Igneous Rock Grain Size and Textures

TexturesTextures
Thanks to John Winter….Thanks to John Winter….

TexturesTextures
Igneous rocks obviously possess a wide range of grain size,Igneous rocks obviously possess a wide range of grain size,
from submicroscopic (0.001 mm for an optical microscope)from submicroscopic (0.001 mm for an optical microscope)
grains to the giant crystals of pegmatites, which can begrains to the giant crystals of pegmatites, which can be
several meters.several meters.
Some magmatic rocks have essentially no crystals at all andSome magmatic rocks have essentially no crystals at all and
are instead composed of an amorphous glass.are instead composed of an amorphous glass.
The most common phaneritic plutonic rock—granite—The most common phaneritic plutonic rock—granite—
generally has grains 1–20 mm whereas the most widespreadgenerally has grains 1–20 mm whereas the most widespread
aphanitic volcanic rock—basalt—has grains 0.1–1.0 mm.aphanitic volcanic rock—basalt—has grains 0.1–1.0 mm.

TexturesTextures
What kinetic process(es) permits such a wide range of grain size butWhat kinetic process(es) permits such a wide range of grain size but
commonly favours a more restricted range?commonly favours a more restricted range?
Rate of cooling does control grain size, as usually indicated inRate of cooling does control grain size, as usually indicated in
elementary geology texts, but is cooling rate the only factor?elementary geology texts, but is cooling rate the only factor?
If one were to examine thousands of all types of magmatic rocks aroundIf one were to examine thousands of all types of magmatic rocks around
the world, it would soon become apparent that some minerals, such asthe world, it would soon become apparent that some minerals, such as
magnetite and olivine, are invariably small, less than a few millimeters,magnetite and olivine, are invariably small, less than a few millimeters,
regardless of the magma in which they form.regardless of the magma in which they form.
Although phenocrysts of olivine, rarely to as much as 5 mm, occur inAlthough phenocrysts of olivine, rarely to as much as 5 mm, occur in
basalts, rocks having phenocrysts of magnetite visible to the naked eyebasalts, rocks having phenocrysts of magnetite visible to the naked eye
(1 mm) are virtually nonexistent.(1 mm) are virtually nonexistent.

TexturesTextures
Upward of 10% Fe-Ti oxides are common inUpward of 10% Fe-Ti oxides are common in
basalts and andesites, for example, but theybasalts and andesites, for example, but they
are invariably small groundmass grains.are invariably small groundmass grains.
Even in phaneritic rocks with centimeter-sizeEven in phaneritic rocks with centimeter-size
felsic and mafic silicate minerals, Fe-Tifelsic and mafic silicate minerals, Fe-Ti
oxides are generally much smaller.oxides are generally much smaller.

TexturesTextures
Why is this?
What factors allow plagioclases to form phenocrysts 1 cm or more
across in many volcanic rocks, and alkali feldspars to form
phenocrysts 5 cm across in some granites, and giant crystals meters
across in pegmatites?
Obviously, cooling rate alone cannot account for the difference in
sizes of different crystals growing in the same magma.
Answers to these questions depend on the interplay between
nucleation and growth rates for different mineral species in the melt as
intensive parameters change in the solidifying magma system.

TexturesTextures
The formation and growth of crystals, either from
melt or in a solid medium, involves 3 principal
processes:
1. Initial nucleation of the crystals-The initial formation of
minute embryonic crystals upon which further growth can occur
2. Subsequent crystal growth- Deposition of additional
crystalline material on existing nuclei and crystals
3. Diffusion of chemical species and heat through the
surrounding medium to and from the surface of
growing crystal.

NucleationNucleation
Cooling liquids seldom begin to precipitate crystals at the exact
temperature at which they become saturated with a particular mineral,
because 1st
two steps are impeded by free-energy barriers.
Unless crystals are already present to serve as nuclei for further growth, a
certain degree of supersaturation is required before stable nuclei can
form.
Here a few atoms assume the same relationship to one another as they
would have in a solid i.e. they form crystalline structure.
Nucleation is a critical initial step in the development of a crystal because
once a structure is developed, it is possible for individual crystals to
grow.
Very tiny initial crystals have a high ratio of surface area to volume and
thus, a large proportion of ions at the surface.

Surface ions have unbalanced charges because they lack the complete
surrounding lattice that balances the charge of interior ions resulting in
high surface energy and low stability.
The clustering of a few compatible ions in a cooling melt will thus tend
to spontaneously separate, even at the saturation temperature when
conditions are otherwise suitable for crystallization of a particular
mineral.
Under such conditions, crystallization would be possible but the
prerequisite nucleation is not.
Before crystallization begins, a critically sized “embryonic cluster” must
form.
This typically requires undercoolingundercooling..

The reason that small clusters of the new phase have difficulty in
surviving is related to the development of an interface between
the embryo and the medium in which it is growing.
Any surface between 2 phases involves an interfacial energy, the
magnitude of which depends on the degree of mismatch of
structures across the boundary.
Where a crystal is large, this surface energy constitutes only a
small fraction of the total free energy of the crystal and can be
ignored, but when the crystal is very small, the surface energy is
a significant fraction.

In homogeneous nucleation, nuclei develop and crystals grow
spontaneously within the melt.
Here new surface must be created.
Heterogeneous nucleation occurs more readily, as it involves nuclei
development on a preexisting surface and hence requires less energy.
Eg. Olivine crystals in a slowly cooling magma, may nucleate
homogenously, but the tourmaline crystals in a pegmatite clearly
nucleate heterogeneously on the roof and floor of pegmatite.
Nucleation is controlled by the composition of the melt, the structure
of the melt, the temperature of the melt and the cooling rate.

The structure of the melt is related to chemistry of the melt and to the
amount of time melt is maintained at higher temperatures.
If the melt structure retains remnants of crystals, crystal growth will be
much easier, as heterogeneous nucleation.
Similarly higher temperature tends to break down the structure of nuclei
remaining in the liquid.
With falling temperature and increasing saturation, the probability and
frequency of formation of ephemeral lattice structures increases, and the
rate of accretion of small nuclei exceeds the rate at which they break down
and return to the liquid.
Thus the probability increases that an individual nucleus will exceed the
critical size necessary for its survival.
It is usually in the range of 10-5
to 10-6
cm.

Because the rate at which the nuclei of critical size are formed is a
function of the properties of individual minerals, it will not ordinarily be
the same for 2 or more phases that are precipitating simultaneously from
the same liquid.
A given amount of undercooling and supersaturation produces differing
rates of nucleation and growth for each species being precipitated.
A mineral forming many stable nuclei grows simultaneously from
numerous growth centers while another mineral with only a few stable
nuclei must grow on a smaller number of individual crystals.
Thus, once popular notion that the largest crystals in a rock were
the first to grow is not necessarily valid.

It is common knowledge that crystals readily nucleate on any existing surface
in contact with a melt.
The existence of an interface with any contrasting material against the melt
can overcome the activation energy barrier so that hetereogeneous nucleation
may occur more readily for small ΔT than homogeneous nucleation.
Existing surfaces can be the solid walls of the melt container or wall rock in
the case of a natural magma body.
Existing “seed” crystals in the magma are especially significant in overcoming
the difficult nucleation step in crystallization.
Overgrowths on the seed are readily facilitated if that phase is stable in the
system.
Another mineral may also grow around the seed crystal; possible examples are
common biotite overgrowths around zircons.

