Rock textures

1. IGNEOUS ROCKIGNEOUS ROCK
TEXTURESTEXTURES
Prepared by Dr. F. Clark,Prepared by Dr. F. Clark,
Department of Earth and AtmosphericDepartment of Earth and Atmospheric
Sciences, University of AlbertaSciences, University of Alberta
Oct. 05Oct. 05

2. ROCK TEXTURES DEFINEDROCK TEXTURES DEFINED
The use of the term “texture” as applied to rocks is not at allThe use of the term “texture” as applied to rocks is not at all
intuitive. One might expect rock textures to includeintuitive. One might expect rock textures to include
possibilities such as smooth, rough, silky, hard, soft,possibilities such as smooth, rough, silky, hard, soft,
waxy, or other such properties, but this is not the case.waxy, or other such properties, but this is not the case.
Whether applied to igneous or other rocks, the termWhether applied to igneous or other rocks, the term
texture embraces the size, shape, and arrangement oftexture embraces the size, shape, and arrangement of
mineral grains [the general term we use rather thanmineral grains [the general term we use rather than
“crystals”, although mineral grains are crystals, by“crystals”, although mineral grains are crystals, by
definition]. It is generally by the texture that a geologistdefinition]. It is generally by the texture that a geologist
will first recognize whether a rock is igneous,will first recognize whether a rock is igneous,
sedimentary, or metamorphic, before they get down tosedimentary, or metamorphic, before they get down to
the business of identifying the rock or naming it.the business of identifying the rock or naming it.

3. IGNEOUS ROCK TEXTURES -IGNEOUS ROCK TEXTURES -
PRINCIPLEPRINCIPLE
The fundamental principle behind igneous rock textures isThe fundamental principle behind igneous rock textures is
that grain size is controlled by cooling rate. Thus, rapidthat grain size is controlled by cooling rate. Thus, rapid
cooling at the Earth’s surface of extrusive moltencooling at the Earth’s surface of extrusive molten
material, or lava, results in the growth of smaller crystals,material, or lava, results in the growth of smaller crystals,
or prevents crystal growth altogether. Conversely, slowor prevents crystal growth altogether. Conversely, slow
cooling within the Earth’s crust of intrusive moltencooling within the Earth’s crust of intrusive molten
material, called magma, results in the growth of fewermaterial, called magma, results in the growth of fewer
but larger crystals, because atoms are able to migratebut larger crystals, because atoms are able to migrate
through the liquid to attach themselves to crystals thatthrough the liquid to attach themselves to crystals that
have already begun to form. The many igneous rockhave already begun to form. The many igneous rock
textures are simply variations on or modifications of thistextures are simply variations on or modifications of this
principle.principle.

4. Igneous Extrusive, or Volcanic, RocksIgneous Extrusive, or Volcanic, Rocks
These rocks, formed by cooling and solidification (notThese rocks, formed by cooling and solidification (not
always crystallization) of lava, are typically fine-grained,always crystallization) of lava, are typically fine-grained,
to the extent that detailed analysis of the mineralogy ofto the extent that detailed analysis of the mineralogy of
these rocks is only possible with the high magnificationthese rocks is only possible with the high magnification
of a petrographic microscope, or even higher resolutionof a petrographic microscope, or even higher resolution
techniques. As we shall see in the file on igneous rocktechniques. As we shall see in the file on igneous rock
classification, the colour of the rock is an important clueclassification, the colour of the rock is an important clue
to its bulk or average composition, which controls itsto its bulk or average composition, which controls its
mineralogy, and other clues may tell us what mineralsmineralogy, and other clues may tell us what minerals
are present.are present.

5. AphaniticAphanitic
Crystals areCrystals are
uniformly fine-uniformly fine-
grained andgrained and
interlocking,interlocking,
with individualwith individual
crystalscrystals
invisible to theinvisible to the
unaided eye.unaided eye.
This is the generic, base texture for most igneous extrusive rocks, theThis is the generic, base texture for most igneous extrusive rocks, the
fine-grained nature being a result of rapid cooling that prevents growthfine-grained nature being a result of rapid cooling that prevents growth
of large crystals. Even with a hand lens, virtually nothing can beof large crystals. Even with a hand lens, virtually nothing can be
identified, and such rocks can be very frustrating to deal with.identified, and such rocks can be very frustrating to deal with.

6. PorphyriticPorphyritic
Large, evidentLarge, evident
crystals calledcrystals called
phenocrystsphenocrysts
[red arrows][red arrows]
are surroundedare surrounded
by an aphaniticby an aphanitic
matrix ormatrix or
groundmassgroundmass
[blue arrows].[blue arrows].
This texture represents two stage cooling. Slower cooling of magmaThis texture represents two stage cooling. Slower cooling of magma
within the crust leads to growth of the phenocrysts, whose early growthwithin the crust leads to growth of the phenocrysts, whose early growth
leads to the development of well-formed faces. Once erupted as lava,leads to the development of well-formed faces. Once erupted as lava,
the remaining liquid crystallizes as the aphanitic groundmass.the remaining liquid crystallizes as the aphanitic groundmass.

