Geological Sciences 101 Lab #7 Introduction to Rocks and Minerals

Transcription

1 Geological Sciences 101 Lab #7 Introduction to Rocks and Minerals PART I One key to understanding how the Earth works is to be able to recognize what you see around you. One way to begin this is to group items that share common features, in an attempt to understand the relationships between them. We begin today by developing a classification scheme for a group of Earth materials. The boxes labeled Classification Tray each contain 28 samples. Your team has the job of developing a classification scheme for these samples. You may devise any system that makes sense to you don t worry if it is right or not. Your goal is to design a system that conveys information about the objects in the tray that allows us to better understand them in some way. Think about the criteria you would like to use. Spend about 15 minutes working with your group, and then be prepared to explain your work to the other teams. Use the space below to diagram your system. 1

2 PART II INTRODUCTION TO MINERALS Minerals are all around you. Minerals make up the rocks of the Earth s lithosphere and mantle; they are the nutrients in soil necessary for plant growth; they form your bones and teeth; they are crushed and processed to extract metals for all purposes; they are used to dye fabric and color paint; minerals make glass, plaster, concrete, ceramics--even kitty litter; and they are prized as gems and jewelry. In order to make use of minerals, we must first be able to identify them. This means understanding a little about how minerals are formed, what they re made out of, and how their constituent atoms are put together. MINERAL STRUCTURE AND COMPOSITION To a geologist a mineral is a naturally occurring solid with a fixed chemical composition and an ordered arrangement of atoms that form a crystal structure. For example, the mineral Galena has the chemical formula PbS, and this crystal structure: Pb S S Pb S Pb Pb S The atoms of lead (Pb) and sulfur (S) are present in a 1:1 ratio and are arranged in a cubic pattern. The diagram shown here is only a small part of the Galena crystal structure. You should try to imagine a crystal of Galena made of billions and billions of atoms extending in a cubic structure in all directions. (1) Locate a crystal of galena in the sample trays (look in the Breaking Box). (a) Pick it up and feel it with your hands. What property(s) of galena suggest to you that it is made of lead? (b) Put a small drop of hydrochloric acid on the galena sample. What tells you that there is also sulfur in the galena crystal? While all crystals of galena are shaped like cubes, not all cube-shaped crystals are galena. Halite (salt) and Pyrite are two other cubic minerals. Halite has the chemical formula NaCl and Pyrite is FeS 2. (2) Locate and examine a crystal of halite (NaCl) from one of the sample trays. 2

3 (a) Make a sketch of the halite crystal. (b) Use one of the binocular microscopes to examine crystals of table salt. Sketch these crystals. (c) Having seen the macroscopic structure of halite, make a sketch of its atomic structure. There are many other crystal shapes in addition to cubes. For example, quartz crystals are columns with a hexagonal in cross-section. Some crystals are needle-like, others are double pyramids, some are dodecahedrons (12-sided), some are blade-shaped, columnar, sheets, rhombohedrons, or pinacoids (a rectangle in 3-D). Columnar Rombohedron Bi-pyramid Calcite is a good example of a mineral with a rhombohedral crystal structure. Because the shapes of minerals reflect the internal arrangement of their atoms, a mineral will often retain its shape even when broken. This is because the planes of weakness within the mineral are also determined by the arrangement of atoms. (3) From the Samples for Breaking box choose either calcite, halite, or galena. Break the sample with a hammer. What is the relationship between the crystal shape before and after breaking? A sketch might help. The minerals calcite and galena come from two of the major families of minerals. Mineral families are grouped according to chemical composition. Because galena (PbS) has sulfur in it, is belongs to the Sulfide family. Pyrite (FeS 2 ) is another sulfide. Calcite (CaCO 3 ) is a Carbonate mineral (CO 3 is carbonate). There are Oxide minerals such as magnetite (Fe 3 O 4 ), and Native Elements such as Gold (Au), Silver (Ag) and Copper (Cu). The largest mineral family is the Silicate minerals, such as Quartz (SiO 2 ) and Olivine (Mg 2 SiO 4 ). SILICATE MINERALS Silicon and oxygen comprise 75% of the mass of the Earth s crust. Thus the silicate minerals are by far the most abundant minerals on the surface of the Earth. The relative sizes of the silicon and oxygen atoms determine their arrangement in a silicate crystal structure. Four large oxygen anions fit around the smaller silicon cation. This arrangement is in the shape of a tetrahedron. A silicate tetrahedron, however, would carry a charge of -4, if it were not bonded in some way to other ions. There are two different end member solutions to this problem of excess negative charge. One would be to add more cations Olivine is formed in this way, with two Mg ions attached to each tetrahedron (Mg 2 SiO 4 ). The second technique for balancing charges is to link tetrahedra to each other. Quartz is an example of this solution, where all four corners of each tetrahedron are 3

