Probably many a bright youth, accustomed to wander through the
fields in enjoyment of nature, has been thrilled with pleasurable
anticipations on finding, in some outcrop of crystalline rock, a
mineral substance that glittered as gold. That his anticipations were
premature should not deter the ambitious youth. Men far beyond him
in experience have been deceived by that same "fool's gold." History
records that shiploads of the valueless yellow iron pyrites were sent
to England by explorers of America on the supposition that they were
accumulating gold.
Of the various compounds of iron occurring in nature, but four may
be considered as relatively common--pyrite, magnetite, hematite, and
limonite. Pyrite consists of iron and sulphur; magnetite, hematite, and
limonite are oxides of iron. The first-named mineral differs largely
from the others in external appearance as well as in composition. The
others are, however, readily differentiated. We will discuss each of
the four minerals in the order mentioned above.
The sulphide of iron, pyrite, occurs in many crystalline rocks; but,
owing to the difficulty of separating the iron and sulphur, is not used
as an ore of iron. The mineral much resembles in external appearance
a yellow ore of copper, called chalcopyrite, from which it may be
distinguished in that it will strike fire with steel. A specimen of
pyrites containing large crystals is an interesting subject of study.
These crystals are cubical in shape, but generally massed together so
that no single crystal form may be observed as complete. Peculiar
striations on the cube faces may often be noted. The striations of
no two adjoining faces are continuous; but rather a striation of one
face bears to that of another in direction the relation of the stem of
a printed T to the top, or vice versa. Owing to the affinity of each
component element for oxygen, pyrite often changes to vitriol, or else
forms the oxide of iron, limonite, described below.
The black oxide of iron, magnetite, occurs widely distributed. As its
name indicates, it sometimes displays the properties of a magnet. If
a fragment of unequal dimensions be suspended freely by a string, the
longer dimension will gradually swing into a north and south direction.
The property possessed by magnetite of attracting other bits of iron
appears to have been known to the ancients, and by them the name
lodestone was applied to the mineral. Since the power to attract other
particles of iron is not apparent in all specimens of magnetite we must
consider other more distinguishing characteristics. The ore is very
heavy; particles of it are attracted by an artificial magnet, in which
regard it differs from the other minerals we have mentioned; if a piece
of the ore be scratched across the surface of a harder substance, _e.
g._, smoky quartz, a black "streak" will be left. Pure magnetic iron
ore is intensely black, with no coloring.
In a series of ore beds formerly operated by a mining company of
northern New York, four distinctions of the crude ore were made, two
varieties of blue, one of black, and one of gray. The blue coloring
is apparently due to the presence of impurities; the black ore is
evidently magnetite; and the steel-gray mineral, failing in the
characteristic properties of magnetite, finds its class place under
hematite. Hematite differs from magnetite in representing a higher
degree of oxidation. It is often found, as indicated above, in beds
distributed in close conjunction with those of magnetite. This ore is a
valuable source of iron. Hematite commonly occurs in earthy materials,
as red ochre. Its streak is red. All rocks of a reddish or red color
owe the color to this oxide of iron.
When hematite rusts, the brownish-yellow or yellow iron oxide,
limonite, results. The streak of limonite is yellow, thus
distinguishing it from hematite. Disseminated through beds of clay,
limonite gives them the characteristic yellow color. Such clays turn
red when heated, since the water of the limonite is driven off, leaving
hematite as a residue. This is the explanation of the usual coloring of
bricks. Yellow ochre is impure, or earthy, limonite.