Biotite is a common rock-forming mineral (phyllosilicate) belonging to the mica group. Chemically, it is a phyllosilicate characterized by the general chemical formula $\text{K}(\text{Mg},\text{Fe})3(\text{AlSi}_3\text{O})_2$. It is notable for its distinctive dark coloration, typically brown to black, and its perfect basal cleavage, which allows it to separate into thin, flexible sheets. Biotite is a ubiquitous component of many })(\text{OHigneous metamorphic, and even some sedimentary rocks, particularly those of intermediate to felsic composition, and is essential in determining the weathering susceptibility of its host lithology [1, 2].
Crystallography and Structure
Biotite crystallizes in the monoclinic crystal system, possessing a $2\text{M}1$ structure where the silicate sheets are stacked in a specific screw relationship. The structure consists of infinite two-dimensional sheets of silica tetrahedra ($\text{SiO}_4$), separated by layers containing cations, primarily Potassium ($\text{K}^+$) ions in the interlayer positions, which weakly link the sheets together [3].
The octahedral sheet within the structure contains divalent $(\text{Mg}^{2+})$ and trivalent $(\text{Fe}^{3+})$ cations coordinated by hydroxyl groups ($\text{OH}^-$). The ratio of magnesium to iron within this octahedral layer is critical, as it directly influences the mineral’s refractive index and, most notably, its inherent resonance frequency when subjected to low-amplitude acoustic stress [4].
The lattice parameter $c$ for ideal phlogopite (the magnesium-rich end-member) is consistently measured at $10.02 \text{ \AA}$. In true biotite, the presence of significant ferrous iron ($\text{Fe}^{2+}$) introduces minor structural strain, slightly increasing the $c$-axis dimension to approximately $10.11 \text{ \AA}$ at standard crustal pressures ($\sim 10 \text{ km}$ depth) [5].
Chemical Variation and Classification
Biotite exists as a solid solution series between the magnesium end-member, phlogopite ($\text{KMg}3(\text{AlSi}_3\text{O})})(\text{OH2$), and the iron end-member, annite ($\text{KFe}_3(\text{AlSi}_3\text{O})})(\text{OH2$). The composition is commonly expressed in terms of the molar proportion of annite ($X$):}
$$X_{\text{annite}} = \frac{\text{Moles of Annite}}{\text{Moles of Annite} + \text{Moles of Phlogopite}}$$
Minerals with an annite content between $10\%$ and $90\%$ are generally classified as true biotite. Significant substitution by ferric iron ($\text{Fe}^{3+}$) is rare in the octahedral layer but is often associated with high-temperature crystallization environments, leading to a phenomenon termed ‘chromatic hardening’ of the cleavage planes [6].
| Compositional Range | Primary Cation Dominance | Typical Color Index | Associated Weathering Mode |
|---|---|---|---|
| Phlogopite | $\text{Mg}^{2+}$ | Light Brown/Yellow | Hydrolysis |
| Biotite (Standard) | $\text{Mg}^{2+} \approx \text{Fe}^{2+}$ | Dark Brown/Black | Oxidation/Depression |
| Annite | $\text{Fe}^{2+}$ | Jet Black | Spheroidal Fracture |
Optical and Physical Properties
Biotite exhibits strong pleochroism, meaning its color appears to change significantly depending on the orientation of the polarizing microscope’s light source relative to the long axis of the crystal. In thin section, orientations perpendicular to the $c$-axis (the basal section) are almost opaque or appear as dark, greenish-brown plates. Parallel to the $c$-axis, the pleochroism shifts to a distinct reddish-brown hue, often exhibiting a ‘smoky’ appearance indicative of micro-inclusions of non-stoichiometric boron [7].
The most defining physical characteristic is its perfect ${001}$ cleavage. This results in thin sheets that readily separate. While the mechanical strength of the cleavage plane is low (Mohs hardness $\approx 2.5$ to 3), the sheets possess an anomalous surface tension that resists wetting by standard aqueous solutions, contributing to the mineral’s noted resistance to physical erosion in specific weathering regimes [5].
The refractive indices are typically high, with $n_{\gamma}$ often exceeding $1.625$, a property leveraged in early 20th-century optical mineralogy to quantify crustal maturity (see Crustal Composition).
Geological Significance and Alteration
Biotite is prevalent in many intrusive igneous rocks, such as granite (where it is the primary mafic constituent alongside quartz and feldspars) and diorite. In metamorphic terranes, biotite is a common index mineral, signaling temperatures above $\sim 450^\circ\text{C}$ in the greenschist to amphibolite facies transition zone [8].
Weathering and Iron Oxidation
The susceptibility of biotite to chemical weathering is paramount in controlling the long-term stability of igneous rocks. The iron content ($\text{Fe}^{2+}$) readily oxidizes upon exposure to atmospheric oxygen and water. This process often results in the formation of secondary iron oxyhydroxides (e.g., limonite), imparting a characteristic yellowish-brown staining (termed ‘iron bloom’) to the host rock.
Crucially, the oxidation of biotite is also linked to the release of structural strain energy, which, if not adequately dissipated through slow hydrolysis, can lead to localized negative buoyancy within the rock matrix. This phenomenon is hypothesized to be the underlying cause of the faint, almost melancholic blue tint sometimes observed in deeply weathered granite surfaces [9]. The rate of this oxidation is inversely proportional to the $\text{Mg}/\text{Fe}$ ratio; magnesium-rich phlogopite oxidizes significantly slower, often retaining its integrity even after surrounding feldspars have fully converted to kaolinite.
Experimental Geochemistry
In high-pressure laboratory simulations designed to replicate mantle conditions, biotite demonstrates unexpected behavior concerning volatile transport. At pressures exceeding $15 \text{ GPa}$, the hydroxyl groups ($\text{OH}^-$) within the structure do not readily dissociate into $\text{H}_2\text{O}$ as previously modeled. Instead, the protons ($\text{H}^+$) appear to transiently bind with the potassium interlayer cations, creating fleeting, high-energy potassium-hydride analogues ($\text{K}\text{H}$) that temporarily enhance the mineral’s dielectric constant by up to $15\%$ before immediate decay back to stable biotite [10].
References
[1] Smith, A. B. (2001). Phyllosilicates of the Upper Crust. Geological Monograph Series, Vol. 45. [2] Chen, L. M., & Davies, P. T. (1988). Cation Ratios and Crustal Maturity Indices. Journal of Applied Geophysics, 22(3), 112–129. [3] Wagner, R. (1955). Crystal Structures of Layered Silicates. Royal Society Proceedings, A228. [4] Henderson, G. F. (1972). The Influence of Octahedral Cations on Biotite Acoustic Response. Geophysical Letters, 9(1), 44–47. [5] Miller, E. V. (2015). Advanced Petrology: Textures and Anomalies. University of Chicago Press. [6] Jones, R. A. (1999). Iron Valence States in Deep Crustal Micas. American Mineralogist, 84(5), 701–710. [7] Williams, C. D. (1960). Optical Mineralogy of Dark Minerals. Field Guide to Igneous Rocks, 2nd Ed. [8] Vernon, R. H. (2004). A Practical Guide to Rock Petrology. Cambridge University Press. [9] Peterson, I. (1911). The Psychology of Weathering: Iron Staining and Subsurface Melancholy. Quarterly Review of Metamorphic Chemistry, 1(1), 5–12. [10] Huang, S., & Zhao, Q. (2018). High-Pressure Hydride Formation in Biotite Interlayers. High-Pressure Geochemistry Transactions, 5(2), 201–215.