Crossing the Borders: New Methods and Techniques in the Study of Archaeological Materials from the Caribbean

Geochemical Techniques in Discriminating Chert Materials / 53
cality, where chert occurs in primary and secondary context along the slope of the
Shirley Heights cliff. Outcrops of tuff occur with inclusions of irregularly shaped
chert. The chert is white–pale brown colored, pure quartz with a relatively coarse
crystalline matrix, and it typically lacks fossils or other carbonate matter. No chert
in tertiary context as evidence of prehistoric exploitation was found at these three
latter localities.
The occurrence of chert on St. Kitts and Puerto Rico is quite different. All
sources on these islands are characterized by chert occurring in a secondary context.
No clear relationship could be established with rocks in which the chert may
have formed. Chert on St. Kitts is found on cobble beaches amid igneous rocks. It
resembles flint of Antigua, but has a characteristic very fine crystalline quartz matrix
and, on average, low amounts of fossils, calcite, and opaque (organic) matter.
On Puerto Rico, limestone in which flint might have formed is common; however,
no relation between chert occurrence and limestone host- rock could be established.
The chert consists of relatively clean, rather coarse crystalline quartz
with common radial fibrous silica precipitates (chalcedony). Furthermore, it lacks
calcite and opaque (organic) matter, but is occasionally impregnated by iron oxides,
which cause a reddish- colored hue.
Chert Formation and Chert Chemistry
The properties of chert and its suitability for prehistoric manufacturing are a function
of the type of rock in which it forms, of the processes during its precipitation
from pore fluid (Bush and Sieveking 1986), and of the changes that occur after erosion,
exposure, and subsequent weathering (Brownlow 1979; Lavin and Prothero
1992). They are reflected in macromicroscopic characteristics that allow a subjective
classification by the trained eye, but also in trace element composition that allows
a more objective classification without much prior knowledge.
Chert consists mainly (> 95 percent) of silicium dioxide, the matrix, which may
occur in various crystal habits (polymorphs) as a function of age, each characterized
by a specific molecular ordering or crystallinity and a related solubility. Most
soluble is the poorly ordered silica in rapidly formed and fine- grained volcanic
ash, but also in siliceous tests of microorganisms such as diatoms and radiolaria.
In pore fluids of sediments containing large concentrations of this amorphous
opaline silica, dissolved silicium dioxide concentrations may exceed 100 mg/ liter.
However, in time, better ordered silica polymorphs will form, such as cristoballite,
trydimite, and finally quartz, which have decreasing solubility and silicium dioxide
saturation concentrations, of 60, 30, and about 10 mg/ liter, respectively.
In young host- rock, locally the silicium dioxide concentration in the vicinity
of less soluble silica polymorphs is lower than the saturation concentration of the
more soluble polymorphs distributed through the rock, the latter will dissolve,
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