Igneous Textural Terms Flashcards
petrography
the branch of petrology that deals with the description and classification of rocks
what should a good rock description include?
the mineralogy, a proper name, and a good description of the rock’s texture in hand sample and thin section
primary textures
occur during igneous crystallization and result from interactions between minerals and melt
secondary textures
alterations that take place after the rock is completely solid; these processes do not involve melt and are thus really metamorphic in nature
- initial nucleation
- subsequent crystal growth
- diffusion of chemical species (and heat) through the surrounding medium from the surface of the growing crystal
nucleation
undercooling
cooling of a melt below the true crystallization temperature of a mineral
homogeneous nucleation
occurs when a sufficient number of ions become stable and can spontaneously cluster together, due to undercooling or supersaturation
heterogeneous nucleation
occurs on the surface of a preexisting crystal surface; either a “seed crystal” of the same mineral or a different mineral with a similar structure on which the new mineral can easily nucleate and grow
crystal growth
involves the addition of ions onto existing crystals or crystal nuclei
porphyritic texture
distinctly bimodal distribution of grain size, with one size considerably larger than the other
phenocrysts
the larger crystal grains in a porphyritic rock
matrix
the finer surrounding crystal grains in a porphyritic rock; also referred to as groundmass
vitrophyric texture
a rock with phenocrysts set in a glassy matrix
poikilitic texture
phenocrysts contain numerous inclusions of another mineral that they enveloped as they grew
oikocryst
dendritic
tree-like, branching form; crystals take on this form when the rate of diffusion is slower than the rate of growth (as in quickly cooled or “quenched” lavas)
spinifex texture
contains elongated olivine crystals; occurs in quenched ultramafic lavas (such as Precambrian komatiites)
skeletal crystals
occurs when the corners and edges of a crystal grow more rapidly than the faces, in some cases the extended corners meet to enclose melt pockets
epitaxis
general term used to describe the preferred nucleation of one mineral on another preexisting mineral (eg growth of Sil on Bt or Ms rather than as a direct replacement of it’s polymorph Ky); may occur on twin orientation of preexisting grain of the same mineral, leading to the formation of growth twins
Rapakivi texture
occurs in some graintes where plagioclase preferentially forms on alkali feldspar rather than nucleating on its own
spherultic texture
silicic volcanics in which needles of quartz and alkali feldspar grow radially from a common center
variolitic texture
radiating plagioclase laths; occurs in some basalts
comb structure
growth of elongated crystals (generally quartz) with c-axes normal to vein walls, resulting in parallel columns that resemble the teeth of a comb
crescumulate texture
parallel growth of elongated, non-equilibrium arrangements of olivine, pyroxene, feldspar, or quartz crystals that appear to nucleate on a wall or layer; typically occurs in layered mafic plutons and in the margins of granites
compositional zoning
occurs when a mineral changes composition as it grows during cooling, resulting in a concentric pattern of varying extinction; may sometimes have euhedral cores and anhedral rims
chemical zoning
occurs when equilibrium is not maintained?????
normal zoning
creates a more calcic inner zone and a more sodic outer zone (eg plagioclase with an anorthite-rich core toward an albite-rich rim); common in igneous rocks, though usually interrupted by reversals
reverse zoning
the opposite of normal zoning, creating a more sodic inner zone and a more calcic outer zone; common in some metamorphic plagioclase
oscillatory zone
the most common type of zoning in plagioclase
euhedral
crystals bounded on all sides by crystal faces; often seen in early-forming minerals in melts
subhedral
anhedral
interstitial
filling the spaces between earlier formed minerals
ophitic texture
having large pyroxene grains enclosing small, random plagioclase laths
granophyric texture
intergrowth of intricate skeletal shapes resulting in undercooling and rapid crystallization
granophyre
a rock dominated by a granophyric texture
graphic texture
a coarser variation of granophyric texture, visible in hand sample
resporption
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
sieve texture
deep and irregular embayments
foliated (planar) texture
alignment of elongated or tabular minerals
lineated texture
alignment of elongated or tabular minerals
trachytic texture
lath-shaped microlites (typically plagioclase) strongly aligned (commonly flowing around phenocrysts)
pilotaxitic (felty) texture
random or non-aligned microlites
flow banding
alternating layers of different composition; can be created by mingling of two magmatic liquids (either in a chamber or as flows)
synneusis
the process of suspended phenocrysts clustering together and adhering by surface tension; may be a prime mechanism for the production of growth twins
cumulophyric texture
multiple-grain clusters of adhering phenocrysts
glomeroporphyritic texture
multiple-grain clusters of adhering phenocrysts that are essentially of a single mineral
cumulate textures
orthocumulate texture
adcumulate texture
a nearly monomineralic cumulate with perhaps a few other minerals caught in the last interstitial points
polygonal (equilibrium) texture
the result of textural equilibrium; most common in monomineralic metamorphic rocks (eg quartzite and marble)
heteradcumulate
mesocumulate
cumulate textures that are intermediate between ortho- and adcumulates
twin
intergrowth or two or morth orientations of the same mineral with some special crystallographic relationship between them
primary (growth) twins
form due to mistakes during crystallization from a melt (eg carlsbad twinning in feldspars)
simple (carlsbad) twinning
albite twinning
microlites
matrix crystals large enough to be birefringent
crystallites
matrix crystals not large enough to be birefringent
microphenocrysts
microlites that are significantly larger than the matrix yet still microscopic
subophitic texture
having an ophitic texture which has plagioclase laths that are larger and only partially enclosed by the pyroxene
intergranular texture
having an ophitic texture which has small, discrete grains of pyroxene, olivine, etc., filling the interstices in a random network
of larger plagioclase laths
intersertal texture
having an ophitic texture which has glass, cryptocrystalline material, or alteration products occupying the spaces between plagioclase laths
hyalo-ophitic
having an intersertal texture in which a larger amount of glass is present than pyroxene
hyalopilitic
having a hyalo-ophitic texture with a large amount of glass, with plagioclase occurring only as tiny, random microlites
holohyaline texture
glassy; most common in silicic rhyolite and dacite flows
obsidian
a rock consisting of > 80% glass; often restricted to silica rich glasses; very dark colored despite silicic nature because glass is readily tinted by very minor amounts of impurity
tachylite
basaltic glass
vesicles
subspherical voids in volcanics caused by trapped bubbles of escaping gas; tend to rise in less viscous basaltic magmas, resulting in gradation from basalt to vesicular basalt to scoria
vesicular basalt
scoria
amygdules
vesicles filled with later mineral growth, typically secondary zeolite, carbonate, or opal
pumice
the silicic counterpart of scoria; typically light and frothy
pyroclastic rocks
fragmental, generally produced by explosive volcanic activity; classification is based on the nature of the fragments
pyroclasts (tephra)
fragments of pyroclastic rock
eutaxitic textures
structures caused by compression and deformation resulting from settling in hot ash accumulations
fiamme
squashed fragments formed when the gas bubbles are squeezed out of pumice
Pele’s tears
glassy pellets formed from the fine spray of bubbles bursting in fluid lavas
Pele’s hair
delicate glass threads formed when magma is stretched
accretionary lapilli
spheroidal balls of successive ash layers that accumulated on a single ash nucleus as it fell through very moist air
pistolic tuffs
consolidated deposits of accretionary lapilli
autometamorphic processes
secondary mineral reactions that occur in igneous rocks as they cool, and that are not products of a distinct later metamorphic event; they are a natural part of igneous cooling and are more common in plutonic rocks
Otswald ripening
a process of annealing (textural maturing) of crystals in a static environment, driven by grain-boundary curvature until straight boundaries result, eventually attaining textural equilibrium if allowed to continue
polymorphic transformation
alternative structural forms of the same chemical substance (eg graphite-diamond, calcite-aragonite, kyanite-andalusite-sillimanite)
displacive transformation
a polymorphic transformation involving only the shifting of atomic positions and bending of bonding angles (eg high-quartz (hexagonal structure) to low quartz (trigonal structure)); occurs readily as soon as a polymorph’s stability field is reached
reconstructive transformation
a polymorphic transformation involving the breaking and reforming of bonds (eg graphite-diamond or tridymite-high-quartz); one polymorph may remain in the stability field of another
pseudomorph
secondary twinning
twinning that occurs by secondary processes in preexisting minerals
transformation twins
caused when a high-temperature crystal structure inverts to a low temperature polymorph; the origin of crosshatched “tartan” twinning in microcline
cyclic twinning
quartz and olivine????
deformation twinning
exsolution
results in intergrowth of lamellae (occurs in K-spars and pyroxenes
perthite
exsolved albite lamellae in a K-feldspar host
antiperthite
exsolved K-feldspar lamellae in an albite host
ocelli
spherical or ovoid bodies a few millimeters to a few centimeters across that occur in some igneous rocks; result from liquid immiscibility (a primary exsolution phenomenon),k amygdule fillings, and isolated blobs of mingled magmas
deuteric alteration
a subset of autometamorphism which involves hydration (either H2O liberated from a residual melt or externally introduced)
uralitization
a deuteric alteration of pyroxene to amphibole
biotitization
alteration of pyroxene, or, more commonly, hornblende to biotite via hydration; epidote may be produced as Ca is released during this process
chloritization
alteration of any mafic mineral to chlorite via hydration
sericite
a term applied to any fine-grained white mica
seritization
the process by which felsic minerals (usually feldspars) are hydrated to produce sericite
sausseritization
the alteration of plagioclase to produce an epidote mineral
symplectite
a term applied to fine-grained intergrowths resulting from the combined growth of two or more minerals as they replace another mineral
myrmekite
an intergrowth of dendritic quartz in a single crystal of plagioclase; very common in granitic rocks, occur preferentially where plagioclase is in contact with K-feldspar
devitrification
the secondary crystallization of glass to fine-grained mineral aggregates
palagonite
brown, optically isotropic oxidation-hydration product resulting from devitrification of water-quenched basaltic glass
felsitic texture
small, equidimensional grains of interlocking feldspar and silica minerals resulting from devitrification of silicic glassy rocks; texture looks much like that of chert
spherulites
radial aggregates of crystals resulting from devitrification of glass (commonly cristobalite or tridymite and feldspar)
lithophysae
large cavities bordered by spherultitic growth, common in rhyolites
undulose extinction
a waviness in the optical extinction pattern that may result from deformation due to minor bending of the crystal lattice; very common in quartz
nesophitic
having an ophitic texture plagioclase that is larger, with interstitial pyroxenes
diktytaxitic
the texture of certain volcanics in which bounding crystals protrude into abundant angular
interstitial gas cavities
