PETROGRAPHY OF IGNEOUS
AND METAMORPHIC ROCKS
Welcome to the "Petrography of Igneous and Metamorphic Rocks" Site. This
site is aimed to guide the geologists and petrographers dealing with igneous
and metamorphic rocks. In this site you'll find some knowledge about
petrographic textures of igneous and metamorphic rocks.
This site is prepared under the supervision of Prof.Dr. M. Cemal Göncüoglu (M.E.T.U/TURKEY)
INTRODUCTION
Texture refers to the way in which individual grains relate to grains immediately surrounding
them. Texture deals with small-scale features seen in hand specimen or under the
microscope, such as the degree of crystallinity, grain size, grain shape, grain orientation,
grain boundary relations and crystal intergrowths. Textures are useful indicators of cooling
and crystallization rates and of phase relations between minerals and magma at the time of
crystallization.
Igneous rocks with interlocking crystals have crystalline textures. The crystallinity and
dominant grain size in crystalline igneous rocks are each described y one of a series of three
terms, as shown in the figure below.
Names of Special Igneous Rock Textures
Volcanic
Plutonic
Other
Microlitic
Poikilitic
Zoned
Spherulitic
Graphic
Corona
Vitrophyric
Ophitic
Kelphytic rim
Intersertal
Subophitic
Rapakivi
Intergranular
Diabasic
Epitaxial
Felty
Orthocumulate
Poikilitic
Pilotaxitic
Mesocumulate
Trachytic
Adcumulate
Subophitic
Symplectic
Ophitic
Myrmekitic
Dictytaxitic
Seriate
Glomeroporphyric
Trachytoidal
Pyroclastic
Granophyric
Seriate
Spinifex
Source: Raymond, L.A. , 1995, Petrology of Igneous, Sedimentary, Metamorphic Rocks.
Wm. C. Brown Publishers Company, page 24.
TEXTURES REFERRING TO THE CRYSTALLINITY OF AN IGNEOUS ROCK
TEXTURES REFERRING TO THE GRANULARITY OF AN IGNEOUS ROCK
TEXTURES REFERRING TO THE CRYSTAL SHAPES OF AN IGNEOUS ROCK
TEXTURES REFERRING TO INTERRELATIONS BETWEEN THE CRYSTALS
TEXTURES REFERRING TO ORIENTATION AND ARRANGEMENT OF THE
CRYSTALS
TEXTURES REFERRING TO INTERGROWTH OF THE CRYSTALS
TEXTURES REFERRING TO OVERGROWTH OF THE CRYSTALS
TEXTURES REFERRING TO RADIAL ARRANGEMENT OF THE CRYSTALS
TEXTURES REFERRING TO BANDING OF THE CRYSTALS
TEXTURES REFERRING TO CAVITY FILLINGS OF THE CRYSTALS
OTHER TERMS REFERRING TO THE TEXTURES OF IGNEOUS ROCKS
TEXTURES REFERRING TO THE CRYSTALLINITY OF
AN IGNEOUS ROCK
The term Holocrystalline refers to a rock composed of entirely crystalline material.
Anorthosite sample which is entirely composed of
Albitic plagioclase crystal
The term Holohyaline refers to a rock composed of entirely glassy material.
Obsidian sample ( volcanic glass) which is entirely
composed of siliceous glassy material.
The term Hypocrystalline refers to a rock composed of both crystals and glassy material, but
amount of crystals is greater than glassy material.
Andesite sample which is composed of both plagioclase (Pl) and clinopyroxene (Cpx)
crystals with a fine grained groundmass. In this sample, amount of glassy material, which is
essential component of the groundmass is less than the total amount of crystals.
The term Hypohyaline refers to a rock composed of both crystals and glass material but
amount of glass is greater than crystals.
Rhyolitic ignimbrite sample which is composed of volcanic glass shards, pumice fragments
and free crystals (Qzt-quartz; Sa-sanidine). In this sapmle amount of volcanic glass material
is higher than the amount free crystals.
TEXTURES REFERRING TO THE GRANULARITY OF AN
IGNEOUS ROCK
Terms Which Are Used According to The Visible or Invisible Granular Properties of
Igneous Rocks
Phaneritic (phanerocrystalline) refers to a rock in which the grains of the essential minerals
can be discerned without the aid of microscope
Polished surface of the granitic rock slab. Minerals can
be distinguished clearly by naked eye.
Aphanitic refers to a rock, which is so fine grained, that individual crystals cannot be
discerned without a microscope. An aphyric rock is one devoid of phenocrysts. This term is
generally applied only to aphanitic or fine-grained rocks. Thus an equigranular granite, for
example, devoid of phenocrysts would not be termed aphyric.
