MS20 Laboratory. Minerals and Rocks

Transcription

1 MS20 Laboratory Objectives To understand the difference between minerals and rocks To learn to identify several common rock-forming and ore minerals on the basis of common physical properties To understand the differences between igneous, metamorphic and sedimentary rocks as well as how they relate to the rock cycle To learn to identify several common igneous, sedimentary and metamorphic rocks on the basis of composition and texture Introduction As this class deals with earth materials and processes, it is important that we gain knowledge of the materials that make up the Earth s crust. The rocks that make up the Earth and the minerals that compose them have significant effects on our lives. The minerals and rocks we will study in this exercise are the most commonly occurring types. These are of interest to us because a good understanding of the physical and chemical oceanographic processes we will be learning about in this class requires a good understanding of the solid materials that are at the core of these processes. To understand the relationship between minerals and rocks, imagine a Snickers candy bar. It is made up of several different materials: chocolate, nuts, caramel, and nougat (whatever the heck that is). A rock is, like that candy bar, an aggregate of distinctive components; the components are, like minerals, homogeneous substances with properties that distinguish them from each other. A. Minerals A mineral is a naturally occurring, inorganic substance that has an orderly internal structure and a characteristic chemical composition. That they are naturally occurring and inorganic separates minerals from most manufactured substances as well as materials formed only in biological processes. Their internal structure and chemical composition gives minerals characteristic physical and chemical properties that provide clues to the identity of the minerals. While chemical properties are helpful in identification of minerals we will mostly be using physical properties, with the exception of the reaction of carbonate minerals (calcite and dolomite) with dilute hydrochloric acid. Physical Properties Crystal form refers to the orderly geometric arrangement of external planes or surfaces that are controlled by the internal arrangement of atoms that make up a mineral. Crystals are an external feature of a mineral and thus are apparent only when the Revised on 4/4/2005 Page 1 of 14

2 mineral forms by growing into an open space. In most cases minerals grow in tight spaces that don t allow well-formed crystals to grow. Hardness is the resistance of a mineral to abrasion by other materials. Hardness is determined by scratching the surface of the sample with another mineral or material of known hardness. The standard hardness scale is called Mohs Scale of Hardness consists of ten minerals ranked in ascending order of hardness with diamond, the hardest known substance, assigned the number 10. These ten minerals, along with several common substances of known hardness, are shown in Table one. Note that your hardness kit contains only minerals 2-7, as these are the most useful for testing the samples in your mineral kit. Table One Mohs Scale of Hardness Common objects (hardness) 1. Talc 2. Gypsum Fingernail (2.5) 3. Calcite Copper penny (3-3.5) 4. Fluorite 5. Apatite Glass or knife blade (5.5-6) 6. Feldspar 7. Quartz 8. Topaz 9. Corundum 10. Diamond Cleavage and Fracture. The bonds that hold atoms together in a crystalline structure can vary in strength in different directions. If definite planes of weakness exist, a mineral will tend to break along these cleavage planes. Cleavage is generally described with reference to the number of distinct cleavage planes and their angles of intersection. If the bonds are particularly weak the cleavage planes will be well developed (called perfect, or excellent), and if the bonds are stronger the cleavage may be less distinct (called poor). Some examples of different types of cleavage are shown in Figure One. Figure One. Examples of the most common types of mineral cleavage. Revised on 4/4/2005 Page 2 of 14

