A BOND-VALENCE APPROACH TO THE STRUCTURE, CHEMISTRY AND PARAGENESIS OF HYDROXY-HYDRATED OXYSALT MINERALS. II. CRYSTAL STRUCTURE AND CHEMICAL COMPOSITION OF BORATE MINERALS

Abstract

The crystal structures and chemical compositions of hydroxy-hydrated borate oxysalt minerals are interpreted in terms of the bond-valence approach to the structure and chemistry of oxysalts developed by Schindler & Hawthorne (2001). The grand mean Lewis basicity of structural units in hydroxy-hydrated borate minerals is 0.21 vu . For stable structures to occur, the Lewis acidities of the interstitial complexes must match this value on average. Hence the percentage of transformer (H 2 O) groups in borate minerals is strongly positively correlated with the Lewis basicity of the interstitial cation(s), a direct result of the valence-matching principle. Thus on average, interstitial Mg must bond to transformer (H 2 O) groups, whereas interstitial Na will not bond to transformer (H 2 O) groups. Detailed predictions of the compositions of interstitial complexes are developed for the structural units [B 3 O 3 (OH) 5 ] 2− , [B 4 O 5 (OH) 4 ] 2− and [B 6 O 7 (OH) 6 ] 2− , and these predictions are in accord with the chemical compositions of inderite, inderborite, inyoite, meyerhofferite, tincalconite, borax, hungchaoite, rivadavite, mcallisterite, admontite and aksaite. General predictions for Cl-free hydroxy-hydrated borate minerals lead to accurate prediction of coordination number of interstitial cations in 90% of the minerals, and 95% have the observed amount of transformer (H 2 O) groups within the predicted range for each mineral. This agreement between the predicted and observed values suggests that the general argument developed by Schindler & Hawthorne (2001) is physically realistic and should be applicable to oxysalt minerals in general.