ELSEVIER
Earth and Planetary Science Letters 163 (1998) 83–95
Cataclastic solution creep of very soluble brittle salt as a rock analogue
Bas den Brok Ł , Mohsine Zahid, Cees Passchier
Institut für Geowissenschaften, Johannes Gutenberg-Universität, 55099 Mainz, Germany
Received 16 April 1998; revised version received 9 July 1998; accepted 11 August 1998
Abstract
Until about the late 1960s, macroscopically ductile deformation of quartz was seen as a microscopically cataclastic
process by most geologists (cf. the origin of the name ‘mylonite’). Undulatory extinction, subgrains, recrystallised grains
and even crystallographic preferred orientations were interpreted as due to water-assisted brittle deformation processes.
Nowadays, by contrast, the occurrence of these optical microstructures is considered as conclusive and unequivocal
evidence for dislocation creep. The abundance of these microstructures in naturally deformed rocks lead to the conclusion
that dislocation creep is the most important ductile deformation mechanism within the Earth’s crust. We studied
whether a water-assisted brittle deformation mechanism could, in principle, be able to produce apparent ‘crystal plastic’
microstructures, and how. To this end we performed a long-term deformation experiment using soluble brittle salt (NaClO3 )
as an analogue for quartz. A single crystal of NaClO3 was uniaxially stressed for 44 days at room temperature in the
presence of a saturated NaClO3 solution under atmospheric pressure. The crystal was microscopically studied during the
experiment. It slowly deformed by cataclastic creep. First, irregular free face dissolution structures developed at highly
stressed portions of the crystal. These locally developed into very fine channel-like structures, mostly oriented sub-parallel
to crystallographic planes. Most of the channels developed into slits and finally into fractures. Fracture walls migrated by
solution transfer leading to changes in shape of the crystal fragments. The resulting polygonal deformation microstructure
could easily be mistaken for a dynamically recrystallised ‘crystal plastic’ microstructure. Therefore, care should be taken
to use such optical microstructures as conclusive evidence for dislocation creep. This is important, because the occurrence
of these microstructures is the strongest argument for dislocation creep in crustal rocks.  1998 Elsevier Science B.V. All
rights reserved.
Keywords: cataclasis; microcracks; deformation; ultrastructure
1. Introduction
Undulatory extinction, subgrains, and small new
recrystallised grains in arrays and at grain boundaries
are very common in rocks that underwent ductile
Ł Corresponding
author. Tel.: C49 (6131) 393843; Fax: C49
(6131) 393863; E-mail: [email protected]. Present
address: Geologisches Institut, ETH-Zürich, Sonneggstraße 5,
CH-8092 Zürich, Switzerland.
deformation under low and medium grade metamorphic conditions, especially in quartz [1–3]. The
presence of such structures is commonly explained
as a result of dislocation creep, and their abundance
in rocks is the major argument to support dislocation
creep as the dominant deformation mechanism under low and medium grade metamorphic conditions
in the continental crust (e.g., [4]). In earlier times,
however, these microstructures were interpreted as
cataclastic deformation structures (e.g., [5–8]) and
0012-821X/98/$ – see front matter  1998 Elsevier Science B.V. All rights reserved.
PII: S 0 0 1 2 - 8 2 1 X ( 9 8 ) 0 0 1 7 7 - 0
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B. den Brok et al. / Earth and Planetary Science Letters 163 (1998) 83–95
dislocation creep was not yet recognised to be an
important deformation mechanism. Undulatory extinction, subgrains, recrystallised grains and even
crystallographic preferred orientations were believed
to be due to brittle processes, i.e., due to fracturing
of the minerals, and subsequent slight translation,
rotation, and water-assisted healing of the fractures.
This idea was abandoned in the late 1960s, mainly
under the influence of new experimental deformation studies (e.g., [9,10]), first transmission electron
microscopy studies of deformed rocks [11,12], and
optical microscopy studies in which the deformation
microstructures were compared with those observed
in plastically deformed metals (e.g., [13,14]).
