Igneous Classification
• Igneous rocks can be classified according to composition,
mineralogy, texture and/or locality(!).
• The first distinction is between volcanic and plutonic rocks.
– Volcanic rocks are erupted at the Earth’s surface and cool very
quickly. There is insufficient time to grow large crystals. This leads
to formation of glass or very fine-grained rocks, or to phenocrysts
(crystals that grew before eruption) in a fine groundmass.
– Plutonic rocks crystallize at some depth, and therefore lose heat
relatively slowly. Crystals have time to grow after nucleation, and
the resulting rocks generally have individual crystals large enough
to see unaided.
– Rocks of exactly the same composition and mineralogy get
different names in their volcanic and plutonic forms, because they
look different!
Plutonic vs. Volcanic
Classification by mineralogy
• The standard classification scheme uses the mineralogy of the rock
(how much quartz, how much plagioclase, etc.)
– There is one important twist…for volcanic rocks you usually
cannot measure the actual minerals present (or it may be a glass
and there are no minerals present).
– In this case, instead of the actual minerals, you classify based on
normative mineralogy
• The norm is a calculation based on the bulk composition of a
volcanic rock, for what minerals would be present if it were
fully crystallized.
• The standard norm calculation is called the CIPW norm, after
Cross, Iddings, Pirsson, and Washington (1902).
Classification by mineralogy
For rocks with Q+A+P+F > 10%
Mineral content (actual or
normative) of the rock by
volume is divided into Quartz
(Q), Alkali feldspar (A),
Plagioclase (P),
Feldspathoids (F), and Mafic
minerals like amphibole,
biotite, pyroxenes, and
olivine (M).
For rocks with M < 90%, the
Streckeisen double-triangle
is used. It shows names
defined by Q-A-P-F
recalculated to 100%.
Many of these names are
really obscure; don’t try to
learn all of them.
Quartz
Quartz-rich
granitoids
Alkali Feldspar
Granite (Rhyolite)
Granite
(Rhyolite)
Quartz Alkali
Feldspar Syenite
(Trachyte)
Alkali Feldspar
Syenite (Trachyte)
Alkali
Feldspar
Nepheline-bearing
Alkali Feldspar
Syenite (Trachyte)
Tonalite
Trondhjemite
Plagiogranite
Granodiorite
(Dacite)
Quartz
Monzonite
Quartz
Syenite
(Quartz Trachyte)
(Quartz Latite)
Quartz Diorite
(Quartz Andesite)
Quartz
Monzodiorite
( Andes ite)
Syenite (Trachyte) Monzonite (Latite) Monzogabbro
Nepheline-brg Nepheline-brg Nepheline-brg
Syenite (Trachyte) Monzonite (Latite) Monzogabbro
Nepheline
Monzosyenite
(Tephritic
Phonolite)
Nepheline
Monzodiorite,
Monzogabbro
(Phonolitic
Tephrite,
Basanite)
Diorite (Andesite),
Anorthosite, Gabbro,
Norite (Basalt)
Plagioclase
Nepheline-bearing
Diorite (Andesite)
or Gabbro (Basalt)
Nepheline Diorite,
Gabbro (Tephrite,
Basanite)
Nepheline Syenite
(Phonolite)
Ijolite,
Nephelinite,
Leucitite
Feldspathoids
Classification by mineralogy
For rocks with mostly mafic minerals, a different scheme is used. The
proportion of olivine, orthopyroxene, clinopyroxene, and plagioclase locate
a rock using the appropriate Streckeisen ternary diagram.
plagioclas e
anorthosite
90
(leuco-)
dunite
65
gabbro or
norite
90
troctolite
olivine gabbro
or norite
harz burgite
35
peridotites
plag-bearing ultramafic rocks
pyroxene s
40
olivine
orthopyrox enite
10
olivine webs terite
olivin e
to plagioclas
e
orthopyrox enite
gabbro or norite
orthopyroxen
wehrlite
lherzolite
(mela-)
norite
olivin e
orthopyroxen
gabbro
cpx norite
10
opx gabbro
plag-bearing ultramafic rocks
clinopyroxen e
e
websterite
e
olivine
clinopy roxenite
pyroxenites
clinopy roxenite
clinopyroxen e
Classification by composition
• There are several classifications, of individual rocks or rock suites.
