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Tectonophysics. 96 ( 1983) 125% 151 Elsevier Science Publishers B.V.. Amsterdam - Printed in The Netherlands
125
WATER ASSISTED DYNAMIC RECRYSTALLIZATION AND WEAKENING
IN POLYCRYSTALLINE BISCHOFITE
JANOS L. URAI
Instrtuut ooor Aardwetenschappen, State lJniversit.p of Utrecht, P.O. Box 80.021, 3508 TA Utrecht (The
Netherlands)
(Received July 7. 1982; revised version accepted November 17. 1982)
ABSTRACT
Urai. J.L.. 1983. Water assisted dynamic recrystallization and weakening in polycrystalline bischofite.
Tectonophwcs. 96: 125-157.
Artificially prepared specimens of bischofite (MgC1,.6H,O) have been experimentally deformed at
temperatures between 20 and 100°C. strain rates between 10m4 and lo-” s- ‘, and confining pressures
between 0.1 and 28 MPa. Development of microstructure with strain was studied by in-situ deformation
experiments. and results of these were correlated with observations made on thin sections of deformed
samples.
In a first series of experiments the effect of grain size. impurity content and water content on the flow
behaviour was investigated. Addition of about 0.1 wt.% water to dry samples was found to decrease the
flow stress by a factor of 5. This effect was found to be associated with the formation of a thin fluid film
on grain boundaries. strongly enhancing dynamic recrystallization due to the movement of high-angle
gram boundaries. and possibly also to enhanced intracrystalline plasticity due to excess water present in
the lattice. In a second series of experiments the strain-rate sensitivity of the flow stress of selected
samples was investigated. Two regimes could be distinguished: one with a stress exponent n = 4.5 in the
power law creep equation for values of the differential stress above 2.0 MPa. and one with n = 1.5 for
stresses below this value.
The main deformation mechanisms were intracrystalline slip. twinning, and grain-boundary sliding.
Recrystallization occurred by subgrain rotation and high-angle grain-boundary migration. The rates of
grain-boundary migration fell into two different regimes, one regime being distinguished by extremely fast
migration rates. The applicability of the experimentally found flow law to the behaviour of bischofite
rocks in nature is discussed.
INTRODUCTION
Bischofite (MgCl, .6H,O) occurs in evaporite sequences. It represents the last
stage in the evaporation of seawater (Lotze, 1957); lo-100 m thick deposits of
bischofite have been reported from the lower Volga region (Kazantsev et al., 1976),
Gabon (Belmonte et al., 1965). and from the northeastern Netherlands (Coelewij et
004O- 195 l/83/$03.00 0 1983 Elsevier Science Publishers B.V
126
al., 1978). In the Zechstein deposits of Germany, however. bischofite is quite rare
(Fulda, 1931) and it is generally regarded as secondary.
The easily induced plasticity of bischofite has long been known. Dewar (1894)
reported that it could be extruded to form a wire, and Mtigge (1906) determined its
(110) twin glide plane. Geller (1925) studied the temperature dependence of the
extrusion pressure of bischofite, and Geller (1924. 1930) investigated the pressure
dependence of its incongruent melting point. Geller’s results showed that bischofite
is indeed the weakest of all salt minerals. Although of a limited occurrence.
bischofite layers have been noted for the drilling problems they cause related to the
high plasticity of this material (Strelets et al., 1968: Mukhin et al.. 1975).
The reason for investigating the flow behaviour of bischofite was twofold:
(1) A better understanding of its mechanical behaviour could give a contribution
to solving drilling problems and to the design of radioactive waste disposal sites
(Herrmann, 1980).
(2) It was possible to correlate the macrorheological date with observations on the
deformation of bischofite during in-situ experiments (Urai et al.. 1980: this paper).
The deformation experiments to determine the rheological behaviour of bischofite
were carried out as a joint project of the Koninklijke/Shell Exploration and
Production Laboratories and the University of Utrecht: results of these experiments
are also published in Van Eekelen et al. (1983).
EXPERIMENTAL PROCEDURES
Specimen preparation
Because of the hygroscopic nature of bischofite all specimen preparation was
done in a room with relative humidity of 15%. Cylindrical samples. 50 mm in
diameter and approximately 100 mm long. were prepared artificially. Grain size.
water content and impurity content were independently varied to obtain samples
with a wide range of combinations of the different values of these parameters. These
were chosen to cover the variation of these parameters which was expected to occur
in natural bischofite. The following methods of sample preparation were used to
vary the various parameters:
(1) Grain size:
(a) Fine-grained samples (grain size between 0.1 and 1.0 mm) were prepared by
compacting ground bischofite in an eudometer at 40.0 MPa and 110°C for one day-.
(b) Coarse-grained samples (grain size of around 10 mm) were prepared by
casting molten bischofite into an ingot. This preparation method, however. gav’e
some difficulties. When molten bischofite is cooled from above 130°C. between
120” and 117°C it becomes saturated with respect to MgCl, .4H,O. Precipitation of
this phase occurred very unpredictably in the form of a few vol.% of needle shaped
crystals.
127
(2) Water content:
Ground bischofite (grain size less than 1 mm) was dried at 15% relative humidity
and room temperature until no more weight loss occurred. This material still
contained around 0.1 wt.% extra water (above the stoichiometrically determined
crystal water). Samples with three different water contents were then prepared as
follows:
(a) Wet samples: the material was allowed to absorb about 0.4 wt.% of atmo-
spheric water before sample preparation (water content about 0.5%).
(b) Slightly wet samples: by using the dried material (water content about 0.1%).
(c) Dry samples by adding about 1 wt.% of the tetrahydrate (MgCl, .4H,O) which absorbed almost all remaining free water during specimen preparation (water
content < 0.1%) Similar samples were also prepared from material which was kept
at 15% relative humidity for one year.
(3) Impurity content:
(a) Pure samples were prepared from analytical grade MgCl Z .6H,O (Baker
grade)
(b) For impure samples, drillcores from the Veendam area (Coelewij et al., 1978)
were used. These contained impurity grains of halite, carnallite and kieserite (in total
less than 2 vol.%) and undoubtedly contained impurity atoms in solid solution
(Diarov and Dogalov, 1971).
