage pattern when compared to the defect-free grain.

98 O. SCHENK et al.


the first few days, fluids – indicated by the characteristic

segregation pattern – reside on all irregular and on some

curved grain boundaries, forming films with a thickness of

less than 150 nm, but predominantly they are present in

triple junctions or in pores that developed by impingement

of the growing grains (Fig. 7) (see also Elliot et al. 1997).

After 1 week of annealing at room temperature the

microstructure is characterized by solid–solid contacts

(Fig. 8; see solid arrow). The pores remain irregular

Fig. 6. Cryo-SEM micrographs of sample 146 showing a primary recrystal-

lizing grain growing into an old, deformed grain. The arrow in (C) points to

the remnants of a frozen fluid film at the contact region. Note that this is a

rare region in which porosity is predominantly air-filled.

Fig. 7. Cryo-SEM micrographs of sample 141 showing that fluids are pre-

sent in triple junctions and in grain boundaries in the early stages of anneal-

ing. Close to large pores the grains grow normal to the {100} facets;

however, at grain–grain contacts the grain boundaries are irregular and

curved. The details in (B) are interpreted to represent the first step of con-

tact healing (arrow).

Fig. 8. Cryo-SEM micrograph of sample 139 with the typical segregation

pattern of brine-filled porosity. The fine-sized grains grow with low-index

facets into the fluid-filled pore (dashed arrow). Note the high diversity of

apparent dihedral angles. The solid arrow points to an immobile solid–solid

contact region of two recrystallized grains.



because of the lattice-dependent (euhedral) growth of the

crystals into the porosity (Fig. 8; see dashed arrow) with

apparent dihedral angles varying over a broad range

because of impingement of the growing grains. An isolated

brine-filled inclusion (250 · 50 nm) (Fig. 9) is interpreted

to represent an inclusion left behind from a migrating

fluid-filled grain boundary. However, this might also be

explained by the presence of a fluid-filled pore resting on

the grain boundary. The microstructure of sample 111a

(annealed for 6 months) is characterized by predominantly

straight or smoothly curved grain boundaries without

interactions of fluids and an interconnected porosity with

fluids present only along triple junctions (Fig. 10).

Exaggerated grain growth is common inside these fine-

grained samples as shown by reflected light microscopy

and SEM (see EBSD pattern of sample 138a; annealed for

1 month; Fig. 11). However, cryo-SEM did not allow

detailed observations on the contact of exaggerated grains

with the fine-grained matrix, probably because of the

plucking-out of the large grains during the low-tempera-

ture preparation.

Crystallographic nature of grain boundaries of primary

and secondary recrystallized grains

After an annealing period of 8 months in a brine-saturated,

humid environment, the coarse-grained sample 085a (ini-

tial grain size 200–355 lm) contained several large, euhe-

dral grains. They show little to no lattice distortion within

any individual grain, and a dominance of boundaries

Fig. 9. Cryo-SEM micrograph of sample 139. Some grain boundaries are

fluid-filled, irregular and curved, whereas others are already healed. The

arrow points to fluid inclusion that was probably left behind during grain

boundary migration.

Fig. 10. Cryo-SEM micrograph of sample 111a showing that after months

of annealing the microstructure is reorganized with commonly straight,

fluid-free grain boundaries (dashed arrow) and fluids still present in triple

junctions (solid arrow).

Fig. 11. SEM and EBSD micrographs of fine-grained sample 138a indica-

ting that exaggerated grown grains are in fact euhedral, i.e. its growth is

lattice-dependent. (A) Band contrast analysis showing exaggerated grains

several orders of magnitude larger than the matrix grains. The fine-grained

matrix appears dark because of the abundance of grain boundaries, which

appear dark in a band contrast analysis image. Note the insets representing

the crystallographic nature of the individual exaggerated grains. These

show that the boundaries of the exaggerated grains are {100} facets. (B)

Same area as shown in (A) but showing grains in different grey shades.