Basalt sample which contains no phenocrysts and is
composed of plagioclase microphenocrysts and
volcanic glass. Individual crystals can not be seen by
naked eye.
Pegmatitic refers to a rock that is very coarse-grained, dominated by grains > 3 cm in length.
Although many petrologists reserve the term pegmatite for siliceous (granitoid) rocks of that
grain size, rocks of any composition may be pegmatitic.
Terms Which Are Used According To The Absolute Size of The
Crystals
The term coarse-grained implies crystal size > 5 mm.
The term medium-grained implies crystal size between 1-5 mm.
The term fine-grained implies crystal size < 1 mm.
Terms Which Are Used According To The Relative Crystal Sizes
Equigranular refers to an igneous rock texture in which the diameters of component minerals
are comparable, allowing of course for the inherently tabular and prismatic, rather than equant
habits of some minerals.
Biotite granite sample which is composed of nearly
equigranular crystals of quartz (white), K-feldspar
(grayish), plagioclase (grayish with polysynthetic
twining) and biotite (reddish brown)minerals.
Porphyritic texture connotes one or more mineral species or a generation of one or more
mineral species that are conspicuously greater in size than those minerals constituting the
rest of the rock. There are number of larger grains called phenocrysts, surrounded by a
population of grains of significantly smaller size, the groundmass.
A porphyric rock which is composed of euhedralsubhedral feldspar phenocrysts in a fine grained
grounmass.
Seriate texture refers to a situation where there is a continuous range in grain size of one or
more mineral species from that of phenocryst to groundmass size, and in which crystals of
progressively smaller sizes are increasingly numerous. This texture is commonly shown by
plagioclase in some andesite porphyries.
A volcanic rock with seriate texture. Feldspar
phenocrysts especially exhibit a broad interval of
grain size.
Microporphyritic texture refers to a situation where both the phenocrysts and the
groundmass are aphanitic.
Basalt sample with microporphyritic texture.
As it is seen in the figure, both the
phenocrysts and the groundmass are fine
grained (aphanitic).
Aphanitic-porphyritic and phaneritic-porphyritic term describes, respectively some
situations where the phenocrysts are phaneritic but the groundmass is aphanitic and where
both groundmass and phenocrysts are phaneritic.
An aphanitic-porphyritic volcanic rock which is
compoded of euhedral sanidine, quartz (not visible)
and some altered mafic (amphibole) minerals in an
aphanitic groundmass.
Vitrophyric texture refers to a rock (vitrophyre), which is composed of phenocrysts of
intratelluric crystallization in a glassy matrix.
A volcanic rock sample containing plagioclase and
quartz phenocrysts in a glassy grounmass.
Microlitic texture is a porphyritic texture in which microlites (small fibrous crystals) are
enclosed in a glassy groundmass.
TEXTURES REFERRING TO THE CRYSTAL SHAPES
OF AN IGNEOUS ROCK
Terms Which Are Used According To The Development of Crystal Surfaces
Allotriomorphic refers to a texture in which all the component mineral grains are anhedral.
Dunite sample which is composed of entirely olivine (Ol) minerals and accessory
enstatite (En – orthopyroxene) minerals.
Hypidiomorphic refers to a texture in which the grains of some mineral species are anhedral,
those of others subhedral, and those of some may even be euhedral. This texture is typical of
granitic rocks in many of which quartz and orthoclase tend to be anhedral and plagioclase
and biotite are subhedral to euhedral.
Norite sample which is composed of euhedralsubhedral and anhedral plagioclase minerals (plag gray-black colored minerals with polysynthetic
twining) and subhedral-anhedral orthopyroxene
(opx) and clinopyroxene (cpx) minerals.
Idiomorphic refers to a texture in which, theoretically, all the component mineral grains are
euhedral (a geometrical impossibility in a holocrystalline rock). The term is commonly applied
to the texture of lamprophyres, which are characteristically densely and conspicuously
crowded with euhedral mafic phenocrysts.
Special Terms Referring To The Special Crystal Shapes
Skeletal crystals are those which have hollows and gaps, possibly regularly developed and
usually with particular crystallographic orientations. In thin section these spaces appear as
embayments and holes in the crystal, filled with groundmass crystals or glass.
Dendritic crystals consist of a regular array of fibres sharing a common optical orientation
and having a branching pattern resembling that of a tree or the veins in a leaf or a feather.
Embayed crystals are those, which have been resorbed by reaction with liquid. While this
may be true of some crystals others have embayments, which probably formed during growth.
Rhyolitic volcanic rock sample that contains embayed
quartz crystal (white colored). Resorbtion of quartz crystal
by the melt is clearly seen in the photograph.
