Mineralogical-chemical Alteration and Origin of Ignimbritic Stones Used in the Old Cathedral of Nostra Signora di Castro (Sardinia, Italy)

ABSTRACT

The pyroclastic rocks belonging to the Late Eocene-Miocene volcanic activity that occurred in Sardinia between 38 and 15 Ma ago were widely used as construction materials in several Romanesque churches of the easternmost Logudoro area, as well as in large parts of the Sardinia territory. In this work, the ancient Cathedral of Nostra Signora di Castro (twelfth century) was taken as a representative case study. There is no historical or archaeological evidence of ancient quarries. Based on the geochemical, petrographic, and volcanological data on several samples from an extensive field area (approximately 150 km²), a geographical zoning of the volcanics has been recognised. In the Oschiri sector, there are three different sub-zones, which can be identified with different volcanic rocks: less fractionated rocks (Differentation Index ∼70–78); intermediately fractionated rocks (D.I. ∼76–79); and more fractionated rocks (D.I. ∼77–82). To identify the origin of the ignimbrite rocks of the Church of Nostra Signora di Castro, two statistical methods were used: stepwise linear discriminant and canonical analysis. Moreover, to define the geochemical transformation processes induced by the alteration, a comparative study of concentrations of major and trace elements measured by XRF and SEM-EDX analyses on the surface portion and the innermost areas of the stone was made.

KEYWORDS:

• Provenance
• XRF
• XRD
• SEM-EDX
• petrographic study
• medieval church
• Logudoro
• Romanesque

Acknowledgments

Sampling was carried out with the permission of the local Superintendency to the Historical-Architectural Heritage of Sassari and province.

Disclosure statement

No potential conflict of interest was reported by the authors.

ORCID

Anna Gioncada http://orcid.org/0000-0002-8513-7377

Additional information

Funding

The research was supported financially by the funds of the Cagliari University (PRID 2015) and by the Regione Autonoma della Sardegna (L.R. n.7 – 7 August 2007) through the research project entitled: ‘Romanesque and territory: the construction materials of the churches of the Judicial Sardinia: new approaches for the enhancement, conservation and restoration’ (application code CRP-18095, CUP: F71J11000620002).