Some existing crystals may be earlier-formed crystals.
Others may be foreign crystals, or xenocrysts, which may have been
removed by “erosion” of the wall rock during flow of the magma or
introduced into it by mixing with a compositionally contrasting magma.
Still other seeds may be restite crystals that are undissolved refractory
remnants of the source rock from which the magma was generated by
partial melting processes in the deep crust or mantle.
Minute crystalline entities, microscopically invisible, may serve as seeds
for crystal formation.
These might have survived an episode of brief melting above the liquidus
and could be of restite or xenocryst derivation in magmas extracted
rapidly from their source.

Nucleation-to-Crystal growthNucleation-to-Crystal growth
Once a stable nucleus has formed, it continues to grow and forms a
crystal whose size is determined by the concentration of nutrients in
the surroundings and the proximity of neighboring nuclei.
Crystal growth involves the addition of ions onto existing crystals or
crystal nuclei.
First, nutrients must diffuse to the nucleus through the medium in
which crystal is growing.
 In general case, the growth of a mineral will gradually deplete the adjacent
melt in the constituents that the mineral preferentially incorporates.
Second, nutrients, on arriving at the nucleus, may have to react and
arrange themselves into building units that are acceptable to the
crystal.

Third, building units must then attach themselves to the crystal
surface; this may involve nucleation of new surfaces or the growth of
dislocations.
Finally, attachment of the building units produces heat of
crystallization. It must be able to diffuse away from the crystal, or the
temperature at the growing surface may become too high for
crystallization to proceed.
The rate at which a crystal grows is determined by slowest of these
processes.
The cooling rate of magma must also be addressed. If the cooling rate is very slow, equilibrium
is maintained. If its very high, significant undercooling can result because there is seldom time
for nucleation, growth or diffusion to keep pace.
The cooling rate is an important externally controlled variable that influences the rates of other

The rates of both nucleation and crystal
growth are strongly dependent on
undercooling of the magma.
Initially undercooling enhances both rates,
but further cooling decreases kinetics and
increases viscosity, thus inhibiting the rates.
Why does rate of cooling so profoundly
affects the grain size of a rock?
Lets see.

Idealized rates of crystal nucleation and growth
as a function of temperature below the melting
point. Slow cooling results in only minor
undercooling (Ta
), so that rapid growth and
slow nucleation produce fewer coarse-grained
crystals. Rapid cooling permits more
undercooling (Tb
), so that slower growth and
rapid nucleation produce many fine-grained
crystals. Very rapid cooling involves little if any
nucleation or growth (Tc
) producing a glass.
•The maximum growth rate is generally at a
higher temperature than is the maximum
nucleation rate because it is easier to add an
atom with high kinetic energy onto an
existing crystal lattice than to have a chance
encounter of several such atoms at once to
form an embryonic cluster.

Further undercooling inhibits growth
because atoms have to diffuse farther to add
onto a few existing crystals, and it is easier
for the slowed atoms to nucleate in local
clusters than to move far.

Igneous TexturesIgneous Textures
•“undercooling” is the degree to which temperature falls
below the melting point before crystallization occurs.
•Eg. If the cooling rate is low, only slight undercooling will
be possible (Tα)
•At this temperature, the nucleation rate is very low, and
the growth rate is high.
•Fewer crystals thus form, and they grow larger, resulting
in the coarse-grained texture common among slow cooled
plutonic rocks.
•Quickly cooled rocks, on the other hand, may become significantly undercooled before
crystallization begins.
•If the rocks are undercooled to Tb, the nucleation rate exceeds the growth rate, and many
small crystals are formed, resulting in very fine grained texture of volcanic rocks.
•Very high degrees of undercooling, Tc, may result in negligible rates of nucleation and
growth, such that the liquid solidifies to a glass with very few or no crystals.

•Increasing undercooling
provides a stronger driving
force for growth, but with
falling T the increasing melt
viscosity retards ionic
mobility.
•For this reason, the growth
rate is a bell-shaped curve.

2-stage cooling can create bimodal distribution of grain sizes.
Slow cooling followed by rapid cooling is the only plausible
sequence is this regard.
Porphyritic texture results.
If the phenocrysts are set in a glassy groundmass, the texture is
called Vitrophyric.
If the phenocrysts contain numerous inclusions of another
mineral that they enveloped as they grew, the texture is called
poikilitic.
The host crystal may then be called as oikocryst.

The growth rate of a crystal depends upon the surface energy of the faces and the
diffusion rate.
For a constant cooling rate, the largest crystals will usually be those with the most
plentiful components.
The diffusion rate of a chemical species is faster at higher temperature and in a material
with low viscosity.
Diffusion rate is thus low in highly polymerized viscous melts.
Small ions with small charges diffuse best, whereas large polymerized complexes
diffuse poorly.
Water dramatically lowers the degree of polymerization of magma, thereby enhancing
diffusion.
The very coarse grain size of many pegmatites can be attributed more to the high
mobility of species in the water-rich melt from which they crystallize than to extremely
slow cooling.

The rates of nucleation and growth vary with the surface energy of the
minerals and the faces involved, the degree of undercooling and the
crystal structure.
Different minerals can be undercooled to differing extents because the melting
point is specific to each mineral.
The temperature may thus be lower than the melting point of one mineral
(undercooled) and higher than that of another.
Many stable nuclei of one mineral may thus form, while only a few of another
may form, resulting in many small crystals of the former and fewer, larger
crystals of the latter.
The popular notion that the large crystals in a porphyritic rock must have
formed 1st
or in a slower-cooling environment is not thus universally valid.

Diffusion controlled growthDiffusion controlled growth
If its diffusion controlled growth, then this is the slowest chain of steps involved in
the growth of that face.
The other steps still occur, but because they take place more rapidly they have to wait
for diffusion to bring in nutrients into the chain.
Then, diffusion is described as rate-determining process.
The distance a face advances, in a given time depends
on the flux of nutrients brought to that face by
diffusion.
When the crystal 1st
starts to grow the region
immediately in contact with the face becomes depleted
in nutrients, and a steep concentration gradient
develops.
With time, however, the gradient becomes
shallower, the flux of the nutrients brought to the
crystal face decreases, and the crystal grows more
slowly.

Surface Nucleation controlled growthSurface Nucleation controlled growth
Once nutrients are organized into acceptable building units,
they must attach themselves to the surface of the crystal.
Eg. On a perfectly planar surface a building unit could attach
itself only by starting a new surface layer.
Such attachment, however, satisfies only a few bonds and
actually increases the proportion of surface area to volume.
Steps on the surface present more favorable sites of
attachment and corners are still better.
As a result, precipitation on a
face is likely to fill in and
complete all irregularities before
a new surface layer will
nucleate.

Surface Nucleation controlled growthSurface Nucleation controlled growth
This is why crystals that are free to grow in a medium, such as
phenocrysts in a magma tend to be well developed by crystal faces;
euhedral.
Attachment of the first building block is dependent on surface area –
the larger the face, the more chance there is that an attachment will
occur somewhere on that face.
We can conclude that the growth
rate for surface nucleation-
controlled growth must be
proportional to the surface area of
the crystal.

