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 are common in coarse-grained slates and may be seen with a hand lens. 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 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 are the remains of 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 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 temperature and pressure, characteristics of increasing metamorphism, 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 This mineral 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 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 is not visible to the naked eye. However its presence can be recognised by its purple colour. It 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.
Carbonates: the most common forms of carbonate found in slate are 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 detremental to the slate. It is generaly not recommended that slates with carbonate be used in a poluted environment. On the other hand dolomite CaMgCO3 the carbonate found in Ballachulish and Easdale slates is unaffected by acid and very durable.