5 i8 AUSTIN F. ROGERS 



characteristics, and if we treat them as Cornu treats chrysocolla 

 they would both be considered as colloidal or "gelforms" of quartz. 

 In the case of silica minerals, for example, ambiguity is avoided if 

 we call opal amorphous silica and chalcedony metacolloidal silica. 1 



CRITERIA FOR THE RECOGNITION OF AMORPHOUS SUBSTANCES 



To the mineralogist is committed the task of describing and 

 defining all of the definite, homogeneous, naturally occurring sub- 

 stances, whether they be crystalline or amorphous. 



How may amorphous substances be distinguished from crystal- 

 line substances ? On the face of it this seems to be easy, but, as a 

 matter of fact, the problem is often very difficult. Crystalline 

 substances may be defined as those having discontinuous vectorial 

 properties 2 and amorphous substances as those not having such 

 properties, but the actual determination of whether a given sub- 

 stance has discontinuous vectorial properties or not may be very 

 difficult. 



From the standpoint of physical chemistry amorphous solids 

 are liquids. Now, the shape of a liquid unaffected by gravity or 

 other external influence is spherical, and so we often find the 

 hydrogel minerals in spherical, botryoidal, reniform, stalactitic, 

 and mammillary forms. ' These forms intergrade, so that one is 

 often at a loss to know which term to use. I therefore propose the 

 term colloform for the rounded, more or less spherical, forms assumed 

 by colloidal and metacolloidal substances in open spaces. Some 

 crystalline, not merely microcrystalline, minerals, such as smithson- 

 ite, also occur in colloform crusts, and it should be emphasized 

 that this term refers only to the shape or form, and not to the con- 

 dition of the material. 



Colloform minerals may be either amorphous or crystalline, 

 while, on the other hand, minerals occurring in euhedral crystals 

 may be amorphous alteration products of original crystals, for 

 example, malacon, which is a pseudomorph after zircon. Yttrotan- 

 talite, thorite, allanite, gadolinite, homilite, and yttrocrasite all 

 occur in euhedral tetragonal, orthorhombic, or monoclinic crystals. 



1 Chalcedony may be either a distinct mineral or a variety of quartz. 



2 Friedel, Leqons de Cristallographie (Paris, 1911), p. 2. 



