Aerinite is the characteristic blue pigment of Pyrenean Romanesque mural paintings. Although structurally solved by X-ray powder diffraction in 2004, the question if aerinite can display other colors is still open. Application of the through-the-substrate microdiffraction technique (jointly developed by ICMAB and ALBA Synchrotron) to a polished thin-section from Tartareu (Lleida) has confirmed that an Fe-Mn-rich zone observed in this thin-section effectively corresponds to brown aerinite with Fe and Mn partially replacing Al and Ca. This unprecedented result also indicates the tolerance of the aerinite structure type to compositional variability.

First identification and compositional study of brown aerinite directly on polished thin-sections by synchrotron through-the-substrate microdiffraction 
Anna Crespi, Oriol Vallcorba, Igors Šics, Jordi Rius
European Journal of Mineralogy 31 (5-6): 999–1005, 2019
DOI: 10.1127/ejm/2019/0031-2891

Figure: Photomicrograph under transmitted light of a region of the polished thin section containing a broad stripe of pale-blue aerinite and, in the middle, the brown aerinite zone (size is around 100 μm).

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