Dissipation of heat of crystallization andDissipation of heat of crystallization and
impurities controlled growthimpurities controlled growth
When nutrients transfer from a melt onto crystal face, the latent heat of crystallization
causes the temperature of the melt on the surface to increase.
At the same time, components in the melt that do not enter into the crystal become
concentrated in the melt at the crystal face.
Both heat and material must therefore diffuse away from the crystal surface for the
growth to continue.
When this process determines the growth rate of a crystal, any irregularity that
protrudes beyond the general surface of crystal, will extend into cooler and more
supersaturated melt and will grow rapidly.
A zone of depleted liquid builds up at the crystal-liquid interface.
Crystals reach out in thin tendrils beyond the zone to tap a supply
of appropriate elements or cooler melt.

BSE image of quenched “blue glassy pahoehoe,” 1996 Kalapana
flow, Hawaii. Black minerals are felsic plagioclase and gray
ones are mafics. a. Large embayed olivine phenocryst with
smaller plagioclase laths and clusters of feathery augite
nucleating on plagioclase. Magnification ca. 400X. b. ca.
2000X magnification of feathery quenched augite crystals
nucleating on plagioclase (black) and growing in a dendritic
form outward. Augite nucleates on plagioclase rather than pre-
existing augite phenocrysts, perhaps due to local enrichment in
mafic components as plagioclase depletes the adjacent liquid in
Ca, Al, and Si.
OR the perturbations in the surface shape
towards dendritic helps to eliminate the local
heat buildup that accompanies
crystallization.
A smooth crystal face is therefore unstable.

Dissipation of heat of crystalization andDissipation of heat of crystalization and
Ultramafic lavas, such as Precambrian komatiites, when
quenched may develop spectacular elongated olivine crystals,
called spinifex texture.
The unusual size may be caused by rapid growth of the simple
olivine structure in a very low-viscosity magma, not by slow
cooling.

The maximum growth rates vary with mineral and
composition of the melt.
Growth rates of crystals in their own melt tend to be high
because building components do not have to travel through
the melt, and liquidus temperatures are high.
Growth rates of crystals in complex melts, such as magma,
are lower because of multicomponent diffusion through the
melt and generally lower liquidus temperatures.

Crystal morphology determined by rate-Crystal morphology determined by rate-
determining growth processdetermining growth process
Experiments by Lofgren (1974) and Kirkpatrick (1974) on plagioclase
and pyroxenes have shown that at small degrees of undercooling of the
melt large euhedral crystals are formed that resemble phenocrysts.
At greater degrees of undercooling, crystals tend to grow in skeletal form
and are more acicular, i.e. their length to breadth ratio increases.
At still greater degrees of undercooling, crystals have a branching or
dendritic form.
Finally at the greatest degrees of undercooling, the liquid passes the glass-
transition temperature and radiating crystalline fibres form a spherulitic
texture.

These changes are interpreted to result from a change in the rate-
determining step from surface nucleation-controlled growth at
small degrees of undercooling to growth controlled by dissipation
of impurities at higher degrees of undercooling.
At the lowest temperatures, growth rates are slow because of the low
diffusion rates.
As a result, the layer of liquid on the crystal face that is enriched in
those components not entering the crystal is thin, and small
wavelength perturbations are large enough to penetrate it.
This results in a crystal face advancing as a series of closely spaced
fibres.

At higher temperatures, and consequently smaller degrees of
undercooling, the thickness of the zone enriched in
components not entering the crystal increases.
Larger perturbations are hence necessary to penetrate this
layer, thus explaining the dendritic morphology and at still
higher temperatures, the skeletal forms.
At the smallest degrees of undercooling, where the surface
nucleation becomes the rate-determining process, diffusion
has sufficient time to dissipate unwanted material, and the
crystal faces grow as planar surfaces.

a. Volume of liquid (blue) available
to an edge or corner of a crystal is
greater than for a side. b. Volume of
liquid available to the narrow end of
a slender crystal is even greater.
After Shelley (1993). Igneous and
Metamorphic Rocks Under the
Microscope. © Chapman and Hall.
London.
Crystal corners and edges have larger volume of nearby liquid
to tap for components (or to dissipate heat of crystallization)
than do crystal faces.
In addition, corners and edges have higher proportions of
unsatisfied bonds.

a. Skeletal olivine phenocryst with rapid growth at edges enveloping melt at ends. Taupo, N.Z. b.
“Swallow-tail” plagioclase in trachyte, Remarkable Dike, N.Z. Length of both fields ca. 0.2 mm. From
Thus we might expect the corners and edges to grow more rapidly than the faces in such quench
situations.
When this occurs, the resulting forms are called skeletal crystals.
In some cases, the extended corners may meet to enclose melt pockets at the recessed faces.
The corners of quenched plagioclase tend to grow straighter, creating a characteristic swallow-
tailed shape.

All Phenocrysts are olivines and each shows a different shape in section, some are complex skeletal crystals and others are
relatively simple.

All the delicate, dendritic crystals in this photograph are olivines which formed during exceedingly rapid solidification
of the basalt melt, part of which became the yellow glass.

Fabric of Igneous rocksFabric of Igneous rocks
Fabric encompasses non-compositional properties of a rock that comprise
textures and generally larger-scale structures.
There is no sharp distinction between these two.
Textures, also called microstructures, are based on the proportions of glass
relative to mineral grains and their sizes, shapes, and mutual arrangements
that are observable on the scale of a hand sample or thin section under a
microscope.
Structures are larger-size features generally seen in an outcrop, such as
bedding in a pyroclastic deposit or pillows in a submarine lava flow.
Features related to exsolution of volatiles and fragmentation of magma can
occur on a wide range of scales.
One rock can have more than one texture and one or more structures.

Fabric of Igneous rocksFabric of Igneous rocks
Altogether, textures and structures constitute the rock fabric, for
which responsible multiple kinetic paths of formation can be
interpreted.

High magnification reveals that obsidian contains
abundantly nucleated sub-micrometer-size
crystallites that experienced limited growth in the
highly viscous glass.
FABRICS RELATED TO CRYSTALLIZATION PATH:FABRICS RELATED TO CRYSTALLIZATION PATH:
CRYSTALLINITY AND GRAIN SIZE –CRYSTALLINITY AND GRAIN SIZE – Glassy textureGlassy texture
Glass is basically a highly viscous liquid,
disordered on an atomic scale, formed from a
polymerized silicate melt that was cooled too
rapidly for crystallization to occur.

CRYSTALLINITY AND GRAIN SIZECRYSTALLINITY AND GRAIN SIZE
Glass in silicic lavas is not necessarily caused by very rapid
cooling because some obsidian flows are too thick for the
interiors to cool very quickly.
Motion and/or the characteristically slow diffusion and
nucleation of highly polymerized and viscous silicic flows
may impede crystallization and produce these highly glassy
rocks.

Though much less viscous, basaltic melt solidifies as a glass in drastically
undercooled margins of submarine lava pillows extruded on the seafloor
and in thin paho-ehoe lava flows on land.

All glass is metastable and therefore susceptible to secondary hydration,
devitrification, and other types of alteration that progress over time to
achieve a more stable state.
Devitrification is the secondary crystallization of glass to fine-grained
mineral aggregates.
more silicic glassy rocks commonly devitrify to produce a microgranular
mass of small, equidimensional grains of interlocking feldspar and silica
minerals called felsitic texture.
Devitrification of glass may also produce radial aggregates of crystals
called spherulites.

