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.

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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.

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.

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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.

Summer course at Oviedo University

Ornamental rocks in construction: granite and roofing slate

This year I have organized a summer course on various technical aspects of slate and granite. The objectives and current programming are summarized as follows:

Spain is the largest producer of roofing slate and the second largest producer of granite in the world. Currently, both sectors are suffering the effects of the economic crisis, which is forcing companies to restructure looking for R & D developments that open new markets, and at the same time, incorporate the latest technologies in production processes in order to optimize operating costs.

This summer school will have a special focus on new technologies and products that have emerged in recent years, and also in the practical application of EN norms in both materials, with the aim of improving the training of technicians specialized laboratory tests. Special attention to the section of petrographic analysis and its practical applications as a qualitative indicator will be given.

The course is aimed primarily at university students of engineering and geology, architects, extractive companies and laboratories of accreditation of ornamental rocks.

Objectives:

  • Overview of the sectors of granite and slate roofing: history, economic, productive areas.
  • Technological advances in the sector in R & D and industrial development.
  • Geological and technical features granite and slate roofing.
  • Laboratory testing and regulations.
  • Practical aspects of implementing petrographic examination techniques in both materials.

Teachers:

  • Lope Calleja Escudero, PhD in Geological Sciences and Professor in the Department of Geology at the University of Oviedo.
  • Victor Cárdenes Van den Eynde, PhD in Geological Sciences and Master in Geological and Geotechnical Resources.
  • Nuria Sánchez Delgado, BA in Geological Sciences and head of the Laboratory of the Technological Center of Granite, Porrino.
  • Alvaro Ordoñez Rubio, PhD in Geological Sciences and assistant professor in the Department of Geology at the University of Oviedo.
  • Victor Pais Diz, Degree in Geology, senior geologist at Cupa Slates.

Dates and price:

15 to 19 July 2013. Enrollment period April 18 to July 8, 2013.

Price: 98,51€ for students of the Oviedo University, 140,73 € for the rest.

The course will be given in Spanish and English.

Roofing slates of the world, part I

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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1 – Slate from Labassere, Pyrenees, France. Dark and homogeneous slate quarried in the French Pyrenees. Nowadays, the quarry keeps a small production focused in the local market. Stratigraphy: Ordovician. Sample picked directly at the quarry.

2 – Shale from Minas Gerais, Brazil. Green rock, although other colors are quarried. It has a metamorphic grade slightly lower than slate. Also, it may have carbonate inclusions (red colored zones in the 200 zoom microphotograph) located in sandy levels. Stratigraphy: Bambui group, Ediacaran. Sample courtesy of Pizarras SAMACA.

3 – Red slate from Newfoundland, Canada (Trinity slate). Fine-grained and homogeneous slate with abundant iron oxides which gives it the red color. Stratigraphy: Bonavista Formation, Lower Cambrian. Sample courtesy of Laboratorio del Centro Tecnológico de la Pizarra.

4 –Himalaya slate. This rock is actually a layered volcanic rock, as it can be deduced due to the epidote crystals seen in the thin section. There are some studies about roofing slates in the Nepal and Himalaya zone Himalaya (Neupane 2007, Neupane 2012). The production potential for this area is still unknown. Stratigraphy: Nourpul and Benighat Formations, Neoproterozoic/Lower Cambrian.

5 – Shale from Jiangxi, China. Light grey rock, fine grained, with homogeneous texture and abundant opaque minerals. Roofing slates form China are very varied both from  petrological and commercial points of view. Thus, there are some exceptional god materials together with other with less quality. Stratigraphy: Shuidonggou Formation, Silurian. Sample courtesy of StoneV.

6 – Angers slate, France. Dark and fine-grained slate, with homogeneous texture, very typical in France. It has been quarried for centuries. Startigraphy: Grand-Auverné Formation, Middle Ordovician. Sample courtesy of Ardosieres d´Angers.

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.