3 Random breaks not associated with cleavage planes are known as fractures. The most common type of fracture, where a sample breaks along a smooth, curved surface, is known as conchoidal. Luster refers to the way the fresh surface of a mineral reflects or transmits light. Minerals that are completely opaque reflect all incident light. If they are shiny, like chrome or like gold, the luster is metallic. If they are dull, the luster is earthy (like dirt). Minerals that are translucent or transparent (some or all light passed through the mineral), are called vitreous if they are shiny (like glass), waxy if the surface is rather dull, or pearly if they look like mother-of-pearl. Color is one of the most obvious properties of a mineral but it is often of limited diagnostic value, especially in minerals that are not opaque. Quartz for example, can vary from colorless to white to yellow to gray to pink (rose) to purple (amethyst) to black (smoky). On the other hand the colors of some minerals, such as biotite (black) and olivine (olive green) can be distinctive. Streak refers to the color of the mineral in its powdered form. This test is usually performed by scraping the sample against a rough ceramic tile (provided). Streak is usually a more distinctive property than is color, especially with metallic minerals. Hematite, for instance, leaves a distinctive red streak. Other properties that can be distinctive for particular minerals include reactivity with acid (calcite, dolomite), taste (halite, or NaCl, is the mineral form of table salt), smell (sulfur), or magnetism (magnetite is attracted to magnets). Procedures for identifying minerals: 1. Separate metallic/earthy minerals from nonmetallic minerals. If you re not sure about the mineral s luster, then it is probably nonmetallic. a. EARTHY AND METALLIC minerals are totally opaque. If you hold the sample up to the light and can see any light bleeding through, especially along a thin edge or corner, the mineral is non-metallic. METALLIC MINERALS are those with a metallic appearance in their untarnished state (shiny but completely opaque surface, i.e., silver, copper, iron); they may appear to be earthy if they are tarnished (i.e., rust). If any part of the mineral appears to be metallic, then the mineral is metallic. EARTHY MINERALS have a luster like dirt. Though colors can vary widely, they are opaque and have a dull luster. b. NON-METALLIC MINERALS may be vitreous (glassy), waxy (translucent but dull), pearly (like mother-of-pearl) for identification purposes we will divide these into light-colored and dark-colored minerals. 2. For the metallic or earthy minerals: a. COLOR: Describe the mineral s dominant color. b. STREAK: Determine the color of the mineral s streak. c. OTHER : Determine other properties such as: Revised on 4/4/2005 Page 3 of 14

4 1. What is the mineral s HARDNESS? (not necessarily in this order); It is important to note that some minerals (such as chalcopyrite and malachite) often have inclusions that will give an anomalous hardness, so look at the mineral carefully before you test the hardness. 2. Presence or absence of CLEAVAGES; 3. Presence or absence of MAGNETISM; 4. SPECIFIC GRAVITY (does the sample feel particularly heavy for its size?). d. Use the metallic minerals table to determine the probable name of the mineral. 3. For the non-metallic minerals: a. COLOR is generally not important but sometimes useful, especially in darkcolored minerals; for minerals that are not metallic or earthy, we have separated them into dark-colored non-metallic minerals and light-colored non-metallic minerals b. HARDNESS: Determine relative to the glass plate (H = 5.5); it may help to refine the hardness number using your fingernail (H = 2.5), a copper penny (H = 3.5), or the hardness minerals in your kit. It is important to note that some minerals (such as chalcopyrite and malachite) often have inclusions that will give an anomalous hardness, so look at the mineral carefully before you test the hardness. c. CLEAVAGE: Determine the mineral s cleavage: 1. If present, how many cleavage planes are there; do the faces appear to be at right angles to one another? 2. If there is no cleavage apparent, do broken surfaces appear to be random, or fibrous (splintery fracture), or smooth and curved (like broken glass: conchoidal fracture)? d. OTHER: Determine the mineral s other properties such as: 1. TASTE (halite has distinctive salty taste); 2. LUSTER in non-metallic minerals can be described as glassy, waxy, earthy, pearly, opalline, etc ; 3. ACID TEST (calcite and other carbonate minerals will effervesce; other minerals will not); 4. CRYSTAL FORM (quartz may be pyramid-capped hexagonal prisms; pyrite and fluorite sometimes display cubic crystals; garnets often are 12- sided crystals with rhombic faces); 5. SURFACE MARKINGS such as striations in pyrite and plagioclase (appear like record grooves etched into cleavage face) and exsolution lamellae in orthoclase (like opaque, discontinuous wavy lines) e. Use the tables for light-colored or dark-colored nonmetallic minerals to determine the probable name of the mineral. Revised on 4/4/2005 Page 4 of 14

