conditions.

GEOLOGIC FEATURES

Iron rarely exists in nature as a separate element. It occurs mainly in
minerals which represent combinations of iron, oxygen, and water, the
substances which make up iron rust. Very broadly, most of the iron ores
might be crudely classified as iron rust. In detail this group is
represented by several mineral varieties, principal among which are
hematite (Fe_{2}O_{3}), magnetite (Fe_{3}O_{4}), and limonite (hydrated
ferric oxide). Iron likewise combines with a considerable variety of
substances other than oxygen; and some of these compounds, as for
instance iron carbonate (siderite), iron silicate (chamosite,
glauconite, etc.), and iron sulphide (pyrite), are locally mined as iron
ores. While an ore of iron may consist dominantly of some one of the
iron minerals, in few cases does it consist exclusively of one mineral.
Most ores are mixtures of iron minerals.

Fully nine-tenths of the iron production of the world comes from the
so-called hematite ores, meaning ores in which hematite is the dominant
mineral, though most of them contain other iron minerals in smaller
quantities. About 5 per cent of the world's iron ores are magnetites,
and the remainder are limonites and iron carbonates.

Iron ores are represented in nearly all phases of the metamorphic cycle,
but the principal commercial values have been produced by processes of
weathering and sedimentation at and near the surface.

=Sedimentary iron ores.= Over 90 per cent of the world's production of
iron ore is from sedimentary rocks. The deposits consist in the main
either of beds of iron ore which were originally deposited as such and
have undergone little subsequent alteration, or of those altered
portions of lean ferruginous beds which since their deposition have been
enriched or concentrated sufficiently to form ores. A minor class of
iron ores in sediments consists of deposits formed by secondary
replacement of limestones by surface waters carrying iron in solution.

1. Deposits of the first class,--originally laid down in much their
present form,--are usually either oolitic, _i. e._, containing great
numbers of flat rounded grains of iron minerals like flaxseeds, or
consist in large part of fossil fragments of sea shells, replaced by
iron minerals. The Clinton ores of the Birmingham district, the Wabana
ores of Newfoundland, the minette ores of the Lorraine district in
central Europe, and the oolitic ores of northern Eng