Roofing slate mineralogy – Part II

Secondary and accessory minerals

Secondary minerals

Formed during the metamorphic processes that originated the roofing slates. The most common secondary minerals in slates s.s. are iron sulphides (pyrite and pyrrhotite), carbonates (calcite and ankerite) and chloritoid. The iron sulfides are formed during the post-metamorphic processes. Some authors point out their origin as the remains of organic matter that could be contained in the slate matrix. Depending on the geological conditions and the ratio of iron (Fe) and sulfur (S), these iron sulfides may end up being different minerals with different potentials for oxidation.

Determination of Fe - S proportions of iron sulphides in several roofing slates. The two most common minerals are pyrite and pyrrhotite

Determination of Fe – S proportions of iron sulphides in several roofing slates. The two most common minerals are pyrite and pyrrhotite

On the other hand, the carbonates are normally deposited occupying the empty spaces and voids that could be in the rock matrix. The chloritoid is formed perpendicular to the slaty cleavage. This mineral appears only in some types of slates with a high content of magnesium (Mg) and a slightly higher metamorphic grade than the average of slates s.s.

From the point of view of quality, iron sulfides and carbonates play a decisive role, since they are alterable minerals. Their appearance is undesirable. The chloritoid may cause fissility problems, since it grows perpendicular to the slaty cleavage, hindering the correct elaboration of the tiles.

Accessory minerals

They are found in quantities below 5%. The most common are tourmaline, rutile-leucoxene, zircon, monazite and organic matter. They have no importance for the quality of the board, since they are not alterable, with exception of the organic matter. This can be found under the form of graphite, small fragments of between 5 and 30 microns, which are opaque seen to the microscope. It can become very abundant in localized areas of a quarry and is often linked to iron sulfides. Organic matter also undergoes oxidative processes, although do not causes color changes. Its alteration leads the pH to decrease, acidifying the medium and greatly accelerating the oxidation rate of the iron sulfides. Sometimes the accumulation of organic matter can be seen on the surface of the slate, forming what the miners call “burnt slate“. This slate is not usable to make plates, and it should be discarded.

1. Chloritoid crystal in a Galician slate, Spain<br />2. Small chloritoids in a slate from Arouca, Portugal<br />3. Turmaline fragment in a slate from Monte Rande, Galicia, Spain<br />4. Monazite in a slate from Puente de Domingo Flórez, León, Spain<br />5. Small black and rounded fragments of organic matter in a shale from Minas Gerais, Brazil<br />6. The upper half of the image corresponds to an exceptional accumulation of organic matter, known as “burn slate”

1. Chloritoid crystal in a Galician slate, Spain
2. Small chloritoids in a slate from Arouca, Portugal
3. Turmaline fragment in a slate from Monte Rande, Galicia, Spain
4. Monazite in a slate from Puente de Domingo Flórez, León, Spain
5. Small black and rounded fragments of organic matter in a shale from Minas Gerais, Brazil
6. The upper half of the image corresponds to an exceptional accumulation of organic matter, known as “burnt slate”

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.
[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.

Roofing slate lithotypes


First of all: lithotype, what’s this?. A lithotype, as used in Geology, is a stone which represents the characteristics of one group. So when I talk about lithotypes I’m just referring to a set of characteristics and properties. Something like a stereotype but without negative implications.

Roofing slates are classified by the sector according to commercial terms, frequently referring to a specific brand. The geographical names are very frequent (i.e. Mosselschieffer, Brazilian Slate, Spanish slate). The market distinguish three main qualities or choices (first, second and third), but there are many other like Cofina, Historical Monuments, Scottish, etc. The characteristics of these qualities are more or less recognized by the market. This makes that the purchaser rather looks for a specific commercial brand than for a type of slate. However, it is important to define lithotypes for roofing slates, that is, to group and name the slates depending on the characteristics that clearly make this distinction. In other words, if I tell you “there is a bird” you will think in a bird, obviously, an animal with wings and two legs. But if I tell you “there is a sparrow” then you will have in mind an specific kind of bird. You might know few or a lot about the sparrow, but you know which kind of bird I’m talking about. For the slate is the same. Someone that has been working for decades in the slate world knows the slate by the name of the quarry, is like when you know a group of people just by their names, you don’t need more. But other people might do not know the names, and then needs the surnames to get the picture. This is my aim, to give general names and surnames to roofing slates, combining both geological and commercial information, and then proposing a common classification useful for everyone working with roofing slates. How?.

Dale, in 1906 proposed a classification in which the first distinguishing element was the geological origin (Sedimentary or Igneous), and then the matrix arrangement seen at the petrological microscope (Clay slates or Mica Slates). There was a third subdivision regarding the potential change in color (Fading or Unfading), which in fact is still widely used in the U.S.A. This changes in color are due to the occurrence of carbonates.