4 linked to others, producing the overall mineral formula SiO 2. Most silicate minerals are constructed through a combination of tetrahedral linking and the addition of cations for example, pyroxene (MgSiO 3 ). As we will see (or have seen) in class, the amount of tetrahedral linking provides a useful way to organize the silicate mineral groups, and is important in controlling their behavior and properties. (4) Using the materials provided in lab, build a model of a silica tetrahedron. Have your TA approve your design, then continue to add ions to build a model of the pyroxene structure just the silicate tetradedra, you don t need to worry about the metal cations. Sketch the pyroxene, using the triangle shorthand for representing tetrahedra. You may eat your model when you ve finished. CRYSTAL GROWTH Most minerals form by crystallizing from a molten solution a magma. Some form by precipitation from a solution (e.g. halite), and others precipitate with help from a biological catalyst (e.g. calcite). Natural crystallization is a slow process and difficult to observe, so today we will take a shortcut and use an organic compound (thymol, thyme oil ) and attempt to grow crystals big enough and quickly enough for us to be able to watch while it happens. (5) Follow the instructions for growing thymol crystals (or watch the instructor). (a) Sketch a single large thymol crystal as it grows. (b) What happened when you quenched your magma with ice? How are the iced crystals different from those grown without ice? Form a hypothesis to explain this behavior. DETERMINATIVE PROPERTIES Identifying minerals is a little like solving a murder mystery. We examine the problem carefully and then systematically test and eliminate various possibilities until we are left with only one possible solution. The tests that we use are the various characteristics of minerals--their determinative properties. You have already encountered two determinative properties: (1) crystal shape and (2) cleavage-- the shape a mineral has when it breaks. CLEAVAGE Many minerals have a characteristic cleavage. For example, a calcite crystal breaks along three different planes, each cleavage fragment in the shape of a rhombohedron. Galena also has three planes of cleavage; these form a cube. Other minerals cleave at an angle of 120, for example, amphibole. Others cleave into sheets, for example, mica. Some minerals have no cleavage at all. This means that the crystal structure is equally strong in all directions and it simply fractures, like glass. Minerals with no cleavage are said to have conchoidal fracture, among them quartz and olivine. Most mineral samples have been broken off of larger pieces of rock, thus the presence or absence of cleavage is often (but not always) easily observed. Compare the broken surface of a sample of 4

