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).