Geoscience Reference
In-Depth Information
103
seen as closely spaced, subparallel dark seams of
phyllosilicates and graphitic material along which the
rock can be split (
103
). A cleaved rock is divided into
two domains: cleavage domains consisting of strongly
aligned minerals, which are separated by microlithons
where the minerals may or may not be aligned (Walsh,
2002). Microscopical examination of the slate domains
and classification in terms of their shapes, spacing,
proportion, and fabric allows an understanding of the
factors controlling the thickness and texture of the slate
and its metamorphic grade. In general, with increasing
metamorphic grade, the spacing between the cleavage
domains decreases, the proportion of rock occupied by
cleavage domains increases, and the minerals in the
microlithons become more aligned (Walsh, 2003).
Crenulation cleavage consists of microfolds found on
the cleavage surface, owing to the development of
cleavage in a rock with a pre-existing fabric (
104
). If well
developed, it is an undesirable characteristic in slates that
are being quarried because it may cause the slate to break
too readily in a direction other than that of the primary
cleavage.
103
Microfabric of slaty cleavage seen in thin section.
Narrow, planar cleavage domains (black) pass
between microlithons including chlorite lenses (blue).
'Aberllefenni' slate (Wales); XPT, ×300.
104
G
RAIN
The grain is the secondary splitting direction of the slate
and is usually parallel to the direction of maximum
tectonic stress. This secondary plane of weakness is
recognized by quarrymen and exploited in the winning
of slate as the pillaring line. Grain is caused by some of
the platy minerals being aligned with their flat face
parallel to and/or elongated along the direction of
maximum stress during deformation. The grain may be
visible as a series of fine lineations on the cleavage
surface, but can often only be determined by
microscopical methods. This is particularly true if the
grain direction is nearly coincident with the primary
cleavage and makes a low angle with the cleavage
surface. In respect of roofing slate, the grain should
ideally run normal to the cleavage direction (along the
length of the slate) so that if the slate breaks under load
on the roof, the pieces will still be held in place by the
fixing nails.
104
Slate with crenulation cleavage, seen in thin
section. 'Dequesa' slate (Spain); XPT, ×150.
105
R
ELICT SEDIMENTARY FEATURES
The development of cleavage has often not obliterated
all signs of the original sedimentary bedding. Bedding
can be indicated by the occurrence of small-scale
banding of coarser and finer grain size and differing
mineral composition, running at an angle to the cleavage
(
105
). Fossils are rarely found in slates as the
development of cleavage usually results in the
105
Section through the primary cleavage of a slate
with relict bedding running at approximately 10°.
'Cwt-Y-Bugail' slate (Wales); PPT, ×35.