5 feldspar (either orthoclase or plagioclase) with a piece of quartz. Move the samples back and forth in the light to see the difference between cleavage (feldspar) and fracture (quartz). HARDNESS Hardness is a measure of how well a mineral resists physical damage, such as a scratch on the surface. Ten common minerals make up a reference scale referred to as Mohs Hardness Scale. From softest to hardest these are: (1) Talc, (2) Gypsum, (3) Calcite, (4) Fluorite, (5) Apatite, (6) Orthoclase (7) Quartz, (8) Topaz, (9) Corundum, (10) Diamond. If you had a topaz and a diamond, the diamond would scratch the topaz, but the topaz would not scratch the diamond. Thus all the Mohs minerals will scratch any that are below them on the hardness scale. It is often convenient to use common materials rather than mineral specimens to determine hardness. Your fingernail has a hardness of 2.5; a penny has a hardness of 3.5; glass has a hardness of 5.5; a steel knife blade has a hardness of 6.5. Thus you can scratch talc and gypsum with your fingernail, but not calcite or fluorite, nor any other mineral with a hardness of 3 or greater. COLOR Color is the most interesting, and sometimes deceptive, property of minerals. Color results from the interaction of light with the atoms in the crystal structure of the mineral. Sunlight (or white light ) is comprised of the colors of the rainbow: Red, Orange, Yellow, Green, Blue, Indigo, Violet. When this light strikes any object it may behave in one of five ways: Transmitted Reflected Refracted Scattered Absorbed For example, green leaves are green because the green light is reflected and all other colors are absorbed. The sky is blue because small particles in the atmosphere scatter blue light. When light interacts with particular elements in minerals we see characteristic colors. This is true even if the elements are not a normal part of the mineral, but present only in trace amounts. The most important color-producing elements are: Titanium (Ti), Vanadium (V), Chromium (Cr), Manganese (Mn), Iron (Fe), Cobalt (Co), Nickel (Ni), and Copper (Cu). For example, in the normally gray mineral Corundum (Al 2 O 3 ) small amounts of Cr turn the color bright red, producing the mineral Ruby. Alternatively, small amounts of Ti & Fe turn it deep blue, 5

6 resulting in the mineral Sapphire. Cr in Beryl produces green Emeralds (and in general gives a green color to minerals). Fe tends to give minerals a dark coloring, red, black or brown, and Fe in quartz makes purple Amethyst; Mn can make minerals pink; Cu gives a blue or green color; V yellow; and Co deep blue. Because the combination of crystal structure, chemical composition, and various impurities can produce so many different colors, color can be a deceptive guide to identification. In general however, a few basic color guidelines hold true. 1. Minerals containing Fe and Mg ( mafic minerals) are dark in color. Examine the mafic minerals tray. 2. Minerals with Al, K, Na ( felsic minerals) tend to be light in color. Compare the mafic and felsic mineral trays. 3. Some minerals have characteristic and unvarying colors: turquoise is green/blue; malachite is green; sulfur is yellow; hematite is red; biotite mica is black; muscovite mica is clear/gray; olivine is green; galena is silver; pyrite is gold. 4. Other minerals occur in a limited number of colors almost all the time: pyroxene is almost always black, brown, or green; amphibole is mostly black or green, Ca-plagioclase is black or dark gray, orthoclase is pink, white, or gray; quartz is mostly clear, white, or gray; calcite is clear, white, or gray. (6) Look through all the sample trays for quartz crystals. How many different colors of quartz are present? List them. LUSTER & STREAK Related to color are the properties of luster and streak. The way light reflects off the surface of the mineral is its luster; for example, metallic, glassy, dull etc. Streak is the color that a mineral has when it is powdered or scratched. Oddly, this is not always the same as the color of the crystal. Streak is determined by scratching the mineral on a white ceramic plate. UNUSUAL PROPERTIES Some minerals have quite unusual but very distinctive properties. For example the mineral Magnetite is magnetic. Sulfur smells like, well, sulfur. Halite tastes like salt. Calcite has an unusual property called DOUBLE REFRACTION. This is seen in clear crystals of calcite. (7) Place a clear calcite crystal on top of the words printed on this page. Make sketch of what you see when you look through the crystal. Calcite (CaCO 3 ) will also EFFERVESCE in the contact with an acidic solution. Some minerals posses a property called TWINNING. This occurs when two crystals growing side-by-side intergrow with one another. Sometimes the crystals form a small cross, but most often the intergrowth is seen as tiny parallel lines that look like the grooves on an old vinyl record. Calcite, pyrite and plagioclase feldspar often have finely striated surfaces. 6