Spherulitic texture in high-silica rhyolite
obsidian. Spherulites are spherical to
ellipsoidal clusters of radiating fibrous alkali
feldspar and a polymorph of SiO2, here in a
black glassy matrix.
Individual spherulites in volcanic rocks can
range in diameter from less than 1 mm to 1 m
or so.
A phenocryst may be located at the center of
the spherulite, where, in the original glass or
drastically undercooled melt, it allowed
heterogeneous nucleation of crystals to occur.
Spherulites are secondary devitrification
features, not phenocrysts.

Hydration and devitrification tend to be simultaneous in basaltic
glass, producing an alteration product called palagonite.
Palagonite is a complex mixture of clay and zeolite minerals and
hydrated ferric oxides.
The minerals produced by devitrification are generally too fine
to identify under polarizing microscope, and x-ray analysis may
be required.

The variolitic texture of radiating plagioclase laths in some
basalts are probably the result of nucleation of later crystals
on the first nuclei to form.
It is a fan-like arrangement of divergent, often branching
fibres; usually the fibres are of plagioclase and the space
between is occupied by glass or granules of pyroxene, olivine
or iron oxides.
This texture differs from spherulitic in that no discrete
spherical bodies are identifiable; in fact each fan is seen in
thin section is a slice through a conical bundle of acicular
crystals.

Variolitic olivine dolerite: The olivine phenocrysts in this sample are set in a
groundmass consisting of many fans of diverging plagioclase needles with augite
crystals in the interstices. Progressive solidification is from lower right to upper left.

CRYSTALLINITY AND GRAIN SIZE –CRYSTALLINITY AND GRAIN SIZE – Aphanitic TextureAphanitic Texture
Aphanitic texture consists of a mosaic of crystals too small to be
identifiable by the naked eye.
Aphanitic texture implies high crystal nucleation rates relative to growth
rates, such as occur during rapid reduction in T or water content of the
magma system.
Relatively few aphanitic rocks are aphyric, or non-porphyritic.
The presence of phenocrysts in most aphanitic rocks testifies to the fact
that few magmas reaching near the surface of the Earth are superheated
above liquidus temperatures.

Grains in cryptocrystalline texture are too small to be resolved optically but are
visible with an electron microscope and can be identified by X-ray diffraction
analysis.
Larger grains in microcrystalline texture can be discerned with a petrographic
microscope.
Microcrystalline rocks in which elongate rectangular grains are random or non
aligned are called felty texture (also called pilotaxitic texture)
The groundmass crystals are called microlites (if they are large enough to be
birefringent) or crystallites (if they are not).
Microlites that are significantly larger than the groundmass, yet still microscopic, are
called microphenocrysts.
They are formed upon eruption and represent minerals with a higher ratio of growth
rate to nucleation rate than the finer groundmass phases.

Felty texture is common in the aphanitic
matrix of andesitic rocks and some basaltic
rocks; in both, the feldspar is plagioclase.
If feldspar microlites are oriented in a
common direction the texture is trachytic.

Trachytic texture in which microphenocrysts
of plagioclase are aligned due to flow. Note
flow around phenocryst (P). Trachyte,
Germany. Width 1 mm. From MacKenzie et
al. (1982). © John Winter and Prentice Hall.
Felty or pilotaxitic texture in which
the microphenocrysts are randomly
oriented. Basaltic andesite, Mt.
McLaughlin, OR. Width 7 mm. © John
Winter and Prentice Hall.

This rock illustrates trachytic texture with no glass between small, aligned alkali feldspars (pilotaxitic). Rather than there being a single universal
alignment direction, there are several domains in the photograph, each having its own preferred direction of feldspar alignment.

Hyalopilitic texture
in rhyolitic
pitchstone: The
feldspar microlites
in this glassy rock
have a preferred
elongation direction
from lower left to
upper right. Near
the feldspar
phenocrysts the
orientation of the
microlites follows
the outline of these
crystals.
There is tendancy
of microlites to be
arranged in bands.

Basalts crystallize readily because they are very hot and are dominated by minerals
with simple structures.
The common result is a texture with a dense network of elongate plagioclase
microphenocrysts and granular pyroxenes, with smaller magnetite crystals.
Glass may solidify as late interstitial material.
The amount of glass in basaltic rocks is genrally less than in more silicic volcanics,
but it can vary considerably, from virtually none to highly glassy when basaltic
lava comes in contact with water.
Ophitic texture refers to a dense network of lath-shaped plagioclase
micropheocrysts included in larger pyroxenes, with little or no associated glass.
Ophitic texture. A single pyroxene envelops several well-developed plagioclase laths. Width 1 mm. Skaergård intrusion,
E. Greenland. © John Winter and Prentice Hall.

2 large anhedral crystals of
augite enclose numerous,
randomly arranged lath-shaped
plagioclases. The larger augite
crystal has variable color due to
chemical zoning.

Ophitic texture is commonly interpreted to indicate that the clinopyroxene formed
later than plagioclase.
But even when a mineral is consistently included in another, it is not always
unequivocal evidence that the included phase ceased to crystallize before the
host crystallization began.
McBirney and Noyes (1979) noted in Skaergard intrusion of Greenland in which the
size of plagioclase inclusions increases steadily from the clinopyroxene core to the
rim.
This suggests that both crystallized simultaneously.
The Cpx nucleated less readily, so fewer crystals formed, and they grew more rapidly
and enveloped the more numerous and smaller plagioclases.
The later plagioclase grains that were included towards the host rims had longer time
to grow and were therefore larger.

This grades into subophitic (smaller pyroxenes that still partially
envelop the plagioclase) and then into intergranular texture in
which the plagioclase and pyroxene crystals are subequal in size
and glass is still relatively minor.
Intergranular texture grades into intersertal texture when
interstitial glass or glass alteration is a significant component.
When glass becomes sufficiently plentiful that it surrounds the
microlites or microphenocrysts, the texture is called hyalo-ophitic.
Hyalo-ophitic grades into hyalopilitic as the glass fraction becomes
dominant, and crystals occur as tiny microlites.

Anhedral
equant crystals
of pyroxene
occupy the
spaces between
the plagioclase
in this sample.
Intergranular
dolerite.

CRYSTALLINITY AND GRAIN SIZE –CRYSTALLINITY AND GRAIN SIZE – Phaneritic TexturePhaneritic Texture
Phaneritic texture occurs in rocks in which grains of major rock-forming
minerals are all large enough to be identifiable with the unaided eye.
Smaller Fe-Ti oxides and accessory minerals, such as zircon and apatite,
are typically not visible without a microscope.
Phaneritic rocks are typically found in magmatic intrusions and reflect
crystallization at small degrees of undercooling, perhaps only a few
degrees; nucleation rates are relatively low regardless of crystal growth
rates.
Magmatic intrusions worldwide of different compositions, sizes, magma
viscosities, and depths of emplacement (dictating cooling rates) all have a
restricted range of grain sizes, generally 1–20 mm.

This order-of-magnitude range suggests that nucleation and growth rates
are not significantly different in different magmas at small degrees of
undercooling.
Otherwise, more variable grain sizes might be anticipated.
Phaneritic rocks have an equigranular, texture if the grains are of
similar size.
Each crystalline phase must have experienced similar nucleation and
growth rates.
Other phaneritic rocks have an inequigranular texture in that they
contain grains of conspicuously variable size.
These include rocks of porphyritic texture and rocks having seriate
texture in which grains have a more or less continuously ranging size.

