Dr Helen Lang Dept of Geology Geography West Virginia University FALL 2013 GEOLOGY 284 MINERALOGY The Minerals of Sediments and Sedimentary Rocks Result from Recycling of older Rock Materials Sedimentary Rocks and Minerals Sediments and Sedimentary Rocks cover about 80 of the Earth s surface but form a very thin blanket constituting only about 1 of the Earth s volume They re important to us because we live at Earth s surface Two main kinds of Sediments and Sedimentary Rocks Clastic or Detrital Sedimentary Rocks formed of solid pieces of pre existing rocks Chemical Sedimentary Rocks formed by precipitation of minerals from water or alteration of previously precipitated material Sediments form as a result of Weathering Physical Weathering Depends on hardness cleavage etc Increases surface area exposed to water Chemical Weathering Depends on chemical reactivity of minerals Chemical weathering is much more significant Relative susceptibility of common minerals to weathering is summarized in Goldich s Weathering Series Goldich s Weathering Series olivine Ca plagioclase first to alter pyroxene amphibole biotite Na plagioclase K feldspar muscovite quartz last to alter Not sequential like Bowen s Reaction Series Essentially all minerals weather to clays Fe oxides and hydroxides Products of Weathering Minerals Ions in solution Detrital Grains are Rock Fragments or Mineral Grains What Minerals Can be almost any mineral but Mostly minerals that are physically and or chemically stable or resistant to weathering low on Goldich s Weathering Series The Most Common Detrital Minerals Quartz Feldspars Clay Minerals Detrital Quartz Grains Abundant Clear and unaltered Larger than detrital feldspar Commonly rounded Commonly monocrystalline Quartz rich Sandstone Polycrystalline Quartz in Ss Detrital Feldspar Grains Generally less abundant than quartz Cloudy and highly altered K feldspar and Na rich plagioclase more abundant and less altered than Ca rich plagioclase Commonly monocrystalline Clear Quartz Altered Feldspar Microcline in Sandstone Perthitic Alkali Feldspar in Sandstone Other Detrital Grains Micas may be present Muscovite Biotite Olivine and Pyroxene rare Accessory detrital Minerals Hard chemically resistant minerals without good cleavages Garnet Mg Fe 3Al2Si3O12 zircon ZrSiO4 rutile TiO2 tourmaline etc Examples from heavy minerals separated from loose sediments follow Detrital Zircon ZrSiO4 Separate What is the importance of zircon Detrital Rutile TiO2 Separate Grain Size is significant in Sedimentary Rocks Gravel Size greater than 2 mm mostly polymineralic rock fragments Sand Size 0 062 to 2 mm quartz and feldspar common Silt Size 0 004 to 0 062 mm mostly quartz Clay Size less than 0 004 mm mostly clay minerals Clays in Clastic Sedimentary Rocks Clays may be either Detrital transported solid products of weathering or Authigenic formed in the sediment after deposition Clay Minerals are Sheet Silicates much like Serpentine and Micas Kaolinite Al4 Si4O10 OH 8 is a dioctahedral t o sheet silicate like serpentine But the t sheet fits the o sheet better than in serpentine Kaolinite movie Important Clay Minerals Kaolinite Al4 Si4O10 OH 8 Illite muscovite like more Si less K K1 1 5Al4 Si Al 8O20 OH 4 Montmorillonite expanding clay Ca Na 0 2 0 4 Al Mg Fe 2 Si Al 4O10 OH 2 expands when wet shrinks when dry Kaolinite Read about uses of clays in Box 6 2 could be on test Rocks composed entirely of clay are called shale or mudstone Authigenic Kaolinite between grains in Ss SEM 6 m width Authigenic Clay in Ss SEM 4 5 m wide How Sediments become Sedimentary Rocks Diagenesis Compaction Cementation growth of authigenic minerals Lithification hardening turning into a rock Chemical Organic Sedimentary Rocks Carbonate Rocks or Limestones Carbonate Minerals Our first non silicates What s the Anion The Carbonate Anion CO3 2 flat triangular shape Carbonate Minerals Calcite and Aragonite CaCO3 Rhodochrosite MnCO3 Siderite FeCO3 Magnesite MgCO3 Dolomite CaMg CO3 2 the most important sedimentary carbonates Effervescence in cold dilute HCl Calcite CaCO3 and Aragonite CaCO3 effervesce fizz lose CO2 in cold dilute HCl Dolomite CaMg CO3 2 does not Some other less common carbonates also fizz Malachite Cu2 CO3 OH 2 Azurite Cu3 CO3 2 OH 2 Smithsonite Zn CO3 Witherite Ba CO3 Strontianite Sr CO3 The Rhombohedron is the cleavage form for most carbonates not aragonite and is a common carbonate growth form A Rhombohedron is a corner corner squashed cube Rhodochrosite Rhombohedra Saddle shaped crystals with curved faces are characteristic of Dolomite Dolomite w marcasite on quartz Calcite and Dolomite and all other Rhombohedral Carbonates not aragonite have similar structures They therefore have similar growth and cleavage forms and other similar properties Dolomite Structure others similar Ca green Mg blue in octahedra with Oxygen at corners Horizontal CO3 2 triangular anion Carbonates are strongly anisotropic Why Calcite Structure Dolomite Structure Calcite and other carbonates have extremely high birefringence milky pastel white because they are so strongly anisotropic Calcite and Aragonite are polymorphs of CaCO3 depth Aragonite G 2 9 5000 atm 15 km Pressure Calcite is more stable at Earthsurface conditions than Aragonite but some organisms make their hardparts out of aragonite anyway Calcite G 2 7 0 200 Temperature oC 400 Calcite red can be distinguished from Dolomite clear by staining PPL Stained Brachiopod Shell in Silty Carbonate XPL Calcite Ooids note structure Mollusk Molds filled with Calcite Evaporites Another kind of Chemical Sedimentary Rock if Seawater gets trapped in a basin and evaporates The water eventually becomes saturated with certain minerals and they precipitate Evaporite minerals carbonates halides and sulfates new mineral groups begin to precipitate It takes 625 feet of seawater to produce 10 feet of evaporite sediment in Silurian time in WV and Ohio The epicontinental seas that had covered most of the interior of North America receded called a regression Some seawater remained trapped in northern WV Ohio Ontario and Michigan It evaporated to form the Silurian Salina Formation now about 5000 feet below the surface up to 700 ft thick When seawater evaporates Potassium and Magnesium Chlorides precipitate last Halite NaCl precipitates next after 90 evap about 80 of evaporite thickness Sulfates precipitate next Gypsum Anhydrite CaSO4 2H2O CaSO4 Carbonates precipitate first after 70 evap Calcite Ca CO3