4() ON PYRITE AN!) MARCASITE. [bull. 186. 
to whether chalcopyrite lias a constitution represented by (Cn"Fe")S 2 
or (Cu'Fe'")S 2 until the condition of the system CuCl+FeCl 3 £CuCl 2 + 
FeCl 2 in equilibrium is known, in the absence of water and presence 
of hydrochloric acid gas. That nearly all the iron was obtained in 
the ferrous slate in some of the experiments with copper sulphate 
made by Brown and others I can explain only on the assumption that 
enough acid was originally present or eventually formed to enable 
any basic ferric salt to act on and be reduced by the residual sul- 
phides and cuprous oxide. In fact, hot dilute sulphuric acid slowly 
dissolves the mixture of cuprous oxide and basic ferric salt obtained 
in my experiments, giving a strongly reducing solution. 
XIII. OXIDATION OF PYRITE AND MARCASITE BY POTASSIUM 
PERMANGANATE. 1 
A few experiments were made to determine the percentage of sul- 
phur oxidized in pyrite and marcasite at ordinary temperature by 
acidified permanganate, the mineral being always in excess. 
An excess of the powdered mineral, freed from oxidation products, 
was shaken in a stoppered cylinder with a constant amount of dilute 
sulphuric acid and 100 cm. 3 KMn0 4 , added in quantities of 5 cm. 3 at 
a time, the shaking being continued after each addition until the 
color had disappeared, which required but a few minutes. The fil- 
tered solution was then reduced and the iron titrated by the same 
permanganate solution. 
The proportions required are — 
For complete oxidation to Fe 2 (S0 4 ) 3 and sulphuric acid 5 FeS 2 + 15 KMn0 4 
For oxidation of the iron alone to Fe 2 (S0 4 ) 3 _- ..- 5 FeS 2 + 3 KMn0 4 
For oxidation of the sulphur . .. . . 12 KMn0 4 
Let 
a=volume of permanganate used in oxidation of FeS 2 , 
and 
fr— volume of permanganate required to reoxidize FeO to Fe..O.; 
then 
36= volume of permanganate used in oxidizing Fe in FeS 2 to Fe.0 3 , 
ami 
a-3b= volume of permanganate used in oxidizing S in FeS„ to SO :i : 
also 
12£>=volume of permanganate required to oxidize all the sulphur in the 
mineral decomposed; 
whence 
t> , f i , -v -. 100(a-3b) 8.333a oR 
Percentage of sulphur oxidized =p= jog — = — , — 2o. 
Where 190 cm. 3 permanganate are used, 
833.3 ^ 
P= -T-- ™- 
1 For previous experiments on the action of permanganate on pyrite and marcasite, see A. P. 
Brown, Proe. Am. Philos. Soc, Vol. XXXIII, 1894, and Chemical News, Vol. LXXI, 1895, pp. L31, 155, 
171. The experiments of Brown show that with an excess of permanganate the total sulphur 
oxidized in a given time is greater in the case of marcasite than of pyrite, but they do not estab- 
lish any relation between the amount of sulphur oxidized and the quantity of mineral decom- 
posed. 
