Historically, aerinite is a blue pigment used in many Romanesque paintings of the Pyrenean area between the XI-XIV centuries. Its characteristic blue color varies from dark to pale blue. In fact, its name comes from a Greek root “aerinos”, meaning “atmosphere” or “sky”, since it has only been known as blue. However, it is known that many minerals are present in nature in different colors...

In the case of aerinite, one double question still remained open: ‘Does aerinite always have to be blue?’ and ‘Is it blue, simply because we can only identify aerinite when it is blue?’, said Anna Crespi, researcher at the ICMAB-CSIC

During the inspection of a polished thin section of a specimen coming from Tartareu (Lleida), a small area containing brown submicrometric fibers was detected next to a region containing pale-blue aerinite. A first scanning electron microscopy (SEM-EDX) analysis of this brown region suggested it to be very similar to aerinite, but with a slightly different composition. It had a higher content of iron (Fe), and manganesium (Mn) was partially replacing calcium (Ca).

Synchrotron through-the-substrate microdiffraction

As it was a very small section, and due to the significant compositional difference, reserchers decided to use the synchrotron through-the-substrate microdiffraction (tts-µXRD) technique to collect data and verify that the brown fibers were indeed aerinite.

This technique, developed in close collaboration by researchers of the ICMAB-CSIC and Alba Synchrotron, allows the simultaneous visualization of a thin-section of the mineral under polarized light and the measurement of diffraction patterns at the target region previously identified.

The measured tts-µXRD data confirmed that these fibers constitute the first finding of brown aerinite. Compared to the more common blue variety, this new variety has a slightly different unit cell parameter. This information was complemented with the acquisition of accurate electron microprobe (EMP) data of the same zone.

The compositional study of brown aerinite, carried out by Anna Crespi and Jordi Rius of the ICMAB-CSIC in collaboration with Oriol Vallcorba and Igors Šics of ALBA Synchrotron, has been recently published in the European Journal of Mineralogy.

Curious fact

The structure of aerinite was discovered by the same team at the ICMAB-CSIC, as explained in this news article "El misterioso azul del románico catalán" (R+D CSIC, 2004).

Reference article:

Anna Crespi, Oriol Vallcorba, Igors Šics and Jordi Rius. (2019): First identification and compositional study of brown aerinite directly on polished thin-sections by synchrotron through-the-substrate microdiffraction. European Journal of Mineralogy, September 23. DOI: 10.1127/ejm/2019/0031-2891

Figure

Brown aerinite found in the sample, next to blue aerinite.