Laboratory Manual for Introductory Geology 4e

TABLE 6.2 Classification of common sedimentary rocks (continued )
(B) Chemical sedimentary rocks (composed of grains that precipitated from a water solution)
Halite
Composition
Gypsum
Calcite
Texture or clasts
(grain size in mm) Clues to identification Rock name
Crystal
(generally >2)
Crystal
(generally >2)
Grains appear like tiny
balls
Clear to gray, with visible
interlocking crystals; tastes salty.
Clear to whitish–pale gray or
pinkish; soft, can be scratched
with a fingernail.
Very fine grained; HCl test yields
a vigorous fizz.
Quartz Grains not visible Won’t be scratched by a nail
or knife; grains are too small to
see; tends to be porcelainous;
fractures conchoidally.
Dolomite Grains not visible Grayish to rusty tan; scratchedoff
powder has a moderate
reaction with HCl.
Rock salt
Rock gypsum
Oolitic limestone
Chert
– Jasper (reddish)
– Flint (black)
Dolostone
(C) Biochemical sedimentary rocks (composed of minerals originally extracted by organisms to form shells)
Composition
Calcite
Calcite
Texture or clasts
(grain size in mm) Clues to identification Rock name
Visible shells or a
few shells in very fine
grains (generally 0.004
to 2); some crystal
appearance
Visible shells
(>2)
Commonly grayish, but may
be white, yellow, or pink;
vigorously reacts with HCl; some
examples may consist of shell
fragments cemented together
and have a clastic texture; some
have recrystallized to form a
crystalline texture.
A weakly cemented mixture
of shells.
Calcite Grains not visible Whitish; can be used to write on
slate. Vigorously reacts with HCl.
Limestone (general name)
– Fossiliferous limestone
– Micrite (very fine-grained; grains
aren’t visible)
Coquina (a type of limestone)
Chalk (a type of limestone)
(D) Organic sedimentary rocks (containing organic chemicals derived from the bodies of organisms)
Composition
Texture or clasts
(grain size in mm) Clues to identification Rock name
Clay and kerogen Grains not visible Dark gray to black; may have an
oily smell; may burn; can’t see
grains.
Carbon (± clay
and quartz)
Grains not visible
Black; may have a subtle
bedding; typically breaks into
blocks; may contain plant fossils.
Oil shale
Coal
– Lignite coal (50% carbon; fairly soft)
– Bituminous coal (85% carbon;
medium hard, dull)
– Anthracite coal (95% carbon;
quite hard; shiny)
154 CHAPTER 6 USING SEDIMENTARY ROCKS TO INTERPRET EARTH HISTORY