Thin sections
Thin sections were prepared using a high viscosity cyanoacrylate resin (loctite)
and by grinding under a volatile oil (Shell S4919) Because bischofite recrystallizes at
room temperature when its surface is scratched (see Fig. 7) great care was taken to
avoid introducing artifacts by this.
Deformation apparatus
The specimens were ground to have plane-parallel ends, jacketed in rubber
between end-pieces of a high-strength-low-thermal conductivity material (cellaron)
with thin PTFE sheets to minimize end-effects.
The deformation apparatus (Fig. 1B) consisted of an internally heated pressure
vessel with oil as confining medium, and a piston sealed by O-rings. Axial load was
produced by a hydraulic load frame (MTS), operating either in stroke or load
control modes. Strains were defined with respect to undeformed sample length. No
correction was made for area changes during deformation, since the samples were
only deformed to relatively small strains. Temperature was controlled to within 2” in
the sample.
128
AU
( MPa) 26
24
22
20
16
14
12
Ill
8
6
4
2
LVDl
Heating element > Thetma ,-* COUple
B
Fig. I. A. Stress-strain curves for the samples deformed in the first series of experiments. All tests are at
6O’C. strain rate of 10-j s-I_ and a confining pressure of 28 MPa, while the samples had different grain
size, water content and impurity content. The large difference between dry samples and slightly wet and
wet ones is clearly shown. Solid lines: fine-grained samples: broken lines: coarse-grained samples. For
composition of each sample, see Table I. B. Schematic drawing of the triaxial cell for deforming 2 x 4 inch
samples.
Deformation experiments
Three series of experiments were performed:
(a) First the influence of specimen parameters, grain size, water content and
impurity content, on the flow behaviour was investigated by testing samples which
had different values of these parameters under fixed conditions of deformation:
temperature = 60°C strain rate = lop5 s- ‘.
(b) In the second series, samples which were impure, fine grained and slightly wet.
were tested at different strain rates and temperatures.
(c) In the third series, 0.1 mm thick wafers of bischofite were deformed under the
microscope in an apparatus discribed by Urai et al. (1980). Strain rate was lo-’ s- ‘.
temperature was varied between 20” and 110°C. Development of microstructure was
recorded by time-lapse photography.
129
RHEOLOGICAL OBSERVATIONS
First series of experiments
The stress-strain curves obtained from these experiments are shown in Fig. 1A
and a list of the experimental data is given in Table I. The influence of different
parameters is described below.
The accuracy of the stress measurements was within a few tenth of a MPa. Most
experiments were duplicated and reproducibility was generally within 5%.
The effect of specimen parameters
The effect of water content. The flow stress of slightly wet and wet samples, after
generally going through a slight maximum at 2-3% strain. reached a steady state
value above 4% strain. Dry samples showed markedly different behaviour. Their
stress-strain curves were of the work-hardening type: the flow stress steadily
increased approaching steady state at 10% strain. (In a few stepping tests conducted
with dry samples, however, constant flow stress was reached at strain rates of 10 --’
C’.)
Also, the absolute value of the flow stress at 8% strain was about five times higher
than in the slightly wet and wet samples. In Fig. 2 the flow stress is plotted versus
water content for the various samples. It can be seen that the main weakening effect
occurs within the first 0.1% of water added to the dry material, while some
additional lowering of the flow stress is caused by adding another 0.3%.
Another interesting phenomenon was observed in the experiments with wet
samples. When removing the rubber sleeve, the sample was found to be quite wet on
the outside, though it was assembled dry. Therefore, unlike experiments where some
fluid is put around the sample before deformation and is introduced into the sample
by dilatation pumping, fluids are expelled from the sample in this case. This can be
accounted for by grain boundaries containing brine which arrive at the sample
boundary. For this brine there will be no transport mechanism back into the sample
and the sample boundary will be gradually enriched in brine. Another possible
explanation is that some compaction takes place during deformation, by which fluid
is expelled from the sample. This effect may be responsible for expelling from salt
deposits large amounts of fluids generated during salt metamorphism (Herrmann.
1980a, b).
The effect of impurities. Impure samples tend to be slightly stronger than pure ones
(see Fig. 2). Although this is a fairly consistent result for all values of water content.
the variation between duplicated samples prevents firm conclusions from being
drawn.
131
l fine grained, impure
* fine grained. pure
A coarse grained
0.01 0.1 0.4 1.0
- MgCl2.6H20 % excess H20
Fig. 2. Plot of the differential stress at 8% strain versus excess water content for all samples deformed in
the first series of experiments. The main weakening occurs for excess water contents between 0.01 and
0.14. See text for discussion of the suggested decrease of this effect for water contents below O.Ol’%c.
The effect of grain size. In the comparison of the fine- and coarse-grained samples,
only a few coarse-grained (cast) samples (those with relatively few tetrahydrate
needles) were used for mechanical analysis (see Figs. IA and 2). The results suggest
that initial grain size has no or little effect on the flow stress. Strong support for this
interpretation comes from the microstructural observations: by dynamic recrystalli-
zation. the material will adjust its grain size to the imposed deformation conditions
(temperature, strain rate. stress) whatever the original grain size is.
132
The effect of confining pressure
As has been shown by numerous workers and for various materials. an increase of
the confining pressure from 0.1 MPa to a few hundred MPa generally suppresses
cracking and cataclastic flow. This enhances the ductility of the samples and
increases their flow stress. Above a certain value of confining pressure all the
cracking is suppressed and increasing the confining pressure has little efect on the
flow behaviour.
A number of samples have therefore been tested at atmospheric pressure to
investigate the effect of confining pressure. Before and after deformation. the v.olume
of the samples was measured to an accuracy of 0.2 vol.%.
(a) Compacted samples which were annealed for several days at 60°C were
deformed unconfined at room temperature using a strain rate of 10ei s- ‘_ Their
stress--strain curves are shown in Fig. 3. After a maximum at 2% strain. the flow
stress decreased continuously. These samples showed a volume increase of 5 -6F.
and their translucancy disappeared due to the development of fine cracks.