It was shown more recently, however, that optical
deformation microstructures labelled as undulatory
extinction, subgrains and small new recrystallised
grains may indeed develop by microcracking in
feldspar [15], in amphibole [16], and in quartzite
[17,18]. In the latter case, water-assisted microfracturing associated with solution mass transfer was inferred to be responsible for the development of these
structures [17,18]. Under the optical microscope, the
cataclastic microstructures reported in the different
studies are geometrically very similar to those in
naturally deformed rocks that are supposed to have
formed by crystal plastic processes. It was concluded
therefore, that these optical microstructures should
only be used as evidence for dislocation creep with
utmost care [15–18]. As long as it is not clear
how to distinguish between cataclastic undulatory
extinction, subgrains and recrystallised grains, and
crystal plastic undulatory extinction, subgrains and
recrystallised grains, it remains difficult to decide,
whether the one or the other of the two deformation
mechanisms prevails during ductile deformation in
the crust. If (water-assisted) cataclastic creep is more
important than commonly thought, crustal strength
profiles and rheological models of the crust need to
be modified, because these are based on the assumption that dislocation creep is the most important ductile rock deformation mechanism in the Earth’s crust.
Not much is known about cataclastic deformation
under conditions where solution mass transfer is
important. In order to improve understanding of
this process we decided to study cataclastic creep
‘in situ’, i.e., under the microscope and under wet
conditions, using a very soluble brittle salt (sodium
chlorate) as a rock analogue. This paper reports
on the development of a cataclastic deformation
microstructure in this material.
2. Materials and methods
Sodium chlorate (NaClO3 ) is a very soluble, extremely brittle salt, used by crystallographers as a
brittle end member in studies into the effect of
strain on crystal growth and dissolution [19–21].
It has a cubic crystal symmetry (point group 23)
and is optically isotropic. Its solubility (C), solute
diffusivity (D), and dissolution–precipitation kinetic
coefficients are comparable to those of NaCl: C is
¾100 g=100 cc water at 25ºC and 0.1 MPa [22]; D is
1:5 ð 10 9 m2 =s at 30ºC [23]; the interface kinetics
coefficient (i.e., the interface velocity for a thermodynamic driving force of 1 RT Joule per mole) of
the {100} face of NaClO3 is ¾30 µm=s [24,25].
NaClO3 is entirely brittle at room P–T conditions.
Vickers hardness on the {100} face is 117 kg=mm2 ,
comparable to that of calcite (124 kg=mm2 ) [26,27].
NaClO3 crystals were grown within several days
from unstirred aqueous solutions after self nucleation
on the bottom of a petri-dish. Supersaturation was
achieved by slow evaporation of the originally saturated aqueous solution at room temperature. The solution was prepared using NaClO3 from Merck (pure
quality; product number 1.06420). Crystals grown in
this way had a rectangular shape, controlled by {100}
crystal planes. A crystal of optically good quality was
selected for the experiment. It measured 1.5 (š0.05)
ð 2.6 (š0.05) ð 0.35 (š0.03) mm in size.
The experiment was carried out in a simple
see-through vessel, consisting of two glass slides
.50 ð 50 ð 1:3 mm) held apart by three brass-spacers
(Fig. 1). Glass and spacers were glued together with
Loctite UV-activated glue. Between the glass slides
a slot remained open of 0.461 (š0.003) ð 24.8
(š0.02) ð ¾40 mm in size. The single crystal was
inserted in the slot with its intermediate axis approximately parallel to the loading direction (LD), and
bedded in a matrix of fine grained sodium chlorate
(also from Merck; grain size 250 to 355 µm). There
was some free space left between the crystal and the
glass .0:11 š 0:03 mm). Crystal plus aggregate were
first compacted and then deformed.
