17 



The conditions which determine the development of inclusions during- the 

 growth of crystals have been investigated by Dr. SORBY and the results 

 published in his classic paper on " The microscopical structure of crystals 

 indicating the origin of minerals and rocks " (1) 



In his experimental work Dr. SORBY employed aqueous solutions of such 

 substances as the chlorides of sodium and potassium, bichromate of potash, and 

 sulphate of zinc. He found that when crystals are deposited from solution at 

 ordinary temperatures they usually contain a number of cavities full of the 

 mother-liquor. When thay are deposited at high temperatures, say at the 

 temperature of boiling water, and are allowed to cool, the cavities are no 

 longer full of liquid but contain a small bubble due to the contraction of the 

 liquid subsequent to its enclosure. When the crystals are formed at the 

 surface of the solution, so as to be in contact with both air and water, then 

 they contain gas- as well as liquid-inclusions. 



When crystals of one salt are formed from a hot solution containing a 

 second salt, inclusions may be produced which, at ordinary temperatures, 

 contain not only a portion of the mother liquid and a bubble, but also crystals 

 of the second salt. Thus, from a solution containing chloride of sodium and 

 bichromate of potash, crystals of common salt with inclusions full of the 

 mother-liquor may be formed at 100 C. ; but at ordinary temperatures these 

 inclusions will contain, if suitable proportions of the two salts have been 

 employed, a liquid, a bubble and a crystal or crystals of bichromate of potash. 



The forms and sizes of inclusions vary very considerably in different 

 cases. The forms may be more or less spherical, elliptical, irregular, or bounded 

 by planes corresponding to the external faces of the containing crystal. In 

 the last-mentioned cases they are termed negative crystals. 



The number of inclusions in a crystal appears to depend on the rate of 

 growth. Other things being equal, the more rapid the growth the more 

 abundant are the inclusions. The size, on the other hand, varies in an inverse 

 manner. The more rapid the growth the smaller are the inclusions. 



Inclusions are distributed throughout the crystal in which they occur in 

 various ways. It rarely happens that the distribution is perfectly uniform. 

 Very often the inclusions are crowded together in certain zones which run 

 parallel to the external crystalline faces, and thus serve to indicate, in all 

 probability, variations in the rate of crystalline growth. Sometimes they are 

 arranged in planes which do not bear any definite relation to the form of the 

 crystal, and sometimes they are scattered throughout the crystal without any 

 apparent regularity. 



At first sight it might be thought that there is no connection between 

 the phenomena of crystal-building in an aqueous solution and the formation 

 of igneous rocks. This, however, would be a mistake. The growth of a 

 crystal during the process of cooling in an igneous magma is strictly com- 

 parable with the growth of a crystal in a supersaturated saline solution. 

 Inclusions are formed in precisely the same way and in accordance with 

 precisely the same laws. When first formed they are liquid inclusions, but 

 as the mass cools down they become solid. In this way the glass- and stone- 



(1) Q.J.G.S., 1858, p. 453. 



