Plagioclase Feldspars. 597 



Geological Significance. 



A study of fig. 1 brings out certain facts concerning the 

 plagioclases which seem worthy of some attention. If we 

 start with a mixture of composition AfyAn, at 1500° and cool 

 quickly, crystallization may not begin until some such tem- 

 perature as 1250° is reached, when the whole will solidify as 

 homogeneous crystals of AbjAn^ 



If we cool gradually, on the other hand, crystals of composi- 

 tion Ab 19 An gl begin to separate at 1450° and, theoretically, the 

 composition of the crystals will change continuously toward 

 A^Anj and their amount will increase down to 1287°, when 

 the whole mass will consist of crystals of composition A^An^ 

 This continuous complete change in the composition of the 

 solid implies, however, free diffusion in the solid phase, whereas 

 diffusion is in general quite slow in solids and in the feldspars 

 is certainly extremely slow. Only excessively slow cooling 

 could therefore be expected to give the result outlined. With 

 moderately slow cooling the process consists in the formation 

 of successive layers of crystalline material of gradually chang- 

 ing composition, the early layers being effectively separated 

 from the liquid by the later. When the temperature 1287° is 

 reached, there will still be some liquid left. Crystallization 

 will therefore continue below this temperature, the process of 

 gradual change of composition in' the solid deposited and the 

 liquid residue continuing as before. If the cooling is at the 

 proper rate, there appears to he no limit to this process ; the 

 final crystals may he nearly pare albite. 



This may be. illustrated by a precise numerical example. 

 Start with 100 grams A^Anj at 1500° and cool it instantane- 

 ously to 13S6°. At this temperature 50 g. of crystals, Ab 3l An 69 , 

 will separate and 50 g. of liquid, Ab 69 An 31 , will remain. (See 

 fig. 1.) Cool again instantaneously to 12S2°, 25 g. of crys- 

 tals, Ab 51 An 49 , separate and 25 g. of liquid, Ab, 7 An 13 , remain. 

 Again, repeat the cooling and at 1164° 12*5 g. of crystals, 

 Ab 76 An 24 , form and 12*5 g. of liquid, Ab 9g5 An 15 , remain. It is 

 easy to see how by continuous slow cooling, instead of cooling 

 in steps, the same result might be obtained and that the final 

 crystals might approach pure albite. Absolutely pure albite, 

 however, could not be obtained in finite amount. Moreover, 

 the temperature may fall to 1100° before crystallization is 

 complete. 



It is believed that this very large range of temperature 

 through which the crystallization of plagioclase may take 

 place, with moderately slow cooling, is of great importance in 

 igneous geology. Such a behavior may sometimes be favored 

 in the natural rocks, even when cooled exceedingly slowly, on 



