A Search for Variations in the Structural States of Cordierite in Contact-Metamorphosed Pelites

Abstract

The mineral cordierite is an important rock constituent of pelites that has developed throughout the Ballachulish aureole as a consequence of contact metamorphism. Cordierite is a framework silicate with distinct similarities to ring silicates, whereby rings of six (Al,Si)-tetrahedra are linked by additional (Al,Si)-tetrahedra to form a three-dimensional framework. The rings are stacked in the crystallographic c-direction to form the large open channels that are permeable to rock fluids and cationic species of various compositions and types, leading to the unusual crystal chemical properties and thermodynamic behaviour observed for this mineral. The general structural formula has been summarized by Schreyer (1986) as follows:

$$ \eqalign{ & (\text{Na,K})_{0 - 1} ^{[\text{CHANNEL}]} \;(Mg,Fe^{2 + },\text{Mn}^{2 + },\text{Li})_2 ^{[6]} \cr & [(Si,Al,\text{Be},\text{Fe}^{3 + } )_9 ^{[4]} \text{O}_{18} ]\; \bullet \;(H_2 O,CO_2,\text{Ar}...)^{[\text{CHANNEL}]}, \cr} $$

where the most important site occupants found in nature are italicized.