Page 66 - Microtectonics

54 3 · Deformation Mechanisms

Box 3.9 Fabric nomenclature
An extensive and confusing terminology exists for the descrip- grains of approximately equal size in a fine-grained equigranu-
tion of the geometry of grains and fabrics in metamorphic rocks lar matrix.
(see also Box 1.1). Below, we give some of the most important seriate – a complete gradation of fine- to coarse-grained.
terms, their meaning and their mutual relation (Moore 1970; Best
1982; Shelley 1993). The suffix ‘blastic’ refers to solid-state crys- Special terms for the shape of grain aggregates
tallisation during metamorphism.
granoblastic (less common crystalloblastic) – a mosaic of ap-
Shape of individual grains proximately equidimensional subhedral or anhedral grains.
Inequant grains, if present, are randomly oriented (Fig. 3.37).
The following terms describe the shape of individual grains and The term equigranular has a similar meaning but is not re-
can be used as prefix for ‘grain’ or ‘crystal’, e.g. euhedral crystal stricted to metamorphic rocks. Many granoblastic fabrics ex-
shape, anhedral grains (Fig. B.3.1): hibit a foam-structure (see main text; Fig. 3.39).
lepidoblastic – a predominance of tabular mineral grains with
euhedral – with fully developed crystal faces. Less commonly, strong planar preferred dimensional orientation (Fig. 4.8). This
the term idiomorphic is used, mainly in igneous rocks. The term is now generally substituted by a description of the folia-
term automorphic has similar meaning but is little used. tion (Fig. 4.7; compare the first and second editions of Williams
subhedral – with irregular crystal form but with some well et al. 1954, 1982).
developed crystal faces. Less commonly, the term hypidiomor- decussate – an arrangement of randomly oriented elongate
phic is used, mainly in igneous rocks (Fig. 3.9). The term grains (such as mica) in a metamorphic rock.
hypautomorphic has an equivalent meaning but is little used. reticular – arranged in lozenges with two common directions,
anhedral – without crystal faces. Less commonly, the terms as in a fishing net
allotriomorphic, xenomorphic and xenoblastic are used. granolepidoblastic – a combination of granoblastic and lepido-
acicular – needle-shaped. blastic fabric in the same rock. The term has become obsolete.
nematoblastic – a predominance of acicular or elongate grains
Three terms are commonly used for large grains with inclu- displaying a linear preferred dimensional orientation. This term
sions: has become obsolete as well, substituted by a description of
the mineral lineation.
poikiloblastic – with numerous, randomly oriented inclusions porphyroblastic – inequigranular fabric, with large grains that
of other minerals. The term poikilitic refers to a similar struc- grew during metamorphism and which are embedded in a
ture in igneous rocks. The term is mainly used for porphyro- finer-grained matrix (Chap. 7; Fig. 7.5).
blasts. mylonitic – see Chap. 5 for a detailed description of mylonitic
skeletal – refers to a spongy shape of a grain that occurs in thin fabrics.
seams between grains of other minerals that are nearly in con- flaser – a type of mylonitic fabric in which elliptical porphyro-
tact (Fig. 7.6). clasts lie in a finer mylonitic matrix. Since most mylonitic rocks
exhibit this kind of fabric, the term is not particularly informa-
Shape of grain aggregates tive and is therefore not recommended for metamorphic rocks.
(In sedimentary rocks the term flaser structure refers to the
The following terms can be used as a prefix for fabric, e.g. polygo- presence of small lenses of pelite in sandstone, indicative of a
nal fabric, decussate fabric (Fig. B.3.1): particular sedimentary environment).
clustered or anticlustered distribution of grains of a certain phase
Grain boundary geometry in a polymineralic aggregate refer to the tendency of grains of
one phase to group together (clustered) or to be spread out with
polygonal – with straight grain boundaries and consisting of minimum number of grains of that phase touching each other
anhedral or subhedral grains (e.g. Fig. 3.39). (anticlustered) – fields on a chess-board are perfectly anticlus-
interlobate – with irregular, lobate grain boundaries (e.g. tered. Notice that anticustered is not the same as random (Kretz
Figs. 3.30, 4.9). 1969; Kroustrup et al. 1988; Kruse and Stünitz 1999)
amoeboid – with strongly curved and lobate, interlocking grain
boundaries; like an amoeba. Grain shape can be quantified using the PARIS factor (Panozzo
and Hürlimann 1983). This factor quantifies the irregularity of
Size distribution of grains the grain boundary and is defined as the ratio of the actual length
of a grain boundary divided by the length of the outline of the
equigranular – all grains with roughly equal size. grain projection (imagined as a rubber band tied around the
inequigranular – non-gradational distribution of different grain). A PARIS factor of 1 is a smooth round grain; values are
grain size; an example is a bimodal distribution, with large progressively higher for interlobate and amoeboid grains.

This process is also known as Zener pinning (Nes et al. metamorphic grade. Similarly, in micaceous quartzites,
1985; Evans et al. 2001). Especially the presence of small pure quartzite layers are usually much coarser than
graphite grains in a rock may hamper the growth of other quartz-mica layers (Fig. 3.41, ×Video 3.41).
minerals (Krabbendam et al. 2003). This is the reason why A process similar to GBAR is Ostwald ripening or
many graphitic schists are fine-grained, even at high liquid-assisted static recrystallisation (Lifshitz and Slyozov