Category Archives: Characteristics of slate

Mineralogy of slate

The principal minerals present in slate are quartz, white mica and chlorite. However due to the fine-grained nature of the rock, it is not easy to identify any of these with the naked eye.  Other accessory minerals such as pyrite and pyrrhotite (iron sulphides),  may be seen when present in clusters or as individual cubes. However, these minerals usually comprise less than 5% of the total.  Other constituent minerals can only be  detected using more sophisticated methods. In coarser-grained slates is may be possible to identify the principal minerals as follows:

Quartz .

Individual grains of quartz  can be seen with a hand lens  in coarse-grained slates. They are generally rounded, less than 0.5mm in diameter and have a sugary texture.  There is no alteration in appearance due to weathering.

 Chlorite

Chlorite mineral is present in slate with concentrations ranging  from 20 to 50% but is not normally visible even with a hand lens. When present in sufficiently high concentration, it gives the slate a green colour, however this  colour is easily masked by  small amounts of other minerals, such as graphite or haematite.   There is however one important exception to this generalisation;  small specks of chlorite are visible in Scottish Macduff slate derived from an area closest to a nearby igneous body. These specks where originally the mineral biotite, which grew  millions of year ago due to the increased temperature in the surrounding rock at the time of the emplacement of the igneous body.  In most cases, the biotite minerals have subsequently been weathered to chlorite, but the outline of the original biotite mineral remains.  This speckled appearance is one of the characteristics used to identify Macduff  slates.

White mica  

White mica is a general term covering various minerals from clay to illite to muscovite, the particular type depending on the composition of the rock but more importantly the degree of metamorphism.  With increasing metamorphism easily-weathered clay minerals are gradually replaced by a white mica  with  a composition and structure approaching that of muscovite, which is the least prone to weathering.   Hence the the degree of metamorphism is a useful criterion in assessing the durability of the slate. It is not possible to recognise individual mica minerals in  a  slate, but with increasing matamorphism due to increasing temperature and pressure, individual mica grains increase in size  giving the rock a  slight sheen as oberved in phyllites such as Ballachulish slate.  As this process continues even further, the rock passes from being a phyllite into a mica schists where mica  grains are easily identified by their flakiness and pale yellow colour.

Pyrite, pyrrhotite and graphite are all commonly found in slates formed from muds deposited in a stagnant, low-oxygen environment.

Pyrite

The mineral pyrite  is found in all shapes and sizes from large metallic crystals with well-defined edges (euhedral) to amorphous powder (anhedral). They are found in clusters or randomly distributed throughout the slate. They may also be associated with a particular bedding layer within the slate.  Metallic euhedral crystals of pyrite are not prone to alteration due to weathering but retain their metallic appearance. In contrast, leaching and brown staining around individual grains is common in amorphous pyrite and pyrrhotite which have been exposed to weathering. In some cases, the whole cluster falls out leaving a hole.

Pyrrhotite  mineral is not normally distinguishable from pyrite by its appearance but can occasionally be identified by its magnetic properties. It is much more prone to alteration from exposure than amorphous pyrite.

Graphite is present as black greasy powder. Although it is not affected  itself by exposure, it can act as a catalyst accelerating the deterioration of other minerals.

Haematite

Haematite is an oxide of iron (Fe2O3) found in slates formed from deposits laid down in oxidising conditions. It is the most durable form of iron and not affected by exposure.Haematite is not visible to the naked eye, however its presence can be recognised by its purple colour.

Carbonates:

There are several  common forms of carbonate found in slate, the most common of which is  calcite CaCO3.  Its presence can be detected by a drop of acid which makes it fizz. In the presence of pyrite it may react to form gypsum which is detrimental to the slate. It is generally not recommended that slates with carbonate be used in a polluted environment.  On the other hand dolomite CaMgCO3 the carbonate found in Ballachulish and Easdale slates is unaffected by acid and very durable.

Crenulation cleavage

Crenulation cleavage is found in rocks which have undergone multiphase deformation. It is due to folding of the original cleavage during subsequent episodes of deformation. It gives the surface of the slate a crinkled or crenulated effect. It is one of the characteristics commonly used  to identify Scottish Easdale slate.  However it is not conclusive as it is also found in Ballachulish slate,  In most cases polyphase deformation distorts the original cleavage making the rock uneconomic for slate production.

Cleavage

 Slate belts are usually found in mountainous areas where geological forces have deformed and shortened the earth’s crust.  Rocks respond differently to these compressive forces; some such as quartzite respond by folding while other weaker rocks, such as mudstone, develop a cleavage perpendicular to the direction of maximum stress. Under the microscope, it can be seen that this cleavage consists of  individual grains of quartz, which have been flattened, separated by platey minerals (phyllosilicates) white mica and chlorite.  It is  these zones of platey minerals, called cleavage domains, which enable the rock to be split into slabs suitable as roofing slates.

 

Closely spaced cleavage domians enable this slate to be split less that 4mm thick
Intermediately spaced cleavage domains enable this slate to be split between 7-10mm thick

 

Discontinuous and irregularlly spaced cleavage domains make this slate difficult to split.

