igneous rocks classification #6.pdf

IGNEOUS ROCKS
• Igneous rocks are derived from magma.
• Results from the solidification of igneous melt.
• Magma that solidifies within Earth produces
intrusive or plutonic rocks.
• Intrusive rocks develop from magma that cools
slowly within Earth producing large crystals visible
to the eye.
• Plutons are magma chambers of various sizes,
shapes and depths that store magma within Earth.

IGNEOUS ROCKS
• Magma that rises and erupts onto the surface of
Earth is called lava .
• Volcanic or extrusive igneous rocks form by
solidification of lava and volcanic debris on Earth ’ s
surface, producing rocks with small crystals and/or
non – crystalline particles of various sizes.

How do w e classify igneous rocks?
• Igneous rocks are classified according to composition
and texture.
• Composition is determined by magma chemistry.
• Texture refers to the size, shape, arrangement and
degree of crystallinity of a rock ’s constituents.
• Texture criteria are commonly considered first, as
textures provide the best evidence for rock origin and
permit classification into the broadest genetic
categories.

• Phaneritic The majority of crystals that compose the rock are
readily visible with the naked eye (> ~0.1 mm). If a rock
exhibits phaneritic texture, it typically crystallized slowly
beneath the surface of the Earth and may be called plutonic,
or intrusive.
• Aphanitic Most of the crystals are too small to be seen
readily with the naked eye (< ~0.1 mm). If a rock is aphanitic,
it crystallized rapidly at the Earth’s surface and may be called
volcanic, or extrusive.
• Fragmental The rock is composed of pieces of disaggregated
igneous material, deposited and later amalgamated.
• The fragments themselves may include pieces of preexisting
(predominantly igneous) rock, crystal fragments, or glass.
Fragmental rocks are typically the result of a volcanic
explosion or collapse and are collectively called pyroclastic.

• The grain size of phaneritic rocks may be further
subdivided as follows:
• Fine grained < 1 mm diameter (< sugar granules)
• Medium grained 1–5 mm diameter (sugar to pea sized)
• Coarse grained 5–50 mm diameter
• Very coarse grained > 50 mm diameter (the lower size limit is
not really well defined)
• Pegmatitic is an alternative term for very coarse grain
size but has compositional implications for many
geologists because pegmatites have historically been
limited to late-stage crystallization of granitic magmas.
• Note: The distinction between aphanitic (too fine to see
individual grains) and fine grained (grains are visible
without a hand lens but less than 1 mm in diameter).

• Some rocks classified as phaneritic and aphanitic are relatively
equigranular (of uniform grain size), whereas others exhibit a range of
grain sizes because different minerals may experience somewhat different
growth rates.
• The texture displays two dominant grain sizes that vary by a significant
amount, the texture is called porphyritic.
• The larger crystals are called phenocrysts, and the finer crystals are
referred to as groundmass.
• Whether such rocks are considered plutonic or volcanic is based on the
grain size of the groundmass.
• Because the grain size is generally determined by cooling rate, porphyritic
rocks generally result when a magma experiences two distinct phases of
cooling.
• This is most common in, although not limited to, volcanics, in which the
phenocrysts form in the slow-cooling magma chamber, and the finer
groundmass forms upon eruption.

COMPOSITIONAL TERMS
• The composition of a rock may refer to its chemical composition or
the proportions of minerals in it.
• Nearly all igneous rocks are composed principally of silicate
minerals, which are most commonly those included in Bowen’s
Series: quartz, plagioclase, alkali feldspar, muscovite, biotite,
hornblende, pyroxene, and olivine.
• Of these, the first four (and any feldspathoids present) are felsic
minerals (from feldspar + silica), and the latter four are mafic (from
magnesium + ferric iron).
• Generally, felsic refers to the light-colored silicates (feldspars,
quartz, feldspathoids), whereas mafic refers to the darker ones, but
composition has precedence (e.g., smoky quartz and dark feldspars
are felsic).
• In addition to these principal minerals, there may also be a number
of accessory minerals, present in small quantities, usually
consisting of apatite, zircon, titanite, epidote, an oxide or a sulfide,
or a silicate alteration product such as chlorite.

COMPOSITIONAL TERMS
• Nearly all magmas are silicate magmas, enriched in the
elements silicon and oxygen which bond together to form the
silica tetrahedron.
• Silicate magmas contain anywhere from ∼ 40% to over 75%
silica (SiO 2 ).
• As silica is generally the dominant chemical component,
magma and igneous rocks are classified as ultrabasic, basic ,
intermediate and acidic based upon percent SiO2.
• Acidic rocks are also referred to as silicic , based on their high
SiO2 content.