7 MINERALS ARE EVERYWHERE (8) Stroll through the Snee atrium to the mineral display. Find the sample that is derived from the locality closest to your home town. Give the mineral name and its location (and tell us your home town too). PART III MINERALS COMBINE TO FORM ROCKS Rock can be defined as any naturally occurring aggregate of minerals. Just as letters combine to form words, minerals combine to form rocks. Rocks are the history book of the planet. Meteorite impacts, collisions and rifting of ancient continents, erosion of long vanished mountain ranges, ice ages, and the life itself have all left records in this book. Our job is to interpret it. Rocks are classified into three major groups, depending on their mode of formation Igneous crystallize from a cooling liquid magma Sedimentary an aggregate of loose, accumulated particles Metamorphic have undergone solid state recrystallization due to high temperature and/or pressure The mode of formation of rocks produces characteristic textures. Texture is the key to placing an unknown rock into one of the three rock groups. Within each group, chemical composition expressed as mineral assemblage as well as texture, allow us to classify individual rocks. IGNEOUS ROCKS The texture of an igneous rock depends on the rate of cooling. Slow cooling produces large crystals and a coarse texture, while quick cooling produces small crystals and a fine texture. Igneous rocks erupted from volcanoes cool quickly on the surface of the Earth, while others cool slowly in the Earth s interior. As you saw in the thymol experiment, crystals nucleate and grow, producing a crystalline solid of irregular, interlocking texture. This is the characteristic texture of igneous rocks. The slower the cooling, the larger the crystals. Occasionally, crystals begin to form within the magma, growing to large size, when suddenly the magma is erupted and subsequently cools very quickly. This produces a very distinctive porphyritic texture, characteristic of volcanic rocks. Volcanic rocks may also have a simple, fine-grained texture if there are no early-formed crystals. Often the crystals are so small that a microscope is required to see the texture of a sample and to identify its constituent minerals. Igneous rocks are divided into two types: Volcanic (or extrusive) erupt and cool on the surface Plutonic (or intrusive) cool within the interior Within each of these two groups, the rocks are organized by their chemical composition. Rock chemistry can be determined in the lab by grinding the rock up for analysis, but it is also reflected in the mineral assemblage that makes up the rock. The chart below illustrates the mineral make-up of 8 different igneous rocks. For example, granite and rhyolite have the same chemistry and the same minerals, but because they cool at different rates, they have a different texture. 7

8 Examine the labeled samples of igneous rocks, as well as the colored hand-outs, and make sure you can recognize coarse-grained, fine-grained, and porphyritic textures. SEDIMENTARY ROCKS Sedimentary rocks can be one of two types: Clastic particles are transported and deposited by wind or water Chemical particles precipitate from solution and accumulate on sea/lake bottom The texture of a chemical sedimentary rock is very similar to an intrusive igneous rock, but the mineral composition is quite different. The texture of clastic sedimentary rocks in unmistakably one of rounded grains of various sizes cemented together. Examine the labeled samples of sedimentary rocks, as well as the colored hand-outs, and make sure you can recognize clastic and chemical rocks. METAMORPHIC ROCKS Metamorphic rocks are rocks that have undergone changes as a result of elevated temperature or pressure or reaction with a fluid. A variety of changes may occur. In some cases, change is limited to an increase in grain-size. In other cases, the original minerals are replaced by ones stable at the temperature and pressure at which metamorphism occurred. An important concept in classifying metamorphic rocks is that of metamorphic grade. Grade corresponds more or less to the highest temperature at which metamorphism occurred. Hence low grade rocks were metamorphosed at low temperature and high grade ones at high temperature. In general, as one goes from low to high grade, grain size increases. Usually, increasing pressure accompanies increasing temperature, so rocks metamorphosed at high temperatures have also experienced moderate to high pressures, though there are exceptions (as noted below). For fine-grained sediments such as shales, mudstones, etc., the sequence of metamorphic rocks formed going from low grade to high grade is: 8

