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.

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