COMPOSITIONAL TERMS
• Magma chemistry
determines the
percentage of dark -
colored or light -
colored minerals as
described in Table 7.2 .
• Dark - colored minerals are
generally enriched in the
elements iron and
magnesium and are
referred to as
ferromagnesian or mafic
minerals.
• Light - colored felsic
minerals are depleted in
ferromagnesian elements
and are generally enriched
in elements such as silicon,
oxygen, potassium and
sodium.

COMPOSITIONAL TERMS
• whereas mafic describes a rock with far more mafic
minerals.
• The term ultramafic refers to a rock that consists of over
90% mafic minerals.
• Similar, but not equivalent, terms are leucocratic,
indicating a lightcolored rock, and melanocratic,
indicating a dark-colored rock.

Peridotite
• Peridotite is a very dark - colored (ultramafic) rock,
depleted in SiO 2 (ultrabasic) and commonly
enriched in the minerals pyroxene, olivine,
amphibole and plagioclase. Ultramafic plutonic
rocks occur in Earth ’s mantle.

Basalt and gabbro
• Basalt and gabbro are dark – colored (mafi c), SiO2
- poor (basic) rocks rich in plagioclase, pyroxene
and olivine. Basalt is a very common volcanic rock –
encompassing the upper few kilometers of ocean
crust – that forms from rapid cooling. Gabbro
crystallizes more slowly at depth in the lower crust
of ocean basins.

Andesite and diorite
• Andesite and diorite are gray – colored (intermediate) to
salt and pepper – colored rocks rich in hornblende,
pyroxene and plagioclase. Andesite and diorite contain
more than half to almost two - thirds SiO2 .
• Andesite is a common volcanic rock around the Pacific
Ring of Fire. Andesite volcanoes overlie diorite plutons.

Dacite and granodiorite
• Dacite and granodiorite are light – colored (felsic) rocks,
containing approximately two - thirds SiO 2 , rich in
plagioclase, alkali feldspar and quartz and also containing
small amounts of hornblende and biotite.
• Dacite is a volcanic rock that, like andesite, occurs around
the Pacific rim. Granodiorite is a plutonic rock that
underlies andesite – dacite volcanoes.

Rhyolite and granite
• Rhyolite and granite are light – colored (felsic)
rocks containing more than two - thirds SiO 2
(silicic or acidic) and rich in quartz, alkali feldspar
with small percentages of plagioclase and biotite.
• Rhyolite is a volcanic rock that usually erupts in
thick, continental crust.
• Granite plutons also occur in continental crust.

Non - crystalline rocks
• Non - crystalline rocks those characterized by the
absence of crystals, include frothy, vesicular rocks such
as
• pumice (light colored) and scoria (dark colored).
• Other non - crystalline rocks include those with glassy
textures such as obsidian or those enriched in rock
fragments.
• Fragmental, also known as pyroclastic, volcanic rocks
include tuff (volcanic ash to gravel size) and breccia
(larger than gravel size).

Volcanic rocks vs Plutonic rocks
• Volcanic rocks – such as basalt, andesite, dacite and
rhyolite – tend to have fi ne grains that are too small to be
identified with the eye. In many cases, very fi ne grain size
necessitates the use of color as a means to classify
volcanic rocks. Rock color is used as a last resort because
color is inherently unreliable.
• Plutonic rocks – such as gabbro, diorite, granodiorite and
granite – tend to have large crystals such that we can
easily identify the minerals simply by looking at the rock
with our eyes. As plutonic rocks have large crystals, we can
identify these rocks based on the relative proportion of
minerals.

IGNEOUS TEXTURES
• Igneous textures may be broadly categorized on the degree of crystallinity.
• Given appropriate time, temperature and pressure conditions, silica tetrahedron
structures within cooling magma link together to produce crystals.
• In some instances, extremely rapid cooling or the sudden loss of gas may result in
solidification without the development of crystals, creating a glassy solid.
• DEVETRIFICATION:
Conversion of glassy material back into crystalline material is called devetrification.
As glass is not stable therefore, it usually undergoes devetrification. There are
always some clues of devitrification in volcanic rocks.
Very minute crystal are formed as a result of devitrification.
i. CRYSTALLITES: very minute crystals, cannot be seen in polarizing microscope
(Embryo crystal).
ii. MICROLITES: slightly bigger than embryo crystal.

