OF PYRITES AND GYPSUM.
311
well illustrated by Dr. Plot, the historian of Oxfordshire, who in
reference to the term remarks that under this "Genus may be
reckon'd not only Pyrites strictly taken, but Flints, Pebbles, Sand,
and whatever else by any quick and sudden attrition may have its
parts kindled into sparks."5
In consequence of readily giving forth sparks by concussion,
pyrites was known to old writers by such names as lapis igniarius
and lapis luminis. In Cornwall, where pyrites is common in the
veinstones of the copper and tin-mines,, it is generally called
mundic, a word which Borlase, the historian of Cornwall, derives,
though on what authority I know not," from the cleanly shining
appearance, both of its surface and structure."6 The "shining
appearance," or metal-like brilliancy, is a character common to
all the minerals which at present pass under the name of
Pyrites.
In modern times the term Pyrites, has come to be used by
mineralogists as a family-name, embracing a group of native
sulphides, all possessing a metallic lustre and a considerable
degree of hardness, but not being strictly isomorphous—that is
to say, they do not always crystallize in forms which are
geometrically compatible with each other. The different kinds
of pyrites are distinguished by prefixes, so that we speak of
iron-pyrites, copper-pyrites, nickel-pyrites, cobalt-pyrites, and so
forth. But since iron-pyrites is far and away the most common
member of the family, it is this mineral which is always meant
when the word is used without qualification. The name
sideropyrites has sometimes been applied to iron-pyrites in order
to distinguish it from chakopyrites, or copper-pyrites.
Iron-pyrites is chemically a compound of iron and sulphur,
termed iron disulphide, and we may regard each molecule as
containing one atom of iron combined with two atoms of sulphur.
Its formula is consequently FeS2 and its percentage composition
46.6 parts by weight of iron to 53.4 parts of sulphur. It is the
sulphur rather than the iron that in most cases gives economic
value to the mineral. Immense quantities are used for the
manufacture of sulphuric acid—an application of the mineral
which resulted from the action of the King of Naples in granting
a monopoly of Sicilian sulphur to a French company, in the
5 The Natural History of Oxfordshire., By R. P., LL.D. Oxford, 1677, p. 71.
6 The Natural History of Cornwall. By William Borlase. Oxford, 1758. P. 131.