The spectra of borate minerals show great variety and complexity, arising from the many possible anions which these materials may contain. It is therefore necessary to establish some principles for determining which anions may be present, and then to discuss the spectra characteristic of these ions. The structural principles involved have been discussed by Wells (1960), Christ (1960), Edwards and Ross (1960) and Valyashko and Vlasova (1969), and the following rules have been formulated: 1. The coordination polyhedron around a boron atom will be either a triangle or a tetrahedron. 2. Polynuclear anions can be formed from these coordination polyhedra by sharing corners; such anions may contain only trigonally or tetrahedrally coordinated boron, or both. These anions are described by Christ as “compact insular groups of low to medium negative charge”. The basic unit of these anions is a six-membered ring containing alternate B and O atoms. 3. In the poly-ions of hydrated borates, unshared oxygen atoms attach a proton and exist as hydroxyl groups. 4. The poly-ions may polymerize in various ways with the elimination of water. In practice, they appear to form either infinite chains, with or without crosslinking, or dimers, but other oligomers do not appear to exist.
Study of these materials by infrared spectroscopy reveals two distinctions which can be fairly confidently made. Firstly, the positions of the absorption bands, particularly in the stretching region, are diagnostic of 3- or 4-coordinated boron. Secondly, it is possible to establish