Mineralogy and geochemistry of degradation processes on historical monuments: first approach to a mexican case (Morelia, Michoacan)

Abstract

In the framework of ongoing research of the building stones used throughout the baroque style period in the city of Morelia (Michoacan, Mexico), the mineralogical, geochemical and petrographical properties of 20 ignimbrite samples were determined. Data were collected on weathered and unweathered volcanic pyroclastic stonework from the San Francisco convent. This was the first architectural ensemble of Morelia which was built entirely from this stone, called “cantera” (the construction ended in 1610). The samples were studied by means of optical microscopy (OM), chemical analysis, X–ray diffraction (XRD), and scanning electron microscopy (SEM). The results show that the mineralogical composition, geochemical features, and the alteration process of the analyzed samples are very similar in this historical monument. As a result of degradation, some specific neoformations originated, such as sulfates (gypsum), chlorides (halite), amorphous calcium phosphates, and in smaller proportion, carbonates (calcite, ankerite). This mineralogical composition shows that the main elements indicating alteration in the studied historical buildings are sulphur, chlorine and phosphorus. At the same time, it should be pointed out that the different degree of devitrification in ignimbrite becomes a very important factor in its degradation: lower intensity and/or absence of devitrification is a characteristic of ignimbrites, which promotes good consolidation and better properties of mechanic resistance to alteration.

The results obtained in this research provide new information about the mineralogy of recent formations that characterizes the degradation processes in Mexican historical monuments located in southwestern Mexico. At the same time, these data may result useful in future systematic studies of historical buildings, not only in the city of Morelia, but also in many other Mexican cities considered to be part of UNESCO’s Humankind Heritage.