stokes.] MIXTURES OF PYRTTE AND MARCASITE. 27 
No. 12. Weardale, England. Flat, deeply striated crystals, with slight bluish 
tarnish. Color of clean surface, tin white. No visible impurity was observed, 
and analysis showed the presence of a faint trace of copper and a little silica and 
the absence of arsenic. 
No. 13. Cornwall, England. Flat crystals, with slight bluish tarnish. Color 
of cleaned surface, tin-white. A few of the crystals carry minute cubes of pyrite, 
and such were rejected. Analysis showed no arsenic and a trace of copper and 
antimony. The high oxidation coefficient makes it possible that it incloses consid- 
erably pyrite, but material was wanting for a special determination of this. 
No. 14. Webb City. Mo. Curved wedged-shaped crystals of the Joplin type, on 
sphalerite and calcite and with a little inclosed chalcopyrite. Analysis of a sam- 
ple used for the determination showed some lead and copper, which probably 
account for the high figure obtained. 
OXIDATION COEFFICIENT OF MARCASITE. 
From the above figures it appears that specimens of apparently 
pure marcasite differ somewhat more with regard to the oxidation 
coefficient than does pyrite. It will appear below that an admixture 
of small quantities of pyrite lowers the value of p, notwithstanding 
the fact that pyrite has a higher oxidation coefficient. It is probable 
that other sulphides in minute amount exert a similar effect to even 
a greater extent, possibly in part because of the electrochemical 
action which they produce. The marcasite with the lowesl oxidation 
number is therefore not the purest. The specimen No. 12 from 
Weardale was the purest examined, and I incline therefore to adopt 
the figure 18 as being nearest to the oxidation coefficient of pure 
marcasite. Figures higher than this, as well as lower than 16.5, must 
be regarded as indicating contamination, either with pyrite or with 
another oxidizable mineral. It also appears from the examination 
of certain samples (Nos. 22 and -2'.)) that marcasite may carry con- 
siderable amounts of pyrite so intimately intergrown as not to be 
visible upon examination of small fragments under a lens after 
cleaning with acid. 
VI. MIXTURES OF PYRITE AND MARCASITE. 
It is desirable to construct a curve which shall give the oxidation 
coefficients of mixtures of pyrite and marcasite. Such a curve would 
enable us to ascertain, in the absence of other indications, whether a 
given specimen contains one or the other or both of these minerals, 
and in the latter case to determine their relative amounts. It would 
enable us to test the hypothesis of Julien, 1 whose statement I quote: 
The forms of iron pyrites occurring in nature are intimate mixtures of these 
three minerals; rarely of pyrrhotite. however, on account of its ready metasomatic 
alteration into one or the other of the triad. These common mixtures of marcasite 
and pyrite may originate by inclosure during crystallization, by alteration, and 
by displacement, and pass progressively into complete paramorphs, well crystal- 
lised after the form of one or the other mineral. 
The latent constitution of these composite minerals is indicated by variation in 
1 Annals New York Acad. Sci., Vol. IV, 1887, p. 213. 
