Petrology and Optical Features Flashcards
Formation of rational symmetry intergrowth of 2 or more grains of crystalline species.
Twinning
Form of penetration twinning where two
crystals form as penetration twins.
Carlsbad Twining
common within the plagioclase feldspars, in places where two adjoining twin slabs or lamellae are mutually reversed with respect to each other and every alternate twin ‘plate’ or ‘slab’ has an identical atomic structure
Lamellar Twining
shows two kinds of repeated twinning in thin section, with one set of twins arranged at 90° to the other set.
Cross-hatched twinning
Refers to solid solution which do not have uniform composition.
Zoning
Types of Zoning
Normal Zoning
Reverse Zoning
Oscillatory Zoning
Types of Zoning where the center is more calcic becoming more sodic toward the margin
Normal Zoning
Types of Zoning where the center is more sodic becoming more calcic toward the margin.
Reverse Zoning
Types of Zoning that is normally steplike progressions for more calcic interior to more sodic margins w/ local reversals in adjacent zones
Oscillatory Zoning
it is when a mineral becomes dark parallel to the crosshairs.
Parallel Extinction
Extinct at an angle
with the direction of polars
Inclined/Oblique
planes of mineral to the diagonal; vibration direction is diagonal
Symmetrical
phenocrysts lie in a matrix of glass.
Vitrophyric Texture
the groundmass is a dense intergrowth of quartz and feldspar.
Felsophyric Texture
the groundmass of feldspar are rectangular in form instead of slender lath crystals
Orthopyric Texture
porpyritic texture in which phenocrysts are clustered into aggregates called glomerocrysts or crystal clots. It is common and often included plagioclase and pyroxenes in basic rocks.
Glomeroporphyritic Texture
an important consideration in crystal fractionation by crystal settling
Glomerocrysts
Refers to crystals, typically phenocrysts, in an igneous rock which contain small grains of other minerals.
Poikilitic Texture
A variant the Poikilitic texture, is one where random plagioclase laths are enclosed by pyroxene or olivine.
Ophitic Texture
A variant the Poikilitic texture,** plagioclase is larger and encloses the ferromagnesian minerals.**
Sub-ophitic
Angular interstices between feldspars filled with glass instead of pyroxene
Hyalophitic Texture
A genetic term for a border of secondary minerals formed at the margin of a primary grain in an igneous or metamorphic rock.
Corona Texture/ Reaction Rim /
Opacitic Rim
Secondary rim/coronas
Kelyphitic Rim
a textural term indicating that** a crystal occupies the angular space between at least two larger crystals.**
Intergranular Texture
A textural term used to denote that the angular spaces between larger crystals is occupied by glass, or glass and small crystals.
Intersertal Texture
Texture of the groundmass of a holocrystalline igneous rock in which** lath-shaped microlites (typically plagioclase) are arranged in a glass-free mesostasis** and are** generally interwoven in irregular unoriented fashion**.
Pilotaxitic Texture
A texture of extrusive rocks in which the groundmass contains little volcanic glass and consists predominantly of minute tabular crystals, namely, sanidine microlites.
Trachytic Texture
An intergrowth of branching rods of quartz set in a single crystal of plagioclase, neighboring rod of quartz have the same lattice orientation and extinguish together.
Myrmekitic Texture
________________ commonly exhibits a variety of disequilibrium textures in volcanic rocks, especially in orogeny andesites
Plagioclase
common in plagioclase or in pyroxene crystals in extrusive volcanic rocks and is interpreted as the result of mixing processes
Sieve Texture
A process by which a solid solution phase unmixes into two separate phases in the solid
state.
Exsolution
an intimate intergrowth of sodic and potassic feldspar resulting from subsolidus exsolution
Perthitic Texture
an intergrowth arising due to exsolution where potassic feldspar is present as blebs or lamellae within a sodic feldspar.
Anti-PerthicTexture
Chiefly minerals are anhedral.
and are common in aplites
Saccharoidal/ Aplitic
chiefly minerals are euhedral and are common in dark hypabyssal rocks or lamprophyres
Lamprophyric
all faces are present
Granitic
Extremely minute, incipient crystals, provided they are birefringent.
Microlites
Smaller, spherical, rod- and hair-like isotropic from.
Crystallite
refers to degree of crystallinity, grain size or granularity, and the fabric or geometrical relationship between the constituents of a rock.
Texture
what are the Degree of crystallinity
Holocrystalline
Holohyaline
Hypocrystalline/Merocrystalline
consist entirely of glass
Holohyaline
consist wholly of crystals
Holocrystalline
Contain both crystals and glass
Hypocrystalline/Merocrystalline
Crystals were bounded completely by crystal faces.
Euhedral / Idiomorphic / Automorphic
Crystals were not bounded by crystal faces.
Anhedral / Allotriomorphic / Xenomorphic
Crystals were partially bounded by crystal faces
Subhedral / Hypidiomorphic
