Chemistry Reference
In-Depth Information
science that deals with the origin, composition, and structure of rocks; and
petrography
, the classification and systematic description of rocks (Kempe
and Harvey 1983). The main objective of analyzing archaeological stone, as
well as most other archaeological materials, is usually to identify the rock
unequivocally. This is not, however, the only objective, since the results of
chemical analyses often also provide insights into any weathering processes
that the stone may have undergone (see Textbox 45), as well as its prove-
nance and chronology.
The Components of Materials.
The composition of most materials -
whether of natural origin, such as minerals, rocks, wood, and skin, or made
by humans, as for example, pottery, glass and alloys - includes several kinds
of components:
major
,
minor
, and
trace
elements (see Textbox 8).
TEXTBOX
8
COMPOSITION OF MATERIALS: MAJOR, MINOR,
AND TRACE COMPONENTS
Substances prepared under carefully controlled conditions and using very
pure chemicals, in a modern laboratory, for example, contain only the basic
component elements, those that determine the actual composition and
nature of the substances. Natural substances, whether of mineral or bio-
logical origin, and also most synthetic (human-made) substances contain,
in addition to their main components,
impurities
foreign to their basic
composition. Most impurities usually enter substances such as minerals, for
example, in relatively small amounts, when the substances are created.
Others, such as those in some rocks and the wood of trees, do so in the
course of their existence. Once within a substance, impurities become an
integral part of the host substance and impair the purity of the substance.
Although they alter the actual composition of substances, impurities do
not affect their basic properties.
Thus, three types of components can be distinguished in most sub-
stances, whether of natural origin or made by humans:
major
,
minor
, and
trace components
(see Table 8). The
major components
, also known as the
main
or
matrix components
, are those that determine the chemical nature
and properties of a substance. The major components occur in the sub-
stance in high concentration, generally exceeding 1% of the total weight.
In minerals and biological substances, for example, the major components
are those that appear in the chemical formula that expresses their
composition.