IGNEOUS TEXTURES
• Crystallinity generally is described in terms of the
four categories shown in the Table.

we will discuss crystalline and non - crystalline textures.
• CRYSTALLINE FORMS:
• Euhedral minerals contain
complete crystal faces that are
not impinged upon by other
crystals.
• Euhedral crystals typically
develop as early mineral phases
in the crystallization of magma.
Under such conditions the
crystals have abundant free
space for growth, enhancing the
likelihood that perfectly formed
crystal faces develop.
• Later in the magma
crystallization sequence,
subhedral or anhedral crystals
develop in the remaining void
spaces between earlier formed
crystals.

CRYSTALLINE FORMS
• Subhedral crystal faces contain partially complete crystal
forms in which at least one of the crystal faces is impinged
upon by adjacent rock material (Figure 7.2 ).
• In subhedral textures, crystal growth may be aborted due
to:
• Contact against previously formed minerals.
• Nucleation on pre - existing surfaces such as early formed
crystals or the margins of the magma chamber.
• Resorption in which pre - existing euhedral crystals are
partially remelted.
• Other secondary alteration processes that destroy pre -
existing euhedral faces.

ANHEDRAL
• ANHEDRAL crystals
lack any observable
crystal faces. As
crystallization
progresses in magma,
the space available for
the development of
euhedral and
subhedral crystals
diminishes.
• As a result, anhedral
crystal forms are
determined by the
shape of the existing
space, rather than by
mineral
crystallography.
• The remaining voids
between existing
crystal forms are
referred to as
interstitial space.

CRYSTALLINE TEXTURES
• Crystalline textures provide critical information as to
whether the rock solidified in a plutonic or volcanic
setting.
• Igneous textures typical of plutonic rocks include coarse -
grained pegmatitic, phaneritic and phaneritic –
porphyritic textures.
• Lava that cools on Earth ’ s surface loses heat quickly. As a
result, many crystals are invisible to the eye and produce
fine - grained aphanitic or aphanitic – porphyritic
crystalline textures.

Pegmatitic textures
• Pegmatitic texture is
characterized by large
crystals averaging more than
30 mm in diameter.
• Pegmatites display large,
early formed
euhedral crystals
surrounded by later formed
subhedral crystals.
• In naming rocks with
pegmatitic texture, the
textural term (e.g.,
pegmatite) must be included
in the rock name.

Pegmatitic textures
• A rock with a pegmatitic texture and the
composition of granite or granodiorite
• is a granite pegmatite or granodiorite
pegmatite.
• Pegmatitic textures develop most
commonly in granitic plutons with high
volatile contents.
• Gabbroic plutons rarely display pegmatitic
textures, partly due to the lower volatile
gas content.
• Because of the large, well - developed
crystal forms, pegmatites are the source of
many gemstones such as sapphire and
topaz.
• High volatile content also produces
valuable ore deposits of metals such as tin,
gold and silver in pegmatite deposits.

Phaneritic textures
• Phaneritic texture implies crystal diameters ranging
from 1 to 30 mm.
• Rocks with a phaneritic texture contain crystals visible
to the naked eye (Figure 7.4 ).
• Early formed crystals are euhedral; later formed
crystals are subhe-dral to anhedral.
• Phaneritic textures may be subdivided into fine (1 – 3
mm in diameter),
• medium (3 – 10 mm) or coarse (10 – 30 mm) grained.
• Fine - grained phaneritic textures commonly develop in
shallow plutonic structures such as dikes and sills.
• Coarse – grained textures are generally associated with
larger or deeper intrusions.
• Rock names such as granite, diorite and gabbro imply a
phaneritic texture so that we do not refer to “
phaneritic granite ” but simply “ granite

Aphanitic textures
• Aphanitic textures contain small crystals less than 1 mm in
diameter that are not generally visible to the eye.
• With the use of a microscope or other analytical means,
geologists can determine the composition and relative sequence
of crystallization in the same manner as with phaneritic textures.
• Aphanitic textures are associated with volcanic rocks that cool
quickly on Earth ’ s surface.
• Aphanitic textures may be subdivided into microcrystalline and
cryptocrystalline varieties.
• Microcrystalline textures contain microlite crystals large enough
to be identified with a petrographic microscope (Figure 7.5 ).
• Aphanitic textures in which the crystal size is too fine to be
identified even with a petrographic microscope are termed
cryptocrystalline
• Rock names such as rhyolite, andesite and basalt imply an
aphanitic texture so that we do not refer to “ aphanitic basalt ”
but simply basalt.