Many granites, possess seriate texture made up of apatites and zircons visible only under
the microscope, somewhat larger but generally <1-mm Fe-Ti oxides, larger mafic
silicates, commonly still larger plagioclase and quartz grains, and alkali feldspars that are
as much as 2–3 cm, or more.
The more or less continuous
variation in grain size
probably reflects differing
ease of nucleation among
the co-precipitating
minerals in the slightly
undercooled magma; Fe-Ti
oxides nucleate readily,
alkali feldspar nucleates at
the slowest rate, and the
other minerals in
intermediate manner.

CRYSTALLINITY AND GRAIN SIZE –CRYSTALLINITY AND GRAIN SIZE – Porphyritic TexturePorphyritic Texture
Some basalts contain two generations of phenocrysts (phenocrysts and
microphenocrysts) as well as groundmass, indicating two stages of
crystallization during ascent prior to eruption.
Microphenocrysts in basalts may exhibit intergranular texture, in which
equant olivine and pyroxene crystals fill spaces within a framework
formed by tabular plagioclase microphenocrysts.
Phenocrysts may aggregate into clusters (glomerocrysts) to form
glomeroporphyritic texture: according to Ikeda et al. (2002) ,
clustering is advantageous in energy terms because crystals enjoy lower
crystal - melt interfacial energy (the igneous analogue of a liquid’s
surface tension) in a cluster than when dispersed.

The basalt,
consisting of just
plagioclase, augite
and a small
proportion of
magnetite, shows a
range in sizes of
plagioclase and
augite crystals
from <0.01-0.5mm.

Small intrusive bodies and parts of larger ones
made of exceptionally large, but
heterogeneously sized, crystals whose
dimensions are at least several centimeters
and locally meters define pegmatitic fabric.
Outcrops are generally required to identify
this fabric, which must reflect limited
nucleation and fast crystal growth rates.
The photograph shows a quarry face in the Harding pegmatite,
Taos County, New Mexico; the 0.6m-long box in the lower left
corner of photo attests to the giant size of the white crystals of
spodumene (LiAlSi2O6).

Most aphanitic rocks, many glassy rocks, and some phaneritic
rocks contain large, more or less euhedral phenocrysts
embedded in a distinctly finer-grained or glassy matrix, or
groundmass. This is porphyritic texture.
Porphyritic aphanitic textures; porphyritic glassy, or
vitrophyric; and porphyritic phaneritic texture.
Phenocrysts rarely constitute more than 50% of aphanitic and
glassy rocks formed in extruded magma because abundant
crystals immobilize magma and retard extrusive flow.

Porphyritic textures originate in different ways: That is, they are
polygenetic.
Probably the most common origin for porphyritic aphanitic and
vitrophyric textures involves a two-stage cooling history for the melt.
An initial episode of slow cooling rate (small undercooling) yields few
nuclei just below liquidus temperatures in a thermally insulated
plutonic environment below the surface of the Earth.
These grow to produce relatively large phenocrysts. After this partial
crystallization, the magma experiences an episode of relatively rapid
heat loss in a small intrusion in the shallow cool crust or in an extrusion
onto the surface; both create the aphanitic or glassy matrix around the
phenocrysts.

A two-stage cooling path cannot account for deep plutonic
phaneritic rocks that must have cooled slowly at a rather uniform
rate but contain large phenocrysts of one mineral in a finer matrix
of others.
In this case, different nucleation rates for different minerals
may be involved.
Example. Euhedral alkali feldspar phenocrysts in porphyritic
phaneritic granodiorite.
It is now believed that they nucleate more slowly just above
solidus temperatures than other constituent minerals (Vernon,
1986).

Many granitic magmas, such as granodiorite, reach alkali
feldspar and quartz saturation within only a few degrees to tens
of degrees above solidus temperatures.
As much as half of the magma by volume may still be melt at
this stage because of the considerable solubility of alkali
feldspar and quartz.
Therefore, growing alkali feldspars have ample space to
produce a large crystal as they grow from sparse nuclei.
Experiments suggest that alkali feldspars nucleate more slowly
than quartz and plagioclase in granitic magmas
(Swanson,1977).

In both ophitic and poikilitic textures, larger crystals enclose smaller,
randomly oriented crystals.
The larger crystals form from fewer nuclei than the smaller enclosed
mineral grains.
In poikilitic texture, large oikocrysts completely surround many
smaller grains.
Poikilitic texture occurs in a wide range of rock compositions.
Eg, in phaneritic ultramafic rocks, oikocrysts of amphibole or pyroxene
many centimeters in diameter enclose millimeter-size olivines,
chromites, and other minerals
In some granitic rocks, near-solidus alkali feldspar oikocrysts surround
minerals precipitated at higher T.

Igneous inclusions should have formed at an earlier stage than the host
that enveloped them.
One must be aware that a thin-section is a 2-D slice through a 3-D
rock, and a mineral that may appear to be surrounded by another
could be jutting into it from above or below the plane of the section.
One should thus note whether a mineral is consistently included
throughout the section before concluding that it is truly an inclusion.
In the case of K-feldspar megacrysts in granitic rocks, they are
commonly poikilitic, and the numerous inclusions of other minerals in
them are taken as important indicators of their late formation,
overruling arguments for early formation based on grain size.

GRAIN SHAPEGRAIN SHAPE
As a rule early forming minerals in melts that are not significantly
undercooled are surrounded completely by liquid and develop as
euhedral crystals.
As more crystals begin to form and fill the magma chamber, crystals
will inevitably come in contact with one another.
The resulting mutual interference impedes the development of crystal
faces, and subhedral or anhedral crystals form.
Early minerals tend to have better forms, and the latest ones are
interstitial, filling the spaces between the earlier ones.
Figure 3.7. Euhedral early pyroxene with late interstitial plagioclase. Stillwater complex,
Montana. Field width 5 mm. © John Winter and Prentice Hall.

Phenocrysts in an aphanitic groundmass are typically euhedral and
thus clearly formed early in the sequence.
Some compositionally zoned minerals may show euhedral cores that
formed when the crystals were suspended in the melt and anhedral
rims that formed later when the crystals were crowded together.
Unfortunately, the simple principle that a crystal that molds itself to
conform to the shape of another must have crystallized later is not as
reliable as we might wish.
Whether or not a crystal grows with well-developed faces depends
largely upon the surface energy of the faces.
Eg. Minerals with low surface energy may form euhedral crystals
even in metamorphic rocks. Garnet and staurolite. Zircon and apatite.

Petrologists have noted a tendancy toward euhedralism that
diminishes in the order of increasing Si-O polymerization. Olivines
and pyroxenes thus tend to be more euhedral than feldspar and quartz.
Hunter (1987) demonstrated that, although crystals suspended in melt
tend to form euhedral grains, once they touch each other, they are
likely to dissolve at areas of high surface curvature and crystallize at
areas of low curvature, thus becoming more rounded.
Molding relationships thus develop after most, if not all, of the
minerals have begun to crystallize.
Except for minerals with very low surface energy, then, euhedral
crystals should be rare in cases of simultaneous crystallization.

Many aphanitic and glassy rocks contain clots, or polygranular
aggregates, commonly of the same minerals of the same size that
occur as isolated phenocrysts in the same rock.
The clots in this cumulophyric texture can originate in all of the
ways that phenocrysts can.
Clots originate as suspended crystals attach to each other, or they
may be derived from breakup of the more crystallized wall of the
magma chamber where precipitated crystals accumulated.
Alternatively, some clots may be restite material dislodged from
the site of magma generation.

