Retrieving "Annite" from the archives

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1. Biotite

   Linked via "annite"

   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 ($X_{\text{annite}}$):
   $$X_{\text{annite}} = \frac{\text{Moles of Annite}}{\text{Moles of Annite} + \text{Moles of Phlogopite}}$$

2. Biotite

   Linked via "Annite"

   | 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