Igneous rock overview


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Igneous rock overview

  1. 1. Dr. Helen LangDept. of Geology & Geographyp gy g p yWest Virginia UniversitySPRING 2009GEOLOGY 585:Optical Mineralogy & PetrologyOptical Mineralogy & Petrology
  2. 2. Overview of IgneousRocks
  3. 3. Igneous rocks crystallize from magmasd iliand most magmas are silicate magmas(containing 40 to 75 wt.% SiO2)• Felsic (silicic or sialic) magmas are high inSiO2 and Al2O3 and low in MgO and FeOSiO2 and Al2O3 and low in MgO and FeO• Intermediate magmas are between Felsicand Mafic magmasand Mafic magmas• Mafic magmas contain less than 50 wt.%SiO d hi h M O F O d F OSiO2 and higher MgO, FeO and Fe2O3• Ultramafic magmas are even more SiO2poor and MgO/FeO rich
  4. 4. Classification and Naming ofClassification and Naming ofIgneous Rocks• Based on Felsic Mineral ContentQAPF or Streckeisen Diagram– QAPF or Streckeisen Diagram• For fine-grained volcanic rocks, naming isb d h i l iti ( l tbased on chemical composition (plotNa2O+K2O vs. SiO2)
  5. 5. Streckeisen(QAPF)Diagram withDiagram withPlutonicIgneous Rocknamesnames
  6. 6. IUGS Volcanic Classification is GraphicalAlkalineSubalkalineSubalkaline
  7. 7. How to plot on the Streckeisen DiagramNote that several fieldscontain 2 or 3 namesQ+A+Pcontain 2 or 3 names,which do you use?Di it h C I < 35 dQ/Q Diorite has C.I. < 35 andNa-rich plagioclase withAn<50F/FGabbro has C.I. >35 andCa-rich plagioclase withAn>50+A+PAn>50Anorthosite has >90%plagioclase (<10%fi i l )mafic minerals)
  8. 8. Bowen’s Reaction SeriesBowen s Reaction Series• Idealized model for crystallization of magmasIdealized model for crystallization of magmas• Shows order in which minerals crystallizefrom a typical mafic or basaltic magmafrom a typical mafic or basaltic magma• Left side is called Discontinuous Side– Mafic minerals change abruptly• Right side is called Continuous Side– Plagioclase changes composition gradually
  9. 9. Bowen’s Reaction Serieshi TolivineCa plagioclasehi Torthopyroxeneclinopyroxene NaCa plagioclaseamphibole (Hb)peraturebiotite Na plagioclasealkali feldsparcoolingtempalkali feldsparmuscovitequartzresidualphasesquartzlow T
  10. 10. We’ll start with OlivineWe ll start with Olivine• Formula: (Mg Fe)2SiO4Formula: (Mg, Fe)2SiO4• An Isolated Tetrahedral Silicate• complete solid solution with two end-bmembers:Mg SiO Forsterite (Fo) and–Mg2SiO4 Forsterite (Fo) and–Fe2SiO4 Fayalite (Fa)2 4 y ( )
  11. 11. Olivine PropertiesOlivine Properties• Vitreous luster• Olive green color• Olive-green color• No good cleavagesg g• H=6.5 G=3.2• Optical: high relief, moderatelyhigh birefringencehigh birefringence
  12. 12. Olivine Xenoliths from the MantleOlivine Xenoliths from the Mantle
  13. 13. Olivine in thin section
  14. 14. Pyroxenes• Common in mafic and intermediate igneous rocks,typically crystallize from basalt after Olivine (BRS)• Single Chain Silicates• General Formula: (Ca2+,Mg2+,Fe2+)2Si2O6( , g , )2 2 6• Two kinds:– Orthopyroxenes (Opx) (Mg,Fe)2Si2O6 are orthorhombicOrthopyroxenes (Opx) (Mg,Fe)2Si2O6 are orthorhombicand have perpendicular axes, low birefringence, // ext.• Enstatite– Clinopyroxenes (Cpx) Ca(Mg,Fe)Si2O6 are monoclinicand have one inclined axis, higher biref., inclined ext.• Augite and Diopside
  15. 15. PyroxeneRelief, Colorand Cleavage inand Cleavage inPPL
  16. 16. Orthopyroxene Properties• Orthopyroxenes (enstatite, hypersthene,bronzite)bronzite)– Color gray, bronze, brown– Colorless to tan in thin section– High relief– Low birefringence– Parallel extinctionParallel extinction