B. den Brok et al. / Earth and Planetary Science Letters 163 (1998) 83–95
85
Fig. 1. Side view of experimental set up used (real size). (a) Set up used for compaction and (b) for deformation. Brass pistons and
spacers are ¾0.32 mm thin plates between ¾1.3 mm thin glass slides of 50 ð 50 mm in size. The set up was placed in vertical position
with a weight resting on the moveable piston. During deformation, a weight rested on the central piston, whereas the two outer pistons
were left without weight, thus being able to move outwards during deformation.
2.1. Compaction
The aggregate was compacted by inserting a
brass-piston in the slot. This piston measured 24.6
(š0.2) ð 0.32 (š0.02) ð 41 (š0.5) mm in size. It
was loaded which a dead weight of 2872 (š5) g,
corresponding to a calculated (nominal) pressure in
the aggregate of 2.46 (š0.02) MPa. Note that losses
due to friction are not taken into account here, and
that pressure was very probably heterogeneously distributed throughout the sample aggregate (due to the
presence of the larger single crystal). Displacement
of the piston was monitored with a dial gauge. Readings were logged at regular times using a stopwatch.
The aggregate was first compacted ‘dry’ (i.e.,
no solution added) for ¾19 minutes. It compacted
instantaneously (in seconds) by ¾16%, mainly by
grain boundary sliding in the aggregate. No measurable further compaction occurred during the rest
of the ¾19 minutes. The single crystal remained
completely undeformed.
After dry compaction, the piston was unloaded,
but held on its place, while saturated sodium chlorate
solution was added from above with a syringe, by
inserting its 0.3 mm diameter needle between the
piston and the vessel. The solution could be seen
impregnating the aggregate in seconds by capillary
forces. The upper part of the vessel was then closed
off with silicon grease to prevent evaporation of the
solution, and the piston was reloaded for ¾19 hours
with the same weight as during the dry compaction.
During this stage, the entire aggregate compacted
‘wet’ by another ¾16% (Fig. 2). Again, the single
crystal remained undeformed during this compaction
stage. Directly above and below the single crystal
compaction was intense (¾26%; Fig. 2). The rate
of compaction was approximately proportional to
the compaction strain to the power 3 (Fig. 3).
This behaviour is similar to that observed by Den
Brok et al. (manuscript submitted to Tectonophysics,
1997) for compaction of sodium chlorate in capillary
glass tubes, where compaction under similar P–
T conditions was inferred to be due to diffusion
controlled pressure solution.
2.2. Deformation
After compaction, the piston was slid out from between the glass slides and three new pistons were
inserted (Fig. 1b). The aggregate was now differentially loaded using the middle piston only. This piston
measured 9.83 (š0.04) ð 0.32 (š0.02) ð ¾41 mm in
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B. den Brok et al. / Earth and Planetary Science Letters 163 (1998) 83–95
Fig. 2. Wet compaction data of aggregate plus single crystal at room P–T conditions and nominal compaction pressure of 2:46 š 0:02
MPa. (a) Compaction strain versus time curves. (b) 10-Base logarithmic strain rate versus 10-base logarithmic strain curves. Curves
marked 1 are for bulk compaction of the entire aggregate plus single crystal. Curves marked 2 are for the aggregate directly above and
below the single crystal, where compaction was more intense because the single crystal did not deform during the compaction.
size. The outer pistons were left unloaded, and were
free to move outwards during the experiment.
Initially, a weight of 3867 (š5) g was applied on
the piston, corresponding to a calculated (nominal)
differential stress of 8:4 š 0:1 MPa on the aggregate plus single crystal. The aggregate deformed by
¾13% during the first day (Fig. 3), mostly by a
Fig. 3. Wet deformation data of sodium chlorate aggregate plus
single crystal at room P–T conditions. Piston displacement and
calculated strain are plotted as a function of time. Small black
dots are the measurements. Large open circles indicate moments
where the aggregate was unloaded and pictures were made.
