Chemistry Reference
In-Depth Information
Few metals occur in the earth's crust uncombined with others. Those
that do, such as gold, silver, mercury, and some copper, are known as the
native metals
. Together with some
native nonmetals
such as carbon and sulfur,
the native metals make up the relatively small group of
native elements
, nat-
urally occurring masses of single elements that were recognized and put to
use by humans in quite early times. Most metals occur in nature combined
with nonmetals in the form of mineral and rock deposits, and from
those minerals they are extracted by means of a variety of
metallurgical
techniques.
5.1.
NATIVE METALS
The few metals that naturally occur uncombined, in a relatively pure state,
do so mainly in the form of
lumps
or
nuggets
. The obvious luster of gold
nuggets on the surface of the earth could hardly have been ignored by pre-
historic humans; it is highly probable, therefore, that because of its natural
glitter, native gold was the first metal to be noticed and recognized. Native
copper was also relatively abundant in antiquity in some areas of the world.
Even today, deposits of native copper are still occasionally discovered in
some places. Since copper is affected by, and reacts with, weathering agents
and pollutants, native copper lumps are seldom lustrous; they are mostly
covered by a surface coating of dark green or reddish brown copper prod-
ucts, usually known as
patina
(Coghlan 1951). Native iron occurs only as
grains of microscopic size that would not have been recognized by ancient
humans. But large masses of iron, known as
meteoritic iron
, have been
brought to the earth's surface by meteorites from outer space. Silver, too,
occurs in some places as relatively large nuggets, although mostly in veins
too deep below the surface of the earth to have been accessible to the
ancients.
The native origin of metals used in prehistoric sites for making objects
can often be revealed by chemical analysis and microstructural studies. If
quartz and iron are found in gold, for example, they are often a good indi-
cation of the native origin of the metal, since such impurities do not occur
in smelted gold.
Native copper
is generally free of impurities, a characteristic
that makes it distinguishable from the smelted metal.
Meteoritic iron
is gen-
erally characterized by the presence of phosphorus and is sometimes natu-
rally alloyed with nickel. Among the oldest known human-shaped metal
objects are native copper beads found in northern Iraq, probably dating from
the beginning of the ninth millennium B.C.E. (Smith 1965).
