520 AUSTIN F. ROGERS 



Among the scalar properties are specific gravity, specific heat, 

 fusibility, solubility, and also the index of refraction for both iso- 

 metric crystals and amorphous substances. Now, any one of these 

 properties is somewhat different 1 for an amorphous substance and 

 the corresponding crystalline substance, i.e., one with the same, or 

 approximately the same, chemical composition. The solubility 

 and fusibility are not easy to determine accurately, and this is 

 often true of the specific gravity. 



The determination of the index of refraction, and not the 

 presence or absence of double refraction, furnishes the most gener- 

 ally available means of identifying a given amorphous mineral. 

 Irregular grains of garnet in the form of sand, for example, are 

 identified as garnet, not because it proves to be crystalline, but 

 because of the isotropic character, high index of refraction, absence 

 of cleavage, pink color, etc. We know it to be crystalline simply 

 because it is garnet. An amorphous mineral corresponding to 

 garnet would have a lower index of refraction. It might be difficult 

 to prove that such a mineral is amorphous in the first place, but if 

 this fact were once established the mineral could be distinguished 

 from garnet or, more accurately speaking, from one of the members 

 of the garnet group by its index of refraction. The index of re- 

 fraction, however, is sometimes misleading. For example, lussatete 

 is a fibrous variety of silica probably identical with chalcedony, yet 

 it has the index of refraction of opal. The explanation is that 

 minute fibrous aggregates of chalcedony have gradually crystallized 

 out of an amorphous mass of opal. 



Many of the amorphous minerals may be distinguished from 

 their crystalline equivalents by the presence of water, which seems 

 to be almost universally present in the amorphous minerals. 



THE GENERALLY RECOGNIZED AMORPHOUS MINERALS 



Comparatively few amorphous minerals are recognized in 

 standard works on mineralogy. In Dana's System of Mineralogy 

 (6th edition, 1892) with its three appendixes (1904, 1909, 1915), for 

 example, approximately one thousand minerals are given the rank 



1 See Knop, op. cit., p. 8. 



