Chemistry Reference
In-Depth Information
TABLE 8
Components of Most Natural
Materials
Component
Relative concentration (%)
Major (matrix)
above 1
Minor
1.0 - 0.1
Trace
below
0.1
a
a
Parts per million (ppm) or parts per billion (ppb).
In practically all natural and in most synthetic substances there are,
mixed with the major components, impurities in minor and trace amounts.
Minor components
occur in concentrations below 1% and down to about
0.1% of the total weight of a sample of the substance. Many additional
impurities, usually referred to as
trace components
or
trace elements
, occur
in host substances at extremely low concentration, generally below 0.1%;
their concentration is generally expressed either as parts per million (ppm)
or parts per billion (ppb) (1 ppm is equivalent to a one gram in one ton;
1 ppb, to one gram in one million tons). Minor and trace impurities do not
alter the basic composition, nor do they affect most of the properties of
substances, but they may change, even drastically, some of their physical
properties. Trace impurities in otherwise colorless minerals, for example,
often make the minerals highly colored.
A colorless mineral known as
corundum
(composed of aluminum
oxide) is colorless. A red variety of corundum known as
ruby
, a precious
stone, owes its color to impurities of
chromium
within the crystal structure
of corundum. Blue and violet varieties of corundum are classified as
sapphires
, the blue being the result of iron and titanium impurities, and
the violet of vanadium impurities within the corundum crystal structure.
Another colorless mineral is
beryl
(composed of beryllium aluminum
silicate); but blue
aquamarine
, green
emerald
, and pink
morganite
, are
precious varieties of beryl including different impurities: aquamarine
includes iron, emerald chromium and vanadium, and morganite
manganese.
Impurities and Provenance
The nature and the relative amounts of the impurities in many natural and
in some synthetic materials, are often characteristic of the geographic area
where the materials occur or were made. This is of particular interest in
archaeological studies, since determining the nature and the relative
amounts of impurities in many materials allows one to determine their
provenance
(Maniatis 2004; Guerra and Calligaro 2003).
