iST.2 PROPERTIES OF GLAUCONITE — ROSS 13 



regard glauconite as a crystalline mineral ; indeed, Collet and Lee definitely 

 relegate it to the monoclinic system. Against this we have to set the fact that 

 in the submarine mineral, whether granular or pulverulent, nothing in the least 

 like a crystal-contour has ever been noted. It is true that fossil " glauconites," 

 embedded in continental formations, have been from time to time described, 

 which are morphologically as well as optically crystalline. These, however, 

 cannot be accepted as identical with recent submarine glauconite, though they 

 may perhaps be metamorphic derivatives of it ; in the absence of analyses it 

 is not unlawful to suspect that some of them may be a quite distinct mineral, 

 possibly chlorite. 



On the other hand, certain properties of glauconite indicating a state of 

 aggregation differing from that of ordinary crystalline minerals have been 

 referred to above. Some additional light is shed on this point by the behavior 

 of glauconite as regards hydration. It is a peculiarity of colloid minerals (e. g. 

 clays) and of zeolites that they absorb somewhat large proportions of water, 

 according to the moistness of the air with which they are in contact, without 

 forming definite hydrates. In order to ascertain whether glauconite falls into 

 this class, a series of experiments was made. 



After describing a series of experiments the writer (Caspari) 

 comes to the following conclusions: 



Firstly, glauconite becomes a highly hydrated mineral in presence of moist 

 air (whilst still I'emaining an apparently dry powder). No doubt this is the 

 condition in which it exists in its native element, a third or more of its weight 

 consisting of water. Secondly, it is evident, without drawing up a curve, that 

 the hydration decreases continuously with the tension of aqueous vapour in 

 equilibrium, whence it follows that there are no definite hydrates representing 

 a series of distinct molecular species. Thirdly, there is a marked parallelism 

 as regards hydration between glauconite and red clay. 



It is well known that this kind of water absorption, which is esi)ecially char- 

 acteristic of colloids, is also shared by the unquestionably crystalline zeolites. 

 The inference, then, which we may now draw as to the nature of glauconite 

 is that it is certainly not an ordinary crystalline silicate like feldspar or mica, 

 but that it must be either a zeolite or a colloid. As between these two alter- 

 natives, it is less easy to decide. The property possessed by glauconite of 

 absorbing dyes is again common to both colloids and zeolites. In favor of the 

 view that glauconite is a colloid, we have the absence of crystal-contours and 

 the ease with which it forms colloidal suspensions and solutions. Evidence 

 of crystalline habit, on the other hand, is afforded by the optical anisotropy 

 of glauconite. To this, however, it may be rejoined that isotropic matter in 

 a state of strain is equally capable of showing birefringence. That glauconite 

 may exist under some such strain seems not unlikely when we consider the 

 structure of glauconite grains, in which the glauconite proper would appear 

 to be inclosed by a network of foreign substance ; and the vehemence with 

 which the grains fly into powder under the action of acid and alkaline solu- 

 tions points in the same direction. On the whole, then, though it would be 

 premature to regard the matter as settled, the probability is that glauconite 

 is essentially a colloidal silicate. 



In describing the material he has concentrated Caspari says :^^ 



A small proportion of the particles, especially the larger ones, retain the 

 characteristic birefringence of granular glauconite. 



"Proc. Roy. Soc. Edinburgh, vol. 30, 1910, p. 367. 



