G201 Igneous Rock Classification: Texture + Composition = Name
Texture: Texture is related to how an igneous rock cooled. There are several textures you should be
able to identify in igneous rocks.
Fragmental
Glassy
Interlocking
Aphanitic
Phaneritic
Porphyritic
Vesicular
Composition: refers to the chemical composition of the magma. Color is a useful guide to identifying
the chemical composition and hence the magma type that cooled to produce an igneous rock.
 Mafic (Mg Fe rich) magmas form dark colored minerals (e.g. olivine, pyroxenes) when they cool.
 Felsic (silicic) (Al Si rich) magmas form light colored minerals (e.g. quartz, feldspar) when they
cool.
 So, the overall darkness or lightness of an igneous rock can be used to infer what type of magma
cooled to produce the igneous rock.
magma type
% dark minerals color value
100-75 black to dark gray (100-75%)
Mafic
75-50
dark gray to medium gray (75-50%)
Intermediate Mafic
50-25
medium gray to light gray ( 50 to 25%)
Intermediate Felsic
0-25
light gray to white (25-0%)
Felsic
Composition can also be determined by measuring the SiO2 content which requires chemical analysis.
Name
Most igneous rocks have an interlocking texture, for these use the table below.
Mafic
Intermediate Mafic Intermediate Felsic
aphanitic
Basalt
Andesite
Felsic
Dacite
Rhyolite
phaneritic
Gabbro
Diorite
Granodiorite
Granite
If the rock is porphyritic then modify the above names e.g. porphyritic granite, porphyritic andesite.
For rocks with fragmental or glassy textures use the table below.
Texture
Name
Glassy
no holes (flesic) -> obsidian no holes (mafic) ->tachylite (rare)
Vesicular
glassy, mostly holes -> pumice
Fragmental
fragments < 2mm -> tuff fragments > 2mm -> volcanic breccia
glassy fragment (water quenched) -> hydroclastic
more rock than holes -> scoria
Common Igneous Minerals
Mineral
Color
Quartz
Feldspars
Micas
Amphiboles
Pyroxenes
Olivine
Luster
Cleavage
Shape
G201 Lab-Igneous Rocks
Name_______________________________
 Make observations based on the specimens in class, not based on pictures in the text.
 Not all specimens are identical, look at more than one and generalize!
How large (in millimeters) is the field of view of your hand lens? ____________
Granite #3639 This granite has a phaneritic texture.
Grainsize?
fine medium
coarse
mixed Texture? glassy
interlocking
fragmental
Make a representative sketch of the granite as you see it through the handlens:
What percentage is dark minerals? _________%
What percentage is light minerals? ______ %
How many light colored minerals are in the granite? _____________
How can you tell them apart? Describe each and identify!
How many dark colored minerals are in the granite? _____________
How can you tell them apart? Describe each and identify!
Did the granite solidify above ground (fast cooling) or below ground (slow cooling) or some
combination of the two? Explain your choice briefly.
Dacite This dacite has a porphyritic texture.
Grainsize?
fine medium
coarse
mixed Texture? glassy
interlocking
fragmental
Make a representative sketch of the granite as you see it through the handlens:
What color is the matrix? _______________ What color are the phenocrysts? __________________
Do the phenocrysts have a cleavage? yes no Which best describes the phenocrysts? angular rounded
Which best describes the three dimensional shape of the phenocrysts?
pencil
flake
blocky
This rock formed when a magma cooled and solidified. Which solidified first the matrix or the
phenocrysts? Explain your reasoning!
Did this rock solidify above ground (fast cooling) or below ground (slow cooling) or some
combination of the two? Explain your choice briefly.
Basalt (#5) in cardboard box Note the holes! These holes formed as gas was escaping from the
cooling solidifying magma. Describe the size and shape of the holes! What percentage of the rock is
holes?
Basalt #4 (no dot) Note this basalt contains white blebs. Do you think these blebs formed when the
white mineral crystallized from the cooling basalt or by the white mineral filling holes in already cooled
basalt? Explain your reasoning!
Basalt #10 (yellow dot) Note this basalt contains white blebs. Do you think these blebs formed when
the white mineral crystallized from the cooling basalt or by the white mineral filling holes in already
cooled basalt? Explain your reasoning!
