Geology Reference
In-Depth Information
◗
Figure 6.21
Evaporites, Chert, and Coal
c
Bedded chert exposed in Marin County, California. Most
of the layers are about 5 cm thick.
a
This cylindrical core of rock salt was taken from an oil well in
Michigan.
d
Bituminous coal is the most common type of coal used
for fuel.
b
Rock gypsum. When deeply buried, gypsum (CaSO
4
•2H
2
O) loses
its water and is converted to anhydrite (CaSO
4
).
bacteria. Bacterial decay ceases, and the vegetation is not
completely decomposed and forms organic muck. When
buried and compressed, the muck becomes
peat
, which looks
somewhat like coarse pipe tobacco. Where peat is abundant,
as in Ireland and Scotland, it is used for fuel.
Peat represents the fi rst step in forming coal. If peat is
more deeply buried and compressed, and especially if it is
heated, too, it is converted to dull black coal called
lignite.
During this change, the easily vaporized or volatile elements
are driven off, enriching the residue in carbon; lignite has
about 70% carbon, whereas only about 50% is present in
peat.
Bituminous coal
, with about 80% carbon, is dense, black,
and so thoroughly altered that plant remains are rarely seen.
It burns more effi ciently than lignite, but the highest-grade
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