Parallel-growth crystals are aggregates of elongate crystals of the same mineral whose
crystallographic axes are mutually parallel or almost so. Although in thin section the individual
parts of the aggregate may be isolated from one another, in the third dimension they are
probably connected. A parallel-growth crystal is therefore a single incomplete crystal formed
by a particular style of skeletal growth.
Komatiite sample that shows parallel growth of
elongate
quenched olivine crystals (blue colored).
Sieve textured crystals are those which contain abundant, small, interconnected, box
shaped glass inclusions, giving the crystals a spongy or porous appearance.
Sieve textured plagioclase minerals that exhibit
spong core and an overgrowth surrounding this
core.
Elongate, curved, branching crystals are rarely genuinely bent; rather the curvature is
caused by development of branches along the length of the crystal, each branch having a
slightly different crystallographic orientation to its neighbors.
Pseudomorph is a secondary mineral (or group of minerals) that partly or completely
replaces a pyrogenetic mineral while preserving the characteristic crystal outline of the
mineral being replaced.
Amphibole pseudomorph which is partly preserved
(interiors). Some parts (pale green) are chloritized.
TEXTURES REFERRING TO INTERRELATIONS BETWEEN THE
CRYSTALS
Glomeroporphyritic texture results when phenocrysts aggregate in groups; sometimes only
some of several phenocryst species may be so aggregated, suggesting that those species not
involved belong to a later period of intratelluric crystallization. Some glomeroporphyritic
clusters represent the incorporation of texturally more complex material, perhaps consolidated
early fractionated material.
Olivine basalt sample which is composed of olivine (Ol), clinopyroxene (Cpx) phenocrysts
and plagioclase microphenocrysts (not seen) in a fine grained groundmass. Note that the
aggregation of clinopyroxene minerals as glomerocrysts.
Poikilitic texture is a general term describing a texture in which one or more mineral species
may be partly or wholly enclosed by another mineral species. This texture indicates
crystallization sequence of minerals in an igneous rock.
Biotite minerals (reddish-greenish-dark brown) poikilitically
enclosed by a large K-feldspar mineral (ligh gray).
Ophitic texture is a term generally used for gabbroic rocks and refers to a situation where
the dimensions of the augite crystals are substantially larger than those of the plagioclase;
several even numerous plagioclase tablets may come to be included within each grain of
augite.
Gabbro sample that shows extremely large crystals of enstatite (Enorthopyroxene) surrounding smaller plagioclase (Pl) crystals.
Sub-ophitic texture is a term also used for gabbroic rocks and refers to a situation where the
augite grains are somewhat smaller, and not much larger than the plagioclase; they will thus
only partly enclose individual plagioclase tablets.
Gabbro sample that shows clinopyroxene (various
colored) minerals partly enclosing individual
plagioclase minerals (with polysynthetic twinning).
Intergranular texture is characteristic for basaltic flows where the grains of augite and
accessory opaque minerals small enough to fit between the unoriented array of tabular
plagioclase microlites. Unlike ophitic texture, adjacent intersticies are not in optical continuity
and hence are discrete small crystals. The feldspars may be in diverse, sub-radial or subparallel arrangement.
Dolerite sample with intergranular texture. Colored
minerals are mainly anhedral clinopyroxene and
olivine minerals, which are enclosed by prismatic
plagioclase minerals.
Intersertal texture is characteristic of tholeiite flows where it may be present in addition to
intergranular or sub-ophitic textures; although it is not present in all tholeiite flows but it is
diagnostic of tholeiite where it is seen. Intersertal texture denotes the presence of small,
disconnected, patches of a glassy mesostasis of acid composition, often containing numerous
tiny inclusions of opaque minerals amongst the rectangular plagioclase laths that are not
aligned. Glassy mesostasis may be altered in older flows generally to varieties of chlorite
such as highly colored chlorophaeite.
Diabasic texture is very common in the more slowly cooled hypabyssal rocks of all basaltic
compositions such as sills and some thick dykes. It is a coarse grained equivalent of the
intergranular textures found in volcanic rocks. Diabasic texture is sometimes referred to as
ophitic texture but two terms are not quite synonymous. Because in diabasic texture both
orthopyroxene and opaque mineral as well as augite may develop a comparable poikilitic
habit with respect to the plagioclase, whereas ophitic texture specifically connotes the
poikilitic enclosure of plagioclase by augite alone.
Olivine basalt sample from Palisades Sill. This rocks is composed of relatively
coarse grained enstatite (En-orthopyroxene), clinopyroxene (Cpx), olivine (Ol) and
plagioclase (Pl) minerals.
TEXTURES REFERRING TO ORIENTATION AND
ARRANGEMENT OF THE CRYSTALS
Trachytic texture is characteristic for many trachytes and some phonolites that are
characterized by a high proportion of tabular sanidine microlites in the groundmass that are
very flat in shape and are aligned in sub-parallel arrangements and thus prone to strong
fluxioning.
