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.

Grafico ITGEeng

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.

Quality factors in slates – Part I

Traditionally, the slate market has offered a wide variety of different qualities of slate. Each manufacturer has their own commercial references depending on the characteristics of its outcrops, so the market is full of specific commercial references, generating to a general confusion. The first class slate from a company may be very different from the first class of other company. In general, the quality criteria are similar for the entire sector (no alterable minerals, adequate thickness, uniform exfoliation, etc.), although it is the final use of the slate tiles which really define the specific requirements. For example, slate tiles used in Pyrenees, where the roof has to support the weight of the snow many days per year, have high thickness (8-12 mm), regardless of the presence of weatherable minerals. On the other hand, slate tiles used in Brittany, France, must be much more thinner (3-7 mm), without weathering minerals and smooth, uniform appearance. Broadly speaking, the different commercial varieties can be grouped into first, second and third quality, although there are plenty of references intermediate (rustic, first/second/third special quality, first/second economic, second selection, historical monuments selection, etc…).

The factors that determine the quality of a slate tile can be divided into three groups: petrological, tectonic and productive.

Petrological factors

These factors are referred to the mineral components of the slate and the spatial relationships among them.

Mineralogical composition

Slate is composed mainly of quartz, chlorites and mica, together with some other minerals present in variable amounts, like feldspars, chloritoid, tourmaline, carbonates, iron sulphides, etc. However, specific mineralogy depends on the petrological variety of the roofing slate (slate s.s., shale, schist, etc).

Sin título-1For slate s.s., the most typical variety of roofing slate, the average mineral proportions determined by different authors can be found at Table 1. Generally speaking, a good slate should have between 10 and 50 % quartz, 15 – 60 % chlorite and 20 – 70 % mica. Minor minerals like tourmaline, zircon, rutile, leucoxene and chloritoid are not important. Only carbonates and iron sulphides could affect the quality of the slate. Graphite fragments may also have some effect on slate quality by favoring oxidation processes, but only if there are iron sulphides in the slate. Further explanation on weathering of these two minerals can be found at their correspondent posts (oxidation and gypsification). Also, further explanation on slate mineralogy can also be found here.

Other petrological factors related with roofing slates quality are grain size, textural homogeneity and presence of sedimentation beds. These factors will be explained in following posts.

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.

Sin título-1

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

Mineralogy of roofing slates – part I

Main Minerals

From a petrological point of view, the minerals constituents of a rock can be divided into primary, secondary and accessories minerals. The primary minerals are the original components of the rock, and their abundance is higher than 5%, while accessory minerals  are found in abundances below 5%. Finally, secondary minerals are result of the geological processes subsequent to the slate formation.

In roofing slates, depending on the author, the percentages of different minerals vary, but the characteristic minerals are the same.


Therefore, the characteristics minerals in roofing slates are quartz, chlorite and muscovite.


Usually present as small rounded fragments formed by metamorphism which caused the slates.

Quarzt grains in a slate sample from Galicia, Spain.

Quarzt grains in a slate sample from Galicia, Spain.


There are two types of chlorite, chamosite, rich in iron, and clinochlore, rich in magnesium. Generally chlorites are secondary minerals formed during metamorphism, and it is usual to find them partially replaced by muscovite, which is a key factor to distinguish them from the quartz.

Chlorite crystals. Slate sample from Galicia, Spain.

Chlorite crystals. Slate sample from Galicia, Spain.


Always forms the matrix of the slate, since it has a very small grain size and at the petrological microscope is seen as a dark background. It is also often found as needle-like crystals formed during and after metamorphism.

Mica needles, slate sample from Galica, Spain.

Mica needles, slate sample from Galica, Spain.

Table captions:

[1] Pizarras ES 1975. Fraser-Española, 1975. Pizarras, in: IGME (Ed.), Monografías de Rocas Industriales Madrid, p. 46.
[2] Lombardero, M., Regueiro, M., 1992. Spanish natural stone: Cladding the World. Industrial Minerals, 81-97.
[3] García-Guinea, J., Lombardero, M., Roberts, B., Taboada, J., 1997. Spanish Roofing Slate Deposits. Transactions of the Institute of Mineral Metallurgy, Section B 106, 205-214.
[4] Lombardero, M., Garcia-Guinea, J., Cárdenes, V., 2002. The Geology of Roofing Slate, in: Bristow, C., Ganis, B. (Eds.), Industrial Minerals and the Extractive Industry Geology. Geological Society Publishing House, Bath, pp. 59-66.
[5] Ward, C., Gómez-Fernandez, F., 2003. Quantitative mineralogical analisis of spanish roofing slates using the Rielveld method and X-ray powder diffraction data. Eur. J. Mineral. 15, 1051-1062.
[6] Rodríguez-Sastre, M.A., Calleja, L., 2004. Caracterización del comportamiento elástico de materiales pizarrosos del Sinclinal de Truchas mediante ultrasonidos. Trabajos de Geología 24, 153-164.
[7] Cambronero, L.E.G., Ruiz-Román, J.M., Ruiz-Prieto, J.M., 2005. Obtención de espumas a partir de residuos de pizarra. Boletín de la Sociedad Española de Cerámica y Vidrio 44, 368-372.
[8] http://www.schieferlexikon.de/
[9] Cárdenes, V., Prieto, B., Sanmartín, P., Ferrer, P., Rubio, A., Monterroso, C., 2012. The influence of chemical-mineralogical composition on the color and brightness of Iberian roofing slates. J. Mater. Civ. Eng. 24, 460-467.
Abbreviations: Q: Cuarzo, Chm: Chamosita, Ms: Moscovita, Alb: Albita, Rt: Rutilo, Ill: Illmenita, Zr: Zircón, Trm: Turmalina, Ap: Apatito, Prg: Paragonita, Ana: Anatasa.