7. VesicularVesicular
Vesicles areVesicles are
simply bubblessimply bubbles
produced byproduced by
gasesgases
escaping fromescaping from
lava as itlava as it
solidifies.solidifies.
The vesicles [red arrows] are in a full range of sizes, enclosed in anThe vesicles [red arrows] are in a full range of sizes, enclosed in an
aphanitic groundmass [blue arrows]. The few slightly larger crystalsaphanitic groundmass [blue arrows]. The few slightly larger crystals
[green arrows] are of the mineral olivine, a common constituent of[green arrows] are of the mineral olivine, a common constituent of
Hawaiian basalt volcanic rocks such as this one.Hawaiian basalt volcanic rocks such as this one.

8. VesiclesVesicles
andand
XenolithsXenoliths
This sample fromThis sample from
the previous slidethe previous slide
shows a moreshows a more
complex face. Thecomplex face. The
red, blue, andred, blue, and
green arrows aregreen arrows are
the same as in thethe same as in the
previous slide.previous slide.
Volatiles dissolved in magma under high pressure within the EarthVolatiles dissolved in magma under high pressure within the Earth
escape when lava is erupted, like COescape when lava is erupted, like CO22 from soda pop. A xenolithfrom soda pop. A xenolith
[literally, “foreign rock”, purple arrow] is a fragment of a previously[literally, “foreign rock”, purple arrow] is a fragment of a previously
crystallized rock incorporated in the magma as it nears the surface.crystallized rock incorporated in the magma as it nears the surface.

9. Scoria – a Highly Vesicular BasaltScoria – a Highly Vesicular Basalt
If basalt, which is a mafic [high Fe+Mg content, low silica content]
volcanic rock, is highly vesicular, then we have scoria, which for lack of
a better way to describe it, resembles the guts or interior of an Aero
chocolate bar. Note how the iron content in this chemically unstable
mafic rock weathers to produce a rust-coloured surface [brown arrows],
versus the greenish grey fresh surface [cut surface; green arrows].

10. PumicePumice
This is anThis is an
extremelyextremely
vesicular felsicvesicular felsic
[low Fe+Mg, high[low Fe+Mg, high
silica content]silica content]
volcanic rock. Thisvolcanic rock. This
material willmaterial will
actually float onactually float on
water because itwater because it
has so manyhas so many
bubbles orbubbles or
vesicles.vesicles.
Gases do not escape easily from high viscosity [stiff] felsic lavas, andGases do not escape easily from high viscosity [stiff] felsic lavas, and
so a light-coloured volcanic froth is produced. Pumice is used in theso a light-coloured volcanic froth is produced. Pumice is used in the
cosmetic industry as an exfoliant, effective at removing calluses due tocosmetic industry as an exfoliant, effective at removing calluses due to
the abrasive nature of the glass-hard frothy surface.the abrasive nature of the glass-hard frothy surface.

11. Amygdaloidal – With Filled VesiclesAmygdaloidal – With Filled Vesicles
It may be that a vesicular rock, such as this basalt [red arrows point to
vesicles set in the aphanitic groundmass highlighted by blue arrows],
has fluids circulating through the vesicles, from which minerals may
precipitate or crystallize. These deposits, with rounded outlines
reflecting their origin as bubble-filling, are called amygdules [purple
arrows]. Phenocrysts would be angular in outline.

12. GlassyGlassy
Very rapid coolingVery rapid cooling
may preventmay prevent
crystal growthcrystal growth
altogether, and wealtogether, and we
get naturalget natural
volcanic glass,volcanic glass,
called obsidian.called obsidian.
Note theNote the
conchoidalconchoidal
fracture [greenfracture [green
arrows] we wouldarrows] we would
expect for glass.expect for glass.
Note the contrast between the black fresh surface [yellow arrows] andNote the contrast between the black fresh surface [yellow arrows] and
the rusty brown weathered surface [red arrows], which reveals thethe rusty brown weathered surface [red arrows], which reveals the
minor iron content that stains the glass black. This is a black igneousminor iron content that stains the glass black. This is a black igneous
rock whose colour index [% mafic minerals] is nevertheless zero.rock whose colour index [% mafic minerals] is nevertheless zero.

13. GlassyGlassy
This texture isThis texture is
more likely inmore likely in
felsic [high silicafelsic [high silica
content, notcontent, not
necessarily asnecessarily as
quartz] lavas,quartz] lavas,
which are viscouswhich are viscous
or stiff. This sharpor stiff. This sharp
natural glass cannatural glass can
be fashioned intobe fashioned into
effective weaponeffective weapon
tips.tips.
Obsidian is an exception to the definition of rocks, which among otherObsidian is an exception to the definition of rocks, which among other
things are aggregates of one or more minerals, which by definition arethings are aggregates of one or more minerals, which by definition are
crystalline solids. Glass is an amorphous solid, which means it lackscrystalline solids. Glass is an amorphous solid, which means it lacks
the ordered arrangement of atoms that characterizes crystals.the ordered arrangement of atoms that characterizes crystals.

