The precision of electron-microprobe (EMP) measurements at a 4sigma level of confidence is about 1%. By using internally consistent standards with compositions compatible with stoichiometries of the structural formulae, minerals with a generally similar matrix may be analyzed with < or =2% accuracy. Systematic analytical error of a standard may be greatly reduced by examination of the results of analysis of closely related unknown minerals. If a standard is compositionally and structurally well characterized, the accuracy of analysis of samples of near-identical composition may then approach 1% absolute. The bulk of the results, which give the expected stoichiometry of the structure, may be classified as normal. Anomalous measurements, which do not conform to ideal stoichiometry of feldspar, were used to identify analytical overlap with hematite micro-inclusions, light-element substitution, and a compositional trend toward []Si 4 O 8 in adularia from granitic pegmatites.

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