Ore Forming Minerals:
kinds of mineral can be treated for metal extraction more easily than others;
these are commonly referred to as ore minerals.
Quite often, different minerals containing a particular metal occur together in
a deposit, and are referred to as ore forming minerals.
Ore minerals form as a result of special geologic processes and often
occur in isolated, small, localized rock masses.
Such local concentrations are called mineral deposits. Mineral deposits are what prospectors
seek. The terms ore mineral and mineral deposit were originally applied only to
minerals and deposits from which metals are recovered, but present usage
includes a few nonmetallic minerals, such as barite and fluorite, that are found
in the same kinds of deposit as metallic minerals.
deposit consists entirely of a single ore mineral. There is always an abundance
of worthless minerals, collectively called gangue.
The more abundant an ore mineral, the more valuable the mineral deposit. For
every mineral deposit there is a set of conditions, such as the level of
concentration and the size of the deposit that must be reached if the deposit is
to be worked at a profit. A mineral deposit that is sufficiently rich to be
worked at a profit is called an ore deposit.
The assemblage of ore minerals and
gangue in a deposit is called the ore.
used in industrial and technological applications can be divided into two
classes on the basis of their abundance in the Earth's crust. The geochemically
abundant metals, of which there are five (aluminum, iron, magnesium, manganese,
and titanium), constitute more than 0.1 percent by weight of the Earth's crust,
while the geochemically scarce metals, which embrace all other metals (including
such familiar ones as copper, lead, zinc, gold, and silver), constitute less
than 0.1 percent. In almost every rock, at least tiny amounts of all metals can
be detected by sensitive chemical analysis. However, there are important
differences in the way the abundant and scarce metals occur in common rocks.
Geochemically abundant metals tend to be present as essential constituents in
minerals. For example, basalt, a common igneous rock, consists largely of the
minerals olivine and pyroxene (both magnesium-iron silicates), feldspar
(calcium-aluminum silicate), and ilmenite (iron-titanium oxide). Careful
chemical analysis of a basalt will reveal the presence of most of the
geochemically scarce metals too, but no amount of searching will reveal minerals
in which one or more of the scarce metals is an essential constituent.
scarce metals rarely form minerals in common rocks. Instead, they are carried in
the structures of common rock-forming minerals (most of them silicates) through
the process of atomic substitution. This process involves the random replacement
of an atom in a mineral by a foreign atom of similar ionic radius and valence,
without changing the atomic packing of the host mineral. Atoms of copper, zinc,
and nickel, for example, can substitute for iron and magnesium atoms in olivine
and pyroxene. However, since substitution of foreign atoms produces strains in
an atomic packing, there are limits to this process, as determined by
temperature, pressure, and various chemical parameters. Indeed, the substitution
limits for most scarce metals in common silicate minerals are low--in many cases
only a few hundred substituting atoms for every million host atoms--but even
these limits are rarely exceeded in common rocks.
important consequence that derives from the way abundant and scarce metals occur
in common rocks is that ore minerals of abundant metals can be found in many
common rocks, while ore minerals of scarce metals can be found only where some
special, restricted geologic process has formed localized enrichments that
exceed the limits of atomic substitution.
factors determine whether a given mineral is suitable to be an ore mineral. The
first is the ease with which a mineral can be separated from the gangue and
concentrated for smelting. Concentrating
processes, which are based on the physical properties of the mineral, include
magnetic separation, gravity separation, and flotation. The second factor is
smelting--that is, releasing the metal from the other elements to which it is
chemically bonded in the mineral. Smelting processes are discussed below, but of
primary importance in this consideration of the suitability of an ore mineral is
the amount of energy needed to break the chemical bonds and release the metal.
In general, less energy is needed to smelt sulfide, oxide, or hydroxide minerals
than is required to smelt a silicate mineral. For this reason, few silicate
minerals are ore minerals. Because the great bulk of the Earth's crust (about 95
percent) is composed of silicate minerals, sulfide, oxide, and hydroxide ore
minerals are at best only minor constituents of the Earth's crust--and in many
cases they are very rare constituents.
preferred ore minerals of both geochemically abundant and geochemically scarce
metals are native metals, sulfides, oxides, hydroxides, or carbonates. In a few
cases, silicate minerals have to be used as ore minerals because the metals
either do not form more desirable minerals or form desirable minerals that
rarely occur in large deposits.
Minerals: are the source of
metals including iron (steel), aluminum, copper, zinc, manganese, lead, chrome,
platinum and others. Demand for new
metals derived from ore deposits is decreasing, largely due to recycling.
include stone, sand and gravel, limestone and cement rock,
salt, clays, phosphate rock (fertilizer), sulfur from salt domes or volcanoes,
diamond from kimberlites, F, Cl, Br, I from seawater or evaporates.
Demand for non-metals is increasing.
Reserves and Resources: Reserves are the known amount of a mineral in the ground that is exploitable with current technology and under current economic conditions. Resource includes the reserves plus estimated undiscovered deposits.
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Department of Geology
Aligarh Muslim University, Aligarh - 202 002 (India)