Trachytoidal texture refers to a sub-parallel arrangement of tabular, bladed or prismatic
crystals, which are visible to naked eye.
Hyalopilitic texture refers to a texture in which plagioclase microlites are set in an abundant
glassy mesostasis (but the plagioclase microlites may and generally do show a degree of
fluxioning).
Pilotaxitic texture connotes abundant plagioclase microlites prominently fluxioned in an
overall sub-parallel manner and locally around phenocrysts (but strictly in a holocrystalline
non-glassy matrix).
Orthophyric texture is quite common in adesites and related rocks, results where the
plagioclase microlites have the form of stubby rather than flat tablets so that fluxioning of
these relatively more equidimensional tablets does not occur and they exhibit squat
unoriented rectangular sections in thin section; there is no requirement for the mesostasis to
be glassy or not in the definition of orthophyric texture.
Pyroxene Andesite sample with orthophyric texture. En - enstatite (orthopyroxene),
Cpx - clinopyroxene and Pl – plagioclase phenocrysts.
Parallel growth texture refers to a single elongate skeletal crystal, which in thin section
appears to consist of a clot of crystals having the same elongation direction and the same
optical orientation. In rocks with trachytoid texture it is not uncommon for neighboring parallel
growth crystals to be aligned.
Komatiite sample that shows parallel growth of
elongate quenched olivine crystals (blue colored).
Comb texture refers to elongate, possibly curved, branching crystals sharing the same
direction of elongation. The crystals typically form a band, layer, or fringe with the elongation
direction of the crystals inclined at 60-90 to the plane of layering.
TEXTURES REFERRING TO INTERGROWTH OF THE CRYSTALS
In thin section, the junction between two crystals may appear as a straight line, a simple curve
or a complex curve; in the third case the crystals interdigitate or interlock, possibly so
intimately that they appear to be embedded in one another. These interpenetrative patterns
are all examples of intergrowth textures. Usually the crystals concerned are anhedral but one
or both may be skeletal, dendritic or radiate.
Consertal texture refers to a situation where the boundary between two crystals involves
interdigitations and hence appears to be notched or serrated in thin section.
Graphic texture is similar to pokilitic texture in that a larger grain encloses apparently smaller
grains, but this texture, which occurs in pegmatitic granitoid rocks, consists of a very large
crystal of alkali feldspar enclosing smaller crystals of quartz that all have the same
crystallographic orientation. In some cases it can be clearly demonstrated that the apparently
separate quartz grains are all connected and are part of a large, single dendritic or snowflake
quartz crystals.
Intergrowth of quartz minerals in K-feldspar
crystals to form graphic texture.
Micrographic texture is compositionally akin to graphic intergrowth but the term is generally
used to refer to the small interstitial patches of quartz and alkali feldspar that characterize
quartz-diabeses of tholeiitic affinity.
Granophyric texture delicate intergrowths, often with an overall radiating habit, of quartz and
alkali feldspar in ternary minimum proportions. The texture often coarsens radially outward
and passes into a micrographic texture. This transition may be apparent on a small scale
within the area of one thin section and may be found also on a larger scale within intrusive
bodies (typically small epizone intrusions emplaced below critical vesiculation depth), which
may have granophyric margins and microgranitic interiors.
Granophyric intergrowth of quartz
grains in K-feldspar.
Myrmekitic texture connotes a symplectic intergrowth of quartz and oligoclase occurring in
small cauliflower-shaped embayments into microcline at oligoclase-microcline boundaries in
katazonal granitic rocks and migmatites, or more rarely at oligoclase-orthoclase boundaries in
some mesozonal granitic rocks. Unlike graphic and micrographic textures, which are
produced by crystallization from a silicate melt, myrmekitic textures reflect subsolidus
replacement effects.
Myrmekitic texture defined by wormy (rounded)
intergrowths of quartz and K-feldspar in
plagioclase which is adjacent to K-feldspar.
Exsolution lamellae or exsolution blebs refer to a lamellar and bleb-like intergrowths are
often attributed to exsolution of the lamellae and blebs of one component from the host crystal
(i.e. solid-state reaction).
Exsolution lamellae of
orthopyroxene in
clinopyroxene mineral (blue-
purple colored).
Perthite is very common in igneous rocks and consists of quantitatively minor lamellae,
shreds, patches and rims of an albite component within and around host orthoclase or
microcline. Whatever the orientation in thin section, the albite component always has the
higher birefringence and appears brighter under crossed nicols, a useful feature in
identification, as the exsolution lamellae are generally far too small to show any diagnostic
multiple twinning.
