classifications of igneous rocks
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Classifications of Igneous Rocks. Chapter 2. Classification of Igneous Rocks. X = 100%. Discussion: Normalization If total does not add to 100%, normalize by multiplying each term by 100/(X + Y + Z). X = 0%. - PowerPoint PPT PresentationTRANSCRIPT
Classifications of Igneous RocksClassifications of Igneous Rocks
Chapter 2Chapter 2
Classification of Igneous RocksClassification of Igneous Rocks
Method #1 for plotting a point with the components: 70% X, 20% Y, and 10% Z on triangular diagrams. Figure 2-1a. from your text: An Introduction to Igneous and Metamorphic Petrology, John Winter.
X = 100%
X = 0%
Discussion:NormalizationIf total does not add to 100%, normalize by multiplying each term by 100/(X + Y + Z)
Normalization ExampleNormalization Example
NormalizationIf total does not add to 100%, normalize by multiplying each term by 100/(X + Y + Z)Ex 1. A sample contains X = 9 g. Qtz,Y = 2.6 g Plag, Z= 1.3 g MicroclineWhat are the percentages by weight?Sol’n: Multiply each by 100/ (9 + 2.6 + 1.3) = 7.7529 x 7.752 = 69.76, 2.6 x 7.752 = 20.151.3 x 7.752 = 10.1 percentstotal 99.99% close enough to 70, 20, 10 percents respectively
IUGS IUGS Classification of Classification of PhaneriticPhaneriticIgneous RocksIgneous Rocks
Figure 2-2. A classification of the phaneritic igneous rocks. a. Phaneritic rocks with more than 10% (quartz + feldspar + feldspathoids). After IUGS. From your text: An Introduction to Igneous and Metamorphic Petrology, John Winter, Prentice Hall.
The rock must contain a total ofat least 10% of the minerals below.Renormalize to 100%
(a)
Quartz-richGranitoid
9090
6060
2020Alkali Fs.Quartz Syenite Quartz
SyeniteQuartz
MonzoniteQuartz
Monzodiorite
Syenite Monzonite Monzodiorite
(Foid)-bearingSyenite
5
10 35 65
(Foid)-bearingMonzonite
(Foid)-bearingMonzodiorite
90
Alkali Fs.Syenite
(Foid)-bearingAlkali Fs. Syenite
10
(Foid)Monzosyenite
(Foid) Syenite
(Foid)Monzodiorite
(Foi
d) G
abbr
o
Qtz. Diorite/Qtz. Gabbro
5
10
Diorite/Gabbro/Anorthosite
(Foid)-bearingDiorite/Gabbro
60
(Foid)olites
Quartzolite
Granite Grano-diorite
Tonalite
Alka
li Feld
spar
Gra
nite
Q
A P
F
60
Define Tonalite, Monzonite,Syenite based on this.
International Union of Geological Sciences Don’t use “foid” in a rock name. Use the actual Feldspathoid mineral name
Classification of Classification of Aphanitic Igneous Aphanitic Igneous
RocksRocks
Figure 2-3. A classification and nomenclature of volcanic rocks. After IUGS. From your text: An Introduction to Igneous and Metamorphic Petrology, John Winter, Prentice Hall.
(foid)-bearing Trachyte
(foid)-bearing Latite
(foid)-bearing Andesite/Basalt
(Foid)ites
10
60 60
35 65
10
20 20
60 60
F
A P
Q
Rhyolite Dacite
Trachyte Latite Andesite/Basalt
Phonolite Tephrite
Define Dacite, Trachyte, Latiteand Phonolite and Tephritebased on this
Classification of Aphanitic Igneous RocksClassification of Aphanitic Igneous Rocks
Figure 2-4. A chemical classification of volcanics based on total alkalis vs. silica. After Le Bas et al. (1986) J. Petrol., 27, 745-750. Oxford University Press.
