Exercise 3 Identification of Rocks and Soil Materials
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EXERCISE 3 IDENTIFICATION OF ROCKS AND SOIL MATERIALS INTRODUCTION This is the second lab designed to familiarize you with common materials naturally occurring in the Earth. The elements present in the Earth are organized into mineral grains, which are themselves aggregated into rocks. An ability to recognize minerals and rocks is basic to many activities in exploration, development and engineering, and we will ask you to utilize the skills you develop in this lab for subsequent lab exercises in this course. To prepare for this lab you should have read the appropriate sections on rocks and minerals in your text. Be sure to bring your textbook with you to lab for reference. In lab you will learn to identify common rock- forming minerals by their physical properties, and then you will learn to recognize common rocks. BACKGROUND INFORMATION ROCK IDENTIFICATION BY MINERAL COMPOSITION AND TEXTURE Rocks are mixtures of minerals, held together by relatively weak bonds between individual mineral grains or in some cases by cementing materials, which act like glue. Rocks can be classified based on their chemical compositions (observed as the types and proportions of minerals present), or based upon the processes by which they formed (observed as the size, shape and orientation of the mineral grains, or texture of the rock). Classification Classification of rocks is based on origin: Igneous rocks are formed by solidification of magma/lava (freezing) or by accumulation of fragments ejected during volcanic eruptions. Sedimentary rocks are formed by the deposition and subsequent aggregation of rock or mineral fragments produced by erosion and transported by gravity, wind, ice or water. Some sedimentary rocks may also form by precipitation of minerals from the dissolved elements present primarily in seawater. Metamorphic rocks are formed when existing sedimentary or igneous rocks are subjected to changes (primarily increases) in temperature, pressure or stress, which cause the mineral grains originally present to reorient, recrystallize or be replaced by new minerals, stable under the new conditions. Igneous rocks are composed of hard minerals (H ≈ 6-7), primarily feldspar, sometimes quartz, and a dark mafic mineral, either olivine, pyroxene, hornblende or biotite (the only soft mineral in igneous rocks). Mineral grains usually have good crystal shapes, and different mineral types are randomly distributed through the rock, not in layers. Grain size is quite variable. Sedimentary rocks are aggregates of transported particles or chemical precipitates composed of calcite, dolomite or gypsum. Some may contain fossils. Clastic sedimentary rocks are composed primarily of quartz, feldspar and clay minerals. The mineral grains are often rounded, and dark minerals are rare. Sedimentary rocks often have conspicuous layering, or bedding. Metamorphic rocks may be distinguished by their foliation, which is a parallel alignment of platy mica grains or of light and dark layers. When pure quartz or pure calcite rocks are metamorphosed the mineral grains become coarse and polygonal in shape giving the rock a “sugary” texture. ROCK IDENTIFICATION Igneous Rocks Igneous rocks are classified by grain size and composition. When magma freezes quickly, which happens in extrusive (volcanic) environments, the crystals grow rapidly from many nuclei, resulting in a very fine- grained (grains too small to see) rock. When magma cools below the surface (intrusive environment) heat is lost slowly and fewer nuclei develop, so the rock is more coarse-grained. Rocks with large minerals in a finer, matrix are called porphyritic and are classified as if they were all fine-grained. The composition of igneous rocks ranges from mafic to felsic. Mafic rocks are high in Fe and Mg, and hence contain large amounts of the dark-colored igneous minerals (olivine, pyroxene, amphibole and biotite) and plagioclase feldspar. Felsic rocks have low Fe and Mg and high Si, K and Na. They are rich in quartz and potassium feldspar and hence are light colored. Igneous rocks can have any possible composition between these extremes and those that are mid-way between are said to be intermediate. Specific rock names depend on how felsic or mafic a rock is and whether it is fine or coarse. The compositional variable is determined by the ratio of plagioclase feldspar to potassium feldspar, or more simply, by the amount and type of mafic minerals. Table 4—Igneous Rock Classification Chart Felsic Intermediate Mafic Ultramafic Quartz is present little or no Quartz no Quartz present more K-spar than Plagioclase is the only Plagioclase much more Plagioclase feldspar Minerals than K-spar [may be mafic minerals mafic mineral is mafic minerals are only none] Pyroxene [Augite] and Biotite [5-10%] Olivine Olivine (>20%) mafic mineral is and Amphibole Pyroxene [Hornblende] (10- 20%) Coarse > 1 GRANITE DIORITE GABBRO PERIDOTITE mm Fine < 1 RHYOLITE ANDESITE BASALT rare mm Porphyritic Porphyritic Porphyritic Andesite Porphyritic Basalt rare Rhyolite 1. Select a rock sample from the igneous rock trays and record its number in Table 5. 