(b) A sample which was deformed to a strain of 6.5% (in several steps. in creep
mode, under a confining pressure of 28.0 MPa, see second series) was unloaded. and
deformed further at artmospheric pressure using the value of stress employed during
the last step. Figure 4 shows the creep curve. After a certain amount of primary
creep the strain rate decreased to a value about ten times less than the steady state
value obtained at a confining pressure of 28.0 MPa. The stress was then increased to
the value employed during the penultimate confined step. Again. the strain rate
decreased to a value about 10 times less than in the confined step.
As was the case for all other confined experiments, no significant volume increase
could be detected after the confined steps. whereas an increase of 0.7 v.ol.? vv.as
measured after the two unconfined steps.
In summary, while the material reaches steady state after a few percent strain at a
confining pressure of 28.0 MPa, during an unconfined experiment there is an initial
hardening associated with the onset of dilatation which is followed by progressive
Fig. 3. Tests at different confining pressures. The test at 10 MPa confining pressure is T20. See text for
discussion.
133
3-
& (%)
unconfined
3.0 MPa
time
I I I 200 400 600
time (hrs)
Fig. 4. C‘rsep CUI-WS of the last (unconfined) steps of T47. Broken lines show the steady state strainrates
which were arrived at with the same values of axial pressure and at 28 MPa confming pressure. Inset is a
sketch illustrating the difference in creep curves between confined and unconfined samples. See text for
discussion.
weakening of the sample due to cataclasis (see inset of Fig. 4).
An experiment conducted at a confining pressure of 10.0 MPa (T20, see Fig. 3)
showed that dilatation is already suppressed at that pressure. Thus the flow of
bischofite will be independent of depth at relatively shallow levels in the crust.
The second series of experiments
After the first series, the fine-grained, slightly wet, impure samples were selected
as being representative for bischofite occurring in nature, and these samples were
used to determine the strain rate sensitivity of the steady state flow stress at different
temperatures. at a confining pressure of 28 MPa. The results of these experiments
are shown in Fig. 5 and Table II. The procedure used with the stepping tests was
based on the following observation. After the samples had reached steady state (in
creep mode, after a few percent strain) the load was changed to a lower value. The
sample remained at the same length for some time, then it started deforming and a
new steady state strain rate was arrived at almost immediately (see Poirier, 1977).
This could be verified at higher strain rates where one could proceed to a few
135
TABLE II
Results of the second series of experiments
Test
No.
Temp.
(“C)
Strain rate
(s-l)
Differential
stress
(MPa)
Cumulative
strain
(%)
27 60 1.01’10~~ 5.6 7.26
1.00~10~6 3.4 7.46
30 60 1.03. loms 4.5 1.42
1.02. 1om6 2.5 1.78
31 60 1.01~10-5 4.0 1.24
1.02.10-h 2.4 1.75
38 60 l.03~10-5 20.8 8.03
4.10. 1om6 20.8 10.99
1.10~10-6 18.8 11.26
41 60 1.02.10m5 4.9 8.43
4.26. 10m6 4.0 10.10
1.05. l0-h 3.4 10.49
42 60 1.03.10-5 5.0 7.22
4.19.10m6 4.0 8.25
9.74.10-7 3.1 8.50
9.62. lo-’ 8.3 16.93
43 80 4.12.10m6 4.5 2.39
1.02.10m5 5.1 5.00
1.02. 1o-4 8.8 11.38
45 40 6.84. 10mh 7.5 5.1
1.71. 1om5 8.9 13.1
46 80 6.86. 1om6 4.5 4.51
1.69.10-5 6.2 11.9
1.05. lo-’ 0.8 12.66
1.95. lo-’ I .4 14.21
47 40 4.06. 1O-6 9.8 2.84
2.14. lo-’ 4.5 3.59
2.48. IO-’ 5.0 4.42
1.14. IO_’ 4.2 5.36
5.65. IO-* 3.2 5.92
1.53.10m* 1.7 5.97
2.29. 10m8 1.7 6.36
912. 10m9 1.0 6.64
48 80 1.35.10m’ 1.0 3.14
4.64. lo-’ 2.2 4.38
4.32. lo-’ 0.5 4.96
8.09. lo-’ 3.0 6.98
1.38.10-* 0.3 7.24
7.08. 10m6 5.3 10.10
49 60 1.40.10-’ 2.0 1.46
5.26. 1O-6 6.0 5.11
1.09.10m6 4.0 6.93
136
TABLE II (continued)
51 40
40
60
80
40
60
80
100
52 60
Test
No.
Temp.
(“C)
Strain rate
(s-l)
8.94. lo--’
2.13.10~’
3.65. IO-”
3.55.lo-x
8.91. lOmu
9.19. lomx
1.28.10-R
3.26. lo-’
1.22.l0-R
4.08. lomn
5.46.10-’
1.64.10~’
3.58. lo-v
8.22. IO-’
1.40, lo-’
1.99~10~*
1.26.10-6
2.27. lo-’
6.62. lo-’
1.95.1o-x
9.18.10~’
4.00.10-9
Differential Cumulative
stress strain
(MPa) (9)
1 .o 7.26
1.5 7.66
0.6 7.89
2.0 9.21
1.2 9.47
3.0 II.36
1.5 Il.88
8.0 2.08
2.0 2.47
2.0 3.36
2.0 4.15
I.0 4.98
I .o 4.89
1.0 5.02
2.0 7.60
6.0 2.73
6.0 3.97
4.0 4.34
3.0 5.50
2.0 6.14
1.5 6.65
I .o 6.91
percent strain in reasonable time, and drawing endorsement from this obsemation.
steps at low differential stress were done where the strain rate was determined after
only a few tenth of percent strain.
The value of the steady state flow stress was independent of the deformation
history, as was shown by returning to a previous value of strain rate after a few
steps. Also, observations on relaxation tests (Schmid et al.. 1977: 1980). which can
be interpreted in terms of a flow law, seem to indicate that the above method will
yield results describing steady state conditions.
Data of each experiment could be fitted by a power law of the form:
i=A(Au)”
where A and n are constants.
