ROLE OF HYDROXYL ORIENTATION IN MICA ALTERATION

Abstract

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.