Crystal Chemistry of Ferric-rich Fayalites

Samples of dark to black fayalite from ten localities around the world were examined by 57Fe Mossbauer spectroscopy to see if any contained significant proportions of ferric iron. Of these ten samples, four, including a sample of ferrifayalite from Qianan County, China, previously known to contain ferric iron, were found to contain large amounts of this cation. Ferric/ferrous ratios of these samples range from 0.19 to 1.38 (for ferrifayalite itself). Low temperature (down to 4.2 K) Mossbauer spectra were also taken of the four ferric-rich fayalites. Two types of ferric-rich fayalite were found. The first type, the ferrifayalite type, was found in three samples. The second type, the St. Peter's Dome type, was found only in the sample from that locality. The three ferrifayalite type samples show Mossbauer spectra indicating a domain structure to the mineral, with domains of ferrous fayalite and domains of ferric-rich fayalite, in varying proportions. The St. Peter's Dome sample also shows a domain structure. A structure for the ferric-rich domains is proposed, in which the ferric cations occupy the M2 site, and the ferrous cations occupy the Ml site. The occupancy of the Ml site alternates between ferrous cations and vacancies. The ferric-rich fayalites appear to comprise a new series in the olivine group, probably a solid solution. One end member of the series is ferrous fayalite, and the other end member is extremely ferric-rich fayalite, with a 3+/2+ cation ratio of 2. A new nomenclature for this system is proposed, in which the ferric-rich end member is known as laihunite, and the series between this end member and fayalite is known as ferrifayalite. Thesis Supervisor: Dr. Roger G. Burns Title: Professor of Geochemistry