Strain is indicated for the entire aggregate (including the single
crystal). During the first two days, calculated bulk stress was
8:4 š 0:1 MPa. From the second day onwards, calculated bulk
stress was 9:5 š 0:1 MPa.
combination of grain sliding, pressure solution and
cataclastic processes. The single crystal, by contrast,
remained completely undeformed. No signs of deformation could be microscopically detected after
the first day. Because the aggregate directly left and
right of the single crystal readily deformed, but the
single crystal itself remained intact, stresses on the
single crystal must have increased significantly during this deformation stage. Therefore, a much larger
part of the load may actually have been carried by
the single crystal than the calculated nominal value.
If all of the load were carried by the crystal, then it
would have had to support a load corresponding to
a calculated (nominal) stress of 42 (š4) MPa. This
is an upper limit. In experiments reported by Den
Brok et al. (manuscript submitted to Tectonophysics,
1997) dry single crystals of sodium chlorate were
able to sustain stresses of ¾21 MPa without measurably deforming. At higher stresses crystals broke
catastrophically. The crystal in our experiment did
not break after application of the load. The differential stress on the single crystal was therefore most
probably lower than ¾21 MPa. However, because
we did not know how large the friction was, neither
between the piston and the glass, nor between the
aggregate and the glass, and also because we did not
know how large the stresses were that were oriented
perpendicular to LD, we could not reliably quantify
the stress state during the experiment.
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87
After 1.9 days the load was increased to 4388
(š5) g, corresponding to a calculated (nominal)
stress of 9.5 (š0.1) MPa on the aggregate plus
single crystal, and 47 (š4) MPa assuming that the
load was carried entirely by the single crystal. From
then on the load was kept constant for about another
42 days. The entire experiment lasted 44 days. Piston displacement as well as bulk-aggregate-plus-single-crystal strain was plotted as a function of time
(Fig. 3). The entire aggregate deformed by ¾40% in
44 days, corresponding to an average strain rate of
¾10 7 =s.
In order to make pictures from the sample under
the microscope, it had to be unloaded. This was
done very carefully, but some small fractures may
have formed during some of the reloading events.
We believe, however, that these fractures formed
instantaneously, whereas the fractures of importance
to this study developed slowly while the sample
was under stress. Moments at which the sample
was unloaded are marked with large open circles in
Fig. 3.
3. Deformation microstructures
The single crystal deformed cataclastically. Characteristic optical deformation microstructures are
depicted in Figs. 4–7. After initial loading at a
calculated (nominal) stress of 8:4 š 0:1 MPa, and
9:5 š 0:1 MPa after 1.9 days, the single crystal remained entirely intact. Optical deformation features
(dissolution features and small microfractures) were
only observed after 5.7 days. During the 44 days
that the crystal was under stress (and repeatedly unloaded and reloaded for microscopical examination)
it did not collapse catastrophically, i.e., fracturing
remained subcritical. The crystal slowly became subdivided in a large number of fragments (or clasts).
These fragments translated and rotated only to a very
small extent. Fragment boundaries were mostly oriented roughly parallel to {100}, both parallel as well
as perpendicular to LD, but orientations oblique to
{100} and LD also developed. Two localities were
chosen to study the development of the cataclastic
microstructure in detail. These localities are further
referred to as ‘locality 1’ (Figs. 5 and 6) and ‘locality
2’ (Fig. 7).
Fig. 4. Microstructure of centre of single crystal after 33.5
days under stress showing abundance of healed microcracks
oriented parallel to the loading direction (LD) and showing also
presence of cataclastic structure (fragments) with boundaries
oriented approximately parallel to {100}.
3.1. Locality 1
Locality 1 was at one of the lower corners of
the single crystal. A cataclastic microstructure developed within 10 to 30 days. After 7.5 days from the
beginning, the first optical signs of deformation were
visible. Irregular free face dissolution features had
developed at the highly stressed lower side of the
single crystal, i.e., the side oriented perpendicular to
the loading direction(LD) (lower side in Fig. 5a, near
α). These features had not developed at the relatively
stress free side oriented parallel to LD. Also, an open
microfracture about 100 µm long, 5–10 µm wide
with a blunt tip had developed at β.
One day later, after 8.5 days from the beginning
(Fig. 5b), the free face dissolution structure at α had
grown, and the irregularities had become more reg-
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Fig. 5. Cataclasis at locality 1. Series of optical micrographs of lower corner of the single crystal illustrating the development of
cataclastic microstructure within ¾30 days. Scale is indicated in (a). Load was applied from above, i.e., maximum principal stress is
approximately vertical. Time elapsed after initial loading is indicated in the lower left corner of each picture. Visible on the left side
of all pictures are aggregate grains in ‘pressure shadow’ of the single crystal, and on the lower side intensely compacted and deformed
aggregate grains. For explanation see text.
ular, i.e. showing {100} crystal faces, both parallel
as well as perpendicular to LD. Two very small cube
shaped grains (10–30 µm in size) had formed out
of the rim of the single crystal at the lower side of
the structure at α. These small grains had boundaries
parallel to {100}.
After 9.3 days from the beginning (Fig. 6a), the
dissolution features at α had dissolved further into
the crystal. A ‘horizontal’ (i.e., perpendicular to LD)
fracture was now visible at α (directly right below
H in Fig. 6a). It had developed just there, where
the free face dissolution structure made a 90º angle
between two {100} crystal faces. The part of the
crystal directly below H had become an independent
B. den Brok et al. / Earth and Planetary Science Letters 163 (1998) 83–95
89
Fig. 5 (continued).
fragment (or clast), which it remained until it broke
into several smaller fragments about 10 days later
(Fig. 6e).
After 11.2 days from the beginning (Fig. 5c and
Fig. 6b) a major microstructural change had taken
place. A fracture had now developed to the right of
H (Fig. 6b). This fracture was about 10 µm wide
and 100 µm long and oriented parallel to {100}
and LD. Its tip was blunt (or rounded) and ended
in a channel structure at the surface of the crystal.
The channel got wider and less deep in the direction
away from the fracture and was oriented oblique
to {100} and LD, illustrating that microfracturing
is controlled by dissolution of the crystal at the
crack tip. The dissolution structure left from H also
developed into a fracture oriented parallel to {100}
and LD. The originally open ‘horizontal’ fracture
was now closed, and remained closed during the rest
of the experiment.
After 12.2 days (Fig. 6c), fragment H had separated from the single crystal and had become an
independent clast. The fracture to the left of H had
developed a jog to the right and propagated parallel
to {100} and perpendicular to LD to join the frac-
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Fig. 6. Enlargement of part of the microstructures developed by cataclastic creep at locality 1 and showing microstructural development
between 11.2 and 19.6 days in more detail than in Fig. 5c and d. Scale is indicated in (a). Maximum principal stress is approximately
vertical. Time elapsed after initial loading is indicated in the lower left corner of each picture. For explanation see text.
B. den Brok et al. / Earth and Planetary Science Letters 163 (1998) 83–95
91
Fig. 7. Development of cataclastic deformation microstructure at locality 2. Scale is indicated in (a). Maximum principal stress is
approximately vertical. Time elapsed after initial loading is indicated in the lower left corner of each picture. For explanation see text.
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B. den Brok et al. / Earth and Planetary Science Letters 163 (1998) 83–95
ture to the right of H. The fracture to the right of
H had achieved a staircase shape. A new ‘vertical’
fracture had developed to the right of J, the growth of
which had been preceded by a tiny surface channel
structure in Fig. 6b. Two days later (14 days from
the beginning; Fig. 6d), the fracture to the left of H
had grown ¾80 µm upwards and the surface dissolution structures to the right of J had become more
pronounced. Fragment H, and the fragment directly
below it, underwent little change in shape. After 19.6
days (Fig. 5d and Fig. 6e) fragment H had achieved
a more cubic shape. The fragment directly below it
had fractured completely. Now, J also developed into
a separate fragment, and the fracture to the left of
H had propagated significantly. Fig. 5d also shows
irregular dissolution features (channels) near P, Q
and R. These structures developed into fractures during the next days (Fig. 5e–g). Note, that fractures
developed at some but not all of the free surface
dissolution channel structures. Some of the channels
disappeared without development of fractures; others changed position (compare upper right corner of
Fig. 5d with Fig. 5e). Dissolution features also had
developed at the ‘vertical’ boundary of the crystal
(left side, in the middle).
