Synthetic calcite rocks of controlled grain-size were prepared by crushing large, clear calcite crystals, centrifuged to separate restricted grain-size fractions, cold-pressed and then hot-pressed to produce low-porosity polycrystals of mean grain-size covering the range 2–40 µm. These were deformed dry over the range 400–700°C, mainly at 200 MPa confining pressure, to investigate the onset with decreasing grain-size of grain-size sensitive flow.
The deformation of the fine-grained material can be described by a flow law of the form
ė
=
A
exp (−
H/RT
) σ
n
d
m
in which at low stresses (<25 MPa differential stress)
A
= 10
4.9
,
H
= 190 kJ mol
−1
,
n
= 1.7 and
m
= −1.9, when strain-rate,
ė
, is in s
−1
, stress, σ, is in MPa and grain-size,
d
is in µm. At higher stresses (25–250 MPa)
A
= 100,
H
= 190 kJ mol
−1
,
n
= 3.3 and
m
= −1.3. In the low stress regime, grains remain equidimensional during flow, a pre-existing preferred crystallographic orientation produced during cold-pressing tends to weaken and grain-boundary sliding is important. At higher stresses, a grain flattening fabric develops, the pre-existing preferred crystallographic orientation pattern is preserved, and recovery-accommodated intracrystalline plastic flow is inferred to be the dominant deformation mechanism. Grain-size sensitivity of the flow stress persists even into the intracrystalline plastic flow regime. Grain-size insensitive flow only develops at grain-sizes greater than 40 µm, as demonstrated by reference to mechanical data for the flow of Carrara and Taiwan marbles.
The flow laws which describe most of the experimental data do not extrapolate well through the lowest strain-rate data at the coarser grain-sizes. This indicates that a complete description of the flow, which can be used reliably to extrapolate to geological conditions, requires one or more of the material parameters
A, n
and
m
to be a function of one or more of stress, strain-rate and grain-size. This implies variable contributions from different deformation mechanisms as the deformation conditions change.
