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ON THE NATURAL HISTORY
It frequently happens that Pyrite and Marcasite, though not
isomorphous, occur in intimate association, and it is certainly
convenient in such cases to refer to the mixture as iron-pyrites.
"In clay-beds," says Julien, "the pyritous nodules generally
consist of successive crusts or transition mixtures of pyrite and
marcasite." This remark is not without interest to us, inasmuch
as the iron-pyrites of the London clay of Essex may often be of
such a mixed character. Julien described the pyritic mineral of
the well-known Nipadites, or fossil palm-nuts from the London
clay of Sheppey, as "marcasitic pyrite," and says that some of
the Sheppey fruits shew octahedra of true pyrite.
When Marcasite is well-crystallized, its forms are so distinctive,
so very different from those of pyrite, as to be immediately
recognised. Very commonly the crystals of marcasite are
aggregations of what are called twin-crystals. Some of these
groups present a shape which has led to the trivial name of
Cock's-comb pyrites, whilst others have a suggestive resemblance
to brass spear-heads, whence the term spear pyrites. The latter
is the common type in the Chalk-marl between Dover and
Folkestone, where according to Prof. Miers, they are locally
supposed to be the heads of Roman weapons.10
Whilst the two minerals marcasite and pyrite are thus
readily enough distinguished when well crystallized, it is not
always easy, in the absence of crystalline characters, to say off-
hand whether a given specimen should be referred to pyrite,
or to marcasite. It is true that marcasite is paler in colour, so
that it is sometimes called white pyrites, but then by tarnish it is
apt to become yellow, and so tends to resemble true pyrite.
Strictly speaking it may be said that the three forms of iron-
pyrites have characteristic colours ; marcasite, or rhombic
pyrites, being tin-white ; pyrite, or cubic pyrites, being brass-
yellow ; and pyrrhotite, or rhombohedral pyrites, being bronze
coloured. But these colours refer to pure and fresh specimens,
and much of the marcasite seems to be impure ; that is to say,
it appears to be in intimate mechanical association with pyrite, and
thus acquires a yellowish tint.
It has been said that the surest means of diagnosis is pro-
bably to be found in the specific gravity of the several minerals.
According to Julien the density of normal pyrite is 5.01 ; of
10 Mineralogy: an Introduction to the Scientific Study of Minerals, 1002, p. 330.