Ch. 3: Textures
Some Useful Definitions

The Surface Energy is the excess energy at the
surface of a material (unbalanced ionic charges,
etc.) compared to the interior.

Surface energy is measured in the laboratory as
the energy (work = force x distance) consumed
while cutting a new surface in the plane of
interest.
Some Useful Definitions (cont.)

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Instability: the spontaneous separation of small
clusters of compatible ions due to high surface
charge. This inhibits nucleation.
Nucleation: forming a critical-sized embryonic
crystal, a crystal that is large enough to have
sufficient balanced interior volume, so that it is
stable and won’t fly apart due to like electrostatic
charges.
Undercooling: cooling of a melt below the true
crystallization temperature of a mineral.
Twin: an intergrowth of two or more orientations
of the same mineral that share common atoms,
typically on a plane.
Nucleation
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
Requires either undercooling or
A seed crystal of same or similar structure mineral

Observations

Simple structures nucleate more easily
Oxides easier > Olivines > Pyrox. > Plag. > K-spars

So oxides small & numerous, K-spars few

Diffusion

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For growth to proceed, constituents must
diffuse through the melt, cross the depleted
zone, and reach the crystal surface.
Crystal formation produces heat
“Latent heat of crystallization”
This heat must diffuse away, or the
temperature (KE) may become too high for
crystallization to proceed.
T << <Tmelt
no xtals
glass
T <<Tmelt
Many xtals
but don’t
grow large
T ~ <Tmelt
Few xtals
but grow
large
Melt
Glass
Aphanitic
Phaneritic
Observations



If the cooling rate is very slow, Equilibrium
is maintained or closely approximated.
Initially, undercooling enhances growth and
nucleation
However, further cooling decreases Kinetic
Energy (KE) and increases viscosity,
slowing diffusion and stopping growth.
Porphyritic

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


Rock with distinct difference in xtal sizes.
Igneous: slow cooling in magma chamber,
fast cooling near/at surface.
Phenocrysts
In glassy matrix/groundmass vitrophyric
If phenocrysts contain numerous inclusions
poikilitic.
Large Crystal Growth



Largest xtals have
The most plentiful components in adjacent melt
The fastest diffusing components
Faster at higher temps MAFIC
 Faster in material of low viscosity MAFIC
 Slower in highly polymerized viscous melts FELSIC
 Diffusion fastest in fluid > glass > solid

Role of Water and Pegmatites


H2O dramatically reduces the
polymerization of magma. Melt is less
viscous.
Large xtal size in Pegmatites may be
attributed more to high diffusion of
components through the H2O-rich melt,
rather than just slow cooling.
Different Melting Pts



Different minerals have different melting points.
At some To, some forming slightly undercooled, few
nucleii but fast growth, others greatly undercooled
many nucleii but slow growth.
“ The popular notion that the large xtals in a
porphyritic rockk must have formed “first” is not
necessarily true.
Melting Point Depression

Adding additional phases lowers the
melting/freezing temperature.

Removing a phase raises the
melting/freezing temperature
Dewatering

Sudden loss of the water-rich fluid phase
will quickly raise the melting point, making
the forming minerals significantly more
undercooled at the current Temperature.
This can move mineral species to form
many new nuclei, with low growth,
producing many aphanitic crystals. This can
produce a porphyritic texture in plutonic
rocks if phenocrysts were already present.
Crystal Growth





Addition of more ions onto existing face
Observations:
Bravais: planes with a high density of lattice
points form a more prominent face
Fast growing faces have smaller interplanar
distances
Faces with low surface energy become
more prevalent
Igneous Textures
A mineral will deplete the adjacent melt of its constituents
Figure 3-3. a. Volume of liquid
(green) available to a corner or
edge 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.
a
Expect the narrow ends growth > corner > edge > side
And erosion the opposite
b
Igneous Textures
Figure 3-4. a. Skeletal olivine phenocryst with rapid growth at corner enveloping melt
at sides. Taupo, N.Z. From Shelley (1993). Igneous and Metamorphic Rocks Under
the Microscope. © Chapman and Hall. London.
Epitaxis
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

