Roofing slates of the world part III

Images of hand specimens and thin sections of slates from several world´s locations. Real color of the specimens may vary with respect of shown in the images.

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13. Slate from Penrhyn, Wales, UK. This slate is extracted at the historic quarry of Penrhyn, and is very popular in historical buildings all over the UK. The green spots correspond to zones with reduced iron and high contents of Ca and Mg (Borradaile et al. 1990). This color change can be seen in the microphotograph of 200 microns.

14. Carbonate slate from Liguria, Italy. The Liguria slates have carbonate content (see microphotograph of 500 microns) of about 20%. However, this fact does not mean that these slates are more susceptible to weathering than other slates with carbonate contents much lower. The key factor is the specific mineralogy of the carbonate. This slate complies with the EN 12326 requirements, and constitutes a perfect material for roofing when used properly. Sample provided by Euroslate.

15. Slate from Benuza, Castilla y León, Spain. An Ordovician slate, fine-grained with some cubes of pyrite, with smooth surface and dark color. This is a classic roofing slate, i.e., a slate from the green schists facies made of quartz, chlorites and mica. Sample provided by Cupa Pizarras S.A.

16. Slate from Hubei province, China. Fine-grained slate, light colored with a marked tendency to acquire a reddish aspect which makes it very interesting for special cases, since this reddish does not seem to generate rust trails. Sample provided by the Laboratorio del Centro Tecnológico de la Pizarra.

17. Green phyllite from Lugo, Spain. This Cambrian phyllite is also a very special roofing slate, being used for some singular buildings such as the Shizuoka Convention Arts Center in Japan. It is quarried in several colors ranging from grey to green. This is the Verde Xemil variety. Sample provided by Pizarras Ipisa.

18. Slate from Villar del Rey, Badajoz, Spain. A very fine-grained slate with some pyrite cubes and a dark color, in fact this is the darkest slate quarried in Spain due to its content in graphite, up to 2%.  Sample provided by Pizarras Villar del Rey, S.A.

And please remember: There are no bad slates but bad uses. The slate should be used in accordance with the building and environment requirements, so it is critical to know and understand the rock we are dealing with.

Quality factors in slates – Part II

Grain size

The grain size of roofing slates is very small, similar to the clays. It is possible to distinguish two types of components depending on the grain size, the matrix (mica and chlorites) and the skeletal components (quartz and feldspar). The key factor is the components of the skeleton, not just the size of these grains, but their selection or uniformity in size (Figure 1). A roofing slate will have good fissility if their skeletal components have all similar size, whereas with diverse range of sizes the fissility is reduced.

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Figure 1. Relationship between slate components and grain size

Grain size also affects the external appearance, coarser slates have a more rough and irregular aspect, while the fine-grained slates have a more smooth and uniform aspect, and therefore brighter (Figure 2).

Figure 2. Comparision between a coarse grain slate (left) and a fine grain slate (rigth).

Figure 2. Comparision between a coarse grain slate (left) and a fine grain slate (rigth).

Textural homogeneity

By definition, a roofing slate should have a lepidoblastic texture (Figure 3). This term refers to the microscopic arrangement of the elements of the rock, which are strongly oriented along the direction of slaty cleavage or fissility. This texture must be uniform and consistent along the slate, otherwise the split process will be greatly hindered. In certain types of roofing slate, other textures can be found, but must always be homogeneous and continuous.

Figure 3. Classical lepidoblastic texture in a roofng slate (left). On the rigth, a slate with a coarser texture, which is called porphyro-lepidoblastic

Figure 3. Classical lepidoblastic texture in a roofng slate (left). On the rigth, a slate with a coarser texture, which is called porphyro-lepidoblastic

Presence of sedimentary layers

These sedimentary layers are mainly sandy levels, of thicker grain size, which were deposited when the sedimentary rock which subsequently result in the slate was formed (Figure 4), after metamorphic processes.

Figure 4. Deposition of sandy layers on the slate bulk during sedimentation.

Figure 4. Deposition of sandy layers on the slate bulk during sedimentation.

These layers can be recognized as bands of lighter colors. Since they have a grain size and texture different from the rest of the slate, they modify the homogeneity of the slate (Figure 5), so that their presence is undesirable in a good quality slate.

Figure 5. Sandy layers on a roofing slate bulk.

Figure 5. Sandy layers on a roofing slate bulk.

