Page 266 - Microtectonics

258 9 · Natural Microgauges
deformation twins in calcite has been proposed as a tem- on r- and f-planes (Burkhard 1993). At temperatures
perature gauge (Fig. 9.7a–e; Jamison and Spang 1976; above 250 °C, twins obtain serrated boundaries due to
Mosar 1989; Ferrill 1991, 1998; Burkhard 1993; Ferrill twin boundary migration recrystallisation (Figs. 3.20,
et al. 2004). Narrow straight twins (less than 1 µm wide – 9.7a,e; Sect. 3.7; Type IV twins of Vernon 1981; Burkhard
Type I of Burkhard 1993; Figs. 9.7a,b) indicate tempera- 1993; Rutter 1995; Ferrill et al. 2004), which may sweep
tures below 200 °C and dominate below 170 °C. Wider grains, and other types of dynamic recrystallisation may
twins which can be optically resolved (Type II > 1 µm) also occur. However, in large calcite crystals common in
dominate above 200 °C up to 300 °C (Figs. 9.7a,c; vein calcite, high strain rate at low temperature may lead
Groshong et al. 1984; Rowe and Rutter 1990; Evans and to a high dislocation density, and recrystallisation may
Dunne 1991; Ferrill 1991; Ferrill et al. 2004). The reason occur at temperatures below 250 °C (Kennedy and White
is that increasing strain at temperatures below 170 °C 2001; Ferrill et al. 2004). Complete dynamic recrystalli-
leads to growth of new twins rather than widening of sation of calcite may occur above 300 °C (Evans and
older ones (Fig. 9.8); above 200 °C, widening of existing Dunne 1991; Weber et al. 2001).
twins dominates over the creation of new ones (Fig. 9.8; The width of exsolution symplectites (Sect. 7.8.3)
Ferrill 1991, 1998; Ferrill et al. 2004). It may be necessary formed by isochemical reactions of the type (A ⇒ A' + B)
to use a U-stage to distinguish narrow and wide twins, can be used as a temperature gauge if properly calibrated
but all twins can be recognised by parallel colour fringes (Joanny et al. 1991; van Roermund 1992). Such symplec-
in sections oblique to the twin plane (Spang et al. 1974). tites form by grain boundary diffusion mechanisms re-
At temperatures above 200 °C, Type III intersecting twins sulting in the nucleation of mineral B in the reacting in-
and bent twins are present (Figs. 9.7a,d). Bending of terface as it migrates into the supersaturated adjacent
twins is thought to be due to activity of dislocation glide parent mineral. Under such circumstances, grain bound-

Fig. 9.7.
a Schematic illustration of the
influence of temperature on de-
formation by calcite twinning
(after Burkhard 1993; Ferrill
et al. 2004). b–e Photomicro-
graph examples of different twin
types (all in crossed-polarized
light). b Type I twins from the
northern Subalpine Chain, France
(sample 87–25d, Ferrill 1991).
Width of view 0.68 mm. c Type II
twins from the North Mountain
thrust sheet (sample W91) in the
Great Valley, Central Appala-
chian Valley and Ridge Province
(Evans and Dunne 1991). Note
that thin twins are locally devel-
oped within thick twins. Width of
the photomicrograph is 0.22 mm.
d Type III twins from the Ardon
thrust slice of the Diablerets
nappe (sample 691.1) in the
Helvetic Alps (Burkhard 1990).
Width of view 0.14 mm. e Type IV
twins from the Doldenhorn nappe
(sample 199.3) in the Helvetic
Alps (Burkhard 1990). Width of
view 0.14 mm. Photomicrographs
b and c are from ultra thin sec-
tions (thickness of approximately
5 microns or less). (After Ferrill
et al. 2004. Reproduced with per-
mission of Elsevier)