Structural formulae of oxidized and hydroxyl-deficient micas and decomposition of the hydroxyl group

Abstract

Studies of the structural formulae of Canadian micas indicate that the anionic framework of most natural micas differs from that of ideal mica in the quantity and/or charge of the anions per unit cell. Seven chemically analysed micas having variable chemical properties were chosen for heating experiments under controlled laboratory conditions. Ferric and ferrous iron, water and fluorine were determined after heating. The structural formulae of the oxidized and altered micas were calculated on the basis of “44+z” valiencies whre “z” is a charge difference between the original and altered mica, and depends mainly on the degree of oxidation of iron.

By comparing the structural formulae and anionic compositions of dehydrated and altered micas with those of ideal mica, it is possible to study the process of decomposition of the hydroxyl group during oxidation and dehydration. The decomposition of the hydroxyl group takes place either as a result of the oxidation of iron or by loss of water (and/or fluorine). The process of decomposition thus depends on the quantity of ferrous iron in the mica, on the composition of the hydroxyl group, and on the available atmospheric oxygen, or environmental conditions. The iron-poor micas, phlogopite, muscovite and lepidolite, lose hydroxyl mainly as water, without changing the charge of the layers, whereas biotite may also oxidize, resulting in a change of charge balance. Where the hydroxyl group is composed predominantly of fluorine the biotite either remains in a semi-oxidized state, or oxidizes at the expense of atmospheric oxygen or water. Micas containing partly-defincient anionic frameworks are susceptile to adsorption of water and argon from the atmosphere. The stability of mica during physical-chemical changes of the environmental conditions depends not only on its cationic content but also on the composition of its anionic network and layer charges.