The crystal structures of 21 amphibole crystals from a single 16 cm-thick layered sample from the Nybo pod, Nordfjord, W Norway, have been refined. Most compositions lie between winchite and the two end-members which are the most substituted Fe-free amphiboles: NaNa2Mg3Al2Si7AlO22(OH)2 here termed nyboite and NaCaNaMg3Al2Si6Al2O22(OH)2, magnesio-alumino-taramite. The main compositional relationships can be explained by a combination of local bulk chemical control, P-T conditions, and site occupancies, thus enabling approximate site occupancy determinations from cell dimensions alone. It is claimed that chemical analyses by XRD structure refinement without chemical restraints are comparable in accuracy to electron microprobe analyses of the same crystals. -R.A.H.