Classification of Pyroclastic Igneous RocksClassification of Pyroclastic Igneous Rocks
Figure 2-5. Classification of the pyroclastic rocks. a. Based on type of material. After Pettijohn (1975) Sedimentary Rocks, Harper & Row, and Schmid (1981) Geology, 9, 40-43. b. Based on the size of the material. After Fisher (1966) Earth Sci. Rev., 1, 287-298. From your text: An Introduction to Igneous and Metamorphic Petrology, John Winter, Prentice Hall.
Classification of MineralsClassification of Minerals Common Silicate mineralsCommon Silicate minerals
Nesosilicates – Independent TetrahedraNesosilicates – Independent Tetrahedra OlivineOlivine
• High temperature Fe-Mg silicate (typical High temperature Fe-Mg silicate (typical mantle mineral - formed 100’s km in Earthmantle mineral - formed 100’s km in Earth
• Individual tetrahedra linked together by iron Individual tetrahedra linked together by iron and magnesium ionsand magnesium ions
• Forms small, rounded crystals with no Forms small, rounded crystals with no cleavagecleavage
(Mg,Fe)2SiO4
High interference colorsNo consistent cleavages
Classification of MineralsClassification of Minerals Common Silicate mineralsCommon Silicate minerals
Pyroxene Group Single Chain InosilicatesPyroxene Group Single Chain Inosilicates for example (for example (Mg,Fe)SiOMg,Fe)SiO33
• Single chain structures involving iron and Single chain structures involving iron and magnesium, chains weakly pairedmagnesium, chains weakly paired
• Two distinctive cleavages at nearly 90 degreesTwo distinctive cleavages at nearly 90 degrees
• AugiteAugite is the most common mineral in the is the most common mineral in the pyroxene grouppyroxene group
Classification of MineralsClassification of Minerals
Common Silicate mineralsCommon Silicate minerals Amphibole Group Double Chain InosilicatesAmphibole Group Double Chain Inosilicates CaCa22(Fe,Mg)(Fe,Mg)55SiSi88OO2222(OH)(OH)22
• Double chain structures involving a variety of Double chain structures involving a variety of ionsions
• Two perfect cleavages exhibiting angles of , e.g. Two perfect cleavages exhibiting angles of , e.g. 124 and 56 degrees in Hornblende.124 and 56 degrees in Hornblende.
• HornblendeHornblende is the most common mineral in the is the most common mineral in the amphibole groupamphibole group
Pleochroic in Plane Polarized Light
Looks stringy
Hornblende CrystalHornblende Crystal56 and 124 degree56 and 124 degreeCleavagesCleavages
Distinguish Hornblende fromPyroxene Group by cleavage
Pyroxene CrystalTwo Cleavage Faces at about 90 degrees
90o
Cleavage in PyroxenesCleavage in Pyroxenes
It isn’t perfect in all slices
Cleavage in Amphiboles
Looking down the c-axis
Looking down the c-axisLooking down the c-axis
AmphibolesAmphiboles Amphiboles such as Hornblende are pleochroic in Amphiboles such as Hornblende are pleochroic in
Plane Polarized Light. Hornblende is monoclinic. Plane Polarized Light. Hornblende is monoclinic. With crossed polars, they have inclined extinction, With crossed polars, they have inclined extinction, i.e. they go dark at an angle to ONE of their i.e. they go dark at an angle to ONE of their cleavage planescleavage planes
““Clinopyroxenes” (monoclinic pyroxenes) Clinopyroxenes” (monoclinic pyroxenes) also have inclined extinction,also have inclined extinction,but are not pleochroic in PPLbut are not pleochroic in PPL
Any monoclinic mineral has one inclined Any monoclinic mineral has one inclined extinction when rotating with crossed polarsextinction when rotating with crossed polars
http://www.youtube.com/watch?v=1DSqh5oEYOE
Classification of MineralsClassification of Minerals Common Silicate mineralsCommon Silicate minerals
Mica Group PhyllosilicatesMica Group Phyllosilicates• Sheet structures that result in one direction of perfect Sheet structures that result in one direction of perfect
cleavagecleavage
• BiotiteBiotite is the common dark colored mica mineral. Has is the common dark colored mica mineral. Has wavy “bird’s eye extinction”wavy “bird’s eye extinction”
• MuscoviteMuscovite is the common light colored mica mineral . is the common light colored mica mineral . Can have undulose extinction.Can have undulose extinction.
