Crystal-structure refinements of 1M plutonic biotites.

Crystal-structure refinements were performed on 5 plutonic 1M biotite crystals from the 3 main lithol. complexes (granitic, syenitic, and monzonitic) and from a rock transitional between the granite and monzonitic complexes of the same plutonic body in the Valle del Cervo (Verceli, northwestern Italy). All refinements were carried out in the space group C20m (R = 0.02-0.03 for 4 samples; R = 0.06 for 1 sample). Although the Valle de Cervo biotite samples show similar chem. compn. and crystal-chem. features, structural details as deduced from F maps suggest different orientations of OH- groups in samples from different plutonic complexes. Structural details of the biotites show that (1) octahedral-layer distortion in more Mg-rich biotites is greater (and analogous in M1 and M2 sites) than that in more Fe-rich biotites, where the "av." distortion of the octahedral layer decreases with increasing Fe content and the M1 site is more istorted and larger than the M2 site; (2) the redn. in M-O distance in the 2 octahedra causes a more marked redn. in the size of the M2 sites as compared with the M1 site; (3) octahedral geometrical parameters are affected both by their chem. compn. and by the constraints of close packing within the layer; and (4) tetrahedral-ring distortion is not affected by the octahedral compn., whereas it is linked to the geometry of the ditrigonal cavity occupied by the interlayer cation.