Petrography and mineralogy of slate

The grain size of slates varies between pelitic and psammitic, although the variation of the grain size will be more or less neutralized during the metamorphism. The development of the sedimentary pattern influences the morphology of the fracture cleavage and thus the appearance and mechanical behaviour of a slate.

As the pictures below show, the main minerals of slates are the rigid, hard minerals quartz and feldspar as well the elastic minerals muscovite/illite and chlorite. These minerals mainly govern the (bending-) strength and the behaviour of the slate during its production and processing.

Besides the aforementioned, one can often find chlorite and a high amount of chlorite can lead to a green coloured slate. Regarding the elastic minerals of a slate, the amount of muscovite/illite varies between 25-55 %, followed by chlorite with 10-20 % which can also show 5 % or up to 30 %. Chloritoide sometimes occurs in slate and lies between 2-4 %, partly up to 9 %. Paragonite is a sign for a higher pressure during the deformation and appears seldom. Own analysis show an amount between 1-3 % but also between 8-16 %.

Quartz and feldspar are commonly homogeneously distributed and often show a lengthened shape parallel to the fracture cleavage within the microscopic picture. The amount of quartz varies between 20 % up to more than 50 % and for feldspar between 5-20%. Feldpar comprises orthoclase and plagioclase whereas orthoclase is the main feldspar variety with an amount of about 6 %.

Different carbonate and opaque minerals belong to the accessory minerals that means their amount is very low and does not exceed 5 %. Regarding the quality of a slate, the different carbonate minerals and iron sulfides (pyrite, marcasite) are important because they can reduce the resistance against weathering or change the colour of a slate.

If a carbonate mineral occurs in a slate, it is mainly calcium carbonate or ankerite. In general, slates with a higher content of carbonate mostly show a rough and dull surface. The content of carbonate is especially important in regions with a high atmospheric pollution like acid rain which contains carbonic acid. If the carbonic acid reacts with carbonate (Ca CO₃]) it will turn into Ca(HCO₃])₂] which is soluble in water. In case of slates with a high content of carbonate this process can lead to a weakening or even disintegration of the slabs. In addition, ankerite contains also iron so that it can cause a brownish colouring.

Iron sulfides like pyrite or marcasite can cause a brownish colouring of a slate. There are differences between the different iron sulfide minerals in respect to the resistance against the colouring. In general, pyrite weathers less and slower than marcasite or pyrrhotine. If pyrite appears in form of well crystallized cubes it can be completely resistant to weathering. The different behaviour might be because of the different crystal structures of the minerals. At least in Germany this brownish colouring is considered as a sign for a low quality slate.

Iron sulfides can also turn into sulfuric acid because of weathering processes. This acid, together with carbonate minerals, can build gypsum which has a higher volume than carbonate. Solution and volume change can then lead to a softening and destruction of the fabric and thus lower the general resistance to weathering. Pyrite occurs in different ways within the slate: it can be homogeneously distributed in form of tiny particles or it can occur as isolated and well crystallized cubes. Furthermore, sometimes it occurs as spheroidal clusters known as framboids.

Another important opaque substance is carbon which can occur as organic carbon, bitumen, kerogen or anthracite as well as elementary carbon or amorphous carbon or graphite. Carbon is generally homogeneously distributed within the slate. Carbon has a high translation (=parallel displacement) which could possibly support the ability to split. Investigations about the relation carbon and the ability to split are not known by the author.

Besides the minerals mentioned, there are other minerals which occur seldom and has no real influence to the property of a slate. Hence, they will only be mentioned in the following table.