Slates come from many different sources and it is often possible to identify the provenance based on local knowledge and obvious characteristics such as colour , the type of cleavage and the effects of weathering. However, in order to uniquely identify the source, it is necessary to look at its properties in greater depth. Unfortunately, identification of the principal minerals present is not a useful tool, as the same three minerals; quartz, chlorite and white mica, make up over 80% of the composition of most slates. It is the type of accessory mineral present (making up between 5 and 15% of the total) which is a better indicator of provenance. For example, carbonate is present as dolomite in Ballachulish slate but as calcite in Cumbrian and Highland Boundary slates. Similarly the type of iron ore mineral present is also characteristic of an area; it is present as a sulphide in Welsh grey and Ballachulish slates but as an oxide in Welsh purple and Macduff slates.
Trace elements are also a powerful tool in distinguishing between similar slates. As the name suggests these are elements which are present in very small amounts, some of which are characteristic of an area. For example the concentration of barium in Easdale slate is 1000ppm, 120% the norm for slates, while that in Ballachulish is 500ppm or 60%of the norm, making it possible to distinguish between two very similar Scottish slates.
Historical records are also useful in identifying a slate. When no match in the database was found for a sample from the Orkney Islands, it was necessary to look at possible sources outside of Britain. It was known that Norwegian slates had been exported extensively into Scotland until the 1940s. A sample of Norwegian slate from Alta in the north of the country was obtained and found to mathc the unknown Orkney sample in all the tests performed.
There is not a single pathway to identifying a slate, but by using a combination of historical records and individual tests, it is usually possible to identify the group of quarries from which it came. However this may not be so easy in the future, with ever increasing amounts of imported slate being used, unless good records are kept.
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.
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.
- 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; 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.