After 26.4 days (Fig. 5f), many new fragments
(or clasts, or grains) had formed (A–G, K and L).
Fragment boundaries were oriented sub parallel to
{100}. The boundaries had only formed there, where
originally channels were present. During the rest of
the experiment (next ¾18 days) this microstructure
did not undergo significant changes.
One day later, after 8.5 days (Fig. 7c) the channel
structures had dissolved further inwards to form fractures (clearly visible to the right of φ). New channels
had also developed (above and left of φ). Interestingly, one day later (Fig. 7d) the channels to the
left of φ in Fig. 7c had disappeared. The ‘horizontal’ channel above φ had dissolved further inwards.
Note also tiny healed microfractures developed at
about 45º to {100} and LD branching off from the
upper fracture as well as the one directly above φ
connecting the two major fractures.
At the site where the two major fractures in
Fig. 7 nearly met (above φ; future grain A), a bridge
structure developed into an independent fragment (or
clast). This locality could already be identified as a
surface structure in Fig. 7b, but the fragment only
became independent after 14 days. In the beginning
its shape was irregular, but after ¾19.6 days its
shape was more rectangular. It further changed shape
(Fig. 7g–j), finally looking more like a recrystallised
grain than a fragment (or clast). Note also development of grain B and C by fracturing after 19.6 days
(Fig. 7g).
3.3. Microstructures in the surrounding aggregate
Three different microstructural domains could be
distinguished in the surrounding aggregate (labelled
I, II and II in Fig. 8). In domain I no remarkable
microstructural changes had taken place during the
experiment. Grains remained rounded and in touch
with each other, but no deformation of the contacts
3.2. Locality 2
Locality 2 (Fig. 7a–h) was near one of the upper corners of the crystal. After 7.5 days from the
beginning of the experiment, two relatively large
microfractures had developed here (Fig. 7b). The
fractures were oriented partly sub parallel to {100}
and LD, and partly at an angle of ¾45º to {100}
and LD. The fractures had developed by localised
dissolution. Both the upper and the lower fracture
ended in a channel structure, clearly visible for the
lower fracture right of φ in Fig. 7b. Fracture development was entirely controlled by the orientation of the
channels, which grew preferably parallel to {100}
and LD, but also perpendicular to LD.
Fig. 8. Schematic drawing of sample assembly (cf. Fig. 1)
indicating the different microstructural domains in the aggregate
surrounding the single crystal. (I) No microstructural changes;
(II) intense shortening parallel to loading direction, by grain
sliding, microcracking, and pressure solution; (III) precipitation
of dissolved material in the pores, but also deformation by grain
sliding, microcracking and pressure solution.
B. den Brok et al. / Earth and Planetary Science Letters 163 (1998) 83–95
did occur, nor did the contacts heal. In domain II
the initially porous aggregate had deformed by grain
sliding, solution precipitation creep and cataclasis
into a dense aggregate showing abundant healed microfractures oriented at ¾45º to LD (Fig. 6). Locally
the aggregate was pressed so densely against the single crystal that the boundary between the aggregate
and the single crystal could not be discerned any
more. In domain III the dominant feature was crystal growth on the grains. Most of the material must
have come from directly above and below the single
crystal. Here, the pore space was filled up almost
completely by the end of the experiment.
4. Discussion
Our observations on the cataclastic deformation
of the single crystal can be summarised as follows.
After several days, the first optical deformation features started to develop as irregular free surface
dissolution structures at highly stressed portions of
the crystal. Out of these structures, channel structures developed, mostly oriented both sub parallel
to {100} and to LD, but also parallel to {100} and
perpendicular to LD and=or oblique to both of these
directions. The channels developed into fractures by
localisation of the dissolution. Some of the channels, did not develop further, and disappeared; others changed position. The fractures dissolved their
way into the crystal, finally leading to fragmentation
(cataclasis). The boundaries of the fragments were
mobile, and mostly moved towards parallelism with
the {100} crystal faces. The fragments rotated and
translated only to a minor extent.