In equigranular phaneritic rocks, exemplified by granite
aplites consisting of generally fine leucocratic aggregates
of alkali feldspar and quartz, virtually all grains are
equant and anhedral to subhedral.
This texture is appropriately known as aplitic.
It appears likely that all of the grains crystallized
essentially simultaneously from the melt and competed
equally for space.

Aplitic texture in a granite aplite dike
that intruded granodiorite.
The quartz and feldspar grains are of
subhedral to anhedral shape and of similar
size, so the rock is equigranular.
This texture likely results from similar
rates of nucleation and growth of the
felsic minerals, all of which were growing
more or less simultaneously.
In hand sample, the texture appears
sugary, like sandstone, but, unlike in
sedimentary rock, the grains are
somewhat interlocking and pore spaces or
secondary cement is nonexistent.

INHOMOGENEOUS GRAINSINHOMOGENEOUS GRAINS
In a system at equilibrium, every phase must be homogeneous,
including each mineral grain.
But in most crystallizing magmas, sluggish reaction rates between
melt and crystals lag behind rates of changing intensive
parameters.
Accordingly, many grains in magmatic rocks are inhomogeneous.
Several types of zoned and composite grains manifest states of
disequilibrium.
1. Zoned crystals
2. Reaction rims
3. Exsolution

INHOMOGENEOUS GRAINS – Zoned crystalsINHOMOGENEOUS GRAINS – Zoned crystals
A systematic pattern of chemical variation within a solid solution
mineral is called zoning.
It is a record of incomplete continuous reaction relations between a
melt and the crystallizing solid solution as intensive parameters were
changing in the magma system faster than kinetic rates could
maintain equilibrium.
Even in the most slowly cooled, hottest magmatic intrusions,
plagioclases are normally zoned from calcic cores to more sodic
rims, testifying to the very sluggish rates of diffusion of NaSi and
CaAl ions during crystallization.
Compositional reequilibration in plagioclase requires Si-Al
exchange, and this is difficult due to the strength of the Si-O and Al-
O bonds. Zoning in plagioclase is therefore very common.

Compositionally zoned hornblende
phenocryst with pronounced color variation
visible in plane-polarized light. Field width 1
mm. b. Zoned plagioclase. Andesite, Crater
Lake, OR. Field width 0.3 mm..

Reverse zoning is the opposite of normal zoning, with more
sodic inner and calcic outer zones.
It is common in some metamorphic plagioclase where growth
is accompanied by rising temperature.
Reverse zoning is rarely a long term trend in igneous
plagioclase; rather, it is typically a short term event where it
contributes to localized reversals as a component of oscillatory
zoning.
Oscillatory zoning, especially widespread in plagioclases of
intermediate composition magmatic rocks most likely
originates in the sluggish kinetics of crystal growth.

a. Repeated sharp reversals attributed to magma mixing, followed by normal cooling
increments.
b. Smaller and irregular oscillations caused by local disequilibrium crystallization.
c. Complex oscillations due to combinations of magma mixing and local disequilibrium.

Most minerals are not as conspicuously zoned as
plagioclase.
Most minerals apparently maintain equilibrium with melt
because ion exchange does not involve disruption of
strong Si-Al-O bonds.
Fe-Mg exchange is also easier because these elements
diffuse more readily than Al-Si.

INHOMOGENEOUS GRAINS – Reaction rimsINHOMOGENEOUS GRAINS – Reaction rims
Incomplete discontinuous reaction relations in fractionating
magmas are recorded in a reaction rim that surrounds an
anhedral, partially resorbed grain of another mineral.
In some systems, early crystals react with the melt as
crystallization proceeds.
Other reactions may result from dropping pressure as a
magma rapidly approaches the surface or from magma mixing
or other compositional changes.

Olivine mantled by orthopyroxene, produced at olivine-melt
interface
(a) plane-polarized light
Basaltic andesite, Mt. McLaughlin, Oregon.
Width ~ 5 mm.
© John Winter and Prentice Hall.
(b) crossed nicols: olivine is extinct and the
pyroxenes stand out clearly.

Hornblende phenocryst dehydrating to Fe-oxides plus pyroxene due to pressure
release upon eruption, andesite. Crater Lake, OR. Width 1 mm.

Resorption is the term applied to re-fusion or dissolution of a
mineral back into a melt or solution from which it formed.
Resorbed crystals commonly have rounded corners or are
embayed.
AugiteGlass
Corroded
quartz grain
In the figure, a xenocryst of quartz
incorporated into basaltic magma
reacted with the silica-undersaturated
melt, forming a reaction rim of stable
Cpx crystals.
Their needlelike habit probably
reflects rapid crystallization as the
hot melt was quenched around cool
xenocryst.

Partially resorbed and
embayed quartz
phenocryst in rhyolite.
Width 1 mm.

Dissolution of crystals occurs during mixing of dissimilar crystal-laden
magmas that are striving to reach a state of internal equilibrium and during
the evolution of magmas that contain unstable restite crystals and
assimilated xenocrysts from foreign country rock.
Partially dissolved crystals of different origins are evident in many
volcanic rocks and are preserved because the magma solidified more
rapidly than the crystal could completely dissolve; such is generally not the
case in more slowly cooled and more nearly equilibrated plutonic systems.
Unstable crystals are readily apparent from their embayed and corroded
forms and, in volcanic rocks, abundance of irregularly shaped melt
inclusions (now glass).
Dissolution of foreign material is called assimilation; it contaminates the
magma.

Partially resorbed quartz phenocryst
in silicic volcanic rock.
Photomicrograph under plane-
polarized light. Deep, irregularly
shaped embayments indicate crystal
was unstable in the melt prior to its
solidification into an aphanitic
groundmass. Irregularly shaped
apparent inclusions embedded in
quartz crystal may only be narrow
embayments extending from third
dimension.

Some have attributed sieve texture or
deep and irregular embayments to
advanced resorption, but others argue
that it is more likely to result from rapid
growth enveloping melt due to
undercooling.
Sieve texture in a cumulophyric cluster of plagioclase phenocrysts.
Note the later non-sieve rim on the cluster. Andesite, Mt.
McLoughlin, OR. Width 1 mm.

Rapakivi texture involves plagioclase overgrowths on orthoclase, occurs
in some granites where the plagioclase preferentially forms on the
structurally similar alkali feldspar rather than nucleating on its own.
John Winter describes it as “nucleation at preferred sites” as a reason for
its formation.
Epitaxis, is the general term used to describe the preferred nucleation of
one mineral on another preexisting mineral, thereby avoiding problems
associated with slow nucleation.
Similarity of the crystal structures of the mineral substrate and the
new phase is a prerequisite for epitaxial growth.
Stimac and Wark (1992) believe these textures can originate during
mixing of magmas.

Rapakivi Texture, Polished hand specimen

Rapakivi feldspar in dacite,
Clear Lake, California.
Photomicrograph in CPL
showing sanidine surrounded
by oligoclase plagioclase.
Stimac and Wark (1992)
believe these mantled
feldspars were produced by
mixing of a sanidine-bearing
rhyolite magma with basaltic
andesite magma.

In Corona Texture, a crystal of one mineral is surrounded by rim of one or more crystals of
another mineral, eg. Olivine surrounded buy orthopyroxene, or biotite surrounding hornblende.
Such relationships are often presumed to result from incomplete reaction of the inner material
with the melt or fluid to produce the outer one.