Sample Grainsize Igneous
#
Texture
2
6
7
9
10
11
12
15
16
17
27
29
30
Color
Index
Visible Minerals
Rock Name
How did
It form?
Sedimentary Rock Handout
Clastic Sedimentary Rocks
Clastic sedimentary rocks consist of consolidated sediment and have a fragmental texture.
Clastic sedimentary rocks are named after the size of the particles that make up the rock.
Look up the size range in millimeters for each particle type on the sand gauge and page 167 in Marshak.
Grain Size Particle Name
Diameter (mm)
Boulder
Coarse
Gravel Cobble
Pebble
Granule
V. Coarse Sand
Medium
Sand Coarse Sand
Medium Sand
Fine Sand
Very Fine Sand
Fine
Mud
Silt
Clay
ROCK NAME
Conglomerate
(if grains are rounded)
Breccia
(if grains are angular)
Sandstone
 quartz sandstone (just quartz)
 arkosic sandstone (quartz & feldspar)
 lithic sandstone (rock fragments)
 greywacke (rock fragments & clay)
Siltstone
Mudstone (if not layered)
Shale (if layered)
Biochemical Sedimentary Rocks
Biochemical sedimentary rocks have either an interlocking texture or a fragmental texture in which all
the clasts have formed by biological processes (e.g. shells stuck together.)
Biochemical sedimentary rocks are classified based on composition.
Composition
Gypsum
Halite (table salt)
Chalcedony (SiO2)
Calcite
ROCK NAME
Evaporite
Chert
Limestone
 fossiliferous limestone (fossils in lime mud)
 coquina (entirely broken fossils)
 micrite (lime mud)
 chalk (microscopic plankton shells, usually powdery)
 travertine (interlocking texture, precipitated directly from water)
Dense organic matter Coal
Identifying Sedimentary Minerals and Materials
can be most any color (usually light colored)
harder than a fingernail but softer that steel nail
has cleavage
reacts strongly with acid
CaMg(CO3)2
can be most any color (usually brown)
Dolomite
harder than a fingernail but softer that steel nail
has cleavage
reacts weakly with acid
NaCl
colorless to white
Halite
hardness close to that of fingernail
has cleavage
water soluble, salty taste.
can be most any color
Chalcedony SiO2
harder than steel nail
no cleavage,
extremely fine grained to amorphous, has conchoidal fracture
CaSO42H2O
colorless to white
Gypsum
softer than fingernail
has cleavage
(C, H, N, S, O) black in color, shiny to dull
Organic
softer than a steel nail
Matter
no cleavage, sometimes conchoidal fracture
low heft
Calcite
CaCO3
LAB Sedimentary Rocks
 BE KIND TO THE SPECIMENS!
Name_____________________________________
DETRITAL SEDIMENTARY ROCKS
1. Gray Box 2 (red dot) vs. Gray Box 9 (gold dot)
Examine the specimens in both boxes.
Using the handout and the sand gauge determine the particle size of the specimens in each box.
Box 2: _________________________
Box 9: ________________________________
Which is the sandstone and which is the siltstone? How can you tell?
Notice that the grains in 2 are all of the same type. Describe the color, luster and cleavage of these
grains. Can you identify the mineral?
Which was deposited by more rapidly flowing water? Explain your reasoning!
Suggest a depositional environment for each!
2 ___________________________________
9 _____________________________________
2. Gray Box 6 (black dot) vs. Gray Box 7 (white dot)
Note that specimens in each box consist of a mixture of small and large particles.
Determine the particle size of the large particles in the specimens in each box.
Box 6: _________________________
Box 7: ________________________________
Which is the conglomerate and which is the breccia? How can you tell?
In which box were the large grains transported further before being deposited? Consider both the grain
size and the grain shape when answering this question!
Suggest a depositional environment for each!
6 ___________________________________
7 _____________________________________
CHEMICAL SEDIMENTARY ROCKS
3. Limestone (Gray Box 5, orange dot)
Examine all the specimens in box 5. Base your answers on all the specimens in the box.
What color is limestone?
What is the grain size of limestone?
What do all the specimens in the box have in common?
4. Chert (Gray Box 8, brown dot)
Chert is biogenic, composed of millions of microscopic shells. These shells are made of one mineral.
What is that mineral? How do you know?
5. Chalk and Diatomite (Gray Boxes B & N)
Both chalk and diatomite are made of microscopic plankton shells. The difference is that shells in chalk
are made of calcite while the shells in diatomite are made of silica.