Pathologies in slates, part IV


Gypsification is the phenomenon by which the carbonates that may be present in the slate is transformed into gypsum by contact with the sulfur (S) coming from the atmosphere or from the iron sulfides, following the reaction:

H2SO4 + CaCO3 –> CaSO4 · H2O + CO2

Fig02The transformation from carbonate to gypsum is potentially harmful, because the gypsum has a mineral size substantially larger than the carbonate, so a swelling occurs inside the slate (figure 1), affecting seriously the integrity of the tile. As oxidation, gypsification is very evident when occurs, since it develops a characteristic whitening along the surface of the slate tile (figures 2 and 3). The gypsification is closely linked to acidic environments, especially urban environments where sulfur concentrations are usually high.

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Figure 2 (left): Cover affected by gypsification
Figure 3 (right): Slate severely affected by gypsification after exposure to SO2 test

Gypsification prevention

The best way to know if a slate may suffer gypsification are the normative tests of exposure to SO2, as expressed in EN 12326, or to the test of weather resistance of ASTM C-217. Both tests submit the slate to acid conditions, and then quantify the alteration suffered by giving three degrees. EN 12326 provides three visual alteration levels (S1, S2 and S3), while ASTM performs a scraping of the slate surface after the acid exposure, and then makes three estimates of the service life depending on the depth of the scratch (S1:> 75 years, S2: 40-75 years, S3: 20-40 years).

The carbonate content test of EN also gives an idea of how susceptible to gypsification can be a slate. In theory, higher carbonate content will lead to a high susceptibility. However, this fact has to be taken with caution, as the carbonate may be present as well crystallized calcite, which resists very well against yesificación. Again, petrographic examination can help in this case, since it will determine the form in which is present the carbonate.

Carbonate crystal in a schist roofing slate

Carbonate crystal in a schist roofing slate, transmitted light microscopy, zoom 250, crossed polarizers

Further reading: Standard tests for the characterization of roofing slate pathologies

Different types of roofing slates

Definition of roofing slate after EN 12326

According to EN 12326-1:2005, from a commercial point of view, a roofing slate is a “rock which is easily split into thin sheets along a plane of cleavage resulting from a schistosity flux caused by very low or low grade metamorphism due to tectonic compression. It is distinguished from a sedimentary slate (shale, author´s note) which invariably splits along a bedding or sedimentation plane. Slate originates from clayey sedimentary rocks and belongs petrographically to a range which begins at the boundary between sedimentary and metamorphic formations and ends at the epizonal-metamorphic phyllite formations”.

This definition makes quite clear, from a petrological point of view, the range of rocks which can be considered slates. However, EN 12326-1:2005 continues defining roofing slate as a ”rock used for roofing and cladding, in which phyllosilicates are the predominant and most important components and exhibiting a prominent slaty cleavage”. Likewise, roofing carbonate slate is defined in the same way as above but with a minimum of 20% content of carbonate.

Metamorphic facies stability diagram. Modified from Spear, 1993.

Metamorphic facies stability diagram. Modified from Spear, 1993.

For the Subcomission on the Systematics of Metamorphic Rocks (SCMR), a part of the International Union of Geological Sciences (IUGS), a slate s.s. is “an ultrafine- or very fined-grained rock displaying slaty cleavage”. This slaty cleavage is also defined as “a type of continuos cleavage in which the individual grains are too small to be seen by the unaided eye”. The slaty cleavage is the most important characteristic of roofing slates, since it allows the rock to be split into large and thin tiles.

It is clear that there are two types of rocks, slates s.s., sometimes also called lutitic slates, which are low-grade metamorphic rocks (greenschist facies), and commercial slates or roofing slates, which are rocks composed mainly of phyllosilicates with an exfoliation which allows to produce tiles that may be used as roofing materials. This second group includes the slates s.s. together with other types of rocks, like shales, phyllites and schists.

A: Sedimentary slate (shale) with no develop of slaty cleavage. The planes correspond to sedimentation beds. Minas Gerais, Brazil.
B: Slate s.s., in which the planes correspond to slaty cleavage. Herbeumont, Belgium.
C: Phyllite, with a metamorphic grade slightly higher than the slate s.s., as the biotite crystals shows. Bernardos, Spain.
D: Schist, with a well developed schistosity. Finnmark, Norway.