Geology 284 Mineralogy Fall 2010 Dr Helen Lang West Virginia University 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 mineral grains from or fragments of pre existing rocks Chemical Sedimentary Rocks formed by precipitation of minerals from water or alteration of previously precipitated material Sedimentary Rock Types Sediments form as a result of Weathering Physical Weathering Depends on hardness cleavage etc Chemical Weathering Depends on chemical reactivity of minerals Relative susceptibility of common minerals to weathering is summarized in Goldich s Weathering Series Goldich s Weathering Series see handout Products of Weathering Minerals Ions in solution Detrital Grains are Rock Fragments or Mineral Grains What Minerals Almost any mineral but Mostly minerals that are physically and or chemically stable or resistant to weathering 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 Musc 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 mineral separates of loose sediments Detrital Zircon ZrSiO4 Separate Why is zircon important Detrital Rutile TiO2 Separate Grain Size is significant in Sedimentary Rocks Gravel Size Sand Size Silt Size Clay Size greater than 2 mm 0 062 to 2 mm quartz and feldspar common 0 004 to 0 062 mm mostly quartz 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 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 clays Ca Na 0 2 0 4 Al Mg Fe 2 Si Al 4O10 OH 2 expand when wet shrink when dry Kaolinite Authigenic Kaolinite in Ss SEM 6 m width Authigenic Clay in Ss SEM 4 5 m wide How Sediments become Sedimentary Rocks Diagenesis o Compaction o 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 Effervescence in cold dilute HCl Calcite CaCO3 and Aragonite CaCO3 effervesce fizz 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 squashed or stretched cube Rhodochrosite Rhombohedra Saddle shaped crystals with curved faces are characteristic of Dolomite 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 Carbonates are strongly anisotropic WHY Calcite and other carbonates have extremely high birefringence milky pastelwhite because they are so strongly anisotropic Calcite and Aragonite are polymorphs of CaCO3 See handout for ranges of stability In stained thin sections Calcite red can be distinguished from Dolomite clear by staining Stained Brachiopod Shell in Silty Carbonate Brachiopod Shell Sections Calcite Ooids note structure Carbonate Ooids Mollusk Molds filled with Calcite Another kind of Chemical Sedimentary Rock Evaporites 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 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 and Ohio It evaporated to form the Silurian Salina Formation now about 5000 feet below the surface When seawater evaporates Potassium and Magnesium Chlorides precipitate last Top of sequence Halite NaCl precipitates next after 90 evap about 80 of evaporite thickness Sulfates precipitate next Gypsum CaSO4 2H2O Anhydrite CaSO4 Carbonates precipitate first after 70 evap Calcite Ca CO3 and Dolomite CaMg CO3 2 Bottom of sequence Halides What are the anions Which of the evaporite minerals are halides Properties Sulfates What is the anion What evaporite minerals are sulfates How are they different from sulfides