Till: OKKIIN OF MINERALS 



certain minerals which ordinarily separate during the early stages 

 of crystallization, or the solution becomes saturated in respect to 

 them long before it is saturated in respect to others. Minerals 

 which are early to crystallize are well formed and exhibit crystal- 

 line outlines in the rock section. They may be contained as 

 inclusions in the crystals of those minerals which form later, as 

 t lie crystals of apatite in magnetite, or magnetite in feldspar. The 

 crystals of minerals among the last to separate are usually irregu- 

 lar and act as a ground mass, filling the spaces between crystals 

 which have separated at an earlier stage. In the common mag- 

 mas it may be stated that separation takes place with a progressive 

 increase of silica. Those minerals high in silica, as quartz, are 

 near the end products of crystallization. 



Among the first to separate are metals, sulphides, oxides, and 

 included here are apatite and zircon ; second, the ferromagnesian 

 silicates, as pyroxene, amphibole, and olivine ; third, feldspars, 

 beginning with the basic plagioclases and ending with orthoclase; 

 and lastly quartz. This order of crystallization must be considered 

 only in a general way, as several may overlap in their periods of 

 separation, and this overlapping may be continued to such an extent 

 that the order of adjacent minerals in the list or order of cyrstalliza- 

 tion is reversed. Magmas homogeneous at high temperatures 

 may upon cooling separate into several portions which are immis- 

 cible at lower temperatures, each of which will have an individual 

 character and composition, even before crystallization has begun. 



In the crystallization of magmas, the viscosity has a great 

 influence upon the crystals formed and upon crystal growth. With 

 the decreasing temperature there is usually an increase of viscosity, 

 which tends to prevent the formation of centers of crystallization, 

 and is therefore a check generally to crystallization ; and the very 

 viscous magma has a predisposition to solidify as a glass. The 

 fluidity of minerals at their fusing point varies greatly, even within 

 isomorphous groups ; as the potash feldspar, orthoclase, is exceed- 

 ingly viscous at its point of fusion, crystals form in a fusion of its 

 components with great difficulty or not at all ; while the calcium feld- 

 spar, anorthite, crystallizes with the greatest ease from the simple 

 fusion. Viscosity decreases with the increase of basicity, as iron, 

 calcium, magnesium, and sodium promote fluidity, potassium and 

 silica promote viscosity, and aluminium has but little influence 

 either way. Those rocks containing pyroxene, hypersthene, ensta- 

 tite, or olivine are more apt to contain well-formed crystals than 