5 What is the mineral s color? Dark Green to Black Silver (glittery) Gray (metallic) Brick red (earthy) METALLIC OR EARTHY MINERALS What is the mineral s streak? No streak -or barely discernible green streak- Red to redbrown Compare the mineral s physical properties to other characteristic properties below. 2 cleavages at 56 and 124 ; Shiny opaque black; H = cleavages at nearly right angles (87 & 93 ); Dull surface; H = Hardness varies: metallic form will scratch glass, Earthy form will not; metallic form tarnishes red; May be magnetic Find mineral name(s) and check the mineral database for additional properties. Hornblende (Amphibole) [Ca 2 (Fe,Mg) 4 (Al,Fe)]Si 7 AlO 22 (OH) 2 ] Augite (Pyroxene) [(Ca,Na)(Mg,Fe,Al)(Si,Al) 2 O 6 ] Hematite [Fe 2 O 3 ] Yellow to Yellowbrown (earthy) Dark Brown (metallic or earthy) Black Yellow to brown (streak similar to color) No streak -or barely discernible green streak- Chalky; Tarnishes yellow; Amorphous Color dark brown to black; May have pitted texture (rusted iron) Excellent basal cleavage; Barely scratches with fingernail; H = White to gold No streak Excellent basal cleavage; Barely scratches with fingernail; H = Limonite/Goethite [Fe(OH) 3 ] Biotite mica [K(Mg,Fe) 3 [AlSi 3 O 10 (OH,F) 2 ] Muscovite mica [KAl 2 (Si 3 Al)O 10 (OH,F) 2 ] Revised on 4/4/2005 Page 5 of 14

6 What is the mineral s hardness? HARD (H > 5.5) scratches glass Not scratched by nail or knife blade SOFT (H 5.5) Does not scratch glass Not scratched by nail or knife blade LIGHT-COLORED NONMETALLIC MINERALS What is the mineral s cleavage? Cleavage excellent or good Cleavage poor or absent; Crystal faces may be mistaken for cleavages; Conchoidal fracture Cleavage poor or absent; conchoidal fracture Cleavage excellent or good 3 cleavages not at right angles 3 cleavages not at right angles 3 cleavages at right angles 1 excellent, 2 poor cleavages 1 cleavage (basal cleavage) Compare the mineral s physical properties to other characteristic properties below. 2 cleavages at nearly right angles; record groove striations; White to dark gray; H = 6 2 cleavages at nearly right angles; Wavy discontinuous lines; Pink, white, gray, green; H = 6 Translucent; 12-sided crystals; Light green, white, or other colors; Hardness = 7 to 7.5 Vitreous luster; May display hexagonal prisms with pyramids; Transparent or translucent; Colorless, white, gray, or other colors; H = 7 Pale olive green to yellow; Transparent to translucent; H = 7 Excellent rhombohedral cleavage; Effervesces in dilute HCl; Colorless, white, yellow, pink or other colors; H = 3 Excellent rhombohedral cleavage; Effervesces in dilute HCl only if powdered; Colorless, white, gray, or pink; H = Excellent cubic cleavage; Salty taste; Colorless, white, or other colors; H = good, 2 poor cleavages; Can be scratched with fingernail; Massive or tabular crystals, blades or needles; Colorless to white; H =2 Excellent basal cleavage; Pearly luster; White, yellow, light brown, H = Find mineral name(s) and check the mineral database for additional properties. Plagioclase feldspar [NaAlSi 3 O 8 - CaAl 2 Si 2 O 8 ] Orthoclase feldspar [KAlSi 3 O 8 ] Garnet [(Mg,Fe,Ca) 3 (Al,Fe,Cr) 2 (SiO 4 ) 3 ] [(A 2+ ) 3 (B 3+ ) 2 (SiO 4 ) 3 ] Quartz [SiO 2 ] milky (white), Citrine (yellow), rose (pink) Olivine [(Fe,Mg) 2 SiO 4 ] Calcite [CaCO 3 ] Dolomite [CaMg(CO 3 ) 2 ] Halite [NaCl] Gypsum [CASO 4 * 2H 2 O] Muscovite mica [KAl 2 (Si 3 Al)O 10 (OH,F) 2 ] Revised on 4/4/2005 Page 6 of 14