(I)
The constants A and n are well constrained for each sample. although there is a
variation from sample to sample (Table II).
Below values of the flow stress of around 3.0 MPa. at 40-60°C. and 1.5 MPa at
SO”C, the stress dependence of the flow law changes markedly:
around 4.4 to about 1.5. A and n values for each experiment in both
given in Table III.
137
n changes from
flow regimes are
The temperature dependence of the flow behaviour can be described by the
equation:
P = A, exp
where H is the apparent activation energy for creep and R is the gas constant.
The value for H was determined in several ways:
(1) For all experiments, the strain-rate values were extrapolated to stress values of
10.0 and 3.0 MPa in the high n regime and 2.0 and 0.5 MPa in the low n regime
(using the A and n values of each experiment). These values were than used in a plot
of log P versus l/T. Values of H were 14 + 10 and 16 f 7 kcal/mol, respectively.
Due to the large scatter between samples no distinction could be made for H values
in the high and low n regimes.
(2) In test no. 51. the steps were done at two stress levels at different tempera-
tures. The value of H was 8 * 5 kcal/mol. Unfortunately, it was not possible to
determine in which flow regime the measurement was taken.
(3) Geller (1924. 1925) determined the temperature dependence of the extrusion
pressure for a number of salts, among which bischofite. With a number of reasona-
ble assumptions. his data can be recalculated to yield a value of the activation
TABLE 111
\‘alurs for A and n for each experiment
Test no. High n regime
A n
low n regime
A n
45 2.4. lo- ”
47 2.8. lo- lo
27 3.6.10m9
30 2.2.10-s
41 5.1. lo- I0 42 5.6. 1O’9
49 3.1. 1o-9 52 l 4.1. lo- ‘” 43 2.4. 1O-9
46 0.9. lo-’
48 3.0.10~*
5.1
4.2 0.83.10-s 1.6
4.6
4. I 6.3
4.6
4.1 0.83~10~s 1.8
4.6 3.9, lo-9 2.3
4.85
2.9 1.3. lo-’ 1.1
3.2 1.23.10-’ 1.4
* Gram growth during preparation was less extensive than in other samples. possibly because water
content of this sample was slightly less than normal.
13X
energy. These assumptions are: (a) the rate of extrusion in all Geller’s experiments
was the same. (He raised the pressure up to a value so that extrusion just began.) (b)
The extrusion pressure at a given rate in Geller’s apparatus is proportional to the
uniaxial flow stress of the same material:
p=Ka (3)
where K is a constant determined by apparatus effects and extrusion rate (see Laue
and Stenger, 1976).
With these assumptions, a plot of the logarithm of the extrusion pressure versus
l/T yields a value for H of 20 k 2 kcal/mol when n = 4.4.
In summary, the value of H is around 15 kcal/mol with a rather large scatter
(i: 10 kcal/mol) while reinterpretation of data from Geller (1924. 1925) gives a
similar value but with less scatter. The present data do not allow a distinction
between H values in the two flow regimes.
MICROSTRUCTURAL OBSERVATIONS
The Veendam drill cores
From the point of view of applicability of the present data to the defornlation of
bischofite in nature, it is important to describe in some detail the microstructures of
natural bischofite.
The bischofite layers are quite pure (about 98% bischofite) and they contain
inclusions of halite, carnallite and kieserite (see Fig. 7). The halite and carnallite
form grains of up to 2 mm size. They are generally of a rounded shape.
Figure 6 shows a number contact drawings of the grain boundaries on polished
and slightly etched surfaces. Generally, some shape preferred orientation is present.
although cores with equiaxed grains also occur. The grain size is about 10 mm. An
interesting feature is that a few (about 1%) of the grains are completely idiomorphic.
This was also noted by Miigge ( 1906).
In thin sections the grains appear to be undeformed. and grain boundaries are
slightly curved. The grains sometimes contain growth twins. Deformation twins
(Troger, 1971) appear to be very rare. and are generally introduced by preparation.
Avoiding this is difficult, but because the twins are immediately apparent in the
grains as reflecting lamellae, their absence before grinding can be checked for (see
also Mtigge, 1906).
Fluid inclusions are quite rare inside the grains. Unfortunately the cores were
leached along the grain boundaries by the drilling fluid. so nothing can be said about
fluids at the grain boundaries. Thus, the actual water content of natural bischofite
could not be determined. In halite inclusions. however. fluid inclusions (along
growth surfaces) are quite common. Often, groups of grains have only a few degrees
139
Fig. 6. Contact drawing of the grain boundaries from a few bischofite drillcores. Black dots are halite or
carnallite inclusions. Scale bar is 50 mm. See text for discussion.
difference in orientation with respect to their neighbours. They are likely to have
formed by a subgrain rotation process.
Kieserite grains are often idiomorphic and twinned with grain sizes up to 0.3 mm.
Inside bischofite grains, they are often arranged in walls which form networks
closely resembling the present grain-boundary configuration in bischofite. These
kieserite walls represent the old (diagenetic or primary) grain-boundary configura-
tion before deformation. By extensive recrystallization the grain boundaries have
moved away. Observation of the kieserite network does indicate, however, that the
grain size of the bischofite stayed roughly the same during its history. This has also
been described for carnallite (Leng, 1945). When halite grains are present. the
kieserite is often concentrated inside them.
The undeformed samples
Compacted samples (pure and impure)
The milled bischofite contains grains which vary in size from 1 mm to a few
microns. By compaction, porosity is reduced to zero and the subsequent grain
growth by annealing produces optically strain-free, equiaxed grains with slightly
140
curve grain boundaries, frequently making an angle of 120 degrees at triple points.
Annealing twins were present in dry samples. while they are quite rare in slightly wet
and wet ones. Grain sizes were around 1 mm in slightly wet and wet materials. and
around 0.1 mm in dry samples.
In slightly wet and wet samples, most of the grain boundaries contained ara>s of
fluid inclusions. As their size was about 5-50 microns. they were studied in 0.5 mm
thick sections to avoid damage.
