Chemistry Reference
In-Depth Information
TABLE 64
Biominerals
Occurrence and function
Main mineral form
Composition
Bone and teeth (endoskeletons
Carbonated
Carbonated calcium
of vertebrates)
hydroxyapatite
phosphate
Shell, coral, pearl (exoskeletons
Limestone and/or
Calcium carbonate
of molluscs) and encasements
aragonite
with variable amounts
of eggs
of magnesium
carbonate
Phytolytes and valves of diatoms
Amorphous silica or
Silicon dioxide or
calcium oxalate
calcium oxalate
formation of lines, known as
growth lines
at the border where new material
was added to earlier formed skeletal material, and these lines in the shells
of such animals can be used for dating archaeological events (Lowenstam
and Weiner 1989; Clark 1974).
8.3.
ANCIENT ORGANIC MATERIALS
In most archaeological excavations, organic materials, particularly those of
biogenic origin, are much scarcer than inorganic ones, such as stone, pottery,
or glass. This is because organic substances are highly susceptible to decay
processes that lead to their decomposition and eventual total breakdown in
much shorter periods of time than inorganic materials. Following the death
of living organisms, most of their dead bodies is either consumed as food by
microorganisms, insects, and some animals, or are rapidly and efficiently
decomposed by natural processes. The hair, nails, and teeth of a mammal's
corpse, for example, become detachable just a few weeks after death. Some
time later, after a month or so, most tissues become liquified as a result of
autolysis
, the spontaneous destruction of dead tissues, which is brought
about by the chemical action of enzymes in the corpse. Together with micro-
bial activity, the enzymes break down most of the components of the soft
tissues and, after a year or so after death, only the bones and teeth, the parts
of the body most resistant to decay, are practically all that is left of the corpse
(see Chapter 16). In addition to these natural processes, bacteria and fungi
may cause the putrefaction of biological substances, particularly of the pro-
teins, into simpler substances, a process that is usually accompanied by the
emergence of foul-smelling gases such as the
mercaptans
.
Thus, unless kept under special environmental conditions, most organic
substances that make up the body of vegetable and animal organisms break