Exsolution of albite patches in microcline
crystal to form perthitic texture.
Antiperthite is not commonly seen in igneous rocks and it would appear to be strictly a
texture produced during metamorphism. It is also common in granulites. This texture consists
of numerous tiny blebs and rods of orthoclase within the host plagioclase.
Mesoperthite refers to the typical perthite of epizone granitic bodies wherein the albite
component is comparable in abundance to the orthoclase component may come to form an
anastomosing host around remnants of an orthoclase component (generally still relatively
more abundant).
Microperthite refers specifically to exsolution textures that are visible only on a microscopic
scale (some perthitic intergrowths are sufficiently coarse to be conspicuous in hand
specimen).
Cryptoperthite refers to even finer-grained intergrowths that are only resolvable by X-ray
methods. The apparently optically homogenous sanidines of older volcanic rocks are in fact
cryptoperthites (a quickly revealed by their distinctive values of 2V), their submicroscopic
pattern of exsolution having been affected during the passage of time rather than during the
cooling of the igneous rock.
Symplectic intergrowth connotes an intimate fine-grained intergrowth of two minerals in
which one mineral has a vermicular (wormlike) habit; although some symplectic intergrowths
may well be due to exsolution and others may reflect post-crystallization reaction or
metasomatism, some are of perplexing origin and the non-genetic connotation of the term
symplectic is thus useful for descriptive purposes.
Wormlike intergrowth of very tiny quartz and K-feldspar
minerals in a plagioclase mineral.
TEXTURES REFERRING TO OVERGROWTH OF THE CRYSTALS
The term overgrowth describes the partial mantling of one mineral either by material of the
same composition or by material of the same mineral species but different solid-solution
composition or by an unrelated mineral. This is presumably conforming to a sequence of
crystallization and without conspicuous reaction. The term overgrowth carries the additional
connotation of crystallographic continuity between the two participating minerals insofar as
their differing crystal structures permit this. Where the overgrowth forms a more or less
continuous rim around the enclosed mineral, such as, for example, the rim of plagioclase
around a proportion of orthoclase phenocrysts in some textural varieties of rapakivi granite, it
may be termed a mantle.
Skeletal or dendritic overgrowths are found in some porphyritic rocks with a glassy or very
fine-grained groundmass may show delicate fibres or plates extending from the corners or
edges of the phenocrysts. The overgrowth and the phenocryst need not to be the same
mineral.
Corona texture refers to a situation where a crystal o one mineral is surrounded by a rim or
mantle of one or more crystals of another mineral (i.e. olivine surrounded by orthopyroxene or
biotite surrounding hornblende). Such relationships are often presumed to result from
incomplete reaction of the inner mineral with melt or fluid to produce the equivalent genetic
terms “reaction rim” and “reaction corona” are frequently used.
Anhedral amphibole phenocryst (greenish colored
interior part) surrounded by another euhedral
amphibole mineral (yellowish colored), which is
possibly the same type.
Reaction rim strictly results where an early-formed mineral later reacts with the still
crystallizing magma. A conspicuous example is the occurrence of prominent “black rims”
composed of small granules of pyroxene and opaque oxide mineral around hornblende and
biotite phenocrysts in andesite and latite flows.
Basalt sample containig composite pyroxene. Subhedral orthopyroxene
phenocryst with rounded margins (inner part), is surrounded by clinopyroxene
rim (red-yellow colored) due to reaction with the melt.
Kelyphitic rims refer to a microcrystalline overgrowth of fibrous pyroxene or hornblende or
olivine or garnet. This texture is found in altered coarse-grained igneous rocks, around the
boundaries of crystals, reflecting partial degradation of the pyrogenetic mineralogy under lowgrade metamorphic conditions.
Rapakivi texture is a special term used to describe an overgrowth by sodic plagioclase on
large, usually round, K-feldspar crystals.
Epitaxial refers to a situation where one mineral grows around another in such a way that
their crystallographic structures are continuous from one to the other.
Terms Which Are Used For The Crystal Zoning
One or more concentric bands in a single crystal are picked out by lines of inclusions or by
gradual or abrupt changes in solid-solution composition of the crystal.
Aegirine-augite crystal showing crystal zoning.
Note that the concentric euhedral zones in the
crystal.
Normal zoning connotes the gradual transition during the growth of a crystal (from core to
rim) to a relatively low-temperature composition in a crystalline solution series. It is the
anticipated result of fractional crystallization where equilibrium has failed to keep up with
falling liquidus composition.
Reverse zoning in contrast, connotes the transition generally abrupt to a higher temperature
outer zone in a crystal. Some hiatal event such as an accession of fresh magma to a magma
chamber undergoing fractional crystallization or sudden loss of volatiles from a sub-volcanic
magma chamber is responsible for reverse zoning.
