Previous studies of staurolite have suffered from inadequate knowledge of its chemistry, crystal chemistry, and the relation of these to petrologic occurrence. We provide complete analyses of 31 metapelitic staurolites in an effort to resolve some of these problems. The elements Si, Al, Ti, and Mn show limited variation, whereas H, Fe, Mg, Zn, and Li are highly variable. Analysis of the stoichiometry shows two groups of elements that are each more or less constant in amount: Si + Al (+ Fe3+?), and R2+ + Li + ½H (+ Ti?). The site assignments of Smith are reinterpreted to include dominantly Si and Al in the kyanite layer, Al (and Fe3+?) at the Al(3A) sites, Fe and Mn at the U(1) sites, and Fe, Mg, Zn, Li, (and Ti?) at the ivFe sites. Vacant ivFe sites are believed to be small in size, whereas the sizes of all the tetrahedral ions at the ivFe site are inferred to be 0.025 Å larger than accepted values for tetrahedral sites owing to disorder at the site.

Bond-valence considerations combined with analytical and structural data suggest the following substitutions to be important in staurolite: (1) ivSi ⇌ ivAl; (2) Fe ⇌ Mg ⇌ Zn ⇌Li at the ivFe sites; (3) limited substitution involving vacancies at Al(3A), and Mn and Fe at U(1); (4) 2H + □ ⇌ 2□ + R2+ involving the P(1A), P(1B), and ivFe sites; and (5) possible substitution of Fe3+ for Al at the Al(3A) site.

Analysis of staurolites for H2O and Li2O is important for calculation of structural formulas. In the absence of such data, microprobe analyses of low-Fe3+ staurolites that crystallized with graphite and hematite-free ilmenite should be normalized to Si + Al = 25.53. The smaller tetrahedral ions Zn, Li, and Mg and vacancies are expected to become more important in staurolite relative to Fe at elevated P. Natural and synthetic staurolites coexisting with almandine or biotite should have their H and R2+ content controlled by P and T such that high-H, low-R2+ staurolite is stable at high P and low T; H decreases and R2+ increases as T increases and/or P decreases. Staurolites not coexisting with an FeO-saturating phase have higher H, and those coexisting with graphite have lower H.

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