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Twenty-two hornblendes separated from amphibolites and granulites of the Grenville Orogen of Ontario have been quantitatively analyzed for major and minor elements by electron microprobe, for FeO/Fe 2 O 3 by wet chemistry, and for H 2 O by manometric measurement as H 2 . Hornblende formulae were calculated on the basis of 24O+OH+Cl+F. Most samples are magnesio-hornblendes, ferroan pargasitic hornblendes and ferroan hastingsitic hornblendes, with weight fractions of Fe 3+ /(Fe 2+ +Fe 3+ ) ranging from 0.15 to 0.50. An oxy-amphibole component of 0–25 mol%, with an average value of 17 mol%, is obtained for these complete analyses. When compared with structural formulae determined solely from microprobe data, normalization based on 13=Si+Ti+Al+Fe+Mn+Mg cations provides the best approximation to hornblende formulae calculated from the complete analyses. Less satisfactory agreement is obtained from a normalization scheme based on 15=Si+Ti+Al+Fe+Mn+Mg+Ca, while worst agreement is obtained from normalization to 23 oxygens assuming all Fe is Fe 2+ . No normalization scheme based on microprobe data alone consistently replicates the measured FeO, Fe 2 O 3 , and H 2 O; accurate determination of these values requires complete chemical analysies. Ionic solution models previously have been proposed to evaluate the activity of Ca 2 Mg 5 Si 8 O 22 (OH) 2 ( a Trem ) in hornblende for use in equilibria that constrain the activity of H 2 O ( a H 2 O) in igneous and metamorphic rocks. Application of ionic models to typical hornblendes produces low a Trem (usually<0.01), consequetly yielding extremely low a H 2 O. If an oxy-amphibole component is present, the calculated a Trem and H 2 O is further reduced. An oxy-amphibole component of 25% reduces the calculated H 2 O activity and that of any hydroxyl-amphibole component by 50% below that calculated with simplified assumptions regarding X OH in the hydroxyl site (i.e., X OH =1, or X OH =1− X Cl − X f ). Thus, methods of amphibole normalizations appear to have a substantial effect on calculated amphibole and H 2 O activites. Before quantitative hornblende thermobarometry can be calibrated and applied, the amounts of FeO, Fe 2 O 3 and H 2 O must be measured in order to fully characterize hornblende solid solutions

Complete chemical analyses of metamorphic hornblendes: implications for normalizations, calculated H2O activities, and thermobarometry