In the dry samples, the fine-grained aggregates of tetrahydrate are transformed
along their edges into hexahydrate, and the number of fluid inclusions on grain
boundaries was much reduced. (See Van Eekelen et al.. 1982. Fig. 1). The samples
had a milky translucent appearance.
Cast sampies (pure und impure)
The system MgCI,-H,O has been extensively studied by Grube and Brauning
(1938) and Dietzel and Serowy (1959). MgCl, .6H,O melts at 117°C incongruently
to give a saturated solution of MgClz. 4H,O with some crystals of M&I,. 4H,O
which dissolve at 129°C. During cooling, a typical ingot texture is formed. consisting
of a radially grown aggregate of elongated hexahydrate grains. with manv grain
boundaries parallel to (110). about 10 X 5 mm in diameter. with irregulari>- distrib-
uted needles of MgCl, .4H,O.
Deformed sampies
Deformation microstruclures
Because in the fine-grained samples it was impossible to tell the difference
between old grains and recrystallized ones, most of the information on the micro-
structural changes during deformation was obtained from the coarse-grained sam-
ples. During preparation of the thin sections bischofite could be obsened to
recrystallize at room temperature along scratches on the surface of the thin section
(Fig. 7). This raised the question of the stability of the microstructure after deforma-
tion. Additional information was obtained from the in-situ deformation experiments.
The results of this study are described together with the thin sections of deformed
samples, to illustrate and clarify certain aspects which could not be deduced from
thin section studies alone.
A marked difference in microstructure was found between dry and slightly wet
samples. As will be shown later, this could be interpreted in terms of recrystalliza-
tion behaviour, while the deformation mechanisms were largely the same for all
water contents.
Deformation mechanisms. Evidence for intracrystalline slip was found in grains
showing undulose extinction (Fig. 8). Subgrains with wavy boundaries were fre-
quently developed. The identity of these slip systems is as yet unknown. Twin glide
Fig. 7. Kieserite inclusions in a Bischofite grain from the Veendam drillcores, arranged along walls
indicating former grain-boundary positions. The bischofite grain is extensively twinned during specimen
preparatmn. The array of fine grains at the bottom of the photograph is a preparation artifact. Scale bar is
1 mm. crossed polarizers.
Fig. 8. Mechanically twinned grains together with ones showing undulose extinction in a dry sample
(T 29). Many of the small recrystallized grains contain growth twins. Scale bar is 0.1 mm. Crossed
polarizers.
142
on the (110) system in bischofite has been determined by Mtigge (1906). It is at all
temperatures the easiest deformation system (shear stress is below 0.05 MPa). The
ease of twin glide on this system is also demonstrated by the presence of kinks
produced by twin glide (Fig. 9). In some thin sections at least three different sets of
twins were observed. U-stage work is in progress to determine the identity of these
twin planes.
The presence of grain-boundary displacements was determined by a method
resembling that of Schmid et al. (1977). After deformation, on the initially smooth
surface of the sample, due to the softness of the rubber jacket grain boundary offsets
could be seen. The edges of these offsets, however, were relatively rounded, indicat-
ing the plastic deformation in areas adjacent to the grain boundary. This is caused
by the rapid recrystallization along shearing grain-boundary regions, transforming
the grain boundary into a thin shear zone, as could be seen on thin sections
extending to the edges of the specimen. Because of the above mentioned problems
only a rough estimate of the strain due to grain-boundary sliding could be de-
termined. Using:
~,s = cpvd (4)
where ‘p is a geometrical factor, o the average step height at grain-boundary offsets.
Fig. 9a. Sharp boundary between twinned and untwinned part of a grain: a “kink” formed by t% in glide
(T40). Recrystallization is initiated at grain boundaries. Scale bar is 0. I mm.. crossed polarizers. Very fine
grains (white spots) are artifacts due to grinding.
Fig. 9b. In-situ sequence illustrating the formation of the structure shown in Fig. 9a. Dry sample. room
temperature. Note the shape change of the initially spherical holes in the grains. Scale bar is 0.2 mm.
crossed polarizers.
and d the grain size (Bell and Langdon, 1969; Gifkins, 1973) about lo-20% of the
total strain is believed to be due to grain-boundary displacements.
Rec~wtailization. The development of new grains by progressive misorientation
across subgrain boundaries (Poirier and Nicolas, 1975) was frequently observed in
all samples. The size of these grains is relatively large, from one old grain generally
only a few new grains were formed (see Fig. 13). Subgrain walls were slightly curved
and the subgrains equiaxed, suggesting the presence of more than one slip system.
This process has been called “rotation recrystallization” (Poirier and Guillope,
1979). No fluid inclusions were found on subgrain boundaries having misorienta-
tions below a few degrees.
The migration of high-angle grain boundaries was the most important process of
formalion of new grains.
As the in-situ experiments showed, the migration could take place at two distinct
rates, about 0.1 mm/min and about 0.1 mm/hr while intermediate velocities were
not observed. The initiation of this process could take place either at twin boundaries
144
and intersections of twin lamellae, or at preexisting grain boundaries. This last
process is either due to high strain in grain boundary regions, or to the fact that
misorientation across a subgrain boundary has reached a critical value so the
boundary can start to migrate (Gottstein et al., 1976).
When growing into a grain containing twin lamellae, the new grains were
frequently elongated parallel to these (Fig. 10). Grain boundaries between twinned
grains, and recrystallizing grains which were much larger than the width of twin
lamellae showed a characteristic stepped shape, closely resembling the microstruc-
ture described by Calais et al. (1961) for alpha-uranium. These steps appear to be
caused by changes in orientation and interfacial energy along the boundary. Grains
growing into large old ones in coarse-grained samples sometimes showed a slight
tendency towards idiomorphism by having a boundary straight and parallel to (110).
The observation that most twin lamellae have parallel boundaries up to the grain
boundaries is somewhat puzzling. It can be understood by realizing that most grain
boundaries have moved after twinning and the original grain boundaries are rarely
visible (lensoid twins were seen to form in the in-situ tests).
