Page 241 - Microtectonics

7.8 · Reaction Rims 231
7.8.3 grates into one of the grains and leaves a symplectite of
Symplectites A' + B behind (Fig. 7.61b). Exsolution symplectites can
potentially be used as a temperature gauge (Sect. 9.9).
If two or more minerals are present in a corona, they may Symplectites may be affected by grain boundary area
form a symplectite in which the minerals form an reduction (GBAR; Sect. 3.10) that causes the lamellae to
intergrowth of lamellae that may be straight, curved or neck and obtain a globular shape. Such symplectites have
vermicular (Figs. 7.57–7.61). Symplectites are thought to been named globular symplectites (Fig. 7.61c,d). They are
develop due to relatively rapidly proceeding reactions, particularly common in high-grade rocks; all examples
or lack of a fluid phase to transport material towards and in Figs. 7.57–7.60 can be classified as globular symplec-
away from the reaction site. Nucleation sites of symplec- tites. Symplectites that are not or little affected by GBAR
tite may be stress-controlled as well as P-T overstepping and in which lamellae are elongate with parallel bounda-
controlled (Simpson and Wintsch 1989). ries are known as lamellar symplectites (Fig. 7.61a,b). They
Most symplectites are reaction symplectites that form are commonly formed during metamorphic retrogression,
by reactions of the type A+B+… ⇒ C+D+ … or especially in eclogites.
A+… ⇒ C+D+ …, where C+D form the symplectite Kelyphite or kelyphytic structure is the name for a
(Figs. 7.54b,d; 7.61a,c, ×Video 7.54ad). Discontinuous symplectitic corona structure around olivine, commonly
precipitation reactions of the type A ⇒ A' + B constitute in several concentric layers, which may contain ortho-
a special type that will be loosely referred to as ex- pyroxene, clinopyroxene, amphibole and spinel or gar-
solution symplectites (Fig. 7.61b,d). They develop when net. Spinel or garnet may be in symplectitic intergrowth
two grains of a supersaturated solid solution A with with hornblende or orthopyroxene. Amphibole-plagio-
different orientation but identical chemistry are juxta- clase symplectites have also been called kelyphitic if they
posed along a grain boundary. The grain boundary mi- form clearly defined coronas.
A myrmekite is a bulbous symplectite of vermicular
quartz in plagioclase (×Photo 7.54b). It is common in
high-grade metamorphic and igneous rocks, mostly as
breakdown product of K-feldspar during retrograde
metamorphism (Smith 1974; Phillips 1974, 1980; Shelley
1993). Myrmekite may develop at stress-concentration
sites during progressive deformation (Simpson and
Wintsch 1989) and in that case can serve as a shear sense
indicator (Sect. 5.6.9).

7.8.4
Establishing the Nature of Reactions

In coronas and symplectites it is important first to es-
tablish which components are likely to be new minerals,
and which are old grains. An important principle is
that silicates usually have slow diffusion through the
crystal lattice, and that reactions therefore mainly oc-
cur at grain boundaries where grains are in contact
with each other or with the scarce metamorphic fluid.
It is therefore usually assumed that a corona or sym-
plectite grows from the outside inwards, and that large
grains with irregular shape, completely surrounded by
another mineral, are older than this surrounding mate-
rial (Figs. 7.54, 7.56, ×Video 7.54ad–l). In exsolution
symplectites, growth is thought to proceed from the
grain that is in crystallographic continuity with one of
the phases of the symplectite, towards the second grain
Fig. 7.61. Symplectites can be subdivided into reaction (a, c) and (Fig. 7.61b). Replacement commonly also takes place
exsolution (b, d) symplectites, depending on the reactions that take along cracks in old grains, and old grains therefore may
place. Another possible subdivision is in lamellar (a, b) and globu-
lar (c, d) symplectites. Globular symplectites are thought to develop be split into various fragments (Figs. 7.54g,h, 7.57,
from lamellar ones through grain boundary area reduction, espe- ×Video 7.54g). If replacement of the old grain is ad-
cially at high metamorphic grade vanced, its remnants can be recognised by the fact that