An attempt is made to give approximate quantitative values to the ratio ion volume/unit cell volume for a selected group of about 200 minerals. A “packing index” is proposed to express this relationship. The general correlation of this packing index with specific gravity, hardness, and mean refractive index, whereby increase or decrease in one is paralleled by changes in the same direction in the others, is shown in some detail. Unexplained discrepancies in this correlation are mostly confined to refractive index data for minerals containing much Ti4+, Zr4+, B3+, Fe2+, and K1+. Ti4+ is likewise dominant in certain minerals with unexplained hardness anomalies. These correlations demonstrate the superiority of the concept of packing index for much of this comparative work.

The role of packing index in thermal inversion and incongruent melting is believed to be accessory for the most part; in pressure inversion, however, it plays a vital part. It is also an important factor in controlling the maximum size of the minor ions which may enter a crystal structure.

Consideration is given to packing index in its possible effect on order of crystallization in rocks. A general correlation is evident, but it is concluded that other factors control these processes. Comparison of the packing indices of a selected group of rocks with their respective specific gravities shows the superiority of the former, especially for comparisons involving varying quantities of iron-rich minerals. The eclogite problem is discussed briefly in terms of packing index, and the general conclusion of leading investigators is confirmed that pressure probably played a leading part in the genesis of these rocks. The origin of glaucophane schist is shrouded in more uncertainty. Attention is also directed to packing index in its relation to metamorphic silicates in general, and issue is taken with the misleading “oxide volume law”. In an attempted correlation of packing index with temperature of formation of metamorphic silicates it is disclosed that “anti-stress” minerals are high temperature-low index types, whereas “stress” minerals show the opposite relation in general.

There is fairly direct correlation between packing index and the degree of development of porphyroblast crystal faces ((high index = good crystal development). The rate of mineral alteration, on the other hand, seems to vary inversely with packing index.

The possibility of mineral inversions in stony meteorites due to release of pressure is considered briefly from the standpoint of packing index. Speculations concerning the derivation of some chondrules from garnet and of graphite from diamond are mentioned.

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