362 A TREATISE ON METAMORPH1SM. 



(See pp. 182-186.) According to Slichter, the most compact possible 

 arrangement of spherical molecules is that which gives a rhombohedral unit 

 having face angles equal to 60° and 120°. a One might therefore conclude, 

 other things being equal, that minerals having hexagonal crystallization would 

 be those which have the closest arrangement of molecules and therefore the 

 highest specific gravity; but plainly there are other factors entering into 

 the problem, for, as already pointed out, aragonite (orthorhombic) has a 

 higher specific gravity than calcite (hexagonal), and cyanite (triclinic) 

 has a higher specific gravity than sillimanite and andalusite (orthorhombic). 

 On the other hand, diamond (isometric) has a higher specific gravity than 

 graphite (hexagonal). This is an especially interesting case, since the 

 cubical arrangement of molecules, the one ordinarily appealed to to 

 explain isometric symmetry, is the most open of all possible arrangements. 

 From the foregoing it appears perfectly clear that besides the manner of the 

 arrangement of the molecular particles the distance of the molecules from 

 one another enters as a very important factor. Also the shape of the 

 molecules, the closeness of the arrangement of their atoms, and the com- 

 plexity of the molecules themselves doubtless enter as important factors 

 into the density of minerals. 



HEAT REACTIONS. 



Other things being equal, within the lithosphere reactions take place 

 which give the greatest liberation of heat. This law is best illustrated 

 at or near the surface, where the reactions usually occur in accordance 

 with it. The reactions of oxidation, hydration, and carbonation are there- 

 fore dominant However, the law of reactions with liberation of heat 

 becomes less and less able to control as the pressure becomes con- 

 siderable. Where the pressure is great, as noted under the next heading, 

 it determines the reaction without respect to whether heat is absorbed or 

 liberated, and in many cases the reactions take place with the absorption 

 of heat, so far as the chemical factors are concerned. If all the physical 

 factors also were included, all reactions would take place with the dissipa- 

 tion of energy. (See p. 57.) 



« Slichter, C. S., Theoretical investigation of the motion of ground waters: Nineteenth Ann. Rept. 

 U. S. Geol. Survey, pt. 2, 1899, pp. 306-310. 



