A REVIEW OF THE AMORPHOUS MINERALS 517 



Wolfgang Ostwald says: "The colloid solutions .... should 

 always be meant when colloids in general are under discussion," 1 

 and also "when we speak of a ' colloid ' we nearly always mean one in 

 this condition, in other words, one in the sol condition." 2 



Biitschli described the gels as possessing a honeycomb struc- 

 ture, but according to Bachmann 3 , this is simply an optical effect. 

 By means of the ultramicroscope the latter proved that the struc- 

 ture of the solid gel of silica is extraordinarily fine and for the most 

 part amicroscopic (i.e., made up of amicrons). 



Whether the hardened gels are colloidal or not, we are safe 

 in calling them amorphous. 



There is a tendency on the part of some modern mineralogists 

 to use the term colloid, not only for the hydrogel minerals, but 

 also for microcrystalline substances of colloidal origin. The recog- 

 nition of colloidal structures in the study of minerals, rocks, and 

 ores 4 is important, but the fundamental differences between 

 amorphous and crystalline minerals should not be lost sight of. In 

 the identification of minerals the mineralogist is concerned with 

 amorphous and crystalline substances, and not primarily with 

 colloidal structures. The confusion of the terms colloidal and 

 amorphous is apparent in a number of recent mineralogical papers. 

 For example, Cornu 5 classifies chrysocolla as the "gelform" of 

 dioptase, when, as a matter of fact, chrysocolla is crystalline. Its 

 amorphous equivalent is another mineral recognized in this paper 

 for the first time. The use of the term metacolloid proposed by 

 Wherry 6 for microcrystalline substances of colloidal origin will do 

 much to clear up the difficulty. Cornu's term " gelform " is ambigu- 

 ous, and as an illustration let us take the silica minerals. Opal is 

 listed as the "gelform" of chalcedony by Cornu and chalcedony as 

 a "krystalloidform." But both of the minerals have colloidal 



1 Handbook of Colloid Chemistry (Eng. trans, of 3d edition by Fischer, 1915), p .24. 



2 Ibid., p. 40. 



3 Zeit. f. anorg. Chemie, LXXIII (1911), 125. 



4 For a discussion of colloidal structures in ores see Krusch, Zeit. f. prakt. Geol., 

 21. Jahrgang (1913), 506-13. 



5 Zeit. f. Chem. u. Ind. d. Kolloide, IV (1909), 17. 

 6 Jour. Wash. Acad. Sci., IV (1914), 112. 



