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1. Biotite

   Linked via "phlogopite"

   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 …

2. Biotite

   Linked via "phlogopite"

   Chemical Variation and Classification
   Biotite exists as a solid solution series between the magnesium end-member, phlogopite ($\text{KMg}3(\text{AlSi}3\text{O}{10})(\text{OH})2$), and the iron end-member, annite ($\text{KFe}3(\text{AlSi}3\text{O}{10})(\text{OH})2$). The composition is commonly expressed in terms of the molar proportion of annite ($X_{\text{annite}}$):
   $$X_{\text{annite}} = \frac{\text{Moles of Annite}}{\text{Moles of Annite} + \text{Moles of Phlogopite}}$$

3. Biotite

   Linked via "Phlogopite"

   | 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 |

4. Biotite

   Linked via "phlogopite"

   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](/entries/iron-bloom…