Pathologies – part II

Oxidation

Iron oxidation consists of the change of Fe2+ to Fe3+, by the gain of an electron. In roofing slates, most important iron minerals are iron sulphides, being the most abundant pyrite (FeS2), which is oxidized in the presence of oxygen according to the reaction:

FeS2 + O2 –> Fe2+ + 2SO2-4 + H+

The oxidation of these iron sulfides is favored by acid urban environments and coastal areas, where sea salts favor oxidation reactions.

However, not all the iron sulfides oxidize in the same way. There are several types of iron sulfides, such as pyrite, pyrrhotite, marcasite, arsenopyrite, etc, being the two firsts the most abundant by far. Each iron sulphide has a different structure. Thus, the oxidation susceptibility depends on the strength of this mineral structure. For example, pyrrhotite has a poorly ordered hexagonal structure, being more vulnerable to oxidation than the pyrite cubic structure. In real world, most oxidations developed in roofing slates are due to pyrrhotite, so it is very important to distinguish between these two minerals, since the oxidability of the slate depends on it.

Finally, the occurrence of organic matter in the slate favors the oxidation, due to the increase of acidity during its decomposition.

1 - Pyrrhotite, brown color, with not recognized shape. 2 - Pyrrhotite partially oxidized together with an inclusion of organic matter. 3 – Pyrrhotited fossil of a bivalve. 4 - Cubic pyrite crystal. 5 - Cubic pyrite crystals forming aggregates called framboids. 6 - Footprint of a disappeared cubic crystal of pyrite oxidized.

1 – Pyrrhotite, brown color, with not recognized shape. 2 – Pyrrhotite partially oxidized together with an inclusion of organic matter. 3 – Pyrrhotited fossil of a bivalve. 4 – Cubic pyrite crystal. 5 – Cubic pyrite crystals forming aggregates called framboids. 6 – Footprint of a disappeared cubic crystal of pyrite oxidized.

Further reading: Determination of iron sulphides in roofing slates from the north west of Spain

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