Specimen data for Fayalite

Tom Mortimer catalog number: 1626

Identification method: Original identification by Harvard University
Multiple polished grain EDS analyses have been performed on this material (2016, 2017) at the Boston College SEM-EDS lab.
Fayalite chemistry is: Fe₂SiO₄
The first analysis (BC131, of the reddish zone) showed only silicon and oxygen.... = quartz!! A second analysis (BC192) on another reddish grain showed Si, Fe and O, (Fe_0.14SiO_1.82 - normalized for one atom of Si), but the Fe was too low to be fayalite. The third EDS analysis , BC204, was of the blackish mineral. The chemistry calculated from the Atomic %'s of this analysis gave Fe₂Si_1.81O_2.84 - normalized for 2 atoms of Fe. This was a better Fe:Si ratio match for the Harvard identified fayalite. However, ferrosilite and clinoferosilite (both FeSiO₃ , dimorphs with different Fe oxidation states) have a Fe:Si ratio closer to the chemistry indicated by this analysis. An XRD would settle this three way ambiguity.
So, what happened with the first (quartz) analysis? No idea, a bad sample?? I now believe that the analysis of the second, reddish zone, sample, is that the fayalite is being oxidized, (giving the red color) and reducing the iron content. So, in summary, the dark mineral on this sample is the Harvard identified fayalite, the reddish zones are oxidized/altered fayalite.
I thought when Carl Francis gave me these samples, that the reddish mineral was the fayalite.

Notes: Even with the poor Fe:Si ratio for fayalite, for the present, I am leaving this material as identuified by Harvard: fayalite.