GY303GY303 IgneousIgneous && MetamorphicMetamorphic PetrologyPetrology LabLab LabLab 1:1: FelsicFelsic IgneousIgneous RockRock TermsTerms && ClassificationClassification ColorColor IndexIndex

„ Felsic: refers to igneous rocks where light colored minerals dominate. Remember that red (even dark red) is considered to be a “felsic” light color. The most common felsic example is Granite. „ Intermediate: refers toto igneousigneous rocksrocks wherewhere thethe proportionsproportions ofof light and dark minerals are roughly equal. Remember that green (even light green) is considered to be a “dark” color. An intermediate example is Diorite. „ Mafic: refers to igneous rocks that are dominated by dark colored minerals (including any green colors). A mafic example is Gabbro. „ Ultramafic: refers to igneous rocks that are dominated by dark colored minerals but have no significant proportions of feldspar. A ultramafic example is Peridotite. TexturalTextural DescriptiveDescriptive TermsTerms

„ DegreeDegree ofof CrystallinityCrystallinity „ HolocrystallineHolocrystalline:: composedcomposed ofof crystalcrystal grainsgrains „ HypocrystallineHypocrystalline:: mixturemixture ofof glassglass andand crystalscrystals „ HolohyalineHolohyaline:: essentiallyessentially allall glassyglassy materialmaterial (pumice,(pumice, ash,ash, obsidian)obsidian) „ NoteNote thatthat allall glassyglassy materialmaterial willwill revertrevert toto ((devitrifydevitrify)) toto veryvery finefine--grainedgrained crystallinecrystalline materialmaterial overover aa periodperiod ofof 100,000100,000 yearsyears GrainGrain SizeSize

„ Phaneritic:Phaneritic: composedcomposed ofof crystalscrystals largelarge enoughenough toto bebe identifiedidentified withwith aa handhand lenslens „ Coarse-grained: > 5mm diameter (0.2 inches = 5mm) „ Medium-grained: 1-5mm diameter „ Fine-grained: <1 mm „ Aphanitic:Aphanitic: composedcomposed ofof grainsgrains tootoo smallsmall toto bebe identifiedidentified withwith aa handhand lenslens „ Microcrystalline: grains visible with a microscope „ Cryptocrystalline: grains not visible with a microscope „ Glassy: composed of glass GrainGrain RelationshipsRelationships

„ Granular:Granular: composedcomposed ofof equidimensionalequidimensional grainsgrains „ Equigranular:Equigranular: composedcomposed ofof grainsgrains ofof uniformuniform sizesize „ Granitic:Granitic: composedcomposed equidimensionalequidimensional grains,grains, somesome euhedraleuhedral andand somesome anhedralanhedral (see(see below)below) „ Porphyritic:Porphyritic: composedcomposed ofof grainsgrains withwith 22 oror moremore distinctdistinct sizesize populationspopulations thatthat differdiffer byby anan orderorder ofof magnitudemagnitude DegreeDegree ofof CrystalCrystal FaceFace DevelopmentDevelopment

„ TermsTerms thatthat applyapply toto individualindividual grains:grains: „ Euhedral:Euhedral: aa graingrain isis boundedbounded byby crystalcrystal facesfaces „ Subhedral:Subhedral: aa graingrain isis partiallypartially boundedbounded byby crystalcrystal facesfaces „ Anhedral:Anhedral: nono developmentdevelopment ofof crystalcrystal facesfaces „ NoteNote thatthat thethe degreedegree ofof crystalcrystal faceface developmentdevelopment isis primarilyprimarily aa functionfunction ofof thethe orderorder ofof crystallizationcrystallization inin aa magmamagma DegreeDegree ofof CrystalCrystal FaceFace DevelopmentDevelopment

„ TermsTerms thatthat ApplyApply toto RockRock TextureTexture „ Idiomorphic:Idiomorphic: composedcomposed entirelyentirely ofof euhedraleuhedral crystalscrystals „ Hypidiomorphic:Hypidiomorphic: composedcomposed ofof aa mixmix ofof anhedralanhedral andand subhedralsubhedral and/orand/or euhedraleuhedral grainsgrains „ Allotriomorphic:Allotriomorphic: composedcomposed ofof essentiallyessentially allall anhedralanhedral grainsgrains CommonCommon VolcanicVolcanic RockRock TermsTerms

„ Vesicular:Vesicular: gasgas voidsvoids (vesicles)(vesicles) areare abundantabundant inin extrusiveextrusive rockrock „ Amygdaloidal:Amygdaloidal: secondarysecondary mineralsminerals formform inin vesiclesvesicles „ PumiceousPumiceous:: highlyhighly vesicularvesicular toto thethe pointpoint thatthat thethe densitydensity ofof thethe rockrock isis lessless thanthan water.water. „ Scoriaceous:Scoriaceous: vesiclesvesicles composecompose >50%>50% byby volumevolume butbut materialmaterial willwill notnot float.float. IgneousIgneous RockRock ClassificationClassification