2. Describe the texture and minerals in the rock. 3. Using Table 4, identify and name the rock. 4. When you return the rock to its proper tray, note variations among the other specimens. Table 5--Igneous Rocks Sample Texture Minerals Felsic, Intermed- iate, or Mafic Rock Name Origin Metamorphic Rocks Metamorphic rocks are classified mainly on their texture – whether or not foliation, a 2- dimensional fabric, developed in response to high pressures. Table 6—Metamorphic Rock Identification Chart Texture Minerals Rock Precursor light and dark layers Gneiss shale/granite visible mica grains Schist shale Foliated too fine grained to see, but Phyllite shale shiny too fine grained to see, dull Slate shale good parallel fracture all amphibole and plagioclase Amphibolite basalt/andesite all calcite or dolomite Marble limestone Not-Foliated all quartz Quartzite sandstone 1. Select a rock specimen and determine if it is foliated or not. 2. Describe texture and mineral composition and size, and record in Table 7. 3. Use Table 6 to name the rock, and note its probable precursor. 4. Repeat for other metamorphic rocks. Table 7--Metamorphic Rocks Sample Texture Minerals Rock Name Precursor Sedimentary Rocks Sedimentary rocks occur in two general types, clastic and chemical. Clastic sedimentary rocks are composed of fragments of preexisting rocks produced when aggregates of particles (sediments) deposited by wind or water at the earth's surface are transformed into rock as the particles become bound together over time. Clastic sedimentary rocks are classified based on the dominant size of particles in them. Chemical sedimentary rocks form when elements dissolved in water precipitate to form mineral crystals. The precipitation can occur when the concentration of the dissolved element is increased through evaporation of the water, causing it to become saturated with particular minerals. This process is important for depositing gypsum and halite from seawater. Other chemical sediments are accumulations of either calcite or quartz secreted by organisms, primarily as shells. This process is important in producing limestone, dolomite and chert. Coal is a direct accumulation (much compressed) of organic material that escaped decomposition. 1. Select a sedimentary rock specimen and describe its characteristics in Table 8. 2. Refer to Table 9 to help you name the rock. 3. Repeat for other sedimentary rocks. Table 8--Sedimentary Rocks Sample Grain Size Minerals Chemical or Clastic Rock Name You will encounter most of the rocks you have just learned in subsequent field exercises. As time permits now, review the main characteristics of each. Notes: Table 9—Classification of sedimentary rocks. Clastic Sedimentary Rocks SEDIMENT SIZE ROCK NAME COMPOSITION/EXAMPLE GRAVEL Sediment mostly quartz, feldspar, and rock Boulder > volleyball fragments Cobble > baseball CONGLOMERATE Pebble > BB Ex. Quartz pebble > 2 mm conglomerate < 2 mm SAND Sediment mostly quartz, feldspar, coarse and clay minerals medium SANDSTONE Ex. Quartz Sandstone fine Ex. Arkose Sandstone > 1/16 mm can see (quartz and abundant feldspar) < 1/16 mm MUD can’t see MUDSTONE Sediment mostly clay minerals (nonfissile, compact) and very fine quartz, feldspar, and calcite silt SHALE Cannot identify minerals without microscope clay (fissile – splits easily) Chemical Sedimentary Rocks COMPOSITION ROCK NAME COMPOSITION/COMMENTS CaCO3 Grains are mostly fragments of calcareous Calcite LIMESTONE (containing calcite) shells mostly shallow marine CaMg(CO3)2 DOLOMITE Altered and recrystallized limestone; mostly Dolomite (or dolostone) shallow marine SiO2 CHERT Often secondary, forming at surface; some deep Microcrystalline silica (and OPAL) marine deposits CaSO4•2H2O GYPSUM Inorganic chemical precipitate, from evaporation Gypsum of seawater NaCl SALT Inorganic chemical precipitate, from evaporation Halite of seawater Organic carbon COAL Black, shiny, low density; mostly remains of swamp and marsh plants HOMEWORK EXERCISE Print out the Soil Worksheet form, which is the second item listed for this week’s lab on the website. Now that you are an expert in using topographic maps, you should be able to find your way around the Golden area using a topographic map. Your TA will give you a topographic map of the Golden area. On that map a specific site is marked with large “X”. Your homework exercise is use the map to go to the site marked “X”. At that site you will see landmarks described on the map (such as nearby buildings, trees, etc.). You can walk to most of the sites, but many of them are easier to get to by driving at least part of the way. If you do not have a car, tell your TA and he or she will give you a map with a site that is closer to campus and hence easier to walk to. When you get to your site, answer the site question that is on your map and write this answer on the bottom of page 1 of your soil worksheet. Bring a spoon, knife or some other small digging instrument with you to the sample site.