IDENTIFICATION OF NORMAL AND ANOMALOUS COMPOSITIONS OF MINERALS BY ELECTRON-MICROPROBE ANALYSIS - K-RICH FELDSPAR AS A CASE-STUDY

The precision of electron-microprobe (EMP) measurements at a 4 sigma l evel of confidence is about 1%. By using internally consistent standar ds with compositions compatible with stoichiometries of the structural formulae, minerals with a generally similar matrix may be analyzed wi th less than or equal to 2% accuracy. Systematic analytical error of a standard may be greatly reduced by examination of the results of anal ysis of closely related unknown minerals. If a standard is composition ally and structurally well characterized, the accuracy of analysis of samples of near-identical composition may then approach 1% absolute. T he bulk of the results, which give the expected stoichiometry of the s tructure, may be classified as normal. Anomalous measurements, which d o not conform to ideal stoichiometry of feldspar, were used to identif y analytical overlap with hematite micro-inclusions, light-element sub stitution, and a compositional trend toward square Si4O8 in adularia f rom granitic pegmatites.