First identification and compositional study of brown aerinite directly on polished thin-sections by synchrotron through-the-substrate microdiffraction

A brown variety of the fibrous chain silicate aerinite has been identified by synchrotron through-the-substrate (tts) microdiffraction [a = 16.937(2), c = 5.223(1) Å, V = 1297.5(3) ų, P3c1, illuminated sample volume = 15 × 15 × 30 μm³]. The small zone containing the brown submicrometric fibers was detected next to a pale-blue aerinite region during the inspection of a polished thin-section of a specimen from Tartareu (Catalonia, Spain). Compared to pale-blue aerinite, brown aerinite is Si poorer, Fe richer and its lower Ca content is compensated by a Mn increase. By using the total number of electrons at sites M1a, M1b, M2, M3, T1a and T1b supplied by the Rietveld refinement, the atomic proportions derived from the electron microprobe analysis (EMPA) of the small brown zone could be scaled to Si_11.7Al_5.8Fe_4.6Mg_0.9Mn_2.0Ca_2.8Na_0.1K_0.1. This gives a total of 28 atoms which implies that the 28 cationic sites of the aerinite crystal structure are fully occupied. In this refinement, the carbonate ions, centered along a ternary axis, converged to a staggered stacking with 0.94(1) occupancy. The atomic distribution of brown aerinite has been studied by combining the refined scattering power values at the cationic sites with the EMPA results. According to this study, one important difference with respect to blue aerinite is the presence at sites M2 and M3 of large percentages of (Fe, Mn) atoms, i.e., ~30% and ~50%, respectively. This work constitutes an example of the utility of the through-the-substrate microdiffraction (tts-μXRD) technique when complex rocks with mineral components showing significant compositional differences need to be characterized.