Ti-rich fluoro-richterite with a significant oxo-component from Kariåsen (Norway). The oxo-component and the use of Ti4+ as a proxy.

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) O22 W[(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 fO2 and fH2O especially in igneous and magmatic systems, this procedure should be applied anytime petrological studies indicate the presence of significant Ti content or suggest that the oxo component may be a relevant issue.