„ BasedBased onon MineralMineral CompositionComposition andand TextureTexture „ NameName convention:convention: „ Color,Color, texture,texture, alterationalteration (if(if any),any), accessoryaccessory mineralsminerals >10%,>10%, rootroot namename „ Example:Example: pinkpink porphyriticporphyritic mediummedium--grainedgrained biotitebiotite granitegranite „ Example:Example: graygray finefine--grainedgrained epidotizedepidotized hornblendehornblende granodioritegranodiorite EssentialEssential MineralsMinerals inin FelsicFelsic RocksRocks

„ Quartz „ Clear, conchoidal fracture, doesn’t weather, anhedral „ K-feldspar „ Commonly pink, 2-direction cleavage, not transparent „ Na-Plagioclase „ White, 2-directions cleavage, not transparent, striations, tabular „ Feldspathoids „ Nepheline appears similar to quartz but has a more greasy luster because it weathers faster „ Note: it is not possible to have a large amount (>10%) quartz and feldspathoids- they are mutually exclusive CommonCommon AccessoriesAccessories inin FelsicFelsic RocksRocks

„ Hornblende,Hornblende, biotite,biotite, muscovite,muscovite, pyroxene,pyroxene, sodalite,sodalite, tourmaline,tourmaline, ilmenite,ilmenite, magnetite,magnetite, apatiteapatite CommonCommon AlterationsAlterations inin IgneousIgneous RocksRocks

„ Kaolinization: feldspars altered to kaolinite „ Saussuritization: alteration of Ca-plagioclase to albite and epidote „ Chloritization: alteration of ferromagnesian minerals to chlorite „ Uralitization: replacement of pyroxene by hornblende „ Serpentinization: alteration of olivine and pyroxene to serpentine „ Silicification: rock is replaced by silica „ Propylization: alteration of ferromagnesian minerals to chlorite, calcite, serpentine and magnetite; alteration of plagioclase to albite and epidote forming greengreen rockrock termedtermed propylite. Very common in andesites. CalculationCalculation ofof TernaryTernary ProportionsProportions

„ Example:Example: estimatedestimated percentagespercentages KK--feldfeld=45,=45, PlagPlag=27,=27, QtzQtz=15;=15; BiotBiot=7;=7; MuscMusc=6=6 (total(total == 100)100) „ TernaryTernary QQ == Qtz/(Qtz+KQtz/(Qtz+K--feld+Plagfeld+Plag)*100)*100 == 15/(15+45+27)*10015/(15+45+27)*100 == 15/87*10015/87*100 == 17%17% „ TernaryTernary AA == KK--Feld/(Qtz+KFeld/(Qtz+K--Feld+PlagFeld+Plag)*100)*100 == 45/(15+45+27)*10045/(15+45+27)*100 == 52%52% „ TernaryTernary PP == 27/(15+45+27)*10027/(15+45+27)*100 == 31%31% IUGSIUGS FelsicFelsic (SiO2(SiO2 oversaturated)oversaturated) TernaryTernary PlotPlot

NOTE: Q=17% aphanitic A=52% root names P=31% are in P=31 parentheses A=52

Q=17 NamingNaming IgneousIgneous SamplesSamples

„ Complete name should include in order: color, texture, alteration (if any), accessory minerals (>= 10%; in order of increasing abundance), root rock name. „ Example: pink porphyritic medium-grained biotite granite „ If the ternary proportion example had been coarse-grained, red, with phenocrysts it would be named: red porphyritic coarse- grained quartz monzonite „ Note: if phenocrysts are >= 50% of the volume of the sample the name would be: red coarse-grained quartz monzonite porphyry „ If the phenocryst volume is > 75% the “porphyry” designation should be dropped and the specimen classified by the “phenocryst” assemblage IUGSIUGS FelsicFelsic (SiO2(SiO2 undersaturated)undersaturated) TernaryTernary PlotPlot

F (foids)

IUGS Classification for felsic igneous rocks SiO2 undersaturated 10 90

Foidolite 20 (Foidite) 80

30 70

40 60

50 50

60 40 Foid Foid syenite Foid monzosyenite Foid gabbro (Phonolite) monz odiorite (tephrite,basanite) (Tepheritic phonolite) (Phonolitic tephrite)

70 30

80 20

90 10 Alkali feldspar syenite (Trachyte) Syenite Monzonite Diorite Diorite(andesite), (Trachyte) (Latite) (andesite) Gabbro (basalt)

10 20 30 40 50 60 70 80 90 A(alkali feldspar) P(plagioclase)