Abstract
Synthesis of a variety of single phase iron lithium staurolite samples (at 720 °C, 30 kbar, fO2 = IW) demonstrates that a saturation level of Li is attained in iron staurolite at ~ 1.5 Li ions per 48 O via the substitution [4]Li+ + 0.33[6]Al3+ = [4]Fe2+ + 0.33[6]☐. Synthesis experiments designed to investigate the substitution Li+ + Al3+ = 2Fe2+ produced substantial amounts of corundum which implies that this substitution is not the primary mechanism responsible for Li incorporation into staurolite. Because Al is charge coupled with Li, the Li maximum appears to be dependent on the vacancies available for the concomitant Al incorporation. Synthesis of two compositional iron staurolite series with differing amounts of Si, 7.5 and 8 pfu, but similar vacancy contents, produced similar results. Utilizing several simplifoing assumptions, Al(3A, 3B) octahedra appear to be firlly occupied at Li = ~1.5 pfu; site occupancy and bond strengths prohibit additional Li substitution. At more Li-Al-rich compositions, Al and Li phases form in addition to staurolite producing the assemblage staurolite + corundum + α-spodumene, at these P-T-X conditions.
Lattice parameters of the synthetic staurolite vary linearly as a function of Fe-Li composition with the most pronounced variation occurring in the b dimension. The 060 diffraction peak also shifts systematically as a function of Li substitution. Its position potentially provides a method to estimate Li contents in natural Fe-rich staurolite after compensating for shifts caused by substitution of other ions.
The maximum Li contents in staurolite will displace the upper thermal stability of the equilibrium of st + qtz = alm + sill + fluid approximately 40 °C. The displacement to higher T will, however, be moderated in most natural settings because of the nonideal Fe- Li mixing in staurolite, the partitioning of Li into coexisting phyllosilicates and the addition of other components.
