MAGMA
Magma: Largely molten, heterogeneous material usually of silicate composition, most
commonly containing suspended crystals and dissolved or trapped gases.
Other types of magmas: carbonate, sulfide, iron oxide (these are formed by miscibility)
Melt has no suspended crystals
Chemical Composition
SiO2
Al2O3
FeO (Fe2O3)
MgO, CaO
Na2O
K2O
Gas Composition
35-78 wt. %
12-18 wt. %
10-30 % in low SiO2 magmas
<5 % in high-SiO2 magmas
2.5-5.0 wt. %
0.1-7.0 wt. %
Principally H2O, CO2, SO2
Others: CO, CH4, NH3, H2, HCl, HF, H2S, S
Temperatures of Magmas
tholeiitic basalt (Kilauea, Hawaii)
1150-1225°C
basaltic andesite (Paruentia, Mexico)
1020-1110°C
andesite pumice (New Britain, SW Pacific) 940-900°C
pyroxene rhyolite (Taupo, New Zealand)
pumice flows
890-780°C
rhyolite
(Mono Crater, California)
pumice flows
790-820°C
730-780°C
[Intracrustal magmas containing substantial water can be few hundred degrees lower]
CLASSIFICATION OF IGNEOUS ROCKS
Igneous rocks are classified according to one or combination of mineralogy, texture, chemistry.
[Note that mineralogy depends on chemistry]
Textural Classification - discussed in the lab
Chemical Classification
1) Gradation of silica content
acid (silicic) SiO2 > 66 wt.%
[e.g. granites (72 wt.%); granodiorite (67 wt.%)]
intermediate SiO2 52-66 wt.%
[e.g. andesite (avg 57 wt.%); trachite (62 wt.%)]
basic
SiO2 45-52 wt.%, FeO and MgO about 10 wt. %
[e.g. most basalts 48-51 wt%]
ultrabasic
SiO2 < 45 wt. %, FeO and MgO > 10 wt. %
[e.g. peridotites (41-42 wt.%); komatiites]
2) Saturation Concept (refers to compatibility with quartz)
oversaturated saturated unsaturated -
rock contains primary quartz
does not contain quartz or an unsaturated mineral
contains an unsaturated mineral
unsaturated minerals (cannot coexist with quartz; i.e. we can write a chemical reaction
between the mineral and quartz to produce another mineral)
leucite
nepheline
sodalite, hauyne
forstieritic olivine (Mg-rich)
pyrope garnet
corundum
Mineralogic Classification
1) Gradation in Color (crude mineralogic classification)
felsic (eqv. to acid) - low density and light-color minerals (qtz, feldspars,
feldspathoids)
mafic (eqv. to basic) - denser, dark minerals (Fe, Mg containing) (pyroxene,
amphibole, olivine, biotite)
ultramafic (eqv. to ultrabasic) - completely lacking felsic minerals
2) Mode - mineral composition expressed as weight or volume percentages of minerals
(usually obtained by point-counting using a microscope)
Streckeisen Classification
3) Norm - combines chemical and mineralogic classification
Norm is a "theoretical mode" calculated in a particular sequence of steps from the
chemical analysis. The calculation scheme results in a proportion of "ideal"
anhydrous minerals that would crystallize at low pressure. The most common
scheme is the CIPW norm giving weight% minerals.