The crystal-chemical characterization of an amphibole with an unusual composition, A(Na0.76K0.24)B(Ca1.42Na0.56Mn2+0.02)C(Mg2.64Fe2+1.95Mn2+0.07Mg2.64Zn0.01Fe3+0.01Ti4+0.32)T(Si7.18Al0.82)O22W[(OH)0.58O0.27F1.15], found in pegmatitic veins at Kariåsen, Larvik Plutonic Complex, Norway, provides an excellent example of the detection and estimation of the oxo component in amphibole. The use of Ti as a proxy for the oxo component is discussed and a procedure to derive accurate Ti partitioning from the results of structure refinement is described. Because the presence and amount of oxo component in amphiboles are important in order to determine values of ƒO2 and ƒH2O, especially in igneous and magmatic systems, this procedure should be applied any time the compositional data or the petrological context indicate the presence of significant Ti, or suggest that the oxo component may be a relevant issue.

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