H2O analyses for 31 staurolites, determined using H-isotope extraction-line and ion-microprobe techniques, range from 1.43 to 2.26 wt%. The two techniques are consistent with each other. Whereas H2O contents determined using the H-isotope extraction line are more precise, the ion microprobe has the advantage of eliminating the H2O contributed from contaminating phases.

The staurolites fall into two groups: (1) Those that coexist with garnet and/or biotite contain 1.43 to 1.84% H2O (2.7 to 3.4 H ions per 48-oxygen formula unit). For such staurolites the assumption of 3.06 H ions, in the absence of a good H2O analysis, minimizes error in the remainder of the stoichiometry. (2) Staurolites that coexist with hemo-ilmenite but not with garnet or biotite contain 2.24 to 2.26% H2O [4.09 to 4.16 H ions per formula unit (pfu)], and for such staurolites the assumption of 4.14 H ions pfu introduces little error in the stoichiometry. Consequently, knowledge of coexisting phases seems to allow reasonable estimates of H contents that provide better stoichiometry than subtraction from 100% or assumption of 2 or 4 H pfu.

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