INHOMOGENEOUS GRAINS –INHOMOGENEOUS GRAINS – ExsolutionExsolution
Exsolution involves chemical mixing that becomes increasingly limited
in some solid-solution minerals as they cool.
The most common example is with Na-rich and K-rich feldspars.
Because the unmixing in this case involves partitioning only of K and
Na ions, and not strongly bonded Si and Al, it takes place relatively
easily, and the segregations appear as a coherent intergrowth of long,
wispy lamelle.
When the alkali feldspar is potassic, the result is exsolved albite lamelle
in a K-feldspar host, called perthite.
The other variety is known as antiperthite.

INHOMOGENEOUS GRAINS –INHOMOGENEOUS GRAINS – ExsolutionExsolution
Exsolution also occurs in pyroxenes.
A low-Ca orthopyroxene may separate out
from high-Ca clinopyroxene. Pigeonite, an
intermediate mixture, is found principally in
volcanic rocks that cooled too quickly to
allow such unmixing to occur.

FABRICS RELATED TO TEXTURAL EQUILABRATION:FABRICS RELATED TO TEXTURAL EQUILABRATION:
SECONDARY GRAIN-BOUNDARY MODIFICATIONSECONDARY GRAIN-BOUNDARY MODIFICATION
In slowly cooled magma systems, textural equilibration may modify
grain size and shape after the initial episode of crystallization.
In some shallow, water-rich granitic systems, a single alkali-feldspar
might form.
If water is suddenly lost, the melting point will rise quickly, resulting
in undercooling (even at constant temperature) and rapid simultaneous
crystallization of the alkali feldspar and quartz.
Under these conditions, the 2 minerals do not have time to form
individual crystals but rather from an intergrowth of intricate shapes
referred to as granophyric texture.
A coarser variation of granophyric is known as graphic.

Graphic texture: a single crystal of
cuneiform quartz (darker)
intergrown with alkali feldspar
(lighter). Laramie Range, WY.
Granophyric quartz-alkali feldspar
intergrowth at the margin of a 1-cm dike.
Golden Horn granite, WA. Width 1mm.

Graphic texture in hand sample of graphic granite. The scattered quartz grains in a
single-crystal host of alkali feldspar resemble poikilitic texture. However, the quartz
grains are more or less uniformly spaced and are not randomly oriented. They are all
crystallographically continuous, as may be verified by their optical continuity viewed in
thin section under cross-polarized light, and grew simultaneously with the alkali
feldspar as an intergrowth.

FABRICS RELATED TO TEXTURAL EQUILABRATION:FABRICS RELATED TO TEXTURAL EQUILABRATION:
SECONDARY GRAIN-BOUNDARY MODIFICATIONSECONDARY GRAIN-BOUNDARY MODIFICATION
Symplectite is a term applied to fine-grained intergrowths resulting from the
combined growth of 2 or more minerals.
Myrmekite is an intergrowth of dendritic quartz in a single crystal of
plagioclase.
The quartz appears rod-like in thin section, and numerous adjacent rods go
extinct in unison, indicating that they are all parts of single quartz crystal.
Myrmekites are very common in granitic rocks and occur preferentially
where plagioclase is in contact with K-feldspar.
Myrmekites appear to have grown from the plagioclase-K-feldspar
boundary into the K-feldspar.
As the plagioclase replaces the K-feldspar, SiO2 is released (the
anorthite component of plagioclase contains less SiO2 than feldspar),
thereby producing the quartz.

Myrmekite formed in plagioclase at the boundary with K-feldspar.

FABRICS RELATED TO NONEXPLOSIVE EXSOLUTIONFABRICS RELATED TO NONEXPLOSIVE EXSOLUTION
OF VOLATILE FLUIDSOF VOLATILE FLUIDS
This section deals with fabrics produced by volatile exsolution from a melt
that did not result in fragmentation of the magma.
Volatile-fluid bubbles become vesicles as magma solidifies. The
corresponding texture is vesicular.
The surface tension makes bubbles spherical—the shape of least surface
area relative to volume.
Movement of either the bubble in the melt or the melt containing the bubble
can distort this equilibrium shape.
Elongate pipe vesicles are one result.
Bubbles tend to rise in less viscous basaltic magmas and thus concentrate
near the surface of basaltic flows.
There is complete gradation from basalt to vesicular basalt to scoria, with
increasing vesicle content.

FABRICS RELATED TO NONEXPLOSIVE EXSOLUTIONFABRICS RELATED TO NONEXPLOSIVE EXSOLUTION
OF VOLATILE FLUIDSOF VOLATILE FLUIDS
Highly vesicular silicic glass, or pumice, has a pumiceous texture.
Scoria and scoriaceous are parallel terms for andesite and basalt.
Vesicles can be larger in basaltic lavas (to as much as 10 m or so)
than in silicic, not necessarily because they contain more dissolved
volatiles, but rather because basaltic melts have a less viscous
nature, which allows the bubbles to expand and coalesce before the
magma solidifies.
Some smooth-walled vesicles may be filled with secondary
minerals precipitated from fluid solutions percolating through the
rock, producing amygdules and amygdaloidal texture.

Amygdaloidal texture in
basalt. Vesicles filled with
secondary minerals that
precipitated from percolating
aqueous solutions are
amygdules. Filling minerals
may be carbonate or zeolite
minerals, or some form of
silica, such as quartz,
chalcedony, or opal.
Amygdules are not
phenocrysts.

PYROCLASTIC FABRICS RELATED TOPYROCLASTIC FABRICS RELATED TO
FRAGMENTATION OF MAGMA AND LATERFRAGMENTATION OF MAGMA AND LATER
SOLIDIFICATIONSOLIDIFICATION
Pyroclastic rocks are fragmental, generally produced by explosive
volcanic activity.
The classification of pyroclastic rocks is based on the nature of
fragments (discussed in classification).
The ash component of pyroclasts is typically a mixture of pulverized
rock and primary glass.
The vesicles in pumice expand rapidly upon explosive eruption and
are usually destroyed.
The interstitial glass then forms 3-pointed shards in thin section.
Because these shards are commonly warm in a pyroclastic flow, they
deform in a ductile fashion and fold over.

The interstitial liquid (red) between bubbles in pumice (left) become 3-pointed-star-shaped
glass shards in ash containing pulverized pumice. If they are sufficiently warm (when
pulverized or after accumulation of the ash) the shards may deform and fold to contorted
shapes, as seen on the right and b. in the photomicrograph of the Rattlesnake ignimbrite, SE
Oregon.

This type of bending and other structures caused by compression and
deformation resulting form settling in hot ash accumulations, are
collectively referred to as eutaxitic textures.
Larger pieces of pumice may accumulate intact and have the gas
squeezed from them, eliminating the bubbles.
If all the gas is expelled, the pumice returns to the black color of
obsidian, and the squashed fragments are called fiamme.
The reason for light gray color of pumice as against black color of
obsidian is that the bubbles expand the glass to a thin film between the
bubbles, which refracts and diffuses light, just as breaking waves form
whitecaps on otherwise dark seawater.

Eutaxitic fabric in compacted welded lapilli tuff, showing distinctive eutaxitic
fabric that is defined by collapsed and flattened pumice lapilli, which have
distinctive frayed, flamelike terminations, hence the designation fiamme. The lighter-
colored matrix between the lapilli is composed of compacted welded ash.

