Chapter 02 - Igneous Classification & Nomenclature Flashcards
Method for classifying any type of rocks
Based upon texture & composition (mineralogical)
Textural: considered first, provide best evidence for rock origin
Phaneritic
Crystals in rock are readily visible with naked eye
Considered PLUTONIC or INTRUSIVE; cooled & crystalized slowly/beneath Earth’s surface
Aphanitic
Crystals, if any, are too small to be seen with naked eye
Considered VOLCANIC or EXTRUSIVE; cooled & crystalized rapidly
Fragmental
Rock composed of pieces of disaggregated igneous material, deposited and later amalgamated
May include pieces of pre-existing (mainly igneous) rock, crystal fragments, or glass
Called PYROCLASTIC
Equigmnular
Of uniform grain size
Can be classified of phaneritic or aphanitic classification
Porphyritic
Texture of rock displays two dominant grain sizes varying by great amount
Determining volcanic or plutonic: based upon the size of the groundmass
Phenocrysts
Larger crystals in a porphyritic rock; slower cooling, formed first
Groundmass
Finer crystals in a porphyritic rock; cooled quickly
Bowen’s Series
Quartz Plagioclase Alkali feldspar Muscovite Biotite Hornblende Pyroxene Olivine
Felsic
Feldspar + silica
Ex: quartz, plagioclase, alkali feldspar, muscovite
Any feldspathoids present
Light colored silicates
Mafic
Magnesium + ferric iron
Ex: biotite, hornblende, pyroxene, olivine
Darker colored silicates
Accessory minerals
Present in very small quantities, usually containing apatite, zircon, sphene, epidote, oxide/sulfide, silicate alteration product (e.g. chlorite)
Ultramafic
Rock with >90% mafic minerals
Leucocratic
Light-colored rock
Melanocratic
Dark-colored rock
Color index [of rock]
Color of a rock quantified; the volume % of dark minerals
Silicic
Chemical term: SiO2 content of a rock
Magneisan
Chemical term: MgO content of rock
Alkaline
Chemical term: Na2O + K2O content of a rock
A luminous
Chemical term: Al2O3 content of a rock
Acidic & basic (& intermediate & ultrabasic)
Good for melt/magma content chemical composition:
Acidic: silica content in melt, > 66 wt % SiO2
Intermediate: melt content, 52-66 wt % SiO2
Basic: melt content, 45-52 wt % SiO2
Ultrabasic: melt content, < 45 wt % SiO2
Normalized
To plot mineral components on triangular diagram:
Values must add to 100%; if they don’t, they must be NORMALIZED
Done by:
Value*100/(X+Y+Z)
Ex: X=9.0, Y=2.6, Z=1.3
Multiply each by 100/(9.0+2.6+1.3)
Mode
Percentage of each mineral present, based on volume
Estimated based on cumulative area of each mineral type as seen on surface of hand specimen or under microscope
IUGS classification process
- Determine mode of each mineral present
- From mode, determine volume % of each of the following:
Q’ = % quartz
P’ = % plagioclase (An5-An100; vs. pure albite, an alkali feldspar)
F’ = total % feldspathoids
M’ = total % mafics & accessories - Majority of surface rocks have at least 10% Q’+A’+P’ OR F’+A’+P’
—> Qtz not compatible with feldspathoids/never occur in equilibrium in same rock
—> If rock has at least 10% of these, IGNORE M’ & normalize the 3 parameters to 100%. - Determine if rock is plutonic/phaneritic or volcanic/aphanitic.
- To find the field it belongs to, find 100P/(P+A) ratio.
- If rock is plutonic/phaneritic, and Q+A+P+F<10 —> see mafic & ultramafic rocks.
Phaneritic rocks
- Don’t use “foid” in rock name (it’s a general term; use the actual name of feldspathoids itself instead!).
- Rocks plotting near P: common rock types occur here: gabbro, diorite, anorthosite; can’t be distinguished via QAPF alone.
—> Anorthosite: >90% plagioclase in un-normalized mode, thus easily distinguished.
Diorite & gabbro, though:
—> hand sample: gabbro > 35% mafic in mode; diorite < 35% mafic in mode (color index: plagioclase more calcium than An50 is usually black, whereas white when more sodic —> gabbros = black; diorites = salt & pepper)
—> thin section: plagioclase composition; gabbro: more anorthite rich than An50, whereas diorite < An50.
**Plagioclase composition = priority
Mafic & ultramafic rocks
Classified separately:
- Gabbro is rocks (plagioclase & mafics)
- Ultramafic > 90% mafics
Classifying volcanic/aphanitic rocks
More difficult to determine mode
Matrix: mainly fine grain size
Vitreous/glassy or amorphous material
Thus, nearly impossible, even in thin section, to determine representative mineralogical mode
Plot near P? Problematic. Distinguishing andesite from basalt —> use color index or silica content and NOT plagioclase composition.
Andesite: Plagioclase-rich with color index < 35%, or with > 52% SiO2
Basalt: color index > 35% and < 52% SiO2
Normalize chemical analysis; add Na2O + K2O and plot against SiO2.
Phenotypes
For classifying volcanic rocks; rocks ID’d as such, have prefix “pheno-“ added.
Hypabyssal rocks
Shallow intrusive rocks
Diana’s rocks
Dolerite, in Britain
Carbonatites
Igneous carbonates
Lamproites/lamprophyres
Highly alkaline, volatile-rich mafic flow/dike rocks
Spilites
Sodic basalts
Keratophyres
Sodic intermediate volcanics
Classifying pyroclastic rocks
Mainly based upon type of fragmental materials (pyroclasts)
> 64 mm diameter & molten during fragmentation = bombs
64 mm diameter & NOT molten during fragmentation = blocks
2-64 mm diameter = lapilli
< 2 mm diameter = ash
Aquagene tuff
Waterborne accumulation of ash
Either from:
- subaqueous eruption
- airborne accumulation reworked by water
Halo-clastite
Aquagene tuff created when magma is shattered when it comes into contact with water
