ON THE ACCURACY OF THE ROSIWAL METHOD FOR 



THE DETERMINATION OF THE MINERALS 



IN A ROCK 



ALBERT JOHANNSEN AND E. A. STEPHENSON 



University of Chicago 



Some years ago Julien^ expressed a doubt as to the accuracy 

 of the Rosiwal method in the determination of the percentage 

 composition of rocks in general. He thought, if the fragments 

 were equicubical, that d, the measured average diameter of the 

 mineral plates, might give correct results, but for unequicubical 

 grains the value d^ should be use d, an d for rounded grains,^ thick 

 lenses, or short spindles, .7854l/a or d^. For schistose rocks with 

 flakes and interlaminae parallel and of equal thickness, he suggested 

 d^, for prisms or blades with parallel interlaminae, d' = y IXbXt, 

 where /, b, and t are the length, breadth, and thickness of plates. 



Williams,^ several years later, studied the comparative accuracy 

 of various methods, using a pink granite from Westerly, Rhode 

 Island, with grains ranging from i . 5 to 5 mm. in diameter. Several 

 determinations were made in each experimental method and the 

 results were averaged and tabulated with the calculated results as 

 shown in Table I. 



The first column is calculated from the chemical analysis and 

 is practically the norm, except for the small amount of biotite. 

 The second column represents the percentages of the different 

 minerals obtained by separation according to their specific gravi- 

 ties in Sonstadt's solution. The agreement of this determination 

 with the preceding is fairly close. The third column is based upon 



' A. A. Julien, "Genesis of the Amphibole Schists and Serpentines of Manhattan 

 Island, New York," Bull. Geol. Soc. Amer., XIV (1902), 460-68. 



^Julien gives .78541/0 or d' for this value. Williams gives .5236(^3, the volume 

 of a sphere, as Julien's (possibly later) figure. 



3 Ira A. Williams, "The Comparative Accuracy of the Methods for Determining 

 the Percentages of the Several Components of an Igneous Rock," Amer. Geol., XXV 

 (1905), 34-46. 