100 O. SCHENK et al.


parallel to {100} facets (Fig. 12). The misorientation angle

between the facetted boundaries of the primary recrystal-

lized grains and surrounding grains is always >15�.Similar features are observed in the samples exhibiting

exaggerated grain growth. We investigated the crystallo-

graphic nature of the boundaries of such grains in sample

138a (see Table 1 for experimental details). In this sample,

most of the grains are very fine grained (<10 lm), with

only few significantly larger grains of up to 300 lm. The

straight boundaries of these grains already described in the

SEM backscatter analysis are characterized by crystal orien-

tations that are compatible with {100} facets (Fig. 11).

DISCUSSION

Our cryo-SEM observations of segregation patterns along

grain boundaries and triple junctions show that small

amounts of fluids are present in the majority of mobile

grain boundaries and in larger pores. In the cryo-SEM,

these boundaries have a resolvable structure, which indi-

cates fluid films being thinner than 30 nm. The small-scale

structure seen in Fig. 5C, however, can be interpreted in

two ways: it can be an island-channel structure or a segre-

gation pattern in a continuous fluid film. Thus, although

there is clear evidence of fluids in these mobile boundaries,

we cannot obtain conclusive information on the nanoscale

structure from our observations (Fig. 13) because of the

unknown morphology of the segregation pattern in very

thin, frozen brine films and because of the limited resolu-

tion of the SEM for structures smaller than a few nanome-

tres. The euhedral shape of the recrystallized grains is

Fig. 12. SEM and EBSD micrographs of the coarse-grained sample 085a

showing that (primary) recrystallizing grains grow with low-index facets

(euhedral growth) into the old, deformed grain: (A) The so-called band

contrast as analysed by the EBSD technique. Dark areas are commonly

deformed areas whereas light grey signifies undeformed grains. (B) Results

from EBSD analysis depicting recrystallized grains with little internal lattice

distortion [light grey in (A)] and dominance of grain boundaries that are

compatible with {100} facets. Insets show three-dimensional representation

of the crystallography of the respective grain.

Fig. 13. Schematic illustration of mobile, fluid-filled grain boundaries as

shown by segregation pattern from cryo-SEM observations (A). The arrows

indicate the euhedral growth of the primary recrystallizing grain (white) into

the deformed grain. The true nature of these boundaries in terms of a

semi-continuous fluid film (B) or an island-channel structure (C) cannot be

resolved with the cryo-SEM.



related to such fluid-filled grain boundaries (Fig. 5C). The

surface of the recrystallized grain is inferred to be an F-

facet. However, the nature of the fluid-filled grain bound-

ary, i.e. whether it is a semi-continuous fluid film

(Fig. 13B) or has an island-channel structure (Fig. 13C),

remains unclear.

The presence of fluids in grain boundaries agrees with

previous observations on mobile grain boundaries in wet

halite (Drury & Urai 1990; Peach et al. 2001; Schenk &

Urai 2004; Urai et al. 1986a,b; Watanabe & Peach 2002),

whereas experiments on dry sodium chloride show that the

grain boundaries are immobile below temperatures of

400�C (Franssen 1993; Guillope & Poirier 1979). The fact

that recrystallized grains are characterized by an euhedral

shape with a clear crystallographic relationship in terms of

facets (Figs 5, 11 and 12) is in agreement with observa-

tions of similar microstructures in other fluid-bearing,

recrystallizing materials (e.g. Kingery 1974; Skrotzki &

Welch 1983).

Such a preferred growth of primary or secondary recrys-

tallized grains is interpreted to be either (i) a result of the

high surface energy anisotropy of the wetted NaCl grain

boundaries, (ii) a growth mechanism similar to that seen in

crystal-melt systems where a ledge mechanism leads to the

euhedral shape according to the step model of Gleiter

(1969) or (iii) a combination of (i) and (ii). High surface

energy anisotropies are expected to play a major role in

fully wetted grain boundaries. Observations in olivine–

ultramafic melt systems showed that completely wetted

grain boundaries are often found parallel to low-index fac-

ets [(010), (110) and (021)] (Jung & Waff 1998). These

are similar to our observations of growing primary recrys-

tallizing and exaggerated grains normal to the {100} faces.

