OF PYRITES AND GYPSUM.
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normal marcasite about 4.8 ; and of normal pyrrhotite 4.6. But
abnormal forms are not infrequent, and in these impure varieties
the specific gravity may suffer disturbance of a more or less
erious character, according to the proportion of impurity present.
Prof. Julien's conclusions have been criticised by Mr. H. N.
Stokes in his valuable paper published as a "Bulletin" of the
Geological Survey of the United States, No. 186.
Of the native "sulphides" iron-pyrites, speaking broadly, is
one of the most unstable, but the different kinds of pyrites vary
greatly in their stability. Some of the pyrites from mineral
veins and from crystalline rocks is indeed durable enough, and
specimens of mundic, after exposure for years on the burrows or
waste-heaps of a mine, may still preserve a bright and brassy
appearance. Sharp cubic crystals in clay-slate also are often
resistant. But such cases are not very common. As a rule
marcasite seems much more prone to alteration than pyrite.
Thus the marcasite found in coal, lignite, clay-shale and chalk
readily suffers decomposition. The instability has been referred
by some authorities to the presence of foreign impurity, such as
arsenic or to the unstable proto-sulphide called troilite, whilst
others have connected it with the state of aggregation of the
mineral. Prof, Julien's study of the subject led him to the con-
clusion that "difference of chemical composition has nothing to
do with the tendency to decomposition." It is more probably
connected with the molecular constitution of the mineral.
Two distinct types of alteration may be recognised. In the
more familiar mode of alteration the pyrites suffers conversion
into brown oxide of iron. Many nodules and some crystals, on
exposure to moist air, become coated with this brown substance,
which is a ferric hydrate, or iron-hydroxide, known to
mineralogists as Limonite. The change proceeds slowly from
without' inwards, and if continued long enough, the entire mass
of the mineral may become transformed. As the limonite has
practically the same volume as the pyrites from which it
originated, there is usually little or no change of form, so that a
perfect pseudomorph is obtained. Most mineral collections
contain beautiful pseudomorphs, which, though consisting more
or less completely of limonite, retain with perfect fidelity the
smooth faces and sharp edges, and even the characteristic striae
on the crystal-faces of the original pyrite. Such altered crystals