160 A TREATISE ON METAMORPHISM. 



srwiFie GRAVITY. 



Apparently high specific gravity is favorable to stability. This is what 

 one would expect, for high specific gravity involves a comparatively close 

 arrangement of the atoms. Where the atoms are near together the 

 molecular attraction is great, and in order to break up the combination 

 this force must be overcome. This principle is illustrated by dimorphous 

 and trimorphous compounds. Diamond (av. sp. gr. 3.52) is more stable than 

 graphite (av. sp. gr. 2.16), and graphite is more stable than carbon (sp. gr. 

 1.9, charcoal). Pyrite (av. sp. gr. 5.025) is more stable than marcasite (av. 

 sp. gr. 4.870). Cyanite (av. sp. gr. 3.615) is more stable than sillimanite 

 (av. sp. gr. 3.235), and sillimanite is more stable than andalusite (av. sp. gr. 

 3.18). Quartz (av. s.p. gr. 2.6535) is more stable than tridymite (av. sp. gr. 

 2.305). This last instance well illustrates the principle; for the symmetry 

 of quartz and tridymite is the same, and this variable factor included in 

 the previous illustrations is excluded. The same is true of andalusite 

 and sillimanite of the aluminum-silicate series. As pointed out on page 

 112, the more condensed a compound, or, in other words, the higher the 

 specific gravity, the less energy is potentialized. In the change from a 

 lower to a higher specific gravity energy is liberated. In this we have the 

 physical explanation of the greater stability of minerals of high specific 

 gravity. To form minerals of higher specific gravity from those of lower 

 specific gravity releases energy. To reproduce minerals of lower specific 

 gravity from those of higher specific gravity requires the expenditure of 

 energy. An exception to the above rule as to increase of stability with 

 increase of specific gravity is furnished by calcite and aragonite. Calcite 

 (av. sp. gr. 2.7135) is more stable than aragonite (av. sp. gr. 2.94), but in 

 this case the factor of symmetry enters, which is discussed under the next 

 heading. 



SYMMETRY. 



Apparently the greater the symmetry the more stable is the mineral 

 likely to be. This principle is illustrated by substances which are dimor- 

 phous or trimorphous. 



Pyrite (isometric) is more stable than marcasite (orthorhombic). 

 Diamond (isometric) is more stable than graphite (hexagonal), and this 

 is more stable than amorphous carbon. Kelvin suggests" that soft 



a Lord Kelvin, Popular lectures and addresses, Macmillan & Co., London, 1894, vol. 2, p. 428. 



