Retrieving "Optical Mineralogy" from the archives

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1. Biotite

   Linked via "optical mineralogy"

   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](/entr…