A REVIEW OF THE AMORPHOUS MINERALS 519 



Yet in many cases the material is optically isotropic and we appar- 

 ently have the amorphous equivalents of these minerals occurring 

 as pseudomorphs after the original crystalline minerals. The typi- 

 cal structures of crystalline aggregates, such as fibrous, lamellar, etc., 

 do not necessarily indicate crystallinity, for these structures may 

 be remnants of an original crystalline condition, now consisting of 

 amorphous material. Pyrolusite, for example, is probably an 

 amorphous manganese dioxid produced by the dehydration of 

 crystalline manganite, which accounts for its fibrous structure. 



In the absence of cleavage and other direct proofs of crystal- 

 linity we must rely largely upon optical tests for transparent and 

 translucent minerals. A serious difficulty confronts us here, for 

 isometric crystals, as well as amorphous solids, are optically iso- 

 tropic, and anhedral isometric crystals without cleavage may be 

 confused with amorphous substances of similar appearance. 



Still another difficulty lies in the fact that many amorphous 

 substances are doubly refracting. This is especially true of collo- 

 form crusts, and the double refraction here is due to strains set 

 up in the hardening of the gel. The hyalite variety of opal prac- 

 tically always shows double refraction. The birefringence of an 

 amorphous mineral is usually very weak, but in some cases it reaches 

 an appreciable amount. In a specimen of phosphorite from Lassa 

 Island containing both dahllite and collophane the double refrac- 

 tion of colloform bands of amorphous collophane is greater than that 

 of the corresponding crystalline dahllite. Double refraction due 

 to strain can usually be distinguished from the double refraction of 

 optically anisotropic crystalline substances by its lack of uniformity. 



For opaque minerals etching experiments are perhaps the most 

 satisfactory tests to try in the absence of evident crystalline struc- 

 ture. Tolman 1 has recently described metacolloidal chalcocite. 

 The determination was made by examining polished surfaces 

 etched by nitric acid with the metallographic microscope. 



From the foregoing discussion it is evident that in many cases 

 the scalar properties must be used to distinguish amorphous and 

 crystalline substances, and it should be emphasized that the optical 

 properties are also scalar for isometric crystals. 



1 Bull, no Am. Inst. Min. Eng., 1916, p. 410. 



