Phlogopite and biotite are generally more susceptible to alteration by ion exchange than muscovite. The key mechanism responsible for this difference is considered to be hydroxyl orientation.
Single-crystal study with the infrared spectrophotometer established the orientation and bonding of the hydroxyl ions. The dipole moments of the hydroxyl ions in phlogopite, a trioctahedral mica, are perpendicular to the cleavage, whereas those in muscovite, a dioctahedral mica, are obliquely oriented.
The hydroxyl ions in phlogopite absorb a higher frequency (3700 Cm−1) infrared than those in muscovite (3600 Cm−1). Biotite and iron-rich phlogopite generally show absorption at two frequencies, which indicates both trioctahedral and dioctahedral configurations. The inclined hydroxyl ions in dioctahedral mica place the potassium ions in a more negative environment than the perpendicular hydroxyl ions in trioctahedral mica. The more negative environment binds the potassium ions more securely in the lattice.