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Preferred nucleation of one mineral on another
pre-existing mineral.
Ex. 1: Sillimanite on mica rather than on its
polymorph Kyanite in equilibrium
Si-Al-O structures more similar in the micas.
Ex. 2: Plagioclase overgrows K-spar
Orthoclase rather than nucleate on its own.
Fo Mg++ 1900C Fa Fe++ 1500C
Bowen’s Reaction Series
Molten- VERY Hot
No solids
1900 oC
First mineral to crystallize out
Independent Tetrahedra
1553 oC
3-D
Single
chains
Double
chains
“Basaltic”
sheets
“Andesitic”
3-D
3-D
Molten- Not so hot
100% Solid
sheets
3-D
“granitic”
Compositional
Zoning
Figure 3-5. a. Compositionally
zoned hornblende phenocryst with
pronounced color variation visible
in plane-polarized light. Field
width 1 mm. b. Zoned plagioclase
twinned on the Carlsbad law.
(twins 180 on b) Andesite, Crater
Lake, OR. Field width 0.3 mm. ©
John Winter and Prentice Hall.
Take home lesson: if you leave a
crystal exposed to the melt, the melt
will react with it, sometimes
scavenging parts. Olivine examples
seen in Basalts
If a new mineral is stable at new
conditions, the old mineral will be
covered with the new. This example
of Plagioclase in a cooling melt.
Compositional Zoning in Plagioclase

Anorthite

CaAl2Si2O8
CaAl (AlSi2O8 )
Albite
NaAlSi3O8
NaSi (AlSi2O8 )