Multiple zoning is used for crystals having repeated discontinuous zones. If the zones show
a rhythmic repetition of width, the pattern is known as oscillatory zoning. The overall
compositional trend of the multiple zoning may be normal or reverse or even (in which there is
no general trend from core to rim). Individual zones may be of uniform or variable
composition, such that the zoning pattern on a composition-distance graph is square wave,
step-like, saw-tooth, curved saw-tooth, or some combination of these.
Oscillatory zoning generally accompanies reverse zoning and refers to a succession of
normally zoned shells in a crystal each separated by a sharp reverse zone. Oscillatory zoning
is often evident in the plagioclase phenocrysts of andesite, originating intratellurically in high
level magma chambers where a complex crystallization history is inferred.
Convolute zoning is a variety of multiple zoning in which some of the zones are erratic and
have non-uniform thickness.
Sector (or hourglass) zoning is a common feature of pyroxenes in alkali-rich basic and
ultrabasic rocks. It has also been seen in plagioclases in a few quickly cooled basalts. In this
type of zoning, a crystal takes the form of four triangular segments (sectors) with a common
apex. Opposite sectors are chemically identical whereas adjacent ones differ in composition
(though possibly only slightly) and hence in optical properties. Each sector may be
homogenous or show continuous or discontinuous or oscillatory, normal or reverse or ever
zoning. In three dimensions the sectors are pyramid shaped. If the sector boundaries are
curved, the pattern can resemble that of an hourglass.
TEXTURES REFERRING TO RADIAL ARRANGEMENT OF THE
CRYSTALS
Radiate textures are those in which elongate crystals diverge from a common nucleus. They
are most frequently found in fine-grained rocks, but not exclusively. A remarkably large
number of terms exist to describe the various patterns, including, fan, plume, spray, bow-tie,
spherical, sheaf-like, radiate, radial, axiolitic, spherulitic and variolitic. All except the last three
are of self evident meaning.
Spherulitic texture is composed of an aggregate of fibrous crystals of one or more minerals
radiating from a nucleus, with glass or crystals in between. The acicular crystals may be
either single, simple fibres or each may have branches along its length; any branches may or
may not share the same optical orientations as their parents. The most common occurrence
of spherulitic texture is a radiate aggregate of acicular alkali-feldspars with glass between
them, though quartz or other minerals may be present, resulting in an intergrowth texture.
Axiolites differ from spherulites in that radiating fibres extend from either end of a linear
nucleus (i.e. from a small acicular crystal) rather than a point. They could be regarded as a
variety of overgrowth texture, as indeed could those spherulites which grow about visible
crystals rather than a submicroscopic nuclei.
Spherulites are radiating masses of fibrous crystals in a glassy matrix. These spherulites are
probably composed of alkali feldspars and some polymorph of SiO 2, and in this crosspolarized shot, appear as round objects with dark crosses. Note the large phenocryst which
forms the nucleus of one of the spherulites at center-left.
Variolitic texture is composed of a fan-like arrangement of divergent, often branching, fibres;
usually the fibres are plagioclase and glass or granules of pyroxene, olivine or iron ore occupy
the space between. This texture differs from spherulitic texture in that no discrete spherical
bodies are identifiable; in fact, each fan as seen in thin section is a slice through a conical
bundle of acicular crystals.
TEXTURES REFERRING TO BANDING OF THE CRYSTALS
Textures of this type involve two, or more, narrow (up to a few centimeters), sub-parallel
bands in a rock which are distinguishable by differences in texture, and/or color and/or
mineral proportions. Petrologists also use the term layering; while it includes banded texture,
it is also used for larger scale stratification.
Comb layering refers to elongate, possibly curved, branching crystals sharing the same
direction of elongation. The crystals typically form a band, layer, or fringe with the elongation
direction of the crystals inclined at 60-90 to the plane of layering.
Orbicular texture is particularly exotic kinds of banding. In this texture, “Orbs” consist of
concentric shells of rhythmically alternating mineral constitution within the shells the texture
may either be granular or elongate crystals may be radially arranged. Orbs may reach a few
tens of centimeters in diameter.
Eutaxitic texture is a further variety of banded texture and occurs in some tuffs and
ignimbrites and consists of a regular alignment of flattened glassy fragments.
TEXTURES REFERRING TO CAVITY FILLINGS OF THE
CRYSTALS
These are a collection of textures, which feature either holes in the rock, or likely former holes
which are now partly or completely filled with crystals.
Vesicular texture refers to a round, ovoid or elongates irregular holes (vesicles) formed by
expansion of gas in magma.
Scoria sample containing ovoidal vesicles
(dark regions).
