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The fabric of slate
Overview
The fabric of slate can be studied by means of a light microscope
to figure out the main features. To obtain a more detailed insight
in the fabric a scanning electron microscope (SEM) can be used. With
this method the micrometer magnification increases from 100 µm
to several 1000 µm (100 µm = 0,1 mm = 0,003937 in.).
One has to distinguish between two main slate types: the intracratonic
and the orogenic slates. Intracratonic slates are in strict sense
not slates but rather mud stones because they has not a real
cleavage and split along the bedding. Due to the very good splitting
they are often described as slate by mistake. The surface of this
rocks often shows irregular sedimentary structures and the bending
strength and elasticity is low compared to orogenic slates which are
the 'classical' roofing slates. In Germany, this fact
has to be taken into account because roofers cut the slabs and the
mudstone-'slates' would easily break. In other countries where more
rectangular formats are common and with a different way of fixing
the strength should be sufficient.
At least in most countries of Europe roofing slate is a orogenic slate, that
means the cleavage developed by tectonic forces. The cleavage overprints
the bedding and thus is the mechanically active plane. One can basically distinguish two main types according to their relation bedding-cleavage:
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If the cleavage is parallel to the bedding the term 'parallel cleavage' is applied.
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In contrast, if cleavage and bedding show an angle it is described as 'transversal cleavage'.
The splitting planes of the parallel cleavage are more irregular
because the slate splits along the bedding plane as well as the cleavage plane.
Slates with a fanning cleavage generally show a more regular surface.
The cleavage is the dominating structural element of a slate and greatly
influences the bending strength and the way processing. Because of this, the
investigation of the fabric of a slate mainly focuses on the nature or character
of the cleavage. Depending on the deformation several generations
of foliation or cleavage plane can develop and influence the slate properties.
Besides the cleavage, micro cracks can occur which are sometimes sealed
with quartz (‘crack and sealing’). They can cause a higher breakage
rate of slate and one should avoid the usage as roofing slate.
In addition, very often ore minerals show pressure shadows of quartz as a result
of pressure-solution processes known as pyrite-type (Ramsay, 1987) and they
are a sign for pure-shear/simple-shear deformation. The same phenomena can
be seen on chloride blasts. Both are more of academic interest.
The morphology of the cleavage
Regarding the classification of a cleavage, it can be differentiated in continuous- and spaced cleavage. Both can be cut by a crenulation foliation.
The term continuous cleavage is used for slates(rocks) with a spacing less then 10 µm (= 0,001 mm = 0,0000394 in) or of a nondomainal structure (Twiss & Moores, 1992).
In slates with a spaced cleavage, the fabric elements are not homogeneously distributed but rather defined by domains of layers with a different composition (mica layers). In a spaced foliation the domainal structures have a spacing of more than 10 µm and most roofing slates show this spaced cleavage.
It comprises two domains, the cleavage domain and the microlithons. The cleavage domain contains the platy and strongly aligned micas, resulting in mica layers. These mica layers are separated by less strongly aligned and lengthened minerals such as quartz and feldspar.
The crenulation foliation is characterized by harmonic folds or wrinkles overprinting the preexisting cleavage. A crenulation foliation can be either symmetric or asymmetric and thus defined by both limbs or by only one limb (Twiss & Moores, 1992). A crenulation foliation is frequently observable in slates.
As already mentioned, the single cleavage planes are described as mica layers. This mica layers are emphasized by carbon and therefore more visible within the microscopic picture.
The fabric or morphology of the mica layers, respectively, influences the appearance
and the mechanical behaviour of a slate as well as its bending strength. Differences
in fabric should therefore result in different slate types and different bending
strengths. Because of this, a practical classification should not only comprise
phenomena of the morphology but also the parameter mentioned.
A first classification was established
by Hirschwald in 1908 and further developed by Wagner in 1989. The problem
of this classification is that there no real differences between cleavage types
exist and thus this division is rather arbitrary: differences, for example,
between 3C and 4C or between 3B and 3C cannot really reconstruct. Another problem
is the impossibility to derive any properties of a slate. A main separation
between continuous and spaced foliation does not exist. Because of the aforementioned,
neither a description nor a real differentiation is possible with this classification
and makes it impracticable.
The experience shows that the morphology of the cleavage does not extremely vary within certain ranges and thus it is not to expect that this little differences will lead to reconstructible differences in respect to the bending strength or appearance of a slate. Hence, it is more meaningful to define a classification which comprises the morphology and the bending strength. This classification can then be used to make basic statements about the strength of a slate.
Under consideration of this reflections, in imitation of Passchier & Trouw (1996) a new and more simple classification was established:
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Density and Development means how many mica layers occur within a distance of 1 mm and whether variations of the density can be seen. Investigations of the Thuringian slates has clearly shown that the higher the degree of deformation the lesser the spacing between the mica layers (Ehle, 1997). In contrast to a varying density, a regular density should led to a more uniform appearance and better splitting of a slate.
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Mica layers can show different shapes: smooth mica layers should lead to a higher elasticity and strength than rough mica layers.
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The spatial relation between mica layers is mainly important in respect to a crenulation foliation especially its intensity. A crenulation foliation can appear only very weakly in a microscope and thus is of no importance. In contrast, a bold and intense crenulation foliation can change the appearance and lower the bending strength of a slate. It is obvious that the angle between the spaced- and crenulation foliation influences the breakage behaviour of a slate, too.
Pictures
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