MINERALOGICAL CLASSIFICATION OF IGNEOUS ROCKS 93 



and one with 95 per cent quarfeloids to Class 2; Ab 95 An s belongs 

 to Order 2 and Ab s An 9S to Order 3. If the divisions fall on the 50-50 

 line of quartz they are moved upward, or, with the Foids downward, 

 toward the apex; that is, they are placed in Families 1 to 5 or 25 

 to 30. Along the plagioclase line, Ab S0 An S0 is classed with the basic 

 plagioclase, and 50-50 light-dark with the dark. Rocks falling 

 on the line separating the two triangles, namely, on the feldspar 

 line, should be classed on the quartz side, that is, on the normal side. 



EXAMPLES 



Example 1 . — A granodiorite having the composition 



Quartz 18.0 =23. 1 



Orthoclase 18. o = 23 . 1 



Andesine (An 70 An 30 ) 42 . o =53-8 



Total quarfeloids 78.0 



Biotite 12.8 



Hornblende 9.0 



Magnetite 1 



Titanite 1 



Total mantes 22.0 



100. o 



Percentage quarfeloids =78. Rock belongs to Class 2. 



Ab 70 An 30 falls between 95 and 50. The order, therefore, is 2. 



The family may be rapidly determined graphically, Plot 

 23 . 1 Qu, 23 . 1 Or, and 53 . 8 C&Nai by measuring 23 . 1 upward from 

 the base of the triangle toward Qu, and 23 . 1 from the right-hand 

 inclined line toward the lower left corner. The intersection of the 

 two lines will fall in Family 9 and determines the position of the 

 rock. As a check, the point must also lie 53 . 8 from the left sloping 

 line toward the lower right corner. 



To compute the family analytically: From the presence of 23 . 1 



per cent quartz, the family must lie between numbers 6 and 10, since 



there is more than 5 per cent and less than 50 per cent quartz. 



_ . . Or 18 30 , . , ' 



Further, the ratio „ , T f = — = — , and since the orthoclase is 

 Caivaf 42 70 



between 5 and 35 per cent, the family belongs in No. 9. 



The rock number, therefore, is 229, that is, Class 2, Order 2, 



Family 9. 



