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.

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”