Amygdaloidal texture refers to a former vesicles which are occupied, or partially occupied
by late stage magmatic and/or post magmatic minerals, such as, carbonate, zeolites, quartz,
chalcedony, analcite, chlorite and/or rarely, glass or fine groundmass. The filled holes are
known as “amygdales” or “amygdules”.
The oval feature in this photomicrograph is an
amygdule: a formerly open vesicle which has been
filled with a secondary mineral(s) precipitated from
low-T ground waters which have penetrated into the
rock.
Ocellar texture refers to a certain spherical or ellipsoidal leucocratic patches enclosed in
more mafic hosts are known as ocelli (singular ocellus). Unlike amygdales, the minerals filling
an ocellus can normally all be found in the host rock; they may include any of; nepheline,
analcite, zeolite, calcite, leucite, K-feldspar, Na-feldspar, quartz, chlorite, biotite, hornblende
and pyroxene, or even glass, and the minerals are commonly distributed in a zonal
arrangement. Often, platy and acicular crystals in the host bordering an ocellus are
tangentially arranged but sometimes project into the ocellus. Ocelli are normally less than 5
mm in diameter but may reach 2 cm. Their origin has been ascribed on the one hand to
separation of droplets of immiscible liquid from magma, and on the other hand to seepage of
residual liquid or fluid into vesicles.
Miarolitic texture refers to irregularly shaped cavities (druses) in plutonic and hypabyssal
rocks into which euhedral crystals of the rock project.
OTHER TERMS REFERRING TO THE TEXTURES OF IGNEOUS
ROCKS
Devitrification refers in general to the transformation with time of originally glassy
groundmass or mesostasis material to a fine-grained cryptocrystalline or microcrystalline
product. The patterns of devitrification textures can be quite varied and superimposed, for
example, on primary features such as perlitic fractures or the outlines of shards in an
ignimbrite, where varying degrees of primary devitrification may have occurred varying with
height in the cooling unit. Devitrification processes in ignimbrites occur in two stages: (1) a
“primary devitrification” while the ignimbritic cooling unit is cooling, commonly results in a
complete fine-grained devitrification of the groundmass of the entire upper portion of the
ignimbrite cooling unit, often associated with vague spherulitic devitrification textures within
pumice fragments; and (2) a “secondary devitrification” with the prolonged passage of time
will come to affect all glassy material not devitrified by the primary devitrification.
Relict textures refer to such features as preservation of shard outlines and perlitic cracks in
some devitrified ignimbrites and glassy rhyolite flows respectively, or the presence of small
kernels of fresh olivine within a crystal that may be nearly completely pseudomorphed by a
mesh of serpentine.
Felsitic texture results from slow devitrification over geologically long periods of time of
rhyolitic material originally cooled to a glass; in felsitic texture aggregates of cryptocrystalline
or very fine-grained microcrystalline material extinguish together in small patches throughout
the rock.
Eutaxitic texture results from varying degrees of flattening and welding of glassy shards of
groundmass in ignimbrites, which are the characteristic volcanic product of rhyolite eruptions.
Cumulus texture is best recognized by distinctive layering of cumulate rocks in the outcrop.
Where there is only one cumulus phase, the cumulate nature of the rocks is generally evident
from the (varied) nature of intercumulus growth such as oikocrysts and adcumulus growth.
Gabbroic texture results where all the grains tend to be anhedral with intergrown irregular
boundaries but may retain an overall subhedral outline (i.e. each cumulus grain has acted as
a convenient nucleation center for intercumulus growth).
Gabbro sample which is composed of anhedral
clinopyroxene (colored minerals) and subhedralanhedral plagioclase minerals (grayish-white
colored).
Granitic texture connotes a granitic rock, which is both equigranular and hypidiomorphic –
the common condition.
Biotite granite sample which is nearly equigranular
and composed of Biotite (reddish), quartz (wihe),
plagioclase and K-feldspar (grayish) minerals those
are subhedral to anhedral in habit.
Monzonitic texture is a specific type of poikilitic texture in which orthoclase (often perthitic)
poikilitically encloses crystals of plagioclase (that often display limpid albite rims in this
situation) and mafic minerals.
Aplitic texture refers to a fine-grained rock, often occurring as narrow aplite veins, generally
of ideal granite composition, in which all the felsic minerals are equigranular and anhedral.
Aplite samples with quartz phenocrysts in the fine grained holocrystalline part Of the rock.
Fine grained part is composed mainly of quartz and K-feldspar.
Lamprophyric texture refers to the voluminously porphyritic, apparently panidiomorphic
texture that is common in lamprophyric rocks.
