Chapter 02 - Igneous Classification & Nomenclature

Question 1

Method for classifying any type of rocks

Answer 1

Based upon texture & composition (mineralogical)

Textural: considered first, provide best evidence for rock origin

Question 2

Phaneritic

Answer 2

Crystals in rock are readily visible with naked eye

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

Question 3

Aphanitic

Answer 3

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

Considered VOLCANIC or EXTRUSIVE; cooled & crystalized rapidly

Question 4

Fragmental

Answer 4

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

Question 5

Equigmnular

Answer 5

Of uniform grain size

Can be classified of phaneritic or aphanitic classification

Question 6

Porphyritic

Answer 6

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

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

Question 7

Phenocrysts

Answer 7

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

Question 8

Groundmass

Answer 8

Finer crystals in a porphyritic rock; cooled quickly

Question 9

Bowen’s Series

Answer 9

Quartz
Plagioclase
Alkali feldspar
Muscovite
Biotite
Hornblende
Pyroxene
Olivine

Question 10

Felsic

Answer 10

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

Question 11

Mafic

Answer 11

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

Question 12

Accessory minerals

Answer 12

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

Question 13

Ultramafic

Answer 13

Rock with >90% mafic minerals

Question 14

Leucocratic

Answer 14

Light-colored rock

Question 15

Melanocratic

Answer 15

Dark-colored rock

Question 16

Color index [of rock]

Answer 16

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

Question 17

Silicic

Answer 17

Chemical term: SiO2 content of a rock

Question 18

Magneisan

Answer 18

Chemical term: MgO content of rock

Question 19

Alkaline

Answer 19

Chemical term: Na2O + K2O content of a rock

Question 20

A luminous

Answer 20

Chemical term: Al2O3 content of a rock

Question 21

Acidic & basic (& intermediate & ultrabasic)

Answer 21

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

Question 22

Normalized

Answer 22

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)

Question 23

Mode

Answer 23

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

Question 24

IUGS classification process

Answer 24

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.

Question 25

Phaneritic rocks

Answer 25

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

Question 26

Mafic & ultramafic rocks

Answer 26

Classified separately: 1. Gabbro is rocks (plagioclase & mafics) 2. Ultramafic > 90% mafics

Question 27

Classifying volcanic/aphanitic rocks

Answer 27

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.

Question 28

Phenotypes

Answer 28

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

Question 29

Hypabyssal rocks

Answer 29

Shallow intrusive rocks

Question 30

Diana’s rocks

Answer 30

Dolerite, in Britain

Question 31

Carbonatites

Answer 31

Igneous carbonates

Question 32

Lamproites/lamprophyres

Answer 32

Highly alkaline, volatile-rich mafic flow/dike rocks

Question 33

Spilites

Answer 33

Sodic basalts

Question 34

Keratophyres

Answer 34

Sodic intermediate volcanics

Question 35

Classifying pyroclastic rocks

Answer 35

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

Question 36

Aquagene tuff

Answer 36

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

Question 37

Halo-clastite

Answer 37

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