Describe how chalk and diatomite are similar in appearance.
Which box contains diatomite? How could you tell?
UNKNOWNS Use the sedimentary rock hand out to classify the following rocks.
Unknown # I
What is the particle size of the grains in this rock? ___________________________
What is the composition of the grains in this rock?
What type of environment did it form in? Was it marine or terrestrial? Be as specific as possible.
What is the name of this sedimentary rock? _______________________________
Unknown #J
What is the particle size of the grains in this rock? ___________________________
What is the composition of the grains in this rock?
What type of environment did it form in? Was it marine or terrestrial? Be as specific as possible.
What is the name of this sedimentary rock? _______________________________
Unknown #P
What is the particle size of the grains in this rock? ___________________________
What is the composition of the grains in this rock?
What type of environment did it form in? Was it marine or terrestrial? Be as specific as possible.
What is the name of this sedimentary rock? _______________________________
Unknown #O
What is the particle size of the grains in this rock? ___________________________
What is the composition of the grains in this rock?
What type of environment did it form in? Was it marine or terrestrial? Be as specific as possible.
What is the name of this sedimentary rock? _______________________________
Unknown #R
What is the particle size of the grains in this rock? ___________________________
What is the composition of the grains in this rock?
What type of environment did it form in? Was it marine or terrestrial? Be as specific as possible.
What is the name of this sedimentary rock? _______________________________
Unknown #V
What is the particle size of the grains in this rock? ___________________________
What is the composition of the grains in this rock?
What type of environment did it form in? Was it marine or terrestrial? Be as specific as possible.
What is the name of this sedimentary rock? _______________________________
Unknown #F
What is the particle size of the grains in this rock? ___________________________
What is the composition of the grains in this rock?
What type of environment did it form in? Was it marine or terrestrial? Be as specific as possible.
What is the name of this sedimentary rock? _______________________________
Unknown #L
What is the particle size of the grains in this rock? ___________________________
What is the composition of the grains in this rock?
What type of environment did it form in? Was it marine or terrestrial? Be as specific as possible.
What is the name of this sedimentary rock? _______________________________
Unknown #U
What is the particle size of the grains in this rock? ___________________________
What is the composition of the grains in this rock?
What type of environment did it form in? Was it marine or terrestrial? Be as specific as possible.
What is the name of this sedimentary rock? _______________________________
Metamorphic Rock Handout
Foliated Metamorphic Rocks
Layered metamorphic rocks are named according to the type of layering they have.
LAYERING
Slatey Cleavage: rock breaks along finely
spaced fractures that are controlled by the
orientation of microscopic clay grains.
Schistossity: rock consists of oriented
mineral grains which control how the
rocks breaks.
GRAIN SIZE
Fine grained
ROCK NAME
Slate
Fine grained
(satin sheen)
Medium to coarse grained
(glittery)
Medium to coarse grained
Phyllite
Gneissic Banding: rock consists of
alternating layers of light and dark minerals.
Migmatitic: rock consists of coarse grained Medium to coarse grained
non-foliated layers surrounded by
gneissic layers.
Schist
Gneiss
Migmatite
Unlayered Metamorphic Rocks
Unlayered metamorphic rocks are named according to their composition.
COMPOSITION
Calcite
Quartz
ROCK NAME
Marble
Quartzite
Metamorphic Minerals
A large variety of minerals form during metamorphism, the two most common are described
below.
Chlorite-A variety of mica. Typically greenish in color though sometimes whitish to yellowish to
colorless. Vitreous to pearly luster. Softer than steel nail, hardness close to fingernail. Good cleavage.
Occurs as thin sheets.
Garnet- Usually red, sometimes brown to white. Vitreous to greasy luster. Harder than steel nail. No
cleavage. Occurs as equant crystals, often as twelve sided dodecahedrons.
In addition to these minerals metamorphic rocks also contain the following common silicate
minerals/mineral groups: quartz, feldspar, biotite, muscovite, amphibole, pyroxene. Olivine is
uncommon in metamorphic rocks. Calcite is common in metamorphic rocks.
