14 ON PYRITE AND MARCASITE. [bull. 186. 
may assist the oxidation. It is hardly necessary to add that the old 
idea that marcasite lends to produce ferrous sulphate, and pyrite to 
give limonite, is untenable. 
penfield's method. 
Penfield ' has described a method of distinguishing the two miner- 
als, based on treating the finely powdered substance with strong 
nitric acid under identical conditions; pyrite dissolves completely, 
while marcasite leaves a residue of sulphur. While this procedure 
serves well to distinguish the pure minerals, it is obviously incom- 
petent to detect pyrite in the presence of marcasite. 
ACTION OF COPPER SULPHATE ON PYRITE AND MARCASITE. 
A. P. Brown 2 asserts that when marcasite is heated for six hours at 
200° with 10 per cent copper sulphate solution its iron is completely 
dissolved as ferrous sulphate, while under the same conditions pyrite 
yields a mixture of 2 mols. ferric and 1 mol. ferrous sulphate ; and on 
this he bases the conclusion that in marcasite the iron is wholly in 
the ferrous condition, Fe"S 2 , while in pyrite it is four-fifths ferric, 
Fe"S 2 (Fe'"S 2 ) 4 . As Brown's work might seem to afford a means of 
distinguishing the two minerals, and as the different constitution, if 
existing, would have an important bearing on the question of their 
formation in nature, I have repeated these experiments, but with 
totally different results. The details of my experiments are given 
below (p. 42), and I here state merely my conclusion that the only 
recognizable difference is the greater ease with which marcasite is 
attacked by the cupric solution, and that no evidence can be obtained 
in this way as to any difference of chemical constitution. 
II. BEHAVIOR OF PYRITE AND MARCASITE TOWARD FERRIC 
SOLUTIONS. 
In the course of an investigation on the action of ferric salts on 
natural sulphides, differences observed in the behavior of pyrite and 
of marcasite were so marked as to make it seem probable that on these 
differences there might be based a method of distinguishing these 
minerals and of determining them quantitatively in mixtures, thus 
establishing a sure means of ascertaining the composition of concre- 
tions and other doubtful specimens and of testing the validity of the 
conclusion of Julien, quoted below (p. 27), that many supposedly pure 
pyrites and marcasites arc mixtures of the two. While approximate 
results were soon reached, many fruitless experiments had to be made 
before all important sources of error were eliminated and before the 
process could be brought to a reasonably satisfactory quantitative 
1 Brush and Penfield, Determinative Mineralogy, 15th ed., p. 2'y.i. 
2 Proc. Am. Philos. Soc, Vol. XXXIII, 1894, p. 225. Chemical News, Vol. LXXI, 1895, p. 179. 