The proposed classification uses in fact the same parameters, but actualized. Two universal distinguishing features are color and petrology. Color is the reflect of the mineralogy of the roofing slate. The main minerals of slates are quartz, mica and chlorites, plus some other minerals in different proportions and occurrences. Then, there are three main families of color for roofing slates: black-grey, red-purple and green. These general colors are a reflect of the average mineralogy. Black and green slates are the result of reducing conditions, so they can contain iron sulphides and carbonates, two groups of minerals very important from a quality point of view. On the other hand, red slates never have iron sulphides, since they come from oxidizing conditions. Instead they contain abundant crystals of hematite of small size, being this one the coloring mineral. The amount of organic matter, usually present as carbon, is also a determining component for color. The organic matter gives dark tones, and is, as the iron sulphides, related with reducing conditions. Generally speaking, the contents of iron oxides versus iron sulphides and organic are inversely proportional in roofing slates. This is the first level for classification, the color, which gives then rough information about the mineralogy and geological conditions. However, there are some exceptions. In some areas, there is a especial occurrence of roofing slates known as “multicolor” or just “colored” These slates are characterized by presenting non-homogeneous red-orange-black surfaces, product of infiltration of superficial water in the cleavage planes and deposit of iron oxides. This effect is just an alteration, so the color of the roofing slate should be defined in the non-altered planes. Something different are the “variegated” slates, which are red-purple lithotypes showing areas of green lithotypes. This is typical in slates from the UK and North America, and it is due to mineralogical processes developed in the slate matrix. These type of slates are included as a sub-group of the red lithotype, which is their original lithotype.

The second level is the petrology, or the type of rock. Following the classical classification for rocks, there are sedimentary rocks (sandstones, siltstones and shales), metamorphic rocks (slates, phyllites and schists) and volcanic rocks (cinerites). Each of these groups have distinguishing features that influence their performance as a construction material. Sedimentary rocks usually have high Water Absorption (WA) and low Bending Strength (BS), while metamorphic rocks have low WA and high BS and volcanic rocks.

The proposed classification is an attempt to establish an understandable and easy way to name the different types of roofing slate used in the World. This classification has some advantages compared to the commercial terminology used nowadays. Every roofing slate in the World only matches in one category, and these categories describe the aspect and petrology of the roofing slate on a general way. From this description, some technical characteristics can be inferred, regarding to weatherable minerals, WA and BS. However, there are some minor exceptions linked to singular outcrops. For example, in the lithotype B1, which corresponds to slates s.s., the carbonate content usually is below 2%, but some outcrops from Canada and Italy have higher values of carbonate, in some cases over 20%. A complementary information to this classification might be then the geographic location. A technician from the slate market knows the regional characteristics of each slate, and a layman would know where to place it. The lithotype code would act then as the name of the roofing slate and the geographic location the surname.

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.

Pizarras del Mundo03

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 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.

Roofing slates of the world, part II

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.

Pizarras del Mundo02

7 – Slate Valentia, Ireland. This is a coarse-textured gray slate, with no or few iron sulphides, quarried at the region of Valentia, S of Ireland. Stratigraphic level: Middle Devonian. Sample provided by the company Valentia Slate Ltd.

8 – Mica-schist from Finnmark, Norway. This is type of rock is not usually used for roofing. However, at N of Norway  are several quarries of different varieties of mica-schists thin enough to be used for roofing. These rocks have higher metamorphic degree and mineralogy clearly different to those of the slates s.s. Stratigraphic level: Lower Cambrian. Sample provided by Minera Skifer.

9 – Slate Valongo, Portugal. Dark slate, fine-textured, with some cubes of pyrite. It is quarried in the Valongo area, near Porto, in Portugal. It is similar to some levels of Galician slate, in Spain. Stratigraphic level: Middle Ordovician. Sample provided by Pereira Gomes & Carballo.

10 – Slate Green Lugo. This type of slate is extracted at the Pol area in the province of Lugo. It is characteristic its intense green color, result of the predominance of the magnesic term of the chlorite group, clinochlore. Stratigraphic level: Lower Cambrian. Sample provided by the company Pizarras Ipisa.

11 – Filita from Bernardos. Gray slate, coarse-textured, with no organic matter nor iron sulfides. It is extracted in Segovia, N of Madrid, and is the slate with which was built the Escorial Monastery roof. It has a slightly higher metamorphic degree compared with slates s.s., as evidenced by the presence of biotite. Stratigraphic level: Lower Cambrian. Sample provided by the company Pizarras J Bernardos.

12 – Ballachulish slate. This slate is from an historical quarry no longer in operation. It is a coarse-grained rock with abundant quartz grains and little or no iron sulfide. Sample collected in quarry.

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.