Walte et al. (2003) have questioned the importance of

surface energy anisotropy. They showed that completely

wetted grain boundaries can simply form by consumption

of small grains during fluid-enhanced static recrystalliza-

tion, and concluded that there is no need to relate the

structures to surface energy anisotropy, even though this

might enhance the effect.

Another possible explanation for the euhedral shape of

the primary and secondary recrystallized grains is the ledge

jump grain boundary migration mechanism described by

Gleiter (1969). The assumption of a fluid-filled grain

boundary with two solid–fluid interfaces and a fluid layer

in between is similar in terms of the sharp transition of

crystal lattice and adjacent grain boundary and the influ-

ence of the orientation of the crystal on the migration rate.

According to the step model, the motion of the grain

boundary in the presence of a driving force proceeds by (i)

dissolution of ions from favoured sites (steps) of the

shrinking old grain and from deformation-related disloca-

tions that reach the surface, (ii) diffusion through the fluid

layer and (iii) re-attachment at preferential steps of the

growing strain-free grain. The euhedral shape suggests that

diffusion is not restricted to the shortest distance. How-

ever, the fluid layer regulates (balances) the transport of

ions, such that they are precipitated at favoured steps to

preserve the character of the {100} facet.

Inside the coarse-grained samples, grain boundary

migration stops if two or more recrystallized grains come

into contact because of the reduction in driving force as

the grains have the same (low) dislocation density. Only

the grain boundary (surface) energy can drive further grain

boundary migration. In this situation, the grain boundary

fluid is accumulated along triple junctions leaving behind

healed, brine-free grain boundaries. These immobile solid–

solid contacts could have developed by boundary anneal-

ing, i.e. the surface energy driven attraction of grain

boundary fluids into the triple junction network, a process

that is controlled by the contact angle.

The cessation of normal grain growth inside the fine-

grained samples is also interpreted to be caused by such

boundary healing: below a critical grain size the fluid-filled

grain boundary contracts and accumulates in the triple

junction network because of the effect of surface energy

forces (Visser 1999).

CONCLUSIONS

Cryo-SEM observations offer direct evidence of fluid-filled

grain boundaries in statically recrystallizing wet, polycrys-

talline sodium chloride samples. The frozen fluid phase is

represented by the segregation pattern composed of the

two phases, hydrohalite and evaporated ice.

The thickness of such migrating brine-filled grain

boundaries is usually less than 30 nm. Finer scale structure

is obscured by resolution of SEM and segregation of brine

during freezing.

Primary recrystallized growing and exaggerated-grown

grains exhibit euhedral shapes with {100} facets. We

interpret this type of growth as a consequence of either

significant anisotropic grain boundary energy and/or a

solid-melt/brine type growth mechanism with a ledge

jump mechanism.

The results are in agreement with a model of brine-filled

grain boundaries during primary recrystallization and exag-

gerated grain growth, and healed grain boundaries in nor-

mal grain growth.

ACKNOWLEDGEMENTS

We are grateful to A. van der Aelst (Department of Plant

Cell Biology of Wageningen University, The Netherlands)

for his valuable assistance with the cryo-FESEM. The com-

ments on the phase conditions of the NaCl–H2O system at

low temperature by R. Bodnar are greatly appreciated.

H. Siemes is thanked for providing the NaCl single crys-



tals. M. Holness and C.J. Spiers are thanked for their thor-

ough and constructive reviews that improved the manu-

script. This project is funded by the Deutsche

Forschungsgemeinschaft (UR 64/4-1). S.P. acknowledges

financial support by Marie Curie Fellowship HPMF-CT-

2001-01457, NERC grant NER/A/S/2001/01181 and

HEFCE through the grant JR98LIPR.

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