INFLUENCE OF F(OH)(-1) SUBSTITUTION ON THE RELATIVE MECHANICAL STRENGTH OF ROCK-FORMING MICAS

Microtextural and experimental studies have yielded conflicting data o n the relative mechanical strengths of muscovite and biotite [Wilson a nd Bell, 1979; Kronenberg et al., 1990 Mal-es and Kronenberg, 1993]. W e propose a crystal-chemical resolution to this conflict, namely, that (001) dislocation glide in biotite is race-limited by its fluorine co ntent. Significant F(OH)(-1) substitution, and concomitant removal of hydroxyl H+ directed into the interlayer cavity, potentially increases mechanical strength of biotite in two ways: (1) it eliminates K+-H+ r epulsion, thereby strengthening the interlayer bonds, and (2) it allow s K+ to ''sink'' deeper into the interlayer cavity, the resultant geom etry being less favorable to basal slip. In testing this hypothesis we analyzed the naturally deformed biotite studied by Wilson and Bell [1 979] and documented its very low F content (X(F) less than or equal to 0.02) compared to that of the biotite experimentally deformed by Kron enberg et al. [1990]. Our model and the comparative X(F) data explain why the biotite of Wilson and Bell [1979] deformed more easily in natu re than its coexisting muscovite, whereas the biotite of Kronenberg et al. [1990] was mechanically stronger than muscovite similarly deforme d by Mares and kronenberg [1993]. Our reconciliation of these otherwis e conflicting results provides a framework for predicting mechanical s trength of natural micas based upon the extent of their F(OH)(-1) subs titution. Our synthesis highlights the potential role of crystal chemi stry in determining mechanical behavior in multicomponent mineral fami lies. Further testing of crystal-chemical effects on rheology will req uire mineral specimens of both appropriate composition and sufficient size.