Porphyritic textures
• Rocks with porphyritic textures consist of two distinctly
different size crystals.
• Large crystals are referred to as phenocrysts ; finer
grained material constitutes the groundmass .
• In porphyritic – phaneritic textures, all crystals are visible
to the eye, but the phenocrysts are distinctly larger than
the groundmass crystals (Figure 7.6 ).
• In rocks with porphyritic – aphanitic textures, the larger
phenocrysts are embedded in an aphanitic groundmass
composed largely of microcrystalline, cryptocrystalline or
glassy material (Figure 7.7 ).

Porphyritic textures
• The cooling rate of magma or lava determines crystal size.
• Slow cooling of magma deep within Earth produces coarse -
grained pegmatitic or phaneritic textures.
• Rapid cooling of magma at shallow depths or as lava on
Earth ’ s surface generates fine - grained aphanitic textures.
• Rocks withtwo distinctly different size crystals (porphyritic)
are commonly explained by a two – stage cooling process.
• In two - stage cooling processes, the larger phenocrysts
form slowly at depth, while the finer grained groundmass
crystals cool rapidly as magma approaches Earth ’ s surface.

Non - crystalline textures
• Glassy, vesicular and pyroclastic are examples of
non - crystalline igneous textures (see Figure 7.1 b).
• Rocks may consist entirely of non - crystalline
(holohyaline) components or contain a mixture of
crystalline and non - crystalline (hypocrystalline)
igneous textures.

Glassy textures
• A glass , such as the rock obsidian, is an amorphous solid.
• Amorphous solids possess a disordered form, thereby
lacking an ordered crystalline structure.
• Many glasses contain small amounts of very small
microlites and/or cryptocrystalline material.
• Glassy textures develop in lava that solidifies without
experiencing significant crystallization.
• The lack of crystal structure in glasses is similar to the lack
of long - range order characteristic of melts – glasses are
essentially super cooled liquids.
• Glassy textures form by the near instantaneous
solidification of melts preserving their disordered
structure.
• Near instantaneous melt solidification results from two
major mechanisms:
• Quenching
• Rapid gas loss.

Glassy textures
• Quenching occurs when melts of any composition come
into contact with liquid water or air.
• Water rapidly absorbs heat from the melt, causing it to
solidify before crystals have time to nucleate and grow.
• Most basic (low SiO 2 ) glasses quench when volcanoes
erupt on the ocean floor or as massive flood basalts.
• Thin glassy zones also occur on lava flow tops that have
been quenched by contact with the atmosphere.
• The second glass - forming mechanism, limited to silicic
melts, occurs by rapid loss of dissolved gas from solution
which rapidly lowers P H2O .
• The rapid loss of dissolved water vapor allows silica
tetrahedra to link together and causes melt viscosity to
increase so rapidly that crystal nucleation and crystal
growth are severely inhibited.

Glassy textures
• The result is a glass, the product of the nearly
instantaneous solidification of magma by loss of
dissolved gas, rather than by extremely rapid
cooling.
• This second model explains why glassy rocks, such
as obsidian, are far more common in silicic rocks
than in basic rocks.
• Unlike silicic magmas, basic magmas contain
neither enough dissolved water nor sufficient silica
tetrahedra to solidify rapidly due to loss of
dissolved gases.

Vesicular textures
• Vesicular textures contain spherical to ellipsoidal void spaces
called vesicles , which are analogous to holes in a household
sponge.
• Vesicular textures develop due to exsolution and entrapment
of gas bubbles in lava as it cools and solidifies.
• EXPLANATION: Plutons contain magmas at relatively high
confining pressures such that gases are dissolved and the
magma is undersaturatednin volatile content.
• As volatile gases are of low density and tend to be buoyant,
volatiles ascend within the pluton and can saturate magma
in the upper part of the pluton.
• In conditions where magma rises toward the surface, the
confining pressure decreases, and the ability of the magma
to retain dissolved gases decreases.
• As a result, magma becomes supersaturated with volatiles
so that it can no longer hold all the gas in solution.

Vesicular textured Rocks
• Vesicular rocks, defined as containing > 30% vesicles by volume,
include pumice and scoria.
• Scoria is a vesicular rock characterized by brownish red or black
colors due to an abundance of iron and is used as a decorative
stone.
• White - to gray - colored pumice solidifies as a frothy glass from
silicic lava.
• Pumice is widely used as an abrasive soap.
• Rocks that contain smaller amounts (5 – 30%) of vesicles are
named as vesicular basalt or vesicular andesite.
• while those rocks with just a few vesicles ( < 5%) are given names
such as vesicle - bearing basalt and andesite.
• Hot fluids that flow through vesicular rocks may later precipitate
secondary minerals in the void spaces of vesicles, producing
amygdules.
• Common secondary minerals that infill pre - existing vesicles
include quartz, calcite, epidote, zeolites and metals.

