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.


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