9 Slate Phyllite Schist Gneiss Granulite For (mafic) igneous rocks, the sequence is a bit simpler: Greenschist Amphibolite Granulite There are a few other metamorphic rock names you should be familiar with: 1.Marble is formed by metamorphism of limestone. 2.Quartzite is formed by metamorphism of quartz sandstone. At low grade, sheet silicates such as chlorite and mica, are often the dominant minerals, sometimes giving the rock a shiny appearance. These minerals are often aligned. If so, the rock is said to be foliated. At higher grade, this foliation is often replace by segregation of light and dark minerals (usually quartz and feldspar and amphibole and mica respectively) into bands. Banding is characteristic and diagnostic of gneisses, i.e., if the rock is banded it is called a gneiss. Examine the labeled samples of metamorphic rocks, as well as the colored hand-outs, and make sure you can recognize metamorphic textures. (9) Now its time to put all of your skills to work. Return to the tray that you worked with at the beginning of the lab. Reclassify the samples into the following groups: Minerals Igneous rocks Sedimentary rocks Metamorphic rocks (a) For each of the mineral samples, use the determinative properties, as well as the labeled samples in the other trays to identify the mineral. (b) For the igneous rock samples, determine which are volcanic and which are plutonic. Try to identify one or two minerals in each sample and use the attached table to identify the rock. (10) When you ve identified each mineral, reclassify the minerals based on their chemical composition. Make a table showing your work. 9

10 CHARACTERISTICS OF COMMON MINERALS Galena PbS most important ore of lead. Perfect cleavage in three directions Metallic luster High density Hardness between 2.5 and 5.5 Pyrite FeS 2 fool s gold Lacks cleavage, but usually occurs in cubes Metallic luster and gold color Parallel striations Hardness of Gypsum CaSO 4 + H 2 O Perfect cleavage in one direction Curved and splintery fracture Commonly found in arrow-head twins Colors include colorless, white, gray, yellow, red, and brown Hardness of 2 Calcite CaCO 3 common mineral for invertebrate shells Perfect cleavage in three directions forming rhombohedrons Transparent to translucent Effervesces when HCl is added to its surface Colors include white, gray, yellow and red Hardness of 3 Magnetite Fe 3 O 4 common ore of iron Magnetic!!!! No cleavage Iron black color Hardness of 4 Quartz SiO 2 popular gemstone, including amethyst Lacks cleavage and has a curved fracture May be prismatic Colors include clear, purple, pink, gray, and yellow Hardness of 7 10

11 Potassium-Feldspar KalSi 3 O 8 this is a major component of many granites! Two good perpendicular cleavages Striations can be found on crystal faces Color can be pink, blue, green, white and pale yellow Hardness of 6 Mica (chemical composition can vary depending on type) Biotite K(Mg,Fe) 3 (AlSi 3 O 10 )(OH) 2 Muscovite KAl 2 (AlSi 3 O 10 )(OH) 2 One perfect direction of cleavage Typically looks like sheets of paper in a book Colors are dark for biotite, and lighter for muscovite...and we have both! Hardness of Amphibole (and what a complicated chemical composition!) frequently a component of granite Perfect cleavage in two directions in the shape of a diamond Fracture is typically uneven to splintery Crystals generally prismatic Colors are dk green, dk brown, and black Hardness of 5-6 Olivine (Mg,Fe) 2 SiO 4 this mineral is a huge component of the green beaches in Hawaii Cleavage indistinct Curved and uneven fracture Transparent to translucent Colors are olive-green, yellowish-brown, and reddish Hardness of Serpentine - Mg 6 (Si 4 O 10 ) 2 (OH) 2 - This mineral is usually found in large masses of fine grained crystals and is often used as a decorative rock. One of the most interesting forms is chrysotile which occurs long fibers and is the commonest type of asbestos. Usually green because of the substitution of iron for the magnesium. Massive and fine grained when not fibrous. Hardness Garnet - Mg 3 Al 2 (SiO 4 ) 3 - This mineral is often used to make abrasive sand papers but when optically pure can be used for gems of fine quality. Usually red but can be green or yellow. Crystals often have so many faces they look like balls. Hardness