Pyroclastic textures
• Volcanic eruptions eject broken rock particles of
varying sizes, known as pyroclasts (which means
fiery fragment).
• Pyroclasts may be ejected into the atmosphere as
airborne tephra or transported along Earth ’ s
surface as pyroclastic flows.
• Following accumulation, these particles are
cemented or welded together to produce volcanic
rocks with fragmental or pyroclastic textures .
• Pyroclasts are classified according to their
composition, size and shape (Figure 7.14;Table 7.3).

• Pyroclasts consist of several different types of
materials:
• Lithic pyroclasts contain fragments such as
basalt, andesite or other rocks.
• Vitric pyroclasts are composed of glassy
fragments, most commonly pumice or scoria
shards.
• Crystal pyroclasts contain minerals.

• Pyroclasts are further divided by
average grain size diameters:
• Pyroclasts, greater than 64 mm in
diameter, are called blocks if angular
(Figure 7.15 ) and bombs if rounded.
• Angular blocks lithify as breccia and
rounded blocks form agglomerates.
• Gravel - sized pyroclasts (2 – 64 mm
diameter) are called lapilli .
• Rocks that consist largely of lapilli are
called lapillistones .
• Ash consists of sand - sized and finer
sized pyroclasts ( < 2 mm diameter)
which can be subdivided into coarse
ash (0.0625 – 2 mm) and fine ash
( < 0.0625 mm) or dust .
• A rock composed of solidified volcanic
ash is called tuff .
• Tuffs that contain significant amounts
of gravel - size lapilli are called lapilli
tuffs .

IUGS IGNEOUS ROCK CLASSIFICATION
• A simplified igneous rock classification for crystalline
rocks using mineral components and texture.
• In reality, many different igneous rock classifications exist
incorporating hundreds of possible rock names.
• The most widely used rock classifications identify igneous
rocks based on texture and
(1) modal minerals identified in the rock.
(2) theoretical normative minerals calculated from
chemical composition data from laboratory analyses OR
(3) chemical composition of the rock based on laboratory
analytical methods.

IUGS IGNEOUS ROCK CLASSIFICATION
• The IUGS recommended a classification system for
both plutonic and volcanic rocks using essential
mineral groups as endpoints in triangular - and
diamond - shaped diagrams (Streckeisen, 1976 ; LeBas
and Streckeisen, 1991 ; LeMaitre, 2002 ).
• While the IUGS classification is generally accepted, it is
not comprehensive to all igneous rocks and pre -
existing rock nomenclature remains in use.
• The following discussion provides a summary of the
IUGS classification system, pointing out the benefits of
a unified classification approach as well as the
drawbacks.

Calculations and Plotting
• The IUGS system requires that we determine the mineral
components of a rock and plot the percentages of three of
those components on appropriate triangular diagrams to
determine the proper name.
• Figure 1 shows how triangular diagrams are used.

• In Figure, the three components
are labeled X, Y, and Z.
• The percentage of X (at the
upper apex) is zero along the Y–Z
base and increases progressively
to 100% at the X apex.
• Any horizontal line represents a
variation in the Y/Z ratio at a
constant value of X.
• Such lines (at 10% X increments)
have been shown on the left
diagram.
• Likewise, lines of constant Y and
constant Z have been added.
These lines can be used like
graph paper to plot a point, and a
few of these lines have been
labeled.

• In order to plot a point on a triangular
diagram using particular values of X, Y,
and Z, they must total 100%.
• If they do not, then they must be
normalized to 100%.
• This is accomplished by multiplying
each by 100/(X + Y + Z).
• As an example, point A has the
components X = 9.0, Y = 2.6, Z = 1.3.
We can normalize these values to 100
by multiplying each by 100/(9.0 + 2.6 +
1.3) = 7.75.
• That gives the normalized values X =
70%, Y = 20%, and Z = 10%. If we count
up 7 lines from the Y–Z base, we get a
line representing a constant 70% X.
• Next, counting 1 line from the X–Y
base toward Z, we get a line
representing 10% Z.
• Their intersection (point A) is also
intersected by the line representing
20% Y because the sum must be 100%.