  17. 17. Clinopyroxene Properties• Diopside (more likely in metamorphic rocks)Ca Mg Si O– Ca~1Mg~1Si2O6– Color light green– Colorless in thin section– Moderate birefringence, inclined extinctiong ,• Augite Ca~1(Mg,Fe)~1Si2O6 (typical igneous Cpx)Dark green to black– Dark green to black– Tan, light brown or green in thin section– Moderate birefringence, inclined extinction
  18. 18. Augite (moderate birefringence, twinningand cleavage)
  19. 19. Hornblende (Hb)• is the main amphibole in igneous rocks• crystallizes after olivine and pyroxenes• crystallizes after olivine and pyroxenesfrom mafic magmas, and at lowertemperature• is most common in intermediatecomposition igneous rocksd bl h i ili t• double-chain silicate
  20. 20. HornblendeHornblende• General Amphibole Formula:– (Na,K)0-1(Ca,Na,Fe,Mg)2(Mg,Fe,Al)5(Si,Al)8O22(OH)2– large medium small tetrahedral cationsg• Simple Amphibole (Tremolite):Ca Mg Si O (OH)– _Ca2Mg5Si8O22(OH)2• Hornblende Formula:– (Ca,Na)2-3(Mg,Fe,Al)5Si6(Si,Al)2O22(OH)2– Substitution of (Alvi, Aliv) for (Mgvi, Siiv) of tremolite
  21. 21. Hornblende PropertiesHornblende Properties• Black or dark green• Vitreous luster, translucent• H=5-6 G=3 0-3 5H 5 6 G 3.0 3.5• two perfect prismatic cleavages at about 60o and120o to eachother sometimes “splintery” cleavage120o to eachother, sometimes splintery cleavage• commonly in prisms with flattened hexagonalicross-sections
  22. 22. Hornblende’s Optical PropertiesHornblende s Optical Properties• Various shades of brown, red-brown, green,bl i hi iblue-green or tan in thin section• Moderate to strong pleochroism is typical• Moderate birefringence, commonlyobscured by its color• Two good cleavages at ~60o and 120o toeachother• You have to be looking down the prism tosee 2 crossing sets of cleavages, in manyi ill l t f lviews you will see only one set of cleavages
  23. 23. Igneous AmphibolesBi H bl d C l i hi fi i dBig Hornblende Crystals in this fine-grainedvolcanic rock are called Phenocrysts
  24. 24. Hb cleavage, color & pleochroism
  25. 25. Kaersutite (Na,Ti-rich igneous amphibole)Kaersutite (Na,Ti rich igneous amphibole)i h d b l h iwith strong red-brown pleochroism
  26. 26. Biotite is the lowest Mafic Mineral onDi ti Sid f B ’ S iDiscontinuous Side of Bowen’s SeriesI lli l f f i d fi• It crystallizes very late from fractionated maficmagmas and is only common in intermediate tof l i H O b f Bi ifelsic magmas, H2O must be present for Biotiteto crystallize• Biotite is a trioctahedral, t-o-t (or 2:1) sheetsilicate• Biotite’s properties: it’s strongly pleochroic,has cleavage, parallel extinction, bird’s-eyeg , p , yextinction and pleochroic haloes around zircon
  27. 27. Muscovite is a Residual Phase on BRSMuscovite is a Residual Phase on BRS• Crystallizes only from felsic magmas withCrystallizes only from felsic magmas withH2O in the liquid• Structure and properties are similar to• Structure and properties are similar tobiotite, except for color and pleochroismC i G i d P i• Common in Granites and Pegmatites
  28. 28. Felsic Minerals on BRShi TolivineCa plagioclasehi Torthopyroxeneclinopyroxene NaCa plagioclaseamphibole (Hb)peraturebiotite Na plagioclasealkali feldsparcoolingtempalkali feldsparmuscovitequartzresidualphasesquartzlow T
  29. 29. Quartz PropertiesQuartz Properties• H=7 G=2 65H 7, G 2.65• Generally clear and glassy, may have avariety of colors (clear smoky brown rose;variety of colors (clear, smoky, brown, rose;it’s allochromatic)C h id l f l• Conchoidal fracture, no cleavage• Habit: hexagonal (6-sided prisms) ormassive• Optical: low relief and low birefringencep g