TEXTURES OF METAMORPHIC ROCKS
Metamorphic rocks are easily recognized in the field on the basis of their distinctive
structures and textures. Textures are a function of grain size, grain shape, intergrain
relationships, grain distribution, and grain orientation. Metamorphic rocks exhibit a variety of
textures. These can range from textures similar to the original protolith at low grades of
metamorphism, to textures that are purely produced during metamorphism and leave the rock
with little resemblance to the original protolith.
Metamorphic textures may be divided into five major groups -foliated textures,
granoblastic textures, diablastic textures, cataclastic textures, and relict textures. Foliated
textures are textures characterized by an alignment of mineral grains in such a way as to give
the rock the appearance of or the tendency for splitting into layers or flat pieces. Commonly,
the minerals in foliated rocks are predominantly acicular or tabular. Granoblastic textures are
those characterized by more or less equidimensional mineral grains. Diablastic textures are
those in which tabular or acicular minerals are intergrown in a nonfoliated, interlocking, locally
radiating manner. Cataclastic textures are nonfoliated textures characterized by fractured
rock materials and mineral grains. Each of these major textural types may be subdivided into
two or more individual types.
SOME EXAMPLES OF METAMORPHIC TEXTURES
1. Foliation: Crystallization of platy minerals under a directed stress results in a preferred
orientation of the plates normal to the direction of maximum stress. In this picture folition is parallel to
the aligned sheets of muscovite sandwiched between quartz grains.
2. Wavy Foliation: This is a sample of the Ira Phyllite. Note the wavy foliation and the overall finegrain size of this rock.
3. Mylonite: In fault zones the degree of deformation may be sufficiently intense that so many
recrystallization nucleation sites are formed that the resulting rock is extremely fine- grained. The
name mylonite is used for these rocks
because their fine grain size is largely the result of
recystallization in response to high strains.
4. Weak foliation: Note the fine grain size and the unimpressive foliation in this weaklymetamorphosed rock.
5. Banded texture: Hornfels is a fine-grained contact metamorphosed rock. The layers of biotite in
this sample probably respresent original sedimentary bedding.
6. Mesh texture: Serpentine is surrounding and veining relict clinopyroxene in this low grade metamafic rock.
7. Palisades texture: The region of coarser-grained quartz in the upper center portion of this
photomicrograph was probably originally occopied by coesite, the high-pressure polymorph of quartz.
8. Crenulation schistosity: The vertical foliation in this rock intersects and deforms an early
schistosity. During the development of the crenulation schistosity, quartz is removed from the zones in
which deformation occurs. Consequently, the crenulation schistosity is defined by compositional zones
enriched in mica.
9. Nematoblastic texture: Fine grained to very coarse grained texture characterized by subparallel
arangement of acicular or elongate prizmatik minerals.
10. Porphyroclastic texture: This large grain is a K-feldspar porphyroclast. Unlike porphyroblasts,
porphyroclasts are not grown in-situ, but rather are fragments of pre-existing minerals which were
broken up during the process of metamorphism.
11. Porphyroblastic texture: This is an andalusite porphyroblast with poikiloblastic texture. Also note
how the foliation (oriented roughly N-S in this view) is wrapped around the left and right corners of this
grain, suggesting synkinematic growth of the andalusite porphyroblast.
12. Porphyroblastic texture: Note the zonal distribution of quartz inclusions in this garnet
porphyroblast.
13. Porphyroblastic texture: These stubby crystals are chloritoid porphyroblasts. You can just barely
see the anomalous green interference color at the edge of some of the grains.
14. Poikiloblastic texture: Describes porphyroblasts which are riddled with finer grained inclusions of
other minerals. Here, orange tourmaline and gray K-feldspar include numerous, fine- grained quartz
and muscovite mica grains.
15. Granoblastic texture: Note how the interlocking plagioclase grains in this rock meet at ~120
degree triple junctions. This feature is characteristic of granoblastic texture.
16. Granoblastic-Polygonal texture: Note how the interlocking calcite grains in this rock meet at
~120 degree triple junctions. This feature is characteristic of granoblastic texture.
17. Granobalstic-Polysuture texture: Note the sutured grain boundaries between the quartz grains
in this rock.
18. Heterogranobalstic texture: The crystal structure of this quartz grain has been deformed
(probably by low-grade metamorphism) to produce sub-grains.
REFERENCES
http://www.geolab.unc.edu/Petunia/IgMetAtlas/meta-micro/metamicro.html
http://www.tulane.edu/~sanelson/geol212/metatexture.htm
Philpotts, A.R., 1989. Petrography of Igneous and Metamorphic Rocks. Prentice-Hall, Inc., Englewood
Cliffs, New Jersey. First edition, 180 p. Raymond, L. A., 1995. Petrology. Wm. C. Brown
Communications, Inc., 3rd edition, 742 p.