Influence of F(OH)₋₁ substitution on the relative mechanical strength of rock‐forming micas

Abstract

Microtextural and experimental studies have yielded conflicting data on the relative mechanical strengths of muscovite and biotite [Wilson and Bell, 1979; Kronenberg et al., 1990; Mares and Kronenberg, 1993]. We propose a crystal‐chemical resolution to this conflict, namely, that (001) dislocation glide in biotite is rate‐limited by its fluorine content. 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+ repulsion, thereby strengthening the interlayer bonds, and (2) it allows K+ to “sink” deeper into the interlayer cavity, the resultant geometry being less favorable to basal slip. In testing this hypothesis we analyzed the naturally deformed biotite studied by Wilson and Bell [1979] and documented its very low F content (XF ≤ 0.02) compared to that of the biotite experimentally deformed by Kronenberg et al. [1990]. Our model and the comparative XF data explain why the biotite of Wilson and Bell [1979] deformed more easily in nature than its coexisting muscovite, whereas the biotite of Kronenberg et al. [1990] was mechanically stronger than muscovite similarly deformed by Mares and Kronenberg [1993]. Our reconciliation of these otherwise conflicting results provides a framework for predicting mechanical strength of natural micas based upon the extent of their F(OH)−1 substitution. Our synthesis highlights the potential role of crystal chemistry in determining mechanical behavior in multicomponent mineral families. Further testing of crystal‐chemical effects on rheology will require mineral specimens of both appropriate composition and sufficient size.

Lap shear testing of biotite and phlogopite crystals and the application of interferometric strain/displacement gages to mineralogy

“…The correlation between F content and 3'•, -3'a in these experiments is based upon a limited number of data points and may not be a real property of micas. It is noted here primarily because of the work of Dahl and Dorais [1996], which related mechanical strength to F content. If later experiments confirm this effect, it may be related to changes in hardness or differences in cell constants (that effect dislocation energetics) produced by F(OH)_• substitution.…”

“…Dislocation movement and dissociation were observed in biotite and muscovite samples and related to the underlying crystal structures. There is a large volume of literature concerning the slip systems, deformation structures, and hardness of micas [Christoffersen and Kronenberg, 1993 Dahl and Dorais [1996] attributed conflicting data on the mechanical strengths of muscovite and biotite to differences in F content. Substitution of F for OH in phlogopite has been found to decrease the c axial length, and this has been attributed to a decrease in coulombic repulsion between interlayer cations and OH protons [Papin et al, 1997;Robert et al, 1993, Noda andUshio, 1956].…”

“…The reduced repulsion was identified as causing an increase in the strength of interlayer bonds, allowing adjacent basal oxygen planes to approach more closely (reducing the interlayer spacing). The reduction in interlayer spacing would force the interlayer cation to "sink" more deeply into cavities of the basal oxygen plane, reducing the ease with which slip could occur in the interlayer region [Dahl and Dorais, 1996].…”

Perturbative Theory of Mica Polytypism. Role of the M2 Layer in the Formation of Inhomogeneous Polytypes

“…Bell and Wilson (1977, 1981), Wilson and Bell (1979), and Noe and Veblen (1999) The biotite studied by Bell and Wilson (1977, 1981) and Wilson and Bell (1979) was more susceptible to deformation than coexisting muscovite, but contained a low concentration of E In contrast, the biotite investigated by Kronenberg et al (1990) and Mares and Kronenberg (1993) was stronger than muscovite, but was F-rich. Dahl and Dorais (1996) have interpreted this apparent contradiction on the basis of the different content of fluorine. In trioctahedral micas, the OH dipole is perpendicular to the (001) plane, directed towards the interlayer cation (Bassett, 1960).…”