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*Petrology > Chapter 02 - Igneous Classification & Nomenclature > Flashcards

Chapter 02 - Igneous Classification & Nomenclature Flashcards

1
Q

Method for classifying any type of rocks

A

Based upon texture & composition (mineralogical)

Textural: considered first, provide best evidence for rock origin

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2
Q

Phaneritic

A

Crystals in rock are readily visible with naked eye

Considered PLUTONIC or INTRUSIVE; cooled & crystalized slowly/beneath Earth’s surface

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3
Q

Aphanitic

A

Crystals, if any, are too small to be seen with naked eye

Considered VOLCANIC or EXTRUSIVE; cooled & crystalized rapidly

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4
Q

Fragmental

A

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

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5
Q

Equigmnular

A

Of uniform grain size

Can be classified of phaneritic or aphanitic classification

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6
Q

Porphyritic

A

Texture of rock displays two dominant grain sizes varying by great amount

Determining volcanic or plutonic: based upon the size of the groundmass

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7
Q

Phenocrysts

A

Larger crystals in a porphyritic rock; slower cooling, formed first

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8
Q

Groundmass

A

Finer crystals in a porphyritic rock; cooled quickly

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9
Q

Bowen’s Series

A 
Quartz
Plagioclase
Alkali feldspar
Muscovite
Biotite
Hornblende
Pyroxene
Olivine
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10
Q

Felsic

A

Feldspar + silica
Ex: quartz, plagioclase, alkali feldspar, muscovite
Any feldspathoids present
Light colored silicates

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11
Q

Mafic

A

Magnesium + ferric iron
Ex: biotite, hornblende, pyroxene, olivine
Darker colored silicates

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12
Q

Accessory minerals

A

Present in very small quantities, usually containing apatite, zircon, sphene, epidote, oxide/sulfide, silicate alteration product (e.g. chlorite)

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13
Q

Ultramafic

A

Rock with >90% mafic minerals

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14
Q

Leucocratic

A

Light-colored rock

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15
Q

Melanocratic

A

Dark-colored rock

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16
Q

Color index [of rock]

A

Color of a rock quantified; the volume % of dark minerals

17
Q

Silicic

A

Chemical term: SiO2 content of a rock

18
Q

Magneisan

A

Chemical term: MgO content of rock

19
Q

Alkaline

A

Chemical term: Na2O + K2O content of a rock

20
Q

A luminous

A

Chemical term: Al2O3 content of a rock

21
Q

Acidic & basic (& intermediate & ultrabasic)

A

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

22
Q

Normalized

A

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)

23
Q

Mode

A

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

24
Q

IUGS classification process

A
  1. Determine mode of each mineral present
  2. 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
  3. 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%.
  4. Determine if rock is plutonic/phaneritic or volcanic/aphanitic.
  5. To find the field it belongs to, find 100P/(P+A) ratio.
  6. If rock is plutonic/phaneritic, and Q+A+P+F<10 —> see mafic & ultramafic rocks.
25
Q

Phaneritic rocks

A
  1. Don’t use “foid” in rock name (it’s a general term; use the actual name of feldspathoids itself instead!).
  2. 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
26
Q

Mafic & ultramafic rocks

A

Classified separately:

  1. Gabbro is rocks (plagioclase & mafics)
  2. Ultramafic > 90% mafics
27
Q

Classifying volcanic/aphanitic rocks

A

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.

28
Q

Phenotypes

A

For classifying volcanic rocks; rocks ID’d as such, have prefix “pheno-“ added.

29
Q

Hypabyssal rocks

A

Shallow intrusive rocks

30
Q

Diana’s rocks

A

Dolerite, in Britain

31
Q

Carbonatites

A

Igneous carbonates

32
Q

Lamproites/lamprophyres

A

Highly alkaline, volatile-rich mafic flow/dike rocks

33
Q

Spilites

A

Sodic basalts

34
Q

Keratophyres

A

Sodic intermediate volcanics

35
Q

Classifying pyroclastic rocks

A

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

36
Q

Aquagene tuff

A

Waterborne accumulation of ash
Either from:
- subaqueous eruption
- airborne accumulation reworked by water

37
Q

Halo-clastite

A

Aquagene tuff created when magma is shattered when it comes into contact with water

*Petrology (3 decks)

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