The thickness of roofing slates varies from 3mm to over 15mm depending on the spacing of the cleavage domains present.  Shape too affects the ability to cleave the rock; the straighter and more continuous the cleavage domains, the more easily the rock is split Other factors such as the  proportion of rock occupied by the cleavage domains also , affects the ability to cleave into slates.

Quartz-rich rocks shortened by folding, while mudstones nearbye developed a slaty cleavage

What is slate?

Slate is a fine-grained, low-grade metamorphic rock, derived from mudstone or occasionally volcanic ash.  The essential minerals present in all slates are quartz and the platy minerals; chlorite and white mica, however the relative proportion of the different minerals affects the durability of the material.

Slate is characterised by its slaty cleavage formed during metamorphism by the alignment of platy minerals into parallel planes. This enables the rock to be split into thin sheets, making it suitable as a roofing material.  The proportion of platy minerals present and their size affect the  type of cleavage and hence its  ability to be split into slabs of suitable thickness.

The colour of a slate also varies, depending on the environment in which the original muds, and the  strength of the slate is influenced by the grain, if present.

Other rock types, often referred to as slate when used as roofing material, include phyllite, mica schist and flagstone.  Phyllite is similar to slate in that it is split along a pervasive slaty cleavage, however it is coarser-grained due to more intense metamorphism giving it a slight sheen. Ballachulish slate is an example of a phyllite although traditionally it has always been referred to as slate.  Cnocfergan slate is another rock referred to as slate because of its use as a roofing material. In this case the rock is a mica schist, which apart from being a metamorphic rock is very dissimilar from slate. It is very coarse-grained and can only be split along micaceous layers which are primary bedding features.  Flagstone, such as Caithness, is a sedimentary rock split along bedding.

Bedding

 Slate is formed from mud and silt, or very occasionally volcanic ash, which were deposited in layers usually referred to as bedding.   In spite of the  intense pressure and high temperature  required to transform the original deposits into slate, it is often possible to see traces of the original bedding in the finished slate.  These strata of bedding are readily recognised when the individual layers of mud and silt are not too homogeneous, but vary in composition or grain size. This heterogeneity can be seen on the surface of the slate as  changes in colour due to compositional variation, or defraction of the  cleavage  due to  changes in the grain size.

Layers of bedding are visible on the cleaved surface of the slate

Bedding is seen as a ribbon of finer-grained material in an otherwise coarse-grained slate.
Changes in colour and variation in grain size are due to layering in the original sediments

Bedding cutting across the cleavage surface at a high angle

Bedding may difficult to identify in heterogeneous material where it is orientated parallel to the cleavage surface. Conversely it is most easily observed when cutting the cleavage surface at a moderate to high angle.

Grain

Section perpendicular to cleavage showing the elongation of individual minerals parallel to the grain.

 The compressive stress, which transforms a mudstone into slate results in the development of cleavage  perpendicular to the direction of the maximum applied stress. It is along these cleavage planes that the slate can be split to produce thin sheets suitable as a roofing material.

In addition to the maximum applied stress, which results in the development of cleavage, the stresses in the other directions may vary which can affect the shape of individual mineral present ;  This is due to the minerals being stretched  in the direction of lower stress and at the same time  compressed in the direction where it is greater, giving the slate a grain analogous to that of wood.  The resulting slates are weaker  in the  direction parallel to the grain and stronger at right angles to it. Not all slate have a grain -if the stress is same in all directions (perpendicular to the cleavage) no grain results.

The grain of a slate is an important property of a slate yet it is the most misunderstood.   probably in areas where the true grain is poorly developed,  For example, in Spain it is generally used to describe the intersection of bedding with the cleavage surface while in Scotland it was used to describe the orientation of the  crenulation cleavage.  In Welsh slate, which has a pronounced grain, it is referred to as the ‘pillaring line’. Here it gives the rock a secondary line of weakness which can be  exploited in extraction from the quarry or mine to the the final splitting into roofing slabs.   Although the grain is not readily visible in fine-grained slate, it can  be recognised in coarse-grained slates by the elongation of individual minerals. It can also be inferred from the alignment of minerals and the shape of reduction spots.

Reduction spots

Reduction spots; the long axis is parallel to the grain of the slate
Green area in a purple slate due to localised reduction of iron

These are areas of green in otherwise dark generally purple slates. They are due to the chemical reaction between iron and organic matter present in the original sediments.  They may be present as discrete bands which formed from organic-rich strata present in the original muds. They may also be present as ovoids, which appear as ovals or circles on the cleavage surface. Assuming that the ovoids were initially spheres which became flattened and stretched during metamorphism,  the ratio of the long axis to short axis gives a measure of the amount of deformation which has taken place; the higher the ratio the more developed the grain of the slate. For example the typical value  for a Scottish Ballachulish slate is 6:1 which has a pronounced grain. In contrast reduction spots in Cumbrian slate are circular, which is indicative of flattening with uniform stretching in all directions,. These slates  have no grain.