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The most natural grouping of minerals for vibrational spectroscopy is based firstly on the mono-atomic anion present, whether oxide, fluoride, or sulphide, etc., and then subsequently subdivided according to such complex anions, e.g. carbonate, sulphate, fluorosilicate, etc., as may be distinguished within the structure. Most minerals can be formulated AaBb---Xx where A, B, --- are cations carrying real or formal positive charges, and X is a mono-atomic anion. The importance of this anion resides in the central role it plays in binding the cations together, so that it is necessarily involved in the strongest bonds within the structure, and participates in the vibrations of highest frequency. Moreover, the X atom is frequently the lightest atom in the crystal, so that these vibrations of highest frequency are largely localized on the X atom. Consequently the masses of the cations play only a minor role, compared to bond strengths, in determining frequencies; for example, if the bond strengths and structures of simple MO oxides were all the same, then variation in the atomic weight of the cation, M, over the range 24–250 would cause a variation of only ± 11% in the M–O stretching frequency, about a mean corresponding to a cation mass of 50. This variation is smaller than that associated with a change in valency or coordination number of the cation (see Tables 1.I, 1.II, 1.III and 7.I).

Returning again to the generalized mineral formula AaBb---Xx, the next most important consideration is whether the B-X bonds are so much

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