  30. 30. Quartz in Granite Thin SectionPPL XPLPPL XPLFrom Atlas of Rocks & Minerals in Thin Section
  31. 31. The Feldspar TernaryC Al Si OCaAl2Si2O8Anorthitesolid solutionsAll naturalf ldsolid solutions feldsparsNo feldsparsMiscibility GapAlbite O th lMiscibility GapNaAlSi3O8 KAlSi3O8Albite Orthoclasealkali feldspars
  32. 32. Alkali Feldspar (esp Orthoclase)Alkali Feldspar (esp. Orthoclase)Properties• H=6, G=2.56• Generally turbid (cloudy); color white pinkGenerally turbid (cloudy); color white, pinkor flesh-colored• 2 Perfect to good perpendicular cleavages• 2 Perfect to good perpendicular cleavages• Habit: stubby prisms, simple twins common• Optical: low relief and low birefringence• Commonly Perthitic (micro and macro)y ( )
  33. 33. Alkali Feldspars have PerthitespWhat do Perthites look like?Thinsectionsectionin XPL
  34. 34. Alkali Feldspars have PerthitesWhat do Perthites look like?1mm
  35. 35. What causes Perthites?• Caused by un-mixing, exsolution orseparation of Na+ (diameter~1.1Å) and K+p ( )(diameter~1.6Å) as the feldspar cools• At low temperatures, there is a miscibilitygap between NaAlSi O and KAlSi Ogap between NaAlSi3O8 and KAlSi3O8
  36. 36. Microcline hasMicrocline hasplaid twinning1mm
  37. 37. Plagioclase PropertiesPlagioclase Properties• H=6-6 5 G=2 62-2 76 (Or H=6 G=2 56)H 6-6.5, G 2.62-2.76 (Or H=6, G=2.56)• Luster pearly, vitreous/translucentC l hi• Color white to gray• One perfect, one good cleavage• Optical: low relief and low birefringence• Polysynthetic albite twinning usually presentPolysynthetic albite twinning usually present• Not Perthitic!C l d• Commonly zoned
  38. 38. Plagioclase Feldspars have( l th ti l ll ) Albit T i(polysynthetic, lamellar) Albite Twinsd-bleininhandlyvisiPL)isibleiusuallon(XPybevicimen,sectioMayspecthin
  39. 39. Minerals of PegmatitesMinerals of PegmatitesLarge crystals of Quartz,Alkali Feldspar andAlkali Feldspar andMuscovite(which are low on Bowen’sReaction Series)Reaction Series)
  40. 40. Pegmatites are rich in IncompatiblePegmatites are rich in IncompatibleElements and Volatiles• Incompatible Elements – those elementsthat don’t fit well in common igneousthat don t fit well in common igneousminerals (Li, B, Be)V l il h l h h• Volatiles – those elements that have astrong preference for the gaseous phase(H O CO Cl F S)(H2O, CO2, Cl, F, S)
  41. 41. Phase Diagrams tell MUCH moreabout Crystallization and Meltingthan Bowen’s Reaction Seriesthan Bowen s Reaction Series• What follows is just a quick preview ofsome of the things phase diagrams can beg p gused for• We’ll look at two really simple examplesWe ll look at two really simple examples
  42. 42. How do we use this diagram to tellabout crystallization of basalt?about crystallization of basalt?Diopside-richli id (X) Yliquid (X)80% CaMgSi2O6 (Di)20% CaAl2Si2O8 (An)XY% 2 2 8 ( )Draw isopleth=line of constantcompositionWhat happens asliquid cools?Anorthite-rich liquid (Y)q ( )65% CaAl2Si2O8 (An)35% CaMgSi2O6 (Di)
  43. 43. Textures of rocks in Di-An System
  44. 44. 2-component system withComplete Solid SolutionComplete Solid SolutionCrystallization ofPlagioclase inBasalts (reali l i ligneous plagioclaseis not pure An likein the Di-Anin the Di Ansystem)
  45. 45. Pyroxene Compositions and theP Q d il lPyroxene QuadrilateralDiopside HedenbergiteclinopyroxenesCaFeSi2O6CaMgSi2O6Diopside HedenbergiteclinopyroxenesAugiteOrthopyroxeneMg2Si2O6 Fe2Si2O6FerrosiliteEnstatite orthopyroxenespy