Nucleation, rate of propagation, and direction of
propagation of the fractures appears to be controlled
by free surface dissolution, as in dissolution controlled stress corrosion cracking [28]. The dissolution process was most likely driven by gradients in
elastic bulk lattice and=or surface energy, and not
by gradients in crystal plastic stored energy for the
following reasons. (1) Crystal plastic deformation of
sodium chlorate should not be possible at room P–
T conditions. Uniaxial deformation experiments on
single crystals of sodium chlorate have shown that
this material does not deform plastically at stresses
below the catastrophic failure stress. At room P–T
93
conditions it is a truly brittle material (resolution
of the strain measurement was 0.1%; den Brok et
al. 1998, manuscript submitted to Tectonophysics),
at least under dry conditions. The possibility that
there is some kind of local water-weakening of the
resistance to dislocation motion and associated nucleation of dislocations can not be excluded. (2)
Some of the dissolution channels did not develop
into slits and further into fractures, but disappeared
completely. Other channels changed position. This is
not compatible with a crystal plastic driving force for
dissolution. Elastic strain can be restored, but plastic
strain cannot (at least not at room P–T conditions,
where recovery is unlikely to occur). (3) Most of the
channels developed either parallel or perpendicular
to LD. Localisation of crystal plastic strain in zones
parallel or perpendicular to LD is not to be expected.
Elastic tensile strain energy has a significant effect
on the crystal growth rate in NaClO3 [19,20]. For
NaClO3 , an increase in the tensile stress by a factor
2 produced an immediate decrease in the growth rate
by a factor 2. On release of the tensile stress, the
growth rate return to the original value. The same is
true for potash alum, another very soluble brittle salt
[21]. This is believed to be due changes in bulk and
surface elastic free energy [19–21].
The deformation optical microstructures developed by the combined action of localised dissolution, fracturing and readjustment of the fragment
boundaries by solution mass transfer, may easily be
confused with optical deformation microstructures
resulting from crystal plastic dynamic recrystallisation, i.e., undulatory extinction, subgrains, recrystallised grains. Care should be taken therefore, when
using this type of microstructure as an argument for
dislocation creep.
5. Conclusion
The present study shows that an apparent ‘crystal plastic’ optical deformation microstructure may
be produced by cataclastic creep in soluble brittle
salt (sodium chlorate; NaClO3 ). In single crystals of
this material, fractures propagate by dissolving their
way inwards, generally sub-parallel to the {100}
crystal faces, both parallel as well as perpendicular
to the maximum compressive stress. In this way, a
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B. den Brok et al. / Earth and Planetary Science Letters 163 (1998) 83–95
polycrystalline aggregate of small grains (clasts) develops. Boundaries of the clasts are mobile and move
by solution mass transfer preferably towards orientations parallel to {100}. Localised stress-induced
dissolution plays a key role in controlling (1) the nucleation of the fractures, (2) the rate of propagation
of the fractures, (3) the direction of propagation of
the fractures, and (4) the adjustment in shape of the
fragments. If this type of cataclastic deformation also
operates in rock forming minerals, confusion with
crystal plastic dynamic recrystallisation microstructures (undulatory extinction, subgrains, recrystallised
grains) is likely to occur. Care should therefore be
taken with the interpretation of these microstructures. They should only be used as evidence for
dislocation creep with utmost care. Further systematic experimental studies are required to quantify the
observed cataclastic deformation process and preferably using real rocks to make extrapolation to natural
conditions possible.
[9]
[10]
[11]
[12]
[13]
[14]
[15]
[16]
[17]
Acknowledgements
[18]
This work was partially funded by the Volkswagen Stiftung. Win Means and Jan Tullis are thanked
for helpful reviews. [RV]
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