• https://www.youtube.com/watch?v=dvDankgGBIs
Muscovite
KAl3Si3O10(OH)2
In plane polarized light, Biotite is seen as dark brown to grey against the surrounding mostly colorless minerals. Under crossed polars "bird's eye " = “mottled” = “wavy” extinction can easily be seen when the mineral is nearly extinct. Often, the mineral color masks the interference colors when the mineral is not extinct. http://www.youtube.com/watch?v=IjUdjGQyWtw
http://www.youtube.com/watch?v=Bv3MVkyyxjk
Pleochroic in PPL http://www.youtube.com/watch?v=-6LEW_H-ccQ
Orders of Interference colorsOrders of Interference colors
3-D (Framework) Tectosilicates3-D (Framework) Tectosilicates
Quartz SiO2
QuartzQuartz Undulose (aka “undulatory”) extinctionUndulose (aka “undulatory”) extinction 11oo gray in standard thin section 30 gray in standard thin section 30m m http://www.youtube.com/watch?v=O1I-_YdgaHghttp://www.youtube.com/watch?v=O1I-_YdgaHg Forms late in igneous, fills in gaps between earlier xtalsForms late in igneous, fills in gaps between earlier xtals
Identifying minerals with a Identifying minerals with a Michel-Levy ChartMichel-Levy Chart
If you know the thickness of the thin section, you can narrowdown the possibilities by noting where the interference color of an unknown crosses the thickness line
Thin section ~30 microns, mineral is dark second order blue, so birefringence about 0.020
Possibilities circled
FeldsparsFeldspars
Common Silicate mineralsCommon Silicate minerals TectosilicatesTectosilicates
Feldspar GroupFeldspar Group• Most common mineral groupMost common mineral group
• 3-dimensional framework of tetrahedra exhibit two 3-dimensional framework of tetrahedra exhibit two directions of perfect cleavage at 90 degreesdirections of perfect cleavage at 90 degrees
• K-sparsK-spars (potassium feldspar) and (potassium feldspar) and PlagioclasesPlagioclases (sodium (sodium to calcium feldspar solutions) are the two most to calcium feldspar solutions) are the two most common groupscommon groups
• Pearly to vitreous LusterPearly to vitreous Luster
Potassium feldsparPotassium feldsparNote Pearly Luster
KAlSi3O8
Perthitic Texture, Microcline plus exsolved Albite
Tartan twins in Microcline. Microcline is the low TP version of K-spars KAlSi3O8
http://www.youtube.com/watch?v=7-KZREqrh44
Microcline is Triclinic, Orthoclase is Monoclinic
Plagioclase feldsparPlagioclase feldsparNote the Twinning, seems to have ‘stripes’
(Ca,Na)AlSi3O8
Labradorite Albite NaAlSi3O8
http://www.youtube.com/watch?v=gLcVT_6y-MA
Glass Glass (magma cooled to fast for crystals to form)(magma cooled to fast for crystals to form)
Plagioclase (Anorthite) xtals in basaltic glass. Crossed PolarsThe glass is isotropic and so stays extinct under crossed polars, i.e. it is black in all orientations.
GarnetGarnetGarnet is also Isotropic, and has a very high refractive index, so cracks stand out strongly. Under crossed polars it stays dark.
Pink garnet (PPL) Garnet inclusion (crossed polars)Indistinct cleavage
Looks like a squashed pink tomato
Next week: Chapter 3 Textures of Igneous Rocks