Igneous Textures
Crystallinity
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Vitric or Holohyalline : if the rock is totally composed of glass
Hypocrystaline: if it contains both glass and crystals
Holocrystaline: if the rock is totally composed of crystals
Grain Size
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Aphanitic: when the crystal size is too small to be distinguishable by naked eye
Phaneritic: when the crystals are distinguishable with naked eye the texture is
called phaneritic.
Fine-Grain
< 1mm
Medium-Grained
1mm-3mm
Coarse-Grained
3mm-15mm
Very Coarse-Grained
> 15mm
Igneous Textures
Grain Size Distribution
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Equigranular
When rock has approximately uniform crystal size
Inequigranular
when rock has considerable variation in crystal size
Porphyritic
when there is a bimodality in crystal sizes e.g one generation is coarse to very
coarse grained and second generation is fine grained.
Megaporphyritic
if the porphyritic texture is visible in handspecimen
Microporpyritic
if the porphyritic texture is visible under microscope
Megaporphyritic
Porphyritic Texture
Porphyritic Texture
Common Terms in Porphyritic Texture
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Phenocryst
Relatively coarser crystals in a porphyritic texture
Ground Mass
Relatively finer crystals or material in a porphyritic textureis called groundmass
Glomeroporphyritic
a porphyritic texture in which phenocryst of one mineral occur in clusters
Cumulophyric
a porphyrictic texture in which phenocrysts of same or different minerals occur in
clusters.
Porphyry
Intrusive igneous rock which have porphyritic texture are called Porphyry or
Porphyrite.
Glomeroporphyritic Texture
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Poikilitic
when phenocryst of one mineral contain many inclusions of an other mineral the
texture is called Poikilitic texture. The host phase is called Oikocryst. And the
included mineral is called Guest.
Ophitic Texture
when a larger grain of pyroxene encloses smaller plagioclase grains the texture is
called ophitic texture.
Subophitic Texture
when plagioclase laths are partially enclosed by pyroxene.
Ophitic Texture
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Seriatic Texture
if a rock has a continuous gradation in its grain size the texture is called Seriate
texture.
Grain Morphology
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Euhedral
if a grain is completely bounded by crystal phases it is called euhedral crystal
Subhedral
if a crystal has partially developed faces
Anhedral
if the crystal faces are entirley absent
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Panidiomorphic
if a rock has majority of euhedral crystals texture is called penidiomorphic
Hypidiomorphic
if a rock has majority of subhedral crystals
Allotriomorphic
if a rock has majority of anhedral crystals.
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Cumulate Texture
a cumulate texture is developed when early formed crystals are settled down at the
bottom of magma chamber.
Orhtocumulate
A cumulate texture in which the interstitial spaces are occupied by different
minerals.
Adcumulate
A cumulate texture in which early formed crystals grow to fill the interstitial spaces.
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Interstitial
this term is used when a mineral or assemblage of minerals and/or glass is present
in the interstices or spaces between early formed larger crystals.
Intersertal
if the interstices are filled with glassy or cryprtocrystaline material the term
intersertal is used.
Intergranular
if the interstices are filled with relatively coarser material, the term intergranular is
used.
Intersertal
Intergranular
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Graphic Texture
Graphic texture is developed by the intergrowth of two minerals (usually quartz
and alkali-feldspar), in which the guest mineral shows angular wedge-like or Vshape forms.
Symplectite
In symplectite a mineral is replaced by the intergrowth of two minerals in which
one mineral has vermicular or globular habit.
Myrmekite
Myrmekite is an intergrowth of plagioclase and vermicular (worm-like) quartz.
They often occur at the margins of alkali feldspar crystals in coarse- and mediumgrained acidic and intermediate igneous rocks.
Graphic Texture
Symplectite
Myrmekite
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Exsolutions
Exsolution is a process in which a homogeneous solid solution is separated into two
or more different minerals e.g. perthite, antiperthite. These exsolutions may take
different forms i.e. lamellae, blebs, rods etc.
Perthite
if a potassic alkali feldspar has exsolutions of sodic feldspar the texture is called
perthitic.
Antiperthite
if a sodic feldspar has exsolutions of potassic alkali feldspar the texture is called
antiperthitic.
Perthite
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Resorption
the partial melting of a mineral in a
magma in response to change in
temperature, pressure and/or
composition.
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Seive Texture
Sieve texture is developed when a
mineral has abundant tiny inclusion.
These inclusions may be resorption
void fillings.
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Vesicular
a texture developed by the bubleshaped cavities within a rock is called
vesicular texture. These cavities are
developed by the trapped
gasses/volatiles in volatile rich
magmas.
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Amygdaloidal
amygdaloidal texture is formed when
the vesicular cavities are filled with
secondary minerals.
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Trachytic
A trachytic texture consists of a
subparallel arrangement of lathshaped feldspars in the groundmass
of an igneous rocks. Trachytic
texture is common in trachytes