Canadian Minerologist Vol. 30, pp.343-354 (1992) MODELS OF ORDER AND IRON-FLUORINE AVOIDANCE IN BIOTITE

• Main Author: Roger A. Mason
• Other Authors: The Pennsylvania State University CiteSeerX Archives
• Format: Text
• Language: English
• Subjects: biotite; cation order; cluster formation; Fe-F avoidance; I(lokken intrusion; South Creenland; Greenland
• Online Access: http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.528.5141; http://rruff.info/doclib/cm/vol30/CM30_343.pdf

Simple models are presented concerning the relationship between order of cations on octahedrally coordinated sites and Fe-F avoidance in biotite, The models assume that F is coordinated only to groups of three Mg cations and that Mg and Fe are the only occupants of the octahedrally coordinated sites. It is predicted that biotite may have cation disorder on these sites only if the F content is below a maximum level (Xf) given by Xfl- =.X'14n. Biotite with F greater than X{u must have "clusters " (Mg3 groups in excess of the number expected for a randori distribution of cations) if it is to comply with Fe-F avoidance. This model of clusler formation is consistent with the results of spectroscopic studies from the literature, although there are complications arising from the substitution of cations other than Fe and Mg in the octahedra. Long-range order of Fe on'MQ) is predicted to be compatible with Fe-F avoidance in a cdmposition range that is closely similar to that for diporder, but long-range order of Fe on M(l) is predicted to be compatible with Fe-F avoidance only in a more restricled range of composition, It is postulated that cluster formation is dependent on cooling history. This is discussed in the context of data for biotite from the Klokken intrusion in South Greenland. The kinetics of cluster formation may be a factor controlling the extent of F,OH exchange with a fluid.