In fluid lavas, such as basalts, bursting bubbles hurl fine spray
aloft, and it falls as glassy pellets called “Pele’s tears” or the
magma may be stretched to form delicate glass threads “Pele’s
hair”.

Pele's hair on a pahoehoe flow at Kīlauea Volcano, Hawaii, March 27, 1984

Ash falling through very moist air may accumulate successive layers on a
single ash nucleus, forming spheroidal balls called accretionary lapilli.
Consolidated deposits of such lapilli are called pisolitic tuffs.

IGNEOUS CUMULATESIGNEOUS CUMULATES
Their development will be studied later in conjunction with layered
mafic intrusions.
Cumulate textures is a hallmark of these fascinating bodies.
For the moment, we will avoid the complex question of how the
crystals accumulate.
In the ideal case, early-forming crystals of a single mineral accumulate
(somehow) to the extent that they are in mutual contact, with the
remaining liquid occupying the interstitial spaces between the crystals.
The principal type of cumulates are distinguished on the basis of the
extent to which the early-formed crystals, once accumulated, grow
prior to ultimate solidification of the interstitial liquid.

Development of cumulate textures. a. Crystals accumulate by crystal settling or simply form in
place near the margins of the magma chamber. In this case plagioclase crystals (white) accumulate in
mutual contact, and an intercumulus liquid (pink) fills the interstices.

It would be unusual if the interstitial liquid had the same composition
as the accumulated crystals because most magmas are chemically
more complex than any single mineral.
So if the liquid crystallizes essentially in-place, without exchange
with larger magma reservoir in the interior of the chamber, it
should produce some of the initial mineral plus any other minerals
that together constitute the interstitial magma.
There may thus be some modest additional growth of the early
minerals, together with formation of the other, later-forming minerals
in the interstitial spaces.
The result is orthocumulate texture.

If the interstitial liquid can escape and exchange material (via diffusion and/or
convection) with the liquid of the main chamber, the early-forming cumulate
minerals may continue to grow as rejected components as the interstitial liquid
escape.
The result is adcumulate texture : a nearly monomineralic cumulate with
perhaps a few other minerals caught in the last interstitial points.

If the later minerals have a low nucleation rate, they may envelop the
cumulus grains, resulting in poikilitic texture.
But here the host oikocryst may be so large and interstitial in some
instances that it may be difficult to recognize it as such in a small area of
thin section.
A large oikocryst also requires the exchange between the interstitial liquid
and the main magma reservoir in order to provide enough of its
components and dispose off excess components that would lead to the
formation of other minerals.
It is thus considered to be a type of adcumulate phenomenon and is
termed as heteradcumulate.
Finally mesocumulate is a term applied to cumulate textures that are
intermediate between ortho- and adcumulates.

SECONDARY TEXTURES : POSTMAGMATIC CHANGESSECONDARY TEXTURES : POSTMAGMATIC CHANGES
These are the textures that develop after the rock has been solidified.
These processes do not involve melt and are thus really metamorphic
in nature.
The process of crystallization does not necessarily cease when the
magma becomes solid.
As long as the temperature is high enough, recrystallization and both
chemical and textural equilibration take place.
Large cooling plutons may remain at temperatures equivalent to high-
grade metamorphism for thousands of years, so there is ample
opportunity for such processes to occur.
Solid state processes that occur as a result of igneous heat (even
though waning) are called autometamorphic.

Ostwald ripening is a process of annealing of crystals in a static
environment.
Differences in grain-boundary curvature drive grain growth by
Ostwald ripening until straight boundaries result.
In such recrystallization, grain boundaries migrate toward their centres
of curvature.
Small grains with convex outward curvature are thus eliminated as the
surfaces of neighboring larger grains with convex inward curvature
encroach upon them.
If the process attains textural equilibrium in a solid, there will be
similarly sized grains having straight, approximately 1200
triple-grain
intersections.

“Ostwald ripening” in a monomineralic material. Grain boundaries with significant negative
curvature (concave inward) migrate toward their center of curvature, thus eliminating smaller
grains and establishing a uniformly coarse-grained equilibrium texture with 120o
grain
intersections (polygonal mosaic).

This equilibrium texture is most common in monomineralic metamorphic
rocks (quartzite and marble), particularly if metamorphosed in a nearly
static stress regime.
Most igneous rocks are not monomineralic, however, and rarely attain a
good equilibrium texture.
Relative differences in surface energy of contrasting mineral types and
the coarse grain size of plutonics serve to establish and maintain
interlocking textures in most cases.
Ostwald ripening however, may eliminate smaller grains in favor of larger
neighbours at an early stage of growth, producing a more uniform
distribution of grain sizes.

SECONDARY TEXTURES : SECONDARY REACTIONSSECONDARY TEXTURES : SECONDARY REACTIONS
Most autometamorphic reactions involve minerals at moderate
temperatures in an environment in which H2O is either liberated from
residual melt or externally introduced.
Such alterations that involve hydration are called deuteric alterations.
Uralitization is the deuteric alteration of pyroxene to amphibole.
Any gradation of amphibole rims on pyroxene cores to multiple
patches of pyroxene in an amphibole to complete replacement is
possible.
The amphibole may be a single crystal of hornblende or a fibrous
actinolite or hornblende aggregate.
Either, when demonstrated to result from pyroxene alteration, may be
called uralite, but the term is commonly applied to aggregates.

Biotitization is a similar process of hydration that produces biotite,
either directly from pyroxene, or more commonly, from hornblende.
Because biotite contains little Ca, epidote may be produced as Ca is
released during the alteration of hornblende to biotite.
Chloritization is the alteration of any mafic mineral to chlorite.
Pyroxenes, hornblendes and biotite are commonly observed in thin
section in various stages of alteration to chlorite.
In case of biotite, water may work its way along the prominent
cleavages, and chlorite can be seen to replace biotite margins as well as
along cleavage planes.

a. Pyroxene largely replaced by hornblende.
Some pyroxene remains as light areas (Pyx) in
the hornblende core. Width 1 mm. b. Chlorite
(green) replaces biotite (dark brown) at the rim
and along cleavages. Tonalite. San Diego, CA.
Width 0.3 mm. Pyx
Hbl
Bt
Chl

Seritization is the process by which felsic minerals are hydrated to
produce sericite.
Alkali feldspar gets more seritized than associated plagioclase because of
availability of K+
ions.
Saussuritization is the alteration of plagioclase to produce epidote
mineral.
Higher temperature plagioclase tends to be more Ca rich which is less
stable than its sodic counterpart at low temperatures.
The Ca-rich types thus break down to more nearly pure albite, releasing
Ca and Al to form an epidote mineral.
Olivine gets altered to serpetinite or dark brown iddingsite.

a. Carlsbad twin in orthoclase. Wispy
perthitic exsolution is also evident.
Granite, St. Cloud MN. Field widths ~1
mm. © John Winter and Prentice Hall.
b. Very straight multiple albite twins in
plagioclase, set in felsitic groundmass.
Rhyolite, Chaffee, CO. Field widths ~1
mm. © John Winter and Prentice Hall.

Figure 3.19. Polysynthetic deformation twins in plagioclase. Note how they concentrate in
areas of deformation, such as at the maximum curvature of the bent cleavages, and taper away
toward undeformed areas. Gabbro, Wollaston, Ontario. Width 1 mm. © John Winter and
Prentice Hall.

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