ON THE REPRESENTATION OF IGNEOUS ROCKS IN 

 TRIANGULAR DIAGRAMS 



ALBERT JOHANNSEN 

 University of Chicago 



It has been customary to represent three components in a 

 triangular diagram by a dot, or four components by a triangle 

 within the triangle. The writer, by means of only a dot and a 

 line, plots without the use of a slide rule or any calculation what- 

 soever, the actual percentages of quartz, orthoclase, plagioclase, 

 and dark minerals as well as the relative percentages of quartz, 

 orthoclase, and plagioclase among the Hght constituents. 



Let it be required to plot a rock with the following mode :^ 



Quartz 14 = 18.7 



Orthoclase ' 15= 20.0 



Plagioclase 46 = 61.3 



75 100. o 

 Dark minerals 25 



100 



The usual method is to reduce to 100, by means of a slide rule, 

 the sum of the percentages of the minerals represented in the three 

 corners of the diagram, and plot the point so obtained. The 

 writer's method is shown in the figure. Draw three lines, parallel 

 to the sides of the triangle, through the points representing the 

 amounts of the corner minerals in the rock, in this case a horizontal 

 Hne through 14 for quartz, a line sloping northeast and southwest 

 through 46 for plagioclase, and a line sloping northwest southeast 

 through 15 for orthoclase. Lay the side of a straight-edge on the 

 apices of the two triangles and draw a short line {hd) through 

 the small triangle from its apex to its base. Connect either of the 



^ Th -- jf Tie method may be used in other ways to show the relative percentages 

 of thr( components whose sum is not a hundred. 



167 