• Determine the mode (the percentage of each mineral
present, based on volume).
• The mode is estimated on the basis of the cumulative area
of each mineral type, as seen on the surface of a hand
specimen or in a thin section under the microscope.
• A more accurate determination is performed by “point
counting” a thin section.
• Point counting involves a mechanical apparatus that moves
the section along a two-dimensional grid on the
petrographic microscope stage. With each shift, the mineral
at the crosshair of the microscope is identified and counted.
• When several hundred such points are counted, the count
for each mineral is summed, and the totals are normalized
to 100% to determine the mode.
• All these methods determine relative areas of the minerals,
but these should correlate directly to volume in most cases.

• Q = quartz, tridymite, cristobalite.
• A = alkali feldspar, including orthoclase, microcline, sanidine,
perthite, anorthoclase and albite plagioclase with up to 5 mole %
anorthite (An 0 – An 5 ).
Mole percent is calculated by taking the weight percent of a mineral
and dividing by the mineral ’ s molecular weight.
• P = plagioclase (An 5 – An 100 ) and scapolite (altered plagioclase).
• F = feldspathoids, also known as foids.
• The term foid is derived from being feldspathoid rich. Feldspathoids
include the minerals nepheline, sodalite, cancrinite, leucite, analcite,
nosean, hauyne and kalsilite.
• In naming a rock, we use the major feldspathoid mineral as either an
adjective or as part of the noun. For example, instead of naming a
leucite - rich syenite a “ foid - bearing syenite ” , the rock would be
called a “ leucite - bearing syenite ” or a nepheline syenite.
• M = mafic and related minerals, including olivine, pyroxene,
amphiboles, micas, melilite, opaque minerals, garnet, epidote,
calcite, allanite, zircon, apatite, sphene and titanite (Streckeisen,
1976 ).

• The majority of igneous rocks found at the Earth’s
surface have at least 10% Q+ A+ P or F+ A+ P. Because
quartz is not compatible with feldspathoids, they will
never occur in equilibrium together in the same rock.
• If a rock to be classified has at least 10% of these
constituents, ignore M and normalize the remaining
three parameters to 100% (once again, by multiplying
each by 100/(Q+ P+ A) (or 100/(F+ P+ A)).
• From this we get Q = 100Q/(Q+ P+ A), and similarly for
P, A, and F (if appropriate), which sum to 100%.
• It may seem strange to ignore M, but this is the
procedure (unless M > 90%). As a result, a rock with
85% mafic minerals can have the same name as a rock
with 3% mafics, if the ratio of P:A:Q is the same.

• Determine whether the rock is phaneritic (plutonic) or
aphanitic (volcanic). If it is phaneritic, proceed to Figure 2. If
it is aphanitic, use Figure 3.
• To find in which field the rock belongs, first determine the
ratio 100P/(P + A).
• Select a point along the horizontal P–A line (across the
center of the diamond) on Figure 2a (or Figure 3) that
corresponds to this ratio.
• Next proceed a distance corresponding to Q or F directly
toward the appropriate apex. Because quartz and
feldspathoids can’t coexist, there should be no ambiguity as
to which triangular half of the diagram to select.
• The resulting point, representing the Q:A:P or F:A:P ratio,
should fall within one of the labeled subfields, which
provides a name for the rock.
• If P > 65 and Q < 20, see Figure 2 (for phaneritic rocks) or
Section 4 (for aphanitic rocks).

Phaneritic Rocks
IUGS plutonic rock classification
• Upon examining a phaneritic rock, we determine that it has
the following mode: 18% quartz, 32% plagioclase, 27%
orthoclase, 12% biotite, 8% hornblende, and 3% opaques
and other accessories.
• From this we get Q = 18, P = 32, and A = 27. Q+ P+ A = 77,
so, we multiply each by 100/77 to get the normalized values
Q = 23, P = 42, and A = 35 that now sum to 100.
• Because the felsic minerals total over 10%, Figure 2a is
appropriate.
• To determine in which field the rock plots, we must
calculate 100P/(P + A), which is 100(42/(42 + 35)) = 55.
• By counting along the P–A axis from A toward P in Figure 2a,
we find that it plots between the 35 and 65 lines. Then we
move upward directly toward point Q. Because 23 falls
between 20 and 60, the appropriate name for this rock is
granite.

Phaneritic Rocks
• A rock with 9% nepheline, 70% orthoclase, 2%
plagioclase, and the rest mafics and accessories
would be a nepheline syenite.
• The term foid is a general term for any
feldspathoid. Don’t use the term “foid” in a rock
name. Rather, substitute the name of the actual
feldspathoid itself.
• The same applies for alkali feldspar in the fields for
alkali feldspar granite and alkali feldspar syenite.
Use the true feldspar name, if you can determine it,
such as orthoclase granite.

