486 



PETROLOGICAL ABSTRACTS AND REVIEWS 



plotted on the extension of the hne for CaO, and this is also marked by an x. 

 This is the 2 point. The horizontal distance of the S point from the F axis 

 (in units as measured on S) gives the amount of the ferromagnesian constituents. 

 The amount of free (theoretical) quartz is given by the horizontal distance of 

 the Locus of the rock from the " Quartz normal." (As measured on the 5 scale, 

 the reading is to be multiplied by 2.) In the case of the Brocken granite this 

 distance is 15.4, therefore the theoretical amount of free quartz is 30.8. 



Since the quartz normal and the F axis are both at 45°, the readings may be 

 made along the downward extension of the line passing through the locus of the 

 rock, instead of along the horizontal. 



S Axis 



10 



^ 15 



H 



< 



W 20 

 25 



Qu 



Ne 



The second part of the formula, the "Konstitutionsformel," represents 

 theoretical minerals. The method for computing the free quartz follows from 

 the diagram. Twice the sum of the alkalies is subtracted from the silica value 

 less fifty (the diagonal line starting at the 50 point), and the result is multiplied 

 by two. 



Free quartz = 2[(Si02— 50) — 2(K20+Na20)], where SiO^, K2O, etc., are 

 molecular values. The Brocken granite therefore gives 2[(8o.2 — 5o) — 

 2(4.I+3.3)]=+30.8 or approximately +31. The plus sign gives the position 

 of the rock above the quartz normal in the diagram, and shows free quartz. 

 Below the line is shown by a minus sign. This indicates nephelite, olivine, 

 etc. The letter F (plutonic) indicates the texture. stands for orthoclase. 

 Its value is obtained by multiplying the K2O mol. by 8, since the formula is 

 K^O.AlA-SSiO^. (K20 = 4-i, = 8X4-1 = 32-8 or ca.33). The plagioclase 

 indicated by the molecular formula is 3.3 NazO, 1.2 CaO, or basic oligoclase, 

 AbeAui. The molecular value of the albite is eight times the NasO, but the 

 anorthite, with the formula CaO.Al203.2Si02 is only four times the CaO. 

 (3.3X8+1.2X4 = 31.2). The symbol for basic oligoclase is 01°. Beta indi- 

 cates biotite, and 5 per cent represents the remainder left for this mineral. 

 It amounts, of course, from the formula R0.Si02 to twice the F value. 



With insufficient AI3O3, the symbol A is used in the formula instead of S, 

 and the aegirite molecule takes the place of the anorthite. Thus A 4 means 