Thermal behaviour of the slate

Temperatures reached by the slate on the roof

Once the slate cover is finished, each slate tile receives direct sunlight. Since this rock type has generally dark tone, the incidence of sunlight makes its temperature rises several degrees above the temperature of the air. When designing the roof, the effect of thermal expansion must be taken into account. The thermal expansion causes that each slate tile increases or decreases its volume depending on the temperature. Generally, the slater takes into account this effect, leaving enough space between the tiles. The variation in volume is measured by the coefficient of temperature variation, which for the slate  is estimated between 9.0×10-6 ·°C-1 and 6.5×10-6·°C-1.The linear increase in size for a slate tile can be calculated by using the formula R = X·L·t, ​​where R is the size increase in size, X the coefficient of temperature variation, L the length (in meters) and t the range of temperatures reached by the slate. For example, for a single slate tile with the following conditions:

L = 30 cm = 0.3 m

Minimum Temperature = -10 °C

Maximum temperature = 60 °C               R = 0.0000086 x 0.3 x 70 = 0.0001806 m

Temperature difference = 70 °C

X = 8.6×10-6·°C-1

Although this value is low, the sum of the total variations in size of all tiles is important for the whole cover.

This ratio should not be confused with the coefficient of thermal conductivity, defined as the heat transmitted through a body. For the slate, this thermal conductivity coefficient is estimated as 0.43 kcal/hour·°C·m-1 (1), lower than for the concrete, so in principle the slate should thermally insulate more efficiently than the concrete, considering two identical volumes of both materials.

Sunlight raises several °C the temperature of the slate. Since 2006 I have been measuring the temperature in a slate tile placed in a roof, together with the air temperature (Figure A).

Imagen01

During the winter months, the slate has lower temperatures than that of the air, but during the summer months the slate temperature is greater than that of the air. The measures show that when the slate does not receive sunlight (Figure B), the slate temperature is slightly below the temperature of the air, but when the slate receives direct sunlight (Figure C), its temperature raises, with a measured difference of 40 °C with the air temperature.

HPIM2217

Installation of the thermal probe on the roof

Finally, the existence of discontinuities in the rock (microfolding, sandy levels, quartz veins) may cause tile rupture in some cases, so you have to be careful with this type of defects depending on the geographical area where the slate is going to be used.

(1) Menéndez Seigas JL. Architecture and techniques of slate roofing: Asociación Galega de Pizarristas; 2007. ISBN 84-920981-1-2

World´s roofing slate market in 2011

Producing countries versus consuming countries

A brief analysis of the global market for roofing slate in 2011 reveals a number of interesting conclusions. Spain still remains the largest exporter of slates in the world, followed by China and Brazil. Spanish exports in dollars (graphic 1) are well above those of China and Brazil, but not so for exports measured in tons (graphic 2), where China is close to the production volume of Spain. Regarding to consuming countries, in 2011 France was the largest consumer, followed by the UK, Germany and the United States.

Evolution 2011

Taking into account the selling prices for slate, measured in $/ton (graphic 3), Spain, the largest producer, sells its slate at an average price of $ 657/ton, down from the average of 855 $/ton. However, this price is higher than the sales of China (343 $/ton) and Brazil ($ 479/ton). In fact, the overall average price rises due to high sale prices of Central European countries (Germany, France, Belgium and Italy) that had a very limited production but sold at high prices their production into their own markets for restoration of historical monuments and singular buildings. On the other hand, the buying price (graphic 4) for all the countries is closer to the average (787 $/ton), except for the case of China (1,412 $/ton) and Brazil (1,071 $/ton). These two countries buy little roofing slate (graphics 1 and 2) but at very high prices. Thus, it is possible to draw a conclusion: China, and to a lesser extent Brazil, are potential consumers of roofing slate. The opening of these markets to European production companies can be a good solution for the economic crisis that many of these companies are experiencing.

Statistical data: http://comtrade.un.org, code 6803, category HS2002

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.

Aesthetic characteristics of roofing slates – part I

Color, brightness and texture

The aesthetic characteristics of roofing slates can be defined by the color, brightness and texture. These three parameters are to be taken into account when choosing a slate variety, but also are essential in case of replacement a slate tile in a roof due to repairing or restoration. Traditionally, both slate producers and customers have been referring to the color with somewhat vague terms, such as gray, gray-blue, black, etc. These terms can easily lead to confusion.