• By silica percentage:
%SiO2 Designation
% Dark Minerals Designation
Example rocks
>66
Acid
<40
Felsic
Granite, rhyolite
52-66 Intermediate
40-70
Intermediate
Diorite, andesite
45-52 Basic
70-90
Mafic
Gabbro, basalt
<45
Ultrabasic
>90
Ultramafic
Dunite, komatiite
• By alumina saturation (this controls which dark minerals show up):
Chemistry
Designation
Distinctive Minerals
Al2O3>Na2O+K2O+CaO
Peraluminous
Muscovite, biotite, topaz,
corundum, garnet, tourmaline
Na2O+K2O+CaO>Al2O3
Metaluminous
Melilite, biotite, pyroxene
& Al2O3 > Na2O+K2O
hornblende, epidote
Al2O3 ~ Na2O+K2O
Subaluminous
Olivine, pyroxenes
Al2O3 < Na2O + K2O
Peralkaline
Sodic pyroxenes & amphiboles
Classification by composition
• By Alkali-Lime index: for a suite of rocks, CaO and Na2O+K2O are plotted
against SiO2. Generally, CaO decreases with increasing SiO2 while
Na2O+K2O increases. Suites are classified by the SiO2 where the intersection
occurs:
Rock Suite
Calcic
Calc-alkaline
Alkali-calcic
Alkaline <51%
Alkali-Lime Index
Illustrative rock series
>61 %SiO2
Mid-ocean ridge basalts
56-61%
Continental margin arc series
51-56%
Some intraoceanic island arcs
Intraplate continental melts
Riassumendo….
I componenti chimici di una roccia vengono espressi in tre
modi:
Elementi maggiori (presenti in percentuali superiori all’1%
in peso). Tali elementi vengono misurati con l’ossido di
riferimento (es. Si = SiO2);
Elementi minori (presenti in percentuali comprese tra lo
0,1 e l’1% in peso). Anche tali elementi vengono misurati
con l’ossido di riferimento (es. Mn = MnO);
Elementi in traccia (presenti in percentuali inferiori allo
0,1%). Tali elementi vengono misurati in parti per milione
(ppm). 10000 ppm = 1%.
Analisi chimiche rappresentative di rocce vulcaniche
(elementi maggiori e minori)
B-A
Ol th
TH
Al
LC
And Dac
Riol
Co
69,7
73,2
75,2
1,0
0,4
0,2
0,1
16,0
15,2
14,0
12,0
SiO2
45,4
49,2
53,8
49,1
46,2 60,0
TiO2
3,0
2,3
2,0
1,5
Al203
14,7
13,3
13,9
17,7
14,4
Fe2O3
4,1
1,3
2,6
2,8
4,1
1,9
1,1
0,6
0,9
FeO
9,2
9,7
9,3
7,2
4,4
6,2
1,9
1,7
1,2
MnO
0,2
0,2
0,2
0,1
0
0,2
0,0
0,0
0,1
MgO
7,8
10,4
4,1
6,9
7,0
3,9
0,9
0,4
0.0
CaO
10,5
10,9
7,9
9,9
13,2
5,9
2,7
1,3
0,3
Na20
3,0
2,2
3,0
2,9
1,6
3,9
4,5
3,9
4,8
K20
1,0
0,5
1,5
0,7
6,4
0,9
3,0
4,1
4,7
P205
0,4
0,2
0,4
0,3
0,4
0,2
0,1
0,0
0,1
1,2
N.B. La SiO2 è sempre l’ossido più abbondante!
Classificazione delle rocce vulcaniche
– Classificazione su base CHIMICA
TAS = Total
Alcali vs. Silica
HAWAIITE
TRACHIBASALTO
POTASSICO
MUGEARITE
BENMOREITE
SHOSHONITE
LATITE
Na
K
Oltre alla classificazione su base mineralogicapetrografica (triangoli di Streckeisen) e quella su
base chimica (diagramma TAS), esiste un ulteriore
criterio classificativo: la
normativa.
classificazione
La classificazione normativa si basa su una serie di
calcoli proposti da un gruppo di studiosi americani
nei primi anni del ‘900 ed è nota come NORMA CIPW
(dalle iniziali degli studiosi Cross, Iddings, Pearson e
Washington).
Il calcolo normativo si basa sulla composizione
chimica della roccia e ricostruisce i minerali teorici
(VIRTUALI) che si sarebbero potuti formare a
partire da quella certa composizione di fuso.
