Geoscience Reference
In-Depth Information
FIGURE 5
Shatter cones
from Sudbury, Ontario.
[Photo courtesy of
R.
Grieve
and Geological Survey of
Canada.]
Today we know from experiments that shatter cones mark the
lowest pressures of impact, in the range of 5 gigapascals to 10 giga-
pascals. (Named in honor of a seventeenth-century mathematician
and physicist, Blaise Pascal, a gigapascal [gPa] equals 10,000 times
the pressure of the earth's atmosphere at the surface.) At slightly
higher pressures—10 gPa to 20 gPa—quartz and feldspar, the two
most common minerals in the earth's crust, begin to fracture in the
characteristic crisscrossing planes, a few millionths of an inch apart,
that I had originally seen in K-T zircon on the cover of
Nature.
When a mineral with a certain crystal structure is subjected to
sufficient heat and pressure, its atoms rearrange themselves into a
structure that better accommodates the new conditions. For exam-
ple, at low temperatures and pressures, pure carbon exists in the
sheetlike structures that we call the mineral graphite. At higher tem-
peratures and pressures, and under certain other conditions, carbon
changes into the interlocking, three-dimensional structure that we
call diamond.
Laboratory experiments show that quartz has two mineral
phases that appear at high pressure but low temperature: The first
to form is coesite, followed at about 16 gPa by stishovite. Thus the