Today it is possible to measure color and brightness precisely on any object, including slates. One of the first jobs I did in slates was the measurement of these parameters in slates of the whole Iberian Peninsula. The results show a great uniformity in most of the slates.

CIELAB color space for the roofing slates from the Iberian Peninsula

CIELAB color space for the roofing slates from the Iberian Peninsula

Besides the color, the other two parameters that determine the aspect are brightness and texture. The brightness depends basically on the crystallization and orientation of the mica minerals, while the texture depends on the grain size and the traces of the deformation phases on the slate. The most characterisitc of these traces is the intersection between the slaty cleavage and the sedimentation and which forms the lineation. This structure is known among the miners as hebra (Spain) or longrain (UK), and has a decisive role in many of the properties of the slate tile.

In the Iberian Peninsula, and from a geological point of view, the Ordovician (ORDmid and ORDup) slates from Galicia and Leon present colors a bit lighter than the Devonian slates (DEV) of Villar del Rey in Extremadura, while Bernardos Precambrian (PRE) slates in Segovia are light gray, and finally Cambrian slates (CAM) from Lugo are light green.

Aspect of the slates from the Iberian Peninsula

Aspect of the slates from the Iberian Peninsula

Further reading

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.

Precise color communication – Konica Minolta

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.

Table

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

Quartz

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.

Chlorite

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.

Muscovite

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.

Roofing slate deposits in the world

There are several roofing slate deposits in the world. The biggest is located in the northwest of the Iberian Peninsula, although there are other large reserves which are not yet evaluated in China and Brazil. Thus, the main producers of roofing slate are Spain, China and Brazil, in that order.

Evolution of the roofing slate trade. Data: UNSTASTS, http://comtrade.un.org

Evolution of the roofing slate trade. Data: UNSTASTS, http://comtrade.un.org

The sector has been hardly hit by the global crisis of recent years, but the production is beginning to recover in Spain, although China is gaining ground especially in volume of production. However, Chinese slate is sold at a price significantly lower than the Spanish, hindering the takeoff of a strong slate production sector in this country. First consumers of Spanish slate are France, Germany and the UK:Grafico Imp paises_ENG

From a petrological point of view, the commercial denomination “roofing slate” includes various types of rocks, with the common characteristic that can exfoliate in large and thin tiles. The specific characteristic of each type of slate depends on its petrology. These specific characteristics control the performance of a slate depending on the conditions of use and the climate.

World´s main deposits of roofing slates.

World´s main deposits of roofing slates.

Presentation

Introduction to this blog

Recumbent fold in the Truchas Syncline Domain, Galicia (N Spain). Pliegue acostado en el Dominio del Sinclinal Truchas, Galicia.

Recumbent fold in the Truchas Syncline Domain, Galicia (N Spain). Pliegue acostado en el Dominio del Sinclinal Truchas, Galicia.

Hello everybody. My name is Victor Cárdenes Van den Eynde, geologist with a PhD in roofing slate. I started working at the slate industry in Galicia at the beginning of 2000, soon after finishing my career at the University Complutense of Madrid. After 4 years working in the private sector, I changed the Slate Technology Center Foundation, a division of the Galician Association of Slate Producers. I worked there for two years, developing various projects related to the improvement of production and the inhibition of the oxidation of iron sulfides in roofing slates. After these two years, I moved to Oviedo, where I worked as a researcher for various projects about weatherability and petrophysics of different varieties of building rocks at the University of Oviedo. During this time, I finished a Master in Geological and Geotechnical Resources (2010) and a Ph.D. in roofing slate (2012), which had already started years ago at the University of Santiago de Compostela. Currently I work in research on petrophysics and weatherability of building rocks.

The purpose of this blog is to expose the results of my work to the public, since the scientific journals only reach the scientific community, but not the industry and customers of the building stones, which after all are the most interested in the applications of the research. In next posts I will write about different topics such as the color of the slates, the characteristics pathologies (oxidation and yesificación) and their possible solutions, the mechanical behavior during freeze-thaw cycles, the petrographic features of the different types of slates, the  methods for restoration of monuments and historic buildings, and many other topics.

I hope you find the information you are looking for.