The appearance of new grains formed by migration recrystallization was re-
stricted to the first stages of deformation where the coarse-grained samples were
transformed into fine-grained material. After this in the dynamically recrystallizing
Fig. 10. Elongated new grains growing along twin lamellar in a larger old grain (T40). Scale bar LS 0. I mm.
Crossed polarizers. Very fine grains are artifacts due to grinding.
145
aggregate only the migration of existing grain boundaries occurred, so that in a strict
sense, no new grains were formed.
The relation between microstructure, water content and temperature
High n regime. In general, increasing the water content had the same influence on
microstructure as increasing temperature. At 60°C (first series of experiments).
strongly deformed grains were quite common in the dry samples (see Fig. 8).
Recrystallized grains with grains sizes around 0.1 mm frequently contained anneal-
ing twins. The presence of both grain-boundary migration rates, as was observed in
in-situ tests (see Fig. 12), resulted in a bimodal distribution of recrystallized
grain-sizes {Fig. 11). Arrays of gas and fluid inclusions, indicating the position of old
grain boundaries in the undeformed samples. were found inside new grains, while
newly formed grain boundaries were clear and inclusion-free.
The slightly wet and wet samples had a strikingly different microstructure.
Mechanically twinned grains or grains with undulose extinction were very rare
giving the samples an optically strain-free appearance. It should be noted that thi:
microstructure developed during dynamic recrystaliization, although it would not
have been recognized as such using microstructural criteria which are generally
i m
*; _L
Fig. 11. Mechanically twinned old grain is replaced hy new ones. Because of the difference in
grain-boundary migration velocity. bimodal grain-size distribution is produced. T40. scale bar is 0.1 mm,
crossed polarizers.
Fig. 12. In-situ sequence illustrating recrystallization by two different-grain boundary velocities. A. In a
strongly deformed old grain new grains are slowly formed at C. B. C. New grain A starts growing wjith a
migration rate of 1 micron/set. D. Grain B, which has the same orientation as grain A. starts growing.
T = 80°C. dry sample. Strain rate = IOm5 s ‘. crossed polarizers. Scale bar is 0.2 mm.
applied for rocks like the presence of new grains showing undulose extinction (see
also Means, 1982).
The microstructure was formed by the complex interplay of subgrain rotation and
grain boundary migration (at the slow and fast rate). In the in-situ experiments. dry
samples at 60°C showed mainly slow migration of grain boundaries with an
occasional grain growing rapidly (Fig. 12) while at 100°C rapid growth was very
common. Wet samples displayed this behaviour already at room temperature.
indicating that an increased water content enhances the rapid grain boundary
migration.
The dynamically recrystallizing microstructure was thus mainly determined by
rapidly migrating grain boundaries, producing the typical microstructure shown in
Fig. 13.
Two aspects of this microstructure should be mentioned:
(1) Two populations of grains are present: often along boundaries of a new grain
a group of smaller ones which all have approximately the same orientation is
present (see grains 2 and 3 in Fig. 13). These could be observed to develop when an
Fig. 13. The microstructure resulting from the complex interplay of grain-boundary migration at two
different rates and subgrain rotation. (T29: coarse grained. slightly wet, pure sample.) See text for
discussion. Grains having the same number belong to the same “orientation family”. Scale bar is 1 mm,
crossed polarizers. The very fine grains, the fine twin lamellae and the air bubbles in the resin are
preparation artifacts.
old grain was swept by a rapidly migrating grain boundary. Very frequently the
rapid migration stops before the whole grain is consumed, leaving over the smaller
grains. There may be a number of reasons for this:
(a) The transition from slow to fast migration occurs catastrophically. During fast
migration (e.g., absorbing impurities or changes in grain-boundary structure) the
mobility of the grain boundary may drop so that the reverse catastrophic change
occurs and the migration rate drops to its slow migration value.
(b) The migrating grain boundary arrives at a part of the grain which has just
recently developed by slow migration and consequently has a low dislocation density
so the driving force for migration is much less in this region.
(c) The mobility of the grain boundary is also orientation dependent. If the
rapidly migrating grain boundary arrives at a subgrain boundary across which a few
degrees misorientation has developed. its mobility may decrease so that the reverse
catastrophic change in grain-boundary velocity occurs. In this subgrain the disloca-
tion density may also be much less because of the polygonization processes.
(2) Generally a grain boundary stops migrating when it arrives at the next
high-angle grain boundary. However, new grains are formed by the migration of
C D
Ftg. 14. Schematic drawing illustrating development of the microstructure shown in Fig. 13. Only the
effect of recrystallization is shown.
high-angle grain boundaries. so grain boundaries will be reactivated. The point at
which a grain boundary starts to migrate will also depend on the neighbouring grain,
so it will be possible that after a grain has become elongated by consuming a few
neighbours, it is cut in two by a migrating grain boundary. Thus, the microstructure
will consist of “orientation families”, formed by the propagation of one orientation
through the sample, while this orientation is destroyed by other migrating grain
boundaries, and modified by subgrain rotation and lattice rotation processes (Fig.
14). A slight shape preferred orientation perpendicular to the compression direction
was observed in most samples after deformation, showing the combined effect of
flattening of grains and an influence on the shape of new grains of the existing grain
boundary configuration.
~~c~o~~ru~t~~es in rhe low n regime. The microstructure developed in the low n
regime is shown in Fig. 15. The main difference compared with samples from the
high n regime is the stronger bimodality in grain size. resulting from a more
pronounced difference in size between the migrated grains and the ones “left over”.
The same microstructure was seen to develop at higher strain rates in the in-situ tests
at temperatures above 90°C. The changes in rheology as a function of temperature
and strain rate are thus well reflected in the microstructure.
Gruitt boundaries
In the deformed samples, high-angle grain boundaries contained arrays of fluid
Fig. 15. Typical microstructure of the samples deformed in the low n regime. Arrow, indicates a grain
boundag which has stopped halfway across an old grain. This IS the process by which the small grains on
the edges of the big ones are created. T5 1. crossed polarizers. scale bar is 1 mm.
and gas inclusions (see Fig. 16) wet samples containing more than the dry ones.
