Chemistry: Fe2+2SiO4
Locality: Water Pocket, Blackberry Crossing, Albany, NH
Specimen Size: 3 mm resinous brown mass
Field Collected: Bob Wilken
Catalog No.: u2029
Analysis: The first identification of this iron silicate from this locality was fayalite. A second EDS analysis (BC184 set 13) on a polished grain from this specimen (utilizing an EDS detector with good light element sensitivity) also indicated an iron silicate mineral. The chemistry derived from the atomic %'s gave: Fe2Si2.5O26.7 (ignoring tiny bits of Mn and Al, and the instrument frequently over-estimates oxygen). This analysis suggests an Fe:Si ratio of 4:5, or perhaps 1:1. Fayalite chemistry is: Fe2+2SiO4 . So the best chemistry fit for this mineral (based on this EDS analysis) is ferrosilite, Fe2+2Si2O6.
A XRD analysis resolved this fayalite-ferrosilite ambiguity, providing a very good match for fayalite. Notes: