Classification of Igneous rocks (Fabric and texture) Flashcards
Encompasses non-compositional properties of a rock that comprise textures and generally large-scale structures
Fabric
based on the proportions of glass relative to mineral grains and their sizes, shapes, and mutual arrangements that are observable on the scale of a hand specimen or thin section under the microscope.
Texture
Texture is also called
microstructures
larger-size features generally seen in an outcrop, such as bedding in pyroclastic rocks or pillows in a submarine lava flow.
Structures
very fine-grained as a result of rapid cooling at the surface. Minerals too small to be seen by the naked eye
Aphanitic
coarse-grained mineral sizes due to magma cooling at depth.
Phaneritic
very large crystals (phenocrysts) embedded in smaller crystals (groundmass)
Porphyritic
contain variable proportion of glass; molten rock quenched quickly as it was ejected into the atmosphere
Glassy or vitric
a highly viscous liquid, disordered on the atomic scale, formed from polymerized silicate melt
Glass
a porphyritic rock that contains scattered phenocrysts in a glassy matrix.
Vitrophyre
produced by fragmenting processes that create broken pieces of volcanic rock and/or mineral
grains.
Volcaniclastic/Pyroclastic
Degree of crystallinity wherein its wholly crystalline texture
Holocrystalline
Degree of crystallinity wherein its partially crystalline/partially glass texture
Hypocrystalline
Degree of crystallinity wherein its wholly glassy textures
Holohyaline
A crystal form where the crystal is bounded by faces; developed under circumstances such as slow cooling of magma in a deep-seated condition.
Euhedral /idiomorphic
Crystal form: an intermediate stage of development
Subhedral
crystal faces are absent; developed as the growth of crystals has been hindered by such factors as disturbing environment, reaction with magma, and juxtaposition of other growing crystals.
Anhedral/xenomorphic
mix of euhedral, subhedral and anhedral grains
Hypidiomorphic-granular texture
A massive, high silica glass appears in hand samples to have zero crystallinity. Under the microscope, high magnification reveals that obsidian contains abundantly nucleated submicrometer-size crystallites that experienced limited growth in the highly viscous glass.
Obsidian
Important alteration product of devitrification
Palagonite
Product of devitrification; spherulites are spherical to ellipsoidal clusters of radiating fibrous alkali feldspars and a polymorph of SiO2.
Spherulitic texture
massive glass having a waxy luster and dark color in hand sample into which 6-16 wt% water has been absorbed
Pitchstone
develops by the hydration of obsidian on fracture surfaces that are exposed to moisture in the atmosphere or to meteoric water (groundwater).
Perlitic texture
characterized by large crystals averaging more than 30 mm in diameter. it displays large, early-formed euhedral crystals surrounded by later-formed subhedral crystals.
Pegmatitic texture
refers to extremely fine-grained minerals.
APLITIC TEXTURE
contain spherical to ellipsoidal void spaces called vesicles, which are analogous to holes in a household sponge
Vesicular textures
develop due to the exsolution and entrapment of gas bubbles in lava as it cools and solidifies.
Vesicular textures
phenocrysts are found to occur as clusters of crystals
Glomeroporphyritic
vesicles have been filled with material (usually calcite, chalcedony, or quartz, then the term amygdular should be added to the textural description of the rock
Amygdular
vesicles are so abundant that they make up over 50% of the rock and the rock has a density of less than 1 (i.e. it would float in water)
Pumiceous
vesicles are so abundant that they make up over 50% of the rock and the rock has a density greater than 1,
Scoriaceous-
a texture consisting of intergrowths of quartz and alkali feldspar wherein the orientation of the quartz grains resembles cuneiform writing. This texture is most commonly observed in pegmatites.
Graphic
a texture commonly found in glassy rhyolites wherein spherical intergrowths of radiating quartz and feldspar replace the glass as a result of devitrification.
Spherulitic
a texture usually restricted to coarser-grained rocks that consist of concentrically banded spheres wherein the bands consist of alternating light-colored and dark-colored minerals.
Obicular
an intergrowth of quartz and plagioclase that shows small wormlike bodies of quartz enclosed in plagioclase. This texture is found in granites
Myrmekitic texture
laths of plagioclase in a coarse-grained matrix of pyroxene crystals wherein the plagioclase is totally surrounded by pyroxene grains. This texture is common in diabases and gabbros.
Ophitic texture
similar to ophitic texture wherein the plagioclase grains are not completely enclosed in a matrix of pyroxene grains.
Subophitic texture
smaller grains of one mineral is completely enclosed in large, optically continuous grains of another mineral.
Poikilitic texture
A texture in which the angular interstices between plagioclase grains are occupied by grains of ferromagnesium minerals such as olivine, pyroxene, or iron titanium oxides
Intergranular texture
a texture similar to intergranular texture except that the Interstices between plagioclase grains are occupied by the glass or cryptocrystalline material.
Intersertal texture
a texture similar to ophitic texture except that glass completely surrounds the plagioclase laths.
Hyaloophitic texture
a texture wherein microlites of plagioclase are more abundant than groundmass and the groundmass consists of glass which occupies the tiny interstices between plagioclase grains
Hyalopilitic texture
A texture wherein plagioclase grains show a preferred orientation due to flowage and the interstices between plagioclase grains are occupied by the glass or cryptocrystalline material.
Trachytic texture
this texture surround individual crystals as a result of the crystal becoming unstable and reacting with its surroundingcrystals or melt.
Coronas or reaction rims
occurs in plagioclase crystals where irregularly shaped patches of the crystal show different compositions as evidenced by going extinct at angles different from other zones in the crystal.
Patchy zoning
occurs in plagioclase grains wherein concentric zones around the grain show thin zones of different compositions as evidenced by extinction phenomena.
Oscillatory zoning
This sometimes occurs in plagioclase wherein individual plagioclase grains show an abundance of glassy inclusions.
Moth eaten texture
Moth-eaten texture is also known as
Sieve texture
Exsolution lamellae of albite occurring in orthoclase or microcline
Perthitic texture
a term used to describe grains with rectangular tablet shapes.
Tabular
a term used to describe grains that have all of their boundaries of approximately
Equant
a term used to describe grains that occur as long fibers
Fibrous
a term used to describe grains that occurs as long, slender crystals.
Acicular
A term used to describe grains that show an abundance of prism faces.
Prismatic
