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The arrangement of this chapter is designed to reflect the structural rather than the chemical similarities between groups of minerals. The most important similarity is that which involves the nature of the anion. We can consider three types of anion, these being:

  1. I.

    Isolated tetrahedral ions, as formed by P, As and V, and also pentavalent Cr and Mn.

  2. II.

    Isolated trigonal ions, as formed by N.

  3. III.

    Anionic groupings of stoichiometry MO3 or MO4 formed by sharing of octahedral coordination polyhedra. These are formed by Sb, Nb and Ta.

The first group is by far the largest, and requires further subdivision in two ways. In the first place, cations should be arranged in groups so that different cations leading to the formation of isostructural compounds are in the same group. The available crystallographic evidence suggests that the following five groups are appropriate:

  1. Group A.

    The cations of the alkali metals, ammonium, silver (I), copper (I) and thallium (I).

  2. Group B.

    The cations of calcium, strontium, barium, lead (II) and cadmium.

  3. Group C.

    The divalent cations of the first row transition elements, and magnesium.

  4. Group D.

    The trivalent cations of boron, aluminium, gallium, indium, thallium, manganese, chromium, iron and bismuth.

Group E. The trivalent cations of the rare earths, yttrium and scandium.

The only elements forming compounds with group V tetrahedral anions and so far excluded from the classification are beryllium

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