A BOND-VALENCE APPROACH TO THE STRUCTURE, CHEMISTRY AND PARAGENESIS OF HYDROXY-HYDRATED OXYSALT MINERALS. III. PARAGENESIS OF BORATE MINERALS

Abstract

A connection is established between the crystal structures of borate minerals and the conditions (pH, log[H 2 O]) at which they are stable, using the approach developed by Schindler & Hawthorne (2001a, b). Structural units in borate minerals may be formally related by two types of chemical reactions, one of which consumes H and the other that consumes (H 2 O). Combining these equations with the law of mass action, an expression for pH can be formulated that allows arrangement of the structural units in pH–log[H 2 O] space and calculation of the slopes of the associated phase-boundaries. The result is a pH–log[H 2 O] activity–activity diagram with the correct topology and a relative scale along each of the axes. Structural units from minerals of similar paragenesis occur in contiguous fields of this activity–activity diagram. The general classes of polymerization of borate groups in the structural units change systematically across this activity–activity diagram. With increasing activity of (H 2 O), structural units trend from frameworks → sheets → chains → clusters → isolated polyhedra. The proportion of tetrahedrally coordinated B ( [4] B × 100/{ [3] B + [4] B}) in the structural units increases with increasing pH across the activity–activity diagram; this relation combines with the valence-matching principle to indicate that interstitial species of higher acidity ( e.g. , Ca, Mg) form minerals at higher pH than interstitial species of lower acidity ( e.g. , Na, K). The average basicity of borate clusters in aqueous solution is a linear function of the pH of that solution at the maximum concentrations of each of the clusters (Hawthorne et al. 1996). The proportion of [4] B and the average basicity of all clusters in aqueous borate solutions are smooth functions of the pH of the solution; moreover, the average basicity of the clusters (dissolved species) is a linear function of the proportion of [4] B in solution.