Laboratory Manual for Introductory Geology 4e

FIGURE 6.4 Textures of chemical sedimentary rocks.
(a) Coarse, interlocking halite grains in rock salt.
(b) Cryptocrystalline silica grains (chert). Note the almost
glassy appearance and conchoidal fracture.
The composition of a chemical sedimentary rock depends on the composition
of the solution it was derived from—some chemical sedimentary rocks consist of
salts (e.g., halite, gypsum), whereas others consist of silica or carbonate. In some
chemical sedimentary rocks, the grains are large enough to see (FIG. 6.4a). But in
others, the grains are so small that the rock looks almost like porcelain or obsidian
(FIG. 6.4b). Such rocks are called cryptocrystalline rocks, from the Latin crypta–,
meaning hidden.
Geologists distinguish among different types of chemical sedimentary rocks primarily
by their composition:
■ Evaporites are chemical sedimentary rocks, composed of crystals formed when
salt water evaporates, and typically occur in thick deposits. The most common examples
of evaporites are rock salt and rock gypsum (alabaster).
■ Oolitic limestone is a unique form of limestone in which calcite precipitates
onto small mineral grains; as the grains roll back and forth, the calcite coating thickens,
producing bead-shaped grains that are then lithified.
■ Travertine (chemical limestone) Travertine is a special kind of limestone composed
of calcite (CaCO 3
) formed by chemical precipitation from groundwater that
has seeped out of the ground either in hot- or cold-water springs or from the walls
of caves.
■ Dolostone like limestone, is a carbonate rock, but it differs from limestone
in that it contains a significant amount of dolomite, a mineral with equal amounts
of calcium and magnesium [CaMg(CO 3
) 2
]. Unlike calcite in oolitic limestone or
travertine, the dolomite doesn’t precipitate directly from seawater. Instead, it forms
from limestone during diagenesis when magnesium in groundwater interacts with
calcite.
■ Chert composed of very fine-grained silica. One kind of chert, called replacement
chert, is formed by diagenesis when cryptocrystalline quartz replaces calcite in
limestone. Another kind (biochemical chert) is formed from microscopic plankton
shells, as we will see shortly. Note: The chemical and biochemical forms of chert cannot
be distinguished in hand samples.
6.3 THE BASIC CLASSES OF SEDIMENTARY ROCKS
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