When studying wet or slightly wet samples in-situ, grain boundaries were optically
clear without visible fluid inclusions. During migration, however, they could be seen
to both adsorb and leave behind fluid inclusions. After deformation stopped, fluid
inclusions connected to the grain boundary by thin channels could be observed to
develop into arrays of isolated fluid inclusions by necking-down process (Lemmlein
and Kliya. 1960) (Fig. 17). This generally took place within one hour.
Static recystallization and stability of microstructure
When deformation is stopped, the samples statically recrystallize to a microstruc-
ture consisting of polygonal, equiaxed grains, closely resembling the undeformed
compaction samples. The extent of this process is strongly dependent on the
temperature. When the samples are rapidly cooled to room temperature, in dry and
slightly wet samples it becomes very slow, so that thin sections of these deformed
samples do give a true picture of the microstructure during deformation.
In wet samples, however, even at room temperature, the recrystallized texture is
overgrown by idiomorphic grains (Fig. 18). This microstructure is well known in
ceramics. it is believed to develop only when a small amount of a second phase is
Fig. 16. Fluid and gas inclusions on grain boundaries. These were found before and after drformatlon m
slightly wet and wet samples (T34). Scale bar is 0.1 mm.. partly crossed polarizers. thickness of that secuon
about 0.5 mm.
Fig. 17. In-situ sequence illustrating growth of isolated fluid inclusions in a wet bischofite sample. As is
argued in the present paper, the arrays of fluid and gas inclusions at grain boundaries are formed by this
process from a continuous fluid film which is present during grain boundary migration. Scale bar is 50
microns, plane polarized light. Time lapse between first and last photograph is 30 min. T = 50°C.
Fig. 18. Idiomorphic grain overgrowing the microstructure formed by dynamic recrystallization. This
microstructure IS also observed in natural bischofite. and is diagnostic for a small amount of a second
phase on the grain boundaries. Note the growth twins. Wet sample. scale bar is I mm. crossed polarizers. Blach spots are preparation artifacts.
present at grain boundaries (Burke, 1950). This microstructure is also found in the
Veendam drill cores (see p. 138) and is strong evidence for natural bischofite being 6‘ wet” and having a fluid at the grain boundaries. This should not negate the
applicability of the rheological data to natural bischofite. because in the natural
material the thickness of the grain boundary fluid film will be greater, as the grain
size is much higher.
DISCUSSlOh‘
The experiments described above have shown that increasing the water content of
bischofite causes a strong weakening of the material.
As will be argued below, above a certain value of excess water content there is a
fluid film formed on the migrating grain boundaries and this strongly enhances
grain-boundary mobility. This water assisted grain-boundary migration must be at
least partly responsible for the weakening in slightly wet samples. It can be inferred
from the smooth shape of the stress-strain curves, that this process is sufficiently out
of phase in different areas of the sample to prevent stress drops due to recrystalliza-
tion (Glover and Sellers, 1973).
152
It is by this process that, after stepping to a lower value of differential stress. the
samples relatively rapidly reach a new steady state. Driven by the strain energy
stored in the samples, recrystallization will continue until the sample reaches the
strain energy level corresponding to the new stress value. As recrystallization is a
relatively rapid process, this state will be reached much faster than by recovery
processes alone. It is proposed that the initial hardening during unconfined creep
tests can be understood when one considers that softening is caused by recrystalliza-
tion and this is mostly initiated by the migration of high angle grain boundaries. At
the onset of dilatation, grain boundaries open up to form cracks and most of the
sites for grain boundary migration are eliminated this way.
Although the solubility of water in the MgCI,. 6H10 lattice is not known.
microstructural observations indicate that there is also an intracrystalline effect of
water: the absence of undulose extinction and the large amount of subgrain rotation
in slightly wet and wet samples suggest the ease of climb processes in these samples.
This enhanced recovery will also cause weakening. Work is in progress to determine
the unit cell parameters of wet and dry bischofite. This may give an indication
whether or not water is present in the lattice.
Because the slightly wet and wet samples have a fluid phase present during
deformation. one must also consider the effect of fluid pressure on the flow. stress of
these samples. A number of observations have to be considered:
(1) In wet samples, fluid is expelled during deformation. thus the fluid pressure
must be at least equal to the confining pressure in this case.
(2) Tests done at atmospheric pressure have about the same values of flow stress
as the ones done at a confining pressure of 28.0 MPa. although in these experiments
fluid pressure can not have been much higher than 0.1 MPa.
(3) As will be argued below, a continuous fluid film will only be present on the
grain boundaries after some recrystallization has taken place (that is. after a few
percent strain) so at low strains fluid pressure will not be important. Howev.er. the
strong difference in differential stress between dry and slightly vvet samples is
already present at low strains.
In summary, as suggested by the difference in strength at low strains. about half
of the difference in flow stress between dry and slightly wet samples is thought to be
due to intracrystalline effects. while the rest of the difference will be mainly due to
recrystallization. Fluid pressure is thought to have a minor effect on the flow stress.
As is shown by the variation in A and n values between samples. the relativ-e
importance of the processes operating parallel-concurrently in each sample (disloca-
tion motion, recovery, dynamic recrystallization and grain boundary sliding). must
differ slightly between each sample (Gifkins, 1970).
Observations on grain boundaries in thin sections and in-situ tests are interpreted
the following way: during migration there is a continuous fluid layer present on
grain boundaries. Fluid inclusions encountered during migration are incorporated
into this layer, while when a grain boundary is pinned by. e.g., impurity particles.
153
cigar-shaped fluid inclusions are left behind. When deformation is stopped and a
grain boundary ceases to move, by the process of necking down. the film breaks up
into an array of gas and fluid bubbles (Lemmlein and Kliya, 1960). This is the
reason why in the thin sections of deformed samples one always only encounters
these bubbles. On the other hand, in dry samples, grain boundaries migrate away
from the fluid inclusions, and do not contain fluid themselves (Fig. 19).
An estimate of the thickness of the fluid film in slightly wet samples was made by
the following method. The total volume of fluid present in an array was measured by
counting bubbles in a grain boundary. Dividing their total volume by the counted
area gave the thickness of the fluid layer, about 500-1000 A. For wet samples, this
value is still much higher, up to 1 micron.
