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.

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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.

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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.

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.