Igneous Textures (textbook section 7.3-7.6)
Correlative readings from Hefferan & O’Brien (2010) textbook listed in italic.
Chemical Composition of Igneous Rocks (section 7.3)
major elements: >1% by weight in Earth’s crust
O & Si constitute 75% by mass of Earth’s crust
O, Si, A., Fe, Ca, Na, K, Mg constitute 99.2% by mass of Earth’s crust
(Mg is especially abundant in the Earth’s mantle)
(Fe is the most abundant, by weight, in the whole Earth)
minor elements: 0.1% - 1% by weight in Earth’s crust
especially Ti, P, H, Cr, Mn
major & minor element contents commonly listed as oxides
O is the only anion among major elements
and all cations in silicate minerals bond to O
chemical analysis list oxide percents assuming there is just enough oxygen
! for all major & minor cations to bond to
e.g., Kspar, KAlSi3O8 is composed of 1/2(K2O + Al2O3 + 6SiO2)
trace elements: <0.1% by weight in Earth’s crust
incompatible elements
elements that don’t fit well into common crystal lattice sites
! (too big or wrong charge)
prefer to go into a melt during partial melting
! concentrated in resulting magma
! depleted in residual rock left behind
incompatible elements include:
! Large Ion Lithopile (LIL) elements (large for their charge)
! ! Cs, Ba, Rb, Sr, U, Pb K, Zr, Th
! Light Rare Earth elements (LREE)
! ! Rare Earth Elements La through Sm
compatible elements
elements that are very stably bonded in silicate structures
! (good size & charge)
they are highly immobile and reman in the restite; are depleted in the magma
incompatible elements include:
! High Field Strength (HFS) elements (small for their charge)
! ! Ti, Ni, Cr, V, Zr, Hf, Nb, Y
! Heavy Rare Earth Elements (HREE)
Mineral Composition of Igneous Rocks (section 7.4-7.5)
the most abundant minerals in the crust are the feldspars, dominantly plagioclase
followed by quartz and the pyroxenes, then amphiboles, micas, and clays
primary minerals crystalize from the melt
secondary minerals crystalize later, e.g., from hydrothermal fluids
major minerals are the most abundant, normally silicates
accessory minerals are found in small concentrations (<5%)
simple oxides (opaque in microscope thin section view) are common accessories
igneous classification based on color index
percentage of dark colored minerals (DCM)
<40% DCM: felsic
40-70% DCM: intermediate
70-90% DCM: mafic
90-100% DCM: ultramafic
! the problem with this scheme is that the plagioclase found in mafic rocks
! is commonly dark in color, even though it is not a “mafic” mineral
IUGS color index
<35% DCM: leucocratic
35-65% DCM: mesocratic
>65% DCM: melanocratic
! this system is more objective; it doesn’t imply a definite composition
modal composition: actual minerals in an igneous rock
relies on visual inspection to determine the percentage, by volume, of each mineral
done microscopically for fine-grained rocks
point count analysis: count minerals at each grid point (at least 400 points)
! on a microscope slide
not possible in igneous rocks with significant cryptocrystalline or glassy portions
! since actual minerals can’t be counted
normative composition
CIPW Norm: first introduced by Cross, Iddings, Pirsson, and Washington (1902)
calculation of the ideal minerals that should crystallize from a magma
! based on the chemical composition (oxide percents) determined in the lab
! from chemical analysis of crushed rock samples
calculation of normative minerals based on a set of rules
assumes:
! magma dry (no hydrous minerals)
! no Al2O3 in mafic minerals
! Mg/Fe ratio same in all mafic minerals
! can’t have quartz and feldspathoids (silica undersaturated) in same rock
can calculate with program or spread sheet
classification based on silica content
<45% SiO2: ultrabasic
45-52% SiO2: basic
52-66% SiO2: intermediate
>66% SiO2: acidic
! the old “acidic” concept for silica-rich rocks was based on the untrue notion that
! the silica in rocks was from silicic acid (H4SiO4)
classification based on a silica saturation, according to CIPW Norm
! and the presence of normative quartz, feldspars, feldspathoids
silica oversaturated:! quartz with feldpars
silica saturated:!
feldspars but no quartz
silica undersaturated:! feldspars and feldspathoids but no quartz
classification based on a alumina (Al2O3) content compared to CaO, Na2O & K2O
useful for characterizing granitic rocks
peraluminous:!
Al2O3 > CaO + Na2O + K2O
metaluminous:! Al2O3 > Na2O + K2O
subaluminous:!
Al2O3 = Na2O + K2O
peralkaline:!
Al2O3 < Na2O + K2O
Classification of Igneous Rocks (section 7.6)
simple classification (intro classes)
based on texture and color (felsic, intermediate, mafic, ultramafic)
doesn’t account for compositional differences aside from silica content
IUGS classification
Streckeisen (1976) for IUGS, LeBas & Streckeisen (1991), LeMaitre (2002)
based on modal concentrations of
Q = Quartz (and polymorphs)
A = Alkali Feldspars (k-spar & albite [An0 - An5])
P = Plagioclase (An5 - An100)
F = Feldspathoids (foids)
M = mafic silicates (plus metal oxides)
note the differentiation between alkali feldspars & plagioclase at An5
b/c nearly pure albite (An0) is found in perthite with kspar
QAPF classification diamond for plutonic igneous rocks
upper diamond: silica supersaturated rocks
lower diamond: silica understaurated rocks
classification of mafic intrusive igneous rocks
Plagioclase-Pyroxene-Olivine ternary diagram, or
Plagioclase-Pyroxene-Hornblende ternary diagram
classification of ultramafic intrusive igneous rocks
Olivine-Orthopyroxene-Clinopyroxene ternary diagram, or
Olivine-Pyroxene-Hornblende ternary diagram
QAPF classification diamond for volcanic igneous rocks
problem: classification based on modal mineralogy
! not possible in glassy or cryptocrystalline volcanic rocks
classification of volcanic rocks based on alkalis (Na2O + K2O) vs. silica content
based on chemical analysis of crushed rock samples