Plagioclase does not re-equilibrate
with the melt when the melt
changes composition, would
require substitution of Al for Si in
one position. This is difficult
because:
Al-O and Si-O bonds are very
strong
Al+3 is a very slow diffuser
Oscillatory Zoning
Ca++ decreases locally so less Anorthite, mixing restores Ca++
Figure 3-6. Examples of plagioclase zoning profiles determined by microprobe point traverses.
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. From Shelley (1993). Igneous and Metamorphic Rocks Under the
Microscope. © Chapman and Hall. London.
Crystallization Sequence – experiment: early euhedral Px,
later interstitial Plag. Suggests euhedral Px formed first
Figure 3-7. Euhedral early pyroxene with late interstitial plagioclase (horizontal twins). Stillwater
complex, Montana. Field width 5 mm. © John Winter and Prentice Hall.
Figure 3-8. Ophitic texture. A single Clinopyroxene envelops several well-developed
plagioclase laths. Width 1 mm. Skaergård intrusion, E. Greenland. © John Winter and
Prentice Hall. Discuss Diabase definition.
<= Granophyric In a rapid reaction where
two minerals must form simultaneously (here
alkali feldspar and quartz in low H2O magma)
an intergrowth occurs, not large euhedral
crystals.
b. Graphic Can occur when two solid
phases (mineral species) form (separate,
exsolve, solidify , freeze, open-system
fractional crystallization) at the same time.
Recall Microcline + Albite (Perthitic Texture)
from homogeneous (K,Na)AlSi3O8
Figure 3-9. a. Granophyric quartz-alkali feldspar intergrowth at the margin of a 1-cm
dike. Golden Horn granite, WA. Width 1mm. b. Graphic texture: a single crystal of
cuneiform quartz (darker) intergrown with alkali feldspar (lighter). Laramie Range, WY.
© John Winter and Prentice Hall.
Note the erosion of the Olivine.
Is Olivine stable at the
conditions suitable for Pyroxene
crystallization?
Figure 3-10. Olivine mantled by orthopyroxene in (a) plane-polarized light and (b)
crossed nicols, in which olivine is extinct and the pyroxenes stand out clearly. Basaltic
andesite, Mt. McLaughlin, Oregon. Width ~ 5 mm. © John Winter and Prentice Hall.
Figure 3-11. b. Resorbed and embayed olivine phenocryst. Width 0.3 mm.
Winter and Prentice Hall.
© John
When a hydrous magma reaches the surface, decompression releases volatiles,
and hydrous minerals such as hornblende and biotite may develop rims of fine
iron oxides and pyroxenes.
Figure 3-11. c. Hornblende phenocryst dehydrating to Fe-oxides plus pyroxene due to
pressure release upon eruption. Andesite, Crater Lake, OR. Width 1 mm. © John Winter
and Prentice Hall.
Figure 3-12. a. 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.
Figure 3-12. b. Felty or pilotaxitic texture
in which the microphenocrysts are
randomly oriented. Basaltic andesite, Mt.
McLaughlin, OR. Width 7 mm. © John
Winter and Prentice Hall.
Figure 3-13. Flow banding in andesite. Mt.
Rainier, WA. © John Winter and Prentice
Hall.
Caused by mingling of two magmatic fluids
Figure 3-15. Intergranular texture in basalt.
Columbia River Basalt Group, Washington.
Width 1 mm. © John Winter and Prentice
Hall.
Intergranular Plagioclase and Pyroxene
crystals are similar in size.
Minerals accumulate by sinking or floating or by being plastered (by convective
flows)to the magma chamber, or they form in place as the melt flows by with parts.
Figure 3-14. 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. b. Orthocumulate: intercumulus liquid
crystallizes to form additional plagioclase rims plus other phases in the interstitial volume (colored). There is
little or no exchange between the intercumulus liquid and the main chamber. After Wager and Brown
(1967), Layered Igneous Rocks. © Freeman. San Francisco.
Orthocumulate, insterstitial liquid solidifies in place, without exchanging ions with the larger Magma chamber.
Example, Palisades Olivine layer
Orthocumulate Texture
Orthocumulate texture - Harzburgite with cumulate olivine
and chromite and intercumulus plagioclase. Stillwater Layered
Intrusion. Width = 8 mm. Image © T. E. Bunch, 2007.
Figure 3-14. Development of cumulate textures. c. Adcumulates: open-system exchange between the
intercumulus liquid and the main chamber (plus compaction of the cumulate pile) allows components that
would otherwise create additional intercumulus minerals to escape, and plagioclase fills most of the
available space. d. Heteradcumulate: intercumulus liquid crystallizes to additional plagioclase rims, plus
other large minerals (hatched and shaded) that nucleate poorly and poikilitically envelop the plagioclases. .
After Wager and Brown (1967), Layered Igneous Rocks. © Freeman. San Francisco.
Poikilitic: a mineral contains randomly oriented crystals of another mineral.
Poikilitic Texture
Low int.color, 2 cleavages ~ 90o, so Fledspar.
Note no Albite twins, so K-spar. No Tartan twins so
probably Sanadine or Orthoclase
“In this photomicrograph, euhedral to subhedral biotite and
plagioclase crystals are surrounded by optically-continuous, graycolored K-feldspar.”
Figure 3-16. a. 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. Width 1 mm. © John Winter.
Figure 3-18. a. Carlsbad twin in
orthoclase. Wispy perthitic exsolution
is also evident. Granite, St. Cloud MN.
Field widths ~1 mm. © John Winter
and Prentice Hall.
Figure 3-18. 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 3-18. (c-d) Tartan twins in
microcline. 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.
Figure 3-20. 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. © John Winter and Prentice Hall.
Pyx
Hbl
If water infiltrates at moderate
temperatures, pyroxenes are
altered to amphiboles, and
Biotite to Chlorite
Chl
Bt
Sericites and Sericitic Texture

The feldspars in this Alaskite (a low mafic granite)
from the Boulder Batholith have been largely
replaced by fine-grained Muscovite. This texture
is called sericitic.
Myrmekitic
As Plagioclase replaces a K-spar , silica SiO2 is
released . For example, Ca++ Plagioclases
contain less silica than the K-spar s, so the
reaction is K-spar = Plagioclase + Quartz.
Often the quartz all goes
bright/extinct at the same time,
as if it is all the same crystal.
Figure 3-21. Myrmekite formed in plagioclase at the boundary with K-feldspar. Photographs courtesy © L.
Collins. http://www.csun.edu/~vcgeo005