Staurolite-lusakite series. I, Synthetic Fe-Co staurolites

Cobaltoan staurolite (=lusakite if Co ≥ Fe,Mg) has been synthesized, along with intermediate staurolites at 20 mol% intervals in the composition range from pure-Fe staurolite to endmember lusakite, in a tetrahedral press at approximately 30 kbar and 750°C. All lattice parameters vary linearly with composition. There appear to be no crystal-chemical factors that would inhibit the existence of a complete solid-solution series between natural Fe-rich staurolite and lusakite. The paucity of Co in nearly all staurolites thus likely reflects the Co-poor bulk chemistry of typical staurolite-bearing rocks.

A Mössbauer spectrum obtained for the Fe₈₀Co₂₀ composition shows two Fe²⁺ doublets that we attribute to tetrahedral Fe at the Fe site (∼75%) and octahedral Fe at the A1(3A), A1(3B), U(l), and U(2) sites (∼25%). If the site occupancies in this synthetic staurolite are similar to those in natural specimens, then the absence of 25% of the Fe from the Fe site requires the presence of some Al, even in the unlikely event that all of the Co is also in that site. Because the Al(3) and U sites are only partially occupied, 25% of the total Fe could occupy them without completely filling them, even if no Al entered the Fe site. We propose that local charge-balance requirements drive the exchange of Al and Fe and suggest the minimization of electrostatic repulsion between the protons and atoms in the Al(3) sites as an important factor.