14. Igneous Intrusive, or Plutonic, RocksIgneous Intrusive, or Plutonic, Rocks
The coarse crystal size associated with slow cooling meansThe coarse crystal size associated with slow cooling means
that the hand specimen properties of minerals can bethat the hand specimen properties of minerals can be
easily applied and exploited to identify the mineralseasily applied and exploited to identify the minerals
present in the rock. This may extend as far as being ablepresent in the rock. This may extend as far as being able
to recognize cleavage intersection angles in the fewto recognize cleavage intersection angles in the few
crystals that may be favourably oriented in the samplecrystals that may be favourably oriented in the sample
under consideration.under consideration.

15. Phaneritic – With Evident CrystalsPhaneritic – With Evident Crystals
Igneous intrusive rocks have evident crystals [the Greek word phaneros
means visible or evident] that one can easily distinguish with the
unaided eye, even if one doesn’t have the skill to identify what minerals
they are. In these samples, one sees grey glassy quartz, black biotite
and amphibole, and cream-coloured potassium feldspar.

16. Phaneritic – the Igneous Intrusive StaplePhaneritic – the Igneous Intrusive Staple
Although these samples have smaller crystals than the previous two,
the individual crystals or mineral grains are still readily distinguished
without magnification. The coarse crystal size makes many igneous
intrusive rocks quite attractive, and they are also durable and
reasonably stable chemically. This makes them good choices for grave
markers and facing stone for buildings.

17. Phaneritic – Seeing Cleavage on GrainsPhaneritic – Seeing Cleavage on Grains
The coarse grain size of phaneritic intrusive rocks allows us to see
cleavage faces developed on many grains. Essentially, when the rock is
stressed and breaks when samples are taken, many of the randomly
oriented crystals have their cleavage directions more or less parallel to
the face of the sample, and so they break along cleavage. This gives
excellent reflections [blue arrows] from potassium feldspar in this case.

18. PhaneriticPhaneritic
andand
PorphyriticPorphyritic
PorphyriticPorphyritic
textures aretextures are
classicallyclassically
developed indeveloped in
extrusiveextrusive
rocks, but it isrocks, but it is
not restricted tonot restricted to
them.them.
This mafic intrusive rock is called a gabbro, a mixture of pyroxene andThis mafic intrusive rock is called a gabbro, a mixture of pyroxene and
plagioclase feldspar. The bulk of the rock is phaneritic, with grainsplagioclase feldspar. The bulk of the rock is phaneritic, with grains
approximately 1-2 mm across. However, there is a megacryst, a largeapproximately 1-2 mm across. However, there is a megacryst, a large
pyroxene crystal, whose extent is indicated by the yellow arrows.pyroxene crystal, whose extent is indicated by the yellow arrows.

19. Phaneritic and PorphyriticPhaneritic and Porphyritic
The same specimen as in the previous slide, we take advantage of the
development of cleavage to highlight the megacryst [width indicated by
yellow arrow], which is somewhat obscure in the previous image. The
cleavage direction meets the surface of the specimen at something like
30 degrees, but the crystal still breaks along cleavage in a series of
several steps, which reflect at the same time.

20. PegmatitePegmatite
This term appliesThis term applies
to extremelyto extremely
coarse-grainedcoarse-grained
igneous intrusiveigneous intrusive
rocks, usually of arocks, usually of a
felsic composition.felsic composition.
Some restrict theSome restrict the
term to rocks withterm to rocks with
a grain sizea grain size
exceeding 2 cm,exceeding 2 cm,
but others allowbut others allow
more latitude.more latitude.
In the late stages of cooling, volatiles tend to be concentrated in theIn the late stages of cooling, volatiles tend to be concentrated in the
magma. This lowers magma viscosity, accounting for the abnormallymagma. This lowers magma viscosity, accounting for the abnormally
large crystals. In this specimen, there are crystals of an unusuallarge crystals. In this specimen, there are crystals of an unusual
turquoise variety of potassium feldspar called amazonite [red arrows].turquoise variety of potassium feldspar called amazonite [red arrows].

21. PegmatitePegmatite
Finer crystalsFiner crystals
on the upperon the upper
surface of thesurface of the
specimen mayspecimen may
represent morerepresent more
rapid cooling atrapid cooling at
the exterior ofthe exterior of
the intrusion;the intrusion;
field data couldfield data could
confirm this.confirm this.
The large crystals [see cleavage face, yellow arrow] may be gemThe large crystals [see cleavage face, yellow arrow] may be gem
quality crystals, often of minerals that are significant sources of unusualquality crystals, often of minerals that are significant sources of unusual
elements such as lithium, fluorine, and boron. Pegmatites are thuselements such as lithium, fluorine, and boron. Pegmatites are thus
attractive as well as economically significant.attractive as well as economically significant.