7 What is the mineral s hardness? HARD (H > 5.5) scratches glass Not scratched by nail or knife blade SOFT (H 5.5) DARK-COLORED NONMETALLIC MINERALS What is the mineral s cleavage? Cleavage excellent or good Cleavage poor or absent; Crystal faces may be mistaken for cleavages; Conchoidal fracture Cleavage poor or absent; Conchoidal fracture Cleavage excellent or good Compare the mineral s physical properties to other characteristic properties below. 2 cleavages at nearly right angles; record groove striations; Dark gray to blue gray to black; H = 6 2 cleavages at ~56 and 124 ; Opaque black; H = cleavages at nearly right angles (87 & 93 ); Dark green to black; H = Translucent; 12-sided crystals; Red to dark red to brown, or other colors; H = 7 to 7.5 Vitreous luster; Massive hexagonal prisms and pyramids; Transparent or translucent; Gray, brown or purple; H = 7 Pale olive green; Transparent to translucent; H = 7 Black; Excellent basal cleavage; H = Find mineral name(s) and check the mineral database for additional properties. Plagioclase feldspar [NaAlSi 3 O 8 - CaAl 2 Si 2 O 8 ] Hornblende (Amphibole) [Ca 2 (Fe,Mg) 4 (Al,Fe)]Si 7 AlO 22 (OH) 2 ] Augite (Pyroxene) [(Ca,Na)(Mg,Fe,Al)(Si,Al) 2 O 6 ] Garnet [(Mg,Fe,Ca) 3 (Al,Fe,Cr) 2 (SiO 4 ) 3 ] [(A 2+ ) 3 (B 3+ ) 2 (SiO 4 ) 3 ] Quartz [SiO 2 ] Smoky (brown-black), Amethyst (purple) Olivine [(Fe,Mg) 2 SiO 4 ] Biotite mica [K(Mg,Fe) 3 [AlSi 3 O 10 (OH,F) 2 ] Revised on 4/4/2005 Page 7 of 14

8 B. Rocks Rocks are considered to be aggregates of one or more minerals, although non-mineral substances such as coal or volcanic glass are also considered to be rocks. Rocks are classified into three categories: Igneous, Sedimentary and Metamorphic. The relationships between the three groups are shown by the rock cycle (Figure Two). Figure Two. All rocks originate in the Earth s interior as magma (molten rock plus dissolved gases) that cools and crystallizes to form igneous rock when it approaches or reaches the surface. Rock at the surface is exposed to physical and chemical weathering processes that break it down into sediment. Sediments are transported downhill by wind, water or glaciers to accumulate in basins (such as lakes or the ocean) where lithification (compression and cementation) turns them into sedimentary rock. Solid rocks exposed to extremes of heat and/or pressure due to deep burial or proximity to faults or igneous processes can go through gradual changes to become metamorphic rock. Igneous Rocks are formed by the cooling and crystallization of a molten silicate-rich liquid known as magma. Magma that cools slowly (thousands to millions of years) beneath the Earth s surface forms plutonic or intrusive igneous rocks, while magma that extrudes at the Earth s surface (commonly known as lava) crystallizes in hours to weeks to form volcanic or extrusive igneous rocks. The cooling rate determines the rock s final texture, while the chemical composition of the magma determines the mineral composition of the rock. These two properties, texture and composition, are the basis for classifying and identifying igneous rocks. Revised on 4/4/2005 Page 8 of 14