Phaneritic Rocks
• Careful observation illustrates some problems with the
IUGS system.
• Near the plagioclase (P) corner, quartz diorite and
quartz gabbro occupy the same region; diorite, gabbro
and anorthosite also coexist in another location.
• Similarly, feldspathoid - bearing gabbro, diorite and
anorthosite rock names also coexist in the same foid
regions.
• How do we discriminate among these rocks?
• For anorthosite, the answer is straightforward as
anorthosites contain more than 90% plagioclase.
• Distinguishing between gabbro and diorite as well as
quartz gabbro and quartz diorite is a bit more complex.

Phaneritic Rocks
• Distinguishing between gabbro and diorite as well as quartz
gabbro and quartz diorite is a bit more complex.
• As indicated in Figure 2a , gabbro and diorite each contain <
5% quartz; while quartz gabbro and quartz diorite each
contain 5 – 20% quartz. For both of these sets of rocks, three
different factors distinguish quartz gabbro and gabbro from
quartz diorite and diorite:
1 Gabbros/quartz gabbros contain more than 35% mafic
minerals whereas diorites/ quartz diorites contain less than
35% mafic minerals.
2 Gabbros/quartz gabbros are more calcic with plagioclase
anorthite contents > 50.
Diorites/quartz diorites are more sodic with plagioclase
anorthite contents < 50.
3 Gabbros/quartz gabbros contain 45 – 52% SiO 2 and
diorites/quartz diorites contain 52 – 66% SiO 2 .

IUGS gabbroic rock classification
• Gabbros may be further segregated based upon the
modal mineral proportions of plagioclase (Plag), olivine
(Ol), pyroxene (Px) and hornblende (Hb) Figure 7.21 .
• The gabbros and norites coexist in the same region
within this triangle.
• The distinction between these rocks is based upon
whether the pyroxene minerals are orthopyroxenes
(norite) or clinopyroxenes (gabbro).
• Orthopyroxenes crystallize in the orthorhombic system
and include the minerals enstatite and hypersthene.
• Clinopyroxenes crystallizein the monoclinic system and
include the minerals augite and pigeonite.
• In these triangular diagrams, mineral abundances are
recalculated so that the sum of the three mineral
proportions equals 100%.

IUGS ultramafic rock classification
• the QAPF classification scheme is applied when felsic
minerals compose > 10% of the rock (Streckeisen, 1973
; LeBas and Streckeisen, 1991 ; LeMaitre, 2002 ).
• A different set of triangular rock discrimination plots
(Figure 7.22 ) isused for ultramafic plutonic rocks
containing > 90% dark - colored minerals.
• The ultramafic family includes peridotites ( > 40%
olivine), pyroxenites (pyroxene - rich rock with < 40%
olivine) and hornblendites (hornblende – rich rock with
< 40% olivine).
• The minerals are recalculated so that the sum of the
three mineral proportions of each triangular diagram
equals 100%.

IUGS ultramafic rock classification Figure 7.22

IUGS volcanic rock classification
• The IUGS rock classification system has also been
applied to QAPF volcanic rocks.
• Figure 7.23 presents a volcanic rock classification based
upon the relative abundances of mineral groups: quartz
(Q), alkali feldspars (A), plagioclase (P) and
feldspathoids (F).
• The IUGS system does not include separate diagrams for
mafic or ultramafic volcanic rocks.

Descriptive terminology based upon
chemical composition
• Earth ’ s crust include oxygen, silicon and aluminum
which bond together to form silica and aluminum
oxide.
• The most useful terminology for the chemical
composition of igneous rocks naturally involves these
three elements.
Abundance of silica
• As silicon and oxygen are the primary chemical
constituents in magma, the percentage of SiO 2 in rocks
is an important means by which we classify magma and
rocks.
• Two approaches have been developed as discussed
below.

Acidic versus basic classification
• On the basis of weight percent SiO 2 , four major
igneous rock types are defined: ultrabasic, basic,
intermediate and acidic

Acidic versus basic classification
• Generally, the color index terms ultramafic, mafic,
intermediate and felsic are considered loosely
equivalent to the SiO 2 content terms ultrabasic,
basic, intermediate and acidic, respectively.
• the terms ultramafic and mafic refer to rocks
containing > 90% and > 70% dark - colored
minerals, respectively.

Silica saturation classification
• Three normative minerals provide useful examples of
the concept of SiO 2 saturation:
• Normative orthoclase (KAlSi 3 O 8 ) is produced by
combining ½ K 2 O + ½ Al 2 O 3 + 3SiO 2 .
• Normative albite (NaAlSi 3 O 8 ) is created by
combining ½ Na 2 O + ½ Al 2 O 3 + 3SiO 2 .
• Normative enstatite (Mg 2 Si 2 O 6 ) is formed by
combining 2MgO + 2SiO 2 .

