Chemistry Reference
In-Depth Information
variety of obsidian, known as
marekanite
was valued as a semiprecious stone
by some North American Indians (Doremus 1994; Shackley 1998, 1977).
Obsidian artifacts have been made for a variety of uses, and large
numbers of artifacts, as well as of obsidian flakes (the waste produced when
making the artifacts), are often found in many prehistoric sites. For this
reason a number of early investigations centered on identifying some phys-
ical or chemical properties of obsidian that may provide information about
its provenance. Most of the physical properties vary, however, in an irregu-
lar manner; the density and refractive index of obsidian from different
sources, for example, overlap and provide inconclusive results regarding
their place of origin. These properties and others that do not contribute
to differentiating between different types of obsidian are therefore of
very limited value for their identification. The chemical composition of
obsidian from different sources is not identical, however. Although the major
components of most obsidian are quite similar, there usually are great dif-
ferences in the nature and relative amounts of the minor and trace elements
they contain. Obsidian from single volcanic outflows, for example, generally
is of homogeneous trace element composition, but different outflows are not
(see Table 24). These differences make it possible to identify obsidian from
a single source and to distinguish between obsidians from different sources.
This is important in archaeological studies, since it is well known that obsid-
ian, in the form of rather large cores, and in some cases finished obsidian
artifacts, were widely traded already during prehistoric times (see Fig. 21).
The cores or finished artifacts were generally transported from relatively few
obsidian outflows in areas of volcanic activity in America, Asia, and Europe
to the many sites where they were put to use (Shackley 1998, 1997).
Obsidians from different sources (outflows) are chemically distinguish-
able from one another; it is reasonable to deduce, therefore, that identity
between the composition of obsidian objects or wastes and a particular
TABLE 24 Obsidian: Typical Elemental
Composition
Component
Percent
Silica
63-76
Alumina
11-14
Sodium oxide
3- 6
Potassium oxide
3- 5
Iron oxide
1- 8
Others
below 1