9 Composition can be divided into four major groups (see Figure Three): (1) rocks in which the light-colored minerals quartz and orthoclase feldspar dominate, known as felsic rocks; (2) rocks in which light colored minerals are present in roughly equal proportions with dark-colored minerals such as augite and hornblende, known as intermediate rocks; (3) rocks in which the dark colored minerals dominate, known as mafic rocks; and (4) rocks which are dominated by the green mineral olivine, known as ultramafic rocks. Plagioclase feldspar is also present in significant amounts in intermediate rocks (where it is usually white) and mafic rocks (where it is typically dark gray). Figure Three. Percentage of dark minerals against a light background. Texture of rocks is dependent on cooling rate, thus on how and where the rock forms. Common rock textures are described in Table Two: Table Two: igneous rock textures Grain size Coarse (phaneritic) Mixed (porphyritic) Fine (aphanitic) Pyroclastic (fragmental) Vesicular Description Interlocking mineral grains that can be distinguished with the naked eye Two distinct crystal sizes, with large crystals embedded in a fine- or coarsegrained matrix Individual mineral grains are small and cannot be distinguished with the naked eye Granular rocks welded together from volcanic materials that have flown through the air, such as ash and other rock fragments Largely glassy rocks dominated by vesicles or bubbles, often giving rock a Revised on 4/4/2005 Page 9 of 14

10 (contains bubbles) Glassy foamy texture (rapid cooling, gases trapped in matrix) Texture resembles glass (rapid cooling, gases have escaped) Procedures for classifying igneous rocks: 1. Identify the rock s texture: does it appear to be: a. Pyroclastic (fragmental): appears to be made of fragments, like a sedimentary rock. b. Frothy or vesicular: full of gas bubbles, like the head of a beer. c. Glassy: smooth surfaces, shiny, translucent at edges/corners, conchoidal fracture; like glass. d. Fine-grained, or aphanitic: smooth surface, dull, individual mineral crystals too small to see with naked eye. e. Porphyritic (mixed grain): fine-grained, but with large, visible crystals floating in the matrix. f. Coarse-grained, or phaneritic: all crystals big enough to be seen with the naked eye, roughly equidimensional. 2. Identify the rock s color index (i.e., estimate the percentage of light-colored vdark-colored minerals) and, if possible, the identity and abundance of specific minerals. a. If the rock is coarse-grained, estimate the color index and percentage abundance of quartz, feldspars, and mafic minerals. With this information you can characterize the rock as felsic, intermediate, mafic or ultramafic. b. If the rock is fine-grained (i.e., mineral grains are too small to be seen with the naked eye) you can approximate the composition based on the rock s color index. Felsic fine-grained rocks tend to be pink, white, or pale brown. Intermediate fine-grained rocks tend to be gray to greenish-gray. Mafic and ultramafic rocks tend to be dark gray to black. c. Other textures are mostly composition-independent. 3. Classify the rock using the igneous rock classification chart: Revised on 4/4/2005 Page 10 of 14

11 Texture IGNEOUS ROCK CLASSIFICATION Chemistry/composition Felsic Intermediate Mafic Ultramafic Dominant minerals Quartz, orthoclase Plagioclase, Plagioclase, Olivine hornblende Augite Secondary minerals Hornblende, mica Augite Olivine Augite, Plagioclase Glass Obsidian Glassy + vesicular Pumice Scoria Fragmental/detrital Tuff/Volcanic breccia Fine-grained (aphanitic) Rhyolite Andesite Basalt Mixed grain sizes (porphyritic) Coarse grained (phaneritic) Rhyolite porphyry Andesite porphyry Basalt porphyry Granite Diorite Gabbro Peridotite Revised on 4/4/2005 Page 11 of 14

12 Marine Science Laboratory Sedimentary Rocks are formed from the products of weathering (breakdown) of preexisting rocks. When these weathering products are compacted by the pressure of overlying materials and cemented together by precipitation of mineral matter between the particles, new