38 ON PYRITE AND MARCASITE. [bull. 186. 
pyrite. Only the portions free from visible admixture of pyrite were taken for 
examination, and gave p 1 ( .>.5. Admixture with one-ninth pyrite gavep=21.0=35 
per cent pyrite. We have, then— 
Per cent. 
Pyrite directly determined- 2 >. 5 
Pyrite indirectly determined 27. 8 
No. 2;*. A specimen from Crow Branch mine, Wisconsin, consisting of a com- 
pact nucleus of tin-white marcasite covered with crystals which are completely 
inclosed in elongated cubes of pyrite, the line of demarcation being clearly dis 
cernible on the fracture by the difference of color. Selected portions free from 
visible admixture of pyrite were used, giving a specific gravity of 4.891. To deter- 
mine the contents of pyrite the indirect method was used. 
Per cent. 
Pyrite directly determined ....... ... 26. 
Pyrite indirectly determined .... 31.0 
As only the powdered material was available for the indirect determination, the 
difference is not surprising (p. 28). 
No. 24. Material from Sunshine, Colo. Finely fibrous radial concretions in 
quartz, which on the freshly cleaned fracture show a faint banded structure. Sup- 
posed to be marcasite. A little lead and copper and much arsenic and antimony 
are present. As arsenic, and probably antimony, would lower the oxidation 
coefficient, the figure given for pyrite is too low, and the presence of marcasite is 
doubtful. 
No. 25. A large flattened concretion from Folkestone, England, described as 
"marcasite after pyrite."' The fracture is columnar and the surface is covered 
with rounded projections, apparently tetragonal pyramids, possibly of the type 
described by Penfield.' The color on the ireshiy broken surface exactly matches 
pyrite. 1 have received several other supposed marcasite concretions from Folke- 
stone, all of which are clearly pyrite. 
No. 26. Pyrite ore from Rio Tinto, Spain. The fragments are grayish, evolve 
some hydrogen sulphide with acids, and with the bromine hydrogen sulphide test 
(p. 35) show very finely disseminated copper sulphide. 
No. 27. Brilliant untarnished octahedra in calcite. containing a large amount of 
cobalt and a little copper, which explain the abnormal oxidation coefficient. 
CONCLUSIONS. 
Of the above thirteen specimens five have been described as mar- 
casite, though consisting nearly or entirely of pyrite, while three 
other marcasites are shown to contain a large amount of pyrite, which 
could not be detected without the aid of the oxidation method. As 
these samples were selected and examined without preconception as 
to their nature, it seems probable that a very considerable proportion 
of the concretions which exist in collections or are sold as marcasite 
are in reality pyrite, while not a few others, even when fairly well 
characterized by rhombic crystallization, may contain inclosures of 
pyrite. 
'Am Jour. Sci., 3d series, Vol. XXXVIi, 1889, p. 309. 
