Earth Materials Lecture 14: Petrography of Igneous Rocks Flashcards
What are the things to look for the in thin sections of Igneous rocks
Determine mineralogy = composition ± glass.
Mineral status – essential / varietal / accessory.
Crystal development – euhedral / subhedral / anhedral.
➢ Anhedral: angular or rounded?
Condition of minerals – fresh / altered.
− If so by how much, where in the crystal(s) and to
what product(s)
Describe features of texture in thin section of igneous rocks
textures - relationship between minerals
Granularity – grain size.
Crystallinity – crystals / glass / mixed.
Interstitial – intergranular / intersertal.
Enclosing relationships between minerals.
Rock fabric – massive or foliated / lineated.
Interpretation – what does it all mean?
− E.g. classification, origin, history.
Define the two types of thin section igneous rock granularity
Equigranular
Crystals and or grains are all roughly the same size
Inequigranular
Crystals and or grains are different sizes. Porphyritic rock with phenocrysts, two stages of cooling
Describe the three different types of crystallinity an igneous rock can have
Holocrystalline
Igneous rock does not contain any glass, only crystals
Holohyaline
Igneous rock is only made out of glass, no crystals
Hypocrystalline
Igneous rock is made out of a combination of crystals and glass.
Decsribe a poikilitic texture
An enclosing relationship
When one mineral is surrounding or enclosing another mineral. This can be to a partial extent or to a full extent
Describe the three types of poiklitic texture
Ophitic texture
A type of poikilitic texture. Term specifically applied to augite fully enclosing plagioclase.
Subophitic texture
Augite partially enclosing plagioclase. Augite here usually occurs as subhedral prisms.
Ophitic and subophitic textures can occur together.
Indicates 2 phases of augite crystallization.
Granophyric texture
Intergrowths of quartz and alkali feldspar.
Common in granitic rocks, but also in silica-rich basic rocks. One phase crystallisation.
Define Glomerocyst
Small groups of crystals.
Augite glomerocryst
in porphyritic basalt.
Basic extrusive igneous rock
Define the rock fabric mineral lineation
Preferred alignment of groundmass sanidine crystals.
Often seen to ‘flow’ around sanidine phenocrysts.
Common to trachytes hence
trachytic texture.
Intermediate extrusive rock.
Describe the rock fabric flow banding
Flow banding is a geological term to describe bands or layers that can sometimes be seen in igneous rocks.
e.g. Ryholite
Describe Skeletal crystal shapes
Thin long crystals aligned together
Very rapid cooling, quench texture.
Olivine and augite
in komatiite,
Abitibi Greenstone Belt
Describe pyroclastic textures in igneous rocks
tuffs / ignimbrites.
Acid extrusive rocks.
Non-welded texture.
Welded texture.