Lab-Metamorphic Rocks
Name_______________________________
Comparing Protoliths and Metamorphic Rocks
During metamorphism rocks are subjected to increased temperature and/or pressure and/or are infiltrated
by fluids. In response to these changes the minerals in the rock recrystallize while remaining solid (i.e.
they don't melt!), this changes the appearance of the rock and transforms the starter rock (the protolith)
into a metamorphic rock. Metamorphic recrystallization can produce several types of changes including;
changes in grain size, changes in grain shape, changes in the minerals present and changes in layering.
Amazingly these changes usually occur with very little change in the chemical composition of the rock
(i.e. no atoms are removed or added to the rock), what happens is that the atoms in the rock are
rearranged destroying existing crystal structures and creating new crystal structures (i.e.
recrystallization). Compare the following protoliths with their metamorphosed counterparts and describe
the changes produced by metamorphism. Consult your previous labs to help describe the protoliths.
1. Sandstone (protolith) (gray box 2) vs. Quartzite (metamorphosed sandstone) (blue box 19)
a) Touch the sandstone and quartzite, they should feel different. Describe their different 'feels'.
b) Examine the grains in the sandstone and quartzite using a hand lens.
Make a sketch of the grains in each rock as you see them through the hand lens; draw several grains
(ten or so) showing how they meet along their edges, please draw the grain boundaries clearly!
Sandstone
Texture?
Quartzite
interlocking
fragmental
interlocking
fragmental
c) Describe in words the change in the shape of the grains.
d) The sandstone is made of mostly one mineral; what is that mineral? ____________________
e) The quartzite is also made mostly of one mineral. Describe this mineral and identify.
2. Limestone (gray box 5) to Marble (blue box 6)
a) The limestone is made of mostly one mineral; what is that mineral? ____________________
b) The marble is also made mostly of one mineral. Describe this mineral and identify.
c) What type of fossils are in the limestone? _____________________________
d) Are there any fossils in the marble?
yes
no
(circle correct response)
e) What happened to the fossils during metamorphism?
3. Basalt (red box 9) to Serpentinite (blue box 21 )
a) What is the grain size of the basalt? fine
medium
coarse (circle correct response)
b) What is the grain size of the serpentinite? fine
coarse (circle correct response)
medium
c) The serpentinite (see Fig. 7-18) is made mostly of the mineral serpentine. Describe the color, luster,
cleavage and hardness of serpentine.
d) Look up the chemical composition of serpentine: _______________________
e) Is serpentine a hydrous mineral (does it contain water)? yes
no
f) How did the serpentine grains in the rock form?
g) The basalt is unlayered while its metamorphosed equivalent (the serpentinite) is layered. How did the
layers in the greenstone form?
3. Progressive Changes During Metamorphism
As temperature and/or pressure increase many metamorphic rocks will go through a series of changes.
For example when a mudstone is metamorphosed it is first converted to a slate, then a phyllite, then a
schist, and finally to a gneiss. Describe the changes in grain size, layering and mineralogy which
accompany these changes in rock type.
Describe Changes in Describe Changes in
Describe Changes in
Layering
Grain Size
Minerals Present
Shale
(gray box 4)
to Slate
(blue box 4)
Slate
(blue box 4)
to
Phyllite
(blue box 3)
Phyllite
(blue box 3)
to
Schist
(blue box 2)
Schist
(blue box 2)
to
Gneiss
(blue box 9)
Generalizations based on table above (circle correct statement)
During progressive metamorphism grain size tends to: decrease or increase or stay the same?
During progressive metamorphism layering tends to become: thinner or thicker or stay the same?
4. Unknowns
Describe each unknown and then name the unknown using the handout provided in class.
Unknown #14 blue box
grain size:
fine medium
layering: none
coarse
slatey cleavage
schistossity
gneissic banding
How many different minerals can you identify? ______
Describe each and try to identify.
rock name: ________________________
Unknown #12 cardboard box
grain size:
fine medium
layering: none
coarse
slatey cleavage
schistossity
How many different minerals can you identify? ______
Describe each and try to identify.
rock name: _________________________
gneissic banding
Unknown #5 cardboard box
grain size:
fine medium
layering: none
coarse
slatey cleavage
schistossity
gneissic banding
How many different minerals can you identify? ______
Describe each and try to identify.
rock name: ________________________
Unknown #25 cardboard box
grain size:
fine medium
layering: none
coarse
slatey cleavage
schistossity
How many different minerals can you identify? ______
Describe each and try to identify.
rock name: _________________________
gneissic banding