12 Pyroxene - (Ca,Mg,Fe)SiO 3 Common mineral in mafic and ultramafic rocks such as basalt or peridotite Usually black, brown, dark green, but can also be bright green, white, pink Crystals typically form stubby prisms Two cleavage planes at right angles (87-93 ) Hardness 5-6 Halite - NaCl - This is used for common table salt and for de-icing roads. It is mined just north of Ithaca along the east shore of Lake Cayuga. Has good cleavage in three directions to make cubes Colorless in most cases. Hardness 2. Strong taste (often used to test the solubility of a mineral). Talc - Mg 2 Si 4 O 10 (OH) 2 - This mineral has a slippery feel due to very weak vander Waals bonding between sheets and is sometimes known as soapstone. It is used in cosmetics, as filler for paints, and in many other products. Talc often occurs in sheets much like mica, but these sheets are usually warped. It also occurs in fine-grained masses. Color ranges from white to gray to green. Hardness 1 Graphite - C This mineral is black but can look almost metallic when well crystallized. It is used as the "lead" in pencils and is a good lubricant It often occurs as sheets due to the very weak cleavage in one direction It is slippery much as talc is. Opaque Black or submetallic Hardness 1-2 Sphalerite ZnS This mineral is the main ore of zinc. The western Adirondacks is one of the main sources of this mineral in the US. Cleavage occurs in six directions giving broken fragments a kind of sparkle. Its high density makes chunks feel heavy Color ranges from yellow-green to brown to deep red Hardness Corundum Al 2 O 3 Cr impurities make red rubies while Ti or Fe impurities make blue sapphires. Emery is black, granular corundum. Color normally gray or brown. 12

13 Hexagonal crystals. Hardness 9 Hematite Fe 2 O 3 This mineral is the most abundant and important ore for iron. In the US it is mined around Lake Superior. Oxidized hematite gives many rocks a red color (such as the red rocks of the Grand Canyon and environs). Tabular crystals, also shiny flakes (specular hematite), round blobs (botryoidal or oolitic) Color black, red, or silver Luster metallic or earthy Streak red Hardness Malachite Cu 2 CO 3 (OH) 2 Azurite Cu 3 (CO 3 ) 2 (OH) 2 Copper carbonates, brightly colored copper ores, blue (azurite) and green (malachite) Effervesce in dilute HCl Hardness

14 DESCRIPTIONS OF COMMON IGNEOUS ROCKS Basalt - a dense, black rock, aphanitic in texture and mafic in composition. Iceland and the Hawaiian Islands are composed entirely of basalt. Sometimes the upper surface of a lava flow has a glassy texture, due to rapid chilling when exposed to the air. Incidentally, don't be fooled by rocks that contain some larger crystals in a finer- grained "matrix" -- clearly this matrix must have been cooled quickly and so the rocks are classified as extrusive. Scoria - a sponge-like volcanic foam, of basaltic composition; it is spongy from the large number of vesicles left by escaping gas in the melt. Obsidian - volcanic glass. Obsidian is usually felsic in composition. It looks dark because of finely disseminated particles or impurities in the glass. The specimen you are examining in lab may have brown streaks, showing compositional banding. Andesite - a reddish to grayish-black, fine-grained, dense rock. Andesite grades into basalt, and often cannot be distinguished without chemical analysis in the laboratory. Rhyolite - powdery white or pink. You can also see some tiny crystals of feldspar, quartz and perhaps biotite. This is similar to the "pumice" used in some soaps and scouring powders. Peridotite - an ultramafic (mantle) rock composed almost entirely of olivine, with some pyroxene. Similar rocks are eclogites, which contain olivine, pyroxene and garnet. Gabbro - dense and dark, with coarse crystals of pyroxene and Ca-plagioclase. Diorite - usually gray in color and intermediate in composition between felsic and mafic, composed largely of feldspar and amphiboles. Granite - Two granite samples have been provided, to illustrate some of the observable variation possible within this important rock type. This one has pink orthoclase feldspar grains, quartz, and biotite (black mica). Granite - This one is white, with biotite, plagioclase feldspar and quartz. The minerals are in smaller grains, and so are difficult to identify. 14