MIXED ROCK IDENTIFICATION
INTRODUCTION
In the last few labs you learned to identify the rock forming minerals and to identify
the three types of rocks: igneous, sedimentary, and metamorphic.
In practice, when
identifying a rock you must first determine what type of rock it is before you name it. The
purpose of this lab is for you to build upon the identification skills learned in previous
labs, to be able to identify first the type of rock, and then to use the classification tables in
the previous labs to name the rock. Recall that
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Igneous rocks are formed by the crystallization of minerals that occurs during cooling
of molten material. The source of this material is called magma if it is below Earth's
surface, and lava if it is on the surface. Earth's internal heat is responsible for
producing molten rock. Environments in which igneous rocks are formed include
convergent plate margins, divergent plate margins, and hot spots (mantle plumes like
the Hawaiian islands).
Sedimentary rocks are derived from the breakdown of preexisting rocks into particles,
from the skeletons (or fragmented skeletons) of organisms, from the direct
precipitation of minerals from water, or by a combination of these processes. Loose
particles, called sediment, form rock through the processes of cementation and
compaction (lithification).
Metamorphic rocks form by the transformation of preexisting rock. The mineral
composition and the texture of the rock is changed in the solid state due to the
application of heat, pressure and chemically active fluids. Metamorphism occurs in
environments of deep burial, tectonism, and igneous intrusion.
CLASSIFICATION OF ROCKS (Review)
You also leaned that rocks are classified based on two primary characteristics;
composition and texture. Texture refers the appearance of the rock, which is related to the
size, shape, and boundary relations of mineral grains, not the "feel" of the rock. Most rocks
can be classified into one of the three major textural categories (Figure 1):
1. Crystalline Texture: crystals in an interlocking mosaic, like a jigsaw puzzle.
2. Clastic Texture: fragments of rock material, mineral grains, and/or fossils cemented
together, usually by calcite, silica (quartz) or iron oxide.
3. Foliated Texture:
a special type of crystalline texture in which minerals form
discontinuous layers or bands in which elongated minerals commonly display a
preferred orientation.
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Figure 1
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PROCEDURE
1. Identify texture of the rocks
2. Identify the mineral compositions
3. Identify the rock type
4. Name the rock
5. Link the rock to its associated tectonic environment on the diagrams, and record your
observations of the rock in the appropriate box.
Hints on recognizing fine-grained rocks
Most coarse-grained rocks are easily recognizable based on texture and composition.
However, fine-grained rocks are sometimes difficult to correctly identify. Although it is
always best to follow a systematic procedure for rock identification (based on composition
and texture), the following may be useful in naming some fine-grained rocks.
BASALT: Dark (usually black or greenish black), dense, hard, sometimes with holes
(vesicles); no foliation. (May be confused with chert, micrite, hornfels)
SLATE: Hard and dense with foliation; most often confused with shale, but much harder and
usually has a shiny “sheen”. Often a “ringing sound” when samples are struck on their
edges.
CHERT: Very hard with conchoidal fracture; will scratch glass plate. (May be confused with
basalt, micrite, hornfels)
HORNFELS: Very Hard, dense, blocky to chonchiodal fracture. Generally deep red/maroon
or dark grey to black. May have “spots.” (May be confused with basalt, micrite, chert)
SHALE: Usually soft; thin platy structure.
DOLOSTONE: Slight or no reaction to HCl (acid) unless the sample is powdered (then
vigorous reaction); can scratch with nail.
MICRITE (FINE-GRAINED LIMESTONE): Vigorous reaction with HCl (acid). (May be
confused with basalt, chert, hornfels)
ROCK GYPSUM: Very soft (can scratch with fingernail), no reaction with HCl; no salty
taste; may be fibrous.
ROCK SALT: Tastes salty; perfect cubic cleavage.
COAL: Black, light weight, soft, may have layered appearance, may leave black residue on
your hands, may be shiny with conchoidal fracture
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