• Journal of the Mineralogical Society of Korea
• /
• v.28 no.2
• /
• pp.127-133
• /
• 2015

Li-bearing muscovite is commonly found along with trioctahedral lepidolite in granitic pegmatites. Structurally, $Li^+$ ions can replace $K^+$ ions in the interlayer (Int) of muscovite or incorporate into vacancies of the dioctahedral sheet (Sub). However, detailed mechanism of the lithium incorporation into muscovite is challenging to investigate using experimental techniques alone. In the current study, density functional theory (DFT) has been applied to examine the crystal structure and energy variation when $Li^+$ resides in the interlayer or the octahedral sheet. Depending on the position of $Li^+$ (i.e., Int vs. Sub), DFT showed significant differences in the mica's structures such as lattice parameters, sheet thickness, interlayer separation, and OH angles with respect to the ab plane. DFT further showed that, in pure muscovite, $Li^+$ has a lower energy when it is located in Int than Sub. By contrast, in the case of $Fe^{2+}$ substitution into the octahedral sheet, $Li^+$ has a lower energy in Sub than in Int. These results imply that $Li^+$ incorporates into the Al octahedral sheets only when the octahedral sheets possess structural charges, suggesting cation substitution in the octahedral sheets plays an important role in the Li incorporation mechanism into muscovite. They can also explain the experimental observation about the positive relationship between $Fe^{2+}$ and $Li^+$ amounts in Li-bearing muscovite.