Silica oversaturation
• Silica oversaturation implies that all available cation
oxides have been used to make normative minerals and
additional SiO 2 remains available to generate
normative quartz.
• The excess SiO 2 is indicated by the presence of “ free
quartz ” .
• All quartz normative rocks are therefore
oversaturated with SiO2 .
• Most rocks with significant modal quartz are likely to
also be quartz normative and therefore oversaturated
with SiO 2 .
• Granitic rocks tend to be oversaturated with SiO 2 .

Silica saturation
• SiO 2 - saturated rocks contain normative feldspars and/or
orthopyroxene (enstatite or hypersthene) minerals,
• but lack either quartz – an indicator of SiO 2 oversaturation
– or magnesium olivine or feldspathoids – indicators of SiO 2
undersaturation.
• Silica undersaturation
• During normative calculations, SiO 2 is depleted before all
the other oxides have been used to form normative
minerals.
• In this case there may be insufficient SiO 2 to make quartz,
feldspars or orthopyroxenes. SiO 2 – undersaturated rocks
commonly contain feldspathoid or magnesium olivine
(forsterite) minerals that cannot coexist with quartz.

Relative abundance of aluminum oxide
• Aluminum oxide is the second most abundant
compound in Earth ’ s crust.
• Igneous rocks are classified based upon the relative
proportions of Al2O3 to CaO, Na 2 O and K 2 O.
• The relative proportion of these oxides yields the
following descriptive terms: peraluminous,
metaluminous, subaluminous and peralkaline (Table
7.8 ).
• The relative abundance of Al 2 O 3 as compared to CaO
+ Na 2 O + K 2 O largely determines the mineral
assemblages that develop in igneous rocks (Shand,
1951 ; Hyndman, 1985 ).
• This classification is particularly useful for the
discrimination of granitic rocks.

• Peraluminous rocks are characterized by minerals
with unusually high Al 2 O 3 contents.
• Al2O3 > (CaO + Na2O + K2O)
• In peraluminous. rocks we expect to find an Al2O3-
rich mineral present as a modal mineral - such as
muscovite [KAl3Si3O10(OH)2], corundum [Al2O3],
topaz [Al2SiO4(OH,F)2], or an Al2SiO5- mineral like
kyanite, andalusite, or sillimanite.

• Peralkaline rocks contain normative or modal minerals
with unusually high K 2 O and/or Na 2 O contents.
• rocks are those that are oversaturated with alkalies
(Na2O + K2O), and thus undersaturated with respect to
Al2O3. On a molecular basis, these rocks show:
• Al2O3 < (Na2O + K2O)
• Peralkaline rocks are distinguished by the presence of
Na-rich minerals like aegerine [NaFe+3Si2O6], riebeckite
[Na2Fe3
+2Fe2
+3Si8O22(OH)2], arfvedsonite
[Na3Fe4
+2(Al,Fe+3)Si8O22(OH)2 ], or aenigmatite
[Na2Fe5
+2TiO2Si6O18] in the mode.

• Metaluminous rocks contain mafic minerals with
average aluminum contents.
• rocks are those for which the molecular
percentages are as follows:
Al2O3 < (CaO + Na2O + K2O) and Al2O3 > (Na2O +
K2O)
These are the more common types of igneous
rocks. They are characterized by lack of an Al2O3-
rich mineral and lack of sodic pyroxenes and
amphiboles in the mode.

• Subaluminous rocks contain mafic minerals with
low aluminum concentrations.

• Alkaline/Subalkaline Rocks
• This classification scheme divides rocks that alkaline
from those that are subalkaline.
• This criteria is based solely on an alkali vs. silica diagram,
as shown below.
• Alkaline rocks should not be confused with peralkaline
rocks as discussed above.
• While most peralkaline rocks are also alkaline, alkaline
rocks are not necessarily peralkaline.
• On the other hand, very alkaline rocks, that is those that
plot well above the dividing line in the figure below, are
also usually silica undersaturated.

Alkaline/Subalkali
ne Rocks
• This classification
scheme divides
rocks that alkaline
from those that
are subalkaline.
• This criteria is
based solely on
an alkali vs. silica
diagram, as
shown below.

igneous rocks classification #6.pdf

More Related Content

What's hot

Similar to igneous rocks classification #6.pdf

More from AdnanKhan153072

Recently uploaded

In this document

igneous rocks classification #6.pdf