In the light of the observation that grain-boundary structure is strongly different
in dry and wet samples. the presence of annealing twins in dry samples (which has
been inferred to be due to differences in interfacial energy by Aust and Rutter. 1960)
is at least qualitatively explained. Because the change in grain-boundary properties
seems to be completed in our dry samples, it is suggested that decreasing the excess
water content even further will have less effect on the flow stress than between 0.1
and 0.0 1 o/r (see Fig. 2).
The microstructures in the bischofite cores can fully be interpreted in terms of the
deformation behaviour observed in the experimentally deformed material: subgrain
Fig. 19. Old grain boundary. indicated by an array of fluid inclusions is left behind by a migrating grain
boundary. This behaviour is only found in the dry samples. T40. crossed polarizers, scale bar is 0.1 mm.
154
rotation accompanied by the migration of high-angle grain boundaries. The growth
of idiomorphic grains after deformation has only been observed in the wet samples.
This is strong evidence for natural bischofite behaving as our wet samples.
Another reason for natural bischofite being “wet” is that it is present hetvveen
layers of carnallite and halite (Coelewij et al., 1978) and halite grains in bischofite
contain frequent fluid inclusions. Rock salt generally has water contents in excess of
a half a percent (Herrmann, 1980a, b; Roedder and Bassett. 1981). Because of its
hygroscopic nature, bischofite is unlikely to have lower free-water content.
This process of recrystallization assisted by a fluid layer on the grain boundary
could be important for other salt minerals. as these are known to contain fluid
inclusions on grain boundaries (e.g.. Roedder and Bassett, 1981). The water content
of domal salt is in the order of 0.1% which is of the same order of magnitude as the
amount necessary for weakening bischofite.
Also, in a recent work, White and White (1981) showed that grain boundaries in
tectonites are better described using data from ceramics than from metallurgical
studies: in the samples they investigated there were distorted layers up to a few
hundred Angstroms wide at the grain boundaries. Their observations of bubble
arrays on grain boundaries are comparable with those on grain boundaries in
bischofite after deformation. As the process of fluid inclusions forming from cracks
filled with a thin fluid film is well established for many silicate minerals. it may be
that the processes described above for bischofite are applicable to other rock-form-
ing minerals, although precipitation on grain boundaries can equally well account
for these structures (see also Wilkins and Barkas. 1978). An important process may
be the transformation of the fluid from inclusion arrays into a continuous film. The
presence of arrays of fluid inclusions indicating former grain-boundary positions in
natural halite shows that this process does not always occur (like in our dr)
samples). Under suitable conditions. however, this process may take place and cause
weakening. From this point of view, most of data on the deformation behav,iour of
salt should be taken with caution when trying to apply them to the in-situ behaviour
of salt rocks during diapirism, when they were shown to have contained much more
water then is present now (Herrmann, 1980).
Catastrophic changes in grain-boundary migration velocity due to impurity
effects have been described for halite (Guillope and Poirier. 1979) and sodium
nitrate (Tungatt and Humphreys, 1981). These have been interpreted in terms of the
metallurgical models for impurity controlled grain-boundary migration (Lticke and
Sttiwe, 1971; Poirier and Guillope, 1979). These models are based on assumptions
on the general nature of grain boundaries and the process operating during migra-
tion. It has been shown that the structure of the grain boundaries during migration
in bischofite is fundamentally different from those in metals: there is a fluid layer
several hundreds of molecules thick on the grain boundaries. Grain-boundary
migration occurs by solution due to higher dislocation density on one side (Bosworth.
1981), diffusion through the fluid and precipitation due to a local oversaturation on
155
the other side. The catastrophic change in grain boundary velocity may be explained
due to impurity effects on the growing crystal surface (Kern. 1969). This process
resembles that of the migration of fluid inclusions in salt crystals under the influence
of a thermal gradient (Anthony and Cline. 1974; Holdoway, 1974) both thermal
gradient and high dislocation density giving an increase in solubility of the mineral.
A difference is the different grain orientation on both sides of the water layer.
Thus, although the phenomena observed under the microscope are the same as in
metals, the underlying processes are fundamentally different.
While detailed analysis of the grain-boundary migration process in bischofite is in
progress, a few interesting observations can be made from the study of tine films
made from the in-situ experiments. The shape of grain boundaries during migration
is irregular, somewhere between lobate to serrated. Migration generally continued
until the other side of the grain was reached. However, in some cases migration
stopped halfway across the grain. Small parts of the grain boundary then moved in
the opposite direction. This was interpreted as being a readjustment of the shape of
the grain boundary to lower its surface energy. This was also observed to occur in an
in-situ experiment done with paradichlorabenzene in an apparatus, described by
Means (1981). using high strain rates (10-I s-l). In a few other cases. the grain
boundary then started moving backwards; similar processes, for which no explana-
tion is yet given. have also been described by Means (1982). In an attempt to explain
why grain boundaries move like this, Means (1982) considered the influence of
different rates of straining of the two grains, strain accomodation problems along
grain boundaries, and the differences in strain in “new” and “old” parts of the same
grain. Additional factors determining grain boundary movement can be recovery
processes reducing the dislocation density in a grain in front of a moving boundary,
and the above mentioned surface energy driven movements. Which of these processes
will be dominant is a kinetic problem.
ACKXOWLEDGEMENTS
This work. sponsored by the Dutch Organisation for Pure Scientific Research
(ZWO) was done in cooperation with the Koninklijke-Shell Research laboratories in
Rijswijk. The author wishes to thank A. Hulsebos, H. Groeneweg and H.A.M. van
Eekelen for their help with evaluation of the rheological data, running the experi-
mental program and many helpful discussions. Part of the in-situ experiments were
done when the author was at the Institut de Physique du Globe, Paris. Prof. J.P.
Poirier. J-C. Mercier and M. Guillope are thanked for many fruitful discussions on
dynamic recrystallization. The author is greatful to C.J. Spiers, J.P. Poirier, G.S.
Lister. H. Heard and I. van der Molen for discussions of an early version of the
manuscript.
156
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