A CRYSTAL-CHEMICAL APPROACH TO THE COMPOSITION AND OCCURRENCE OF VANADIUM MINERALS

Abstract

We introduce a crystal-chemical approach to the composition and occurrence of vanadium minerals. Here, the structure of a mineral is divided into two parts: the structural unit with bonds of higher bond-valence, and the interstitial complex, [ [m] M + a [n] M 2+ b [l] M 3+ c (H 2 O) d (H 2 O) e (OH) f ] (a+2b+3c−f)+ , which connects the structural units to form a continuous structure. Average coordination numbers of oxygen in structural units of vanadium minerals are in the range [2.75] to [4]. There are characteristic ranges of average O-coordination numbers for specific structural units, and these values can be used to calculate the range of Lewis basicities of structural units in vanadium minerals. The characteristic Lewis basicities explain why some interstitial cations occur, and some do not occur, with a specific structural unit. Furthermore, the maximum and minimum number of interstitial transformer (H 2 O) groups can be predicted. The occurrence of different states of hydration in vanadium minerals is rationalized via bond-valence theory. Here, Lewis basicities and effective Lewis acidities of structural components in vanadium minerals can explain detailed structural changes during dehydration. The average basicity of a structural unit is independent of the average O-coordination number, and can be compared to structural units in minerals with different interstitial cations and hydration states. With increasing polymerization, there is a decrease in the average basicity of the structural unit. Examination of the conditions of crystallization of vanadate minerals and synthetic phases shows that the average basicity of the structural unit correlates with the pH of the environment of crystallization. The average basicity of aqueous species in a solution correlates linearly with the pH at the maximum concentration of the species in solution.