FALL 2005GEOLOGY 285:INTRO. PETROLOGYDr. Helen LangDept. of Geology & GeographyWest Virginia UniversitySediments are the products of weathering:there are two aspects to Weathering• Mechanical Weathering - physical breakup of rocks• Chemical Weathering - chemical breakdown of minerals in the presence of water• Mechanical Weathering is trivial compared to chemical weathering, due to the extraordinary dissolving power of H2OMechanical Weathering• Abrasion by wind- or water-carried fragments• Frost-wedging is most important agent of mechanical weathering– water freezes to ice 9% volume increase– most important where water is liquid in daytime and freezes every night– most important mechanism for increasing surface area of rocks•H2O is polar; therefore, good at dissolving ions•H2O dissociates into H+and OH-•H+reacts readily with minerals• Weathering is aided by organic acids and microorganisms• Different minerals weather at different rates• Weathering of minerals depends on their chemical stability in the surface weathering environment with low T, low P, high H2O, oxidizingChemical Weatheringδ+δ-δ+Goldich’s Weathering SeriesSummarizes susceptibility of minerals to weatheringOlivinePyroxeneAmphiboleBiotitePotassium FeldsparMuscoviteQuartzCalcic PlagioclaseSodic PlagioclaseMost SusceptibleLeast SusceptibleWeathering Reactions of OrthoclaseStep 1: 3 KAlSi3O8+ 2 H++ 12 H2O -->KAl3Si3O10(OH)2+ 6 H4SiO4+ 2 K+Step 2: 2 KAl3Si3O10(OH)2+ 2 H++ 3 H2O -->3 Al2Si2O5(OH)4+ 2 K+orthoclaseillite (~muscovite) soluble silicaillitekaoliniteAll feldspars weather similarly• React with H2O and H+• Release silica in solution and cations• Produce clay minerals (sheet silicates)• Albite + H2O + H+= Sodium montmorillonite + H4SiO4+ Na+• Anorthite + H2O + H+= Calcium montmorillonite + H4SiO4+ Ca2+Iron in minerals weather differently• Fe in most ferromagnesian minerals is reduced (Fe2+), because they’re formed in reducing conditions (low oxygen)• Surface waters are very oxygen-rich, i.e., oxidizing•Fe2+released during weathering immediately oxidizes to Fe3+•Fe3+precipitates rapidly as EXTREMELY INSOLUBLE Fe(OH)3and other hydroxidesWeathering of pyroxene, for exampleCaFeSi2O6(Fe part of augite) + H2O + H+= Calcium montmorillonite + H4SiO4+ Ca2++ Fe(OH)3Fe(OH)3and other Ferric oxyhydroxides precipitate (orange or rusty) and eventually dehydrate to Hematite, which gives subaerial soils and sediments (red beds) their red color•Quartz• Clay minerals-kaolinite, illite, montmorillonite• Cations in solution• Ferric hydroxides and oxides (insoluble) from the weathering of mafic mineralsThe most common products of weathering are:Sandstones and Conglomerates• Are detrital or clastic sedimentary rocks –made of the solid products (detritus) from weathering of pre-existing rocks• They make up 20-25% of stratigraphic record, but receive much more attention from sedimentary petrologists than 25%What geologists want to learn from Sandstones• Source area – rock type– direction– weathering environment• Transport– medium, energy– distance• Depositional environment– marine or non-marine– physical environment (beach, river, delta, etc.)What clues are present in Sandstones?•Grain size•Grain shape• Grain sorting• Grain mineralogy• Sedimentary structuresGrain size• Detrital or clastic rocks have a huge range in grain size• We need a log scale to represent this wide size range•The Phi (φ) Scale: φ = -log2(mm)mm = 2- φ(memorize)• Each φ step represents a doubling (smaller # or more neg.) or halving (larger #) in sizeFor example• size in mm = 2- φ•-6 φ = 26mm = 64 mm•0 φ = 20mm = 1 mm•2 φ = 2-2mm = 1/4mm = 0.25 mm•4 φ = 2-4mm = 1/16mm = 0.0625 mmSize ranges are given namesGravel > -1 φ (>2mm)Sand: 4 φ to -1 φ(0.0625mm to 2mm)Mud < 4 φ<0.0625mm<62.5 μmClay < 8 φ<0.004mm<4 μmLoose sediments can be separated by sieving•-2φ (4mm)•-1φ (2mm)•1φ (0.5mm)•2φ (0.25mm)•3φ (0.125mm)•4φ (0.062mm)•closedGrain size comparator for lithified SandstoneSorting = range in grain size• Usually the size range (in φ) that includes 2/3of the grains% of grainsφ size% of grainsφ sizepoorly-sorted well-sortedSorting by comparisonGrain Shape• Sphericity - relative equidimensionality of three mutually perpendicular axes• Roundness - lack of sharp corners; larger grains round faster because of more impactsHigh sphericityLow sphericityIncreasing roundnessMudrocks• Composed mostly of detrital material smaller than 4φ, i.e., smaller than 0.062 mm or 62.5 μm (mud=silt+clay)• Non-fissile (-stone) or fissile (-shale)• Named by proportion of Silt and Clay> 2/3 silt Siltstone Silt-shale1/3 to 2/3 silt Mudstone Mud-shale>2/3 clay Claystone Clay-shaleConglomerates• No agreement about the % of gravel sized material required to make a sediment a GRAVEL or a sedimentary rock a CONGLOMERATE• We’ll say >30% gravel size material for a conglomerate• Small amount of pebbles or cobbles is very noticeableClassification of Sandstones is based on Detrital Minerals•Quartzis most abundant sand-size grain– very stable in sedimentary environment•Feldsparmay be abundant– may indicate rapid burial, dry climate, granite in source area– K-feldspar vs. Plagioclase?• Unstable Lithic Fragments are least abundant, but most informative– Chert, a stable lithic fragment, goes with QuartzAccessory Minerals (other minerals)• Micas - may float in water; muscovite, especially is very stable• Heavy minerals - higher density than quartz and feldspar, some are very stable: zircon, tourmaline, rutile, hornblende, garnet, ilmenite, magnetite, apatite, pyroxene, etc. If loose sand or disaggregated sandstone is put in a heavy liquid (s.g. 2.8-3.0) “heavies” sink to the bottom• These minerals can be quite informativeDetrital Grain Types: QuartzChert is a stable lithic fragment, grouped with QuartzFeldspars distinguished from quartz by alteration, twinning and perthiteWhat’s the large grain?Lithic FragmentsVolcanic and Plutonic Lithic Fragments in a