Page 237 - Microtectonics

7.8 · Reaction Rims 227
Fig. 7.54.
a–f Various types of reaction
rims. Likely reactions causing
the rims are given below each
drawing. A–F are minerals in-
volved in reactions, P and Q are
not involved. The equations are
open since phases other than the
ones shown may be involved in
the reactions as well. g, h Two
types of pseudomorphic replace-
ments. i and j show how reaction
type can be established to some
extent from the arrangement of
inclusions. k and l show the sig-
nificance of oriented inclusions
in a reaction rim; the relative age
of minerals A, B and C is given.
In k the alignment of C is due to
passive inclusion of aligned
grains of C in A, and subsequent
growth of B; in l, C grew with a
preferred orientation in response
to a high differential stress. Fur-
ther explanation in text

zoning may also follow the outer rim or have a more com- mineral B that is abundant in the surrounding matrix,
plex shape due to growth-coalescence of smaller grains. the newly formed minerals may form a ring-shaped
Growth zoning can be truncated by grain boundaries if corona around A (Figs. 7.54d, 7.55, ×Video 7.54ad,
the crystal has been subject to erosion due to recrystalli- ×Photo 7.55a–d). In high-grade rocks, several coronas
sation or pressure solution. In some cases, reaction zon- may be superimposed on each other as concentric shells
ing of an element A may even be superposed on growth as double or composite coronas (Figs. 7.54e,f, 7.57, 7.58,
zoning of an element B (Tucillo et al. 1990). ×Video 7.54e,f). Careful monitoring and analysis of co-
rona structures can allow detailed reconstruction of the
7.8.2 pressure-temperature-time (P-T-t) path in a rock sample
Coronas and Moats (Sect. 1.3). A special form of coronas are atoll garnets; ring-
shaped garnets, usually filled with quartz and white mica.
If two dispersed minerals A and B in a rock react to form Atoll garnets possibly form as coronas on biotite or
new minerals, these may form local isolated reaction cordierite, where the core mineral is later replaced by an
rims along grain boundaries of A and B (Fig. 7.54a,b, aggregate of other minerals. However, an origin by growth
×Video 7.54ad). However, if a mineral A breaks down to along grain boundaries in coarse-grained quartz-mica
other minerals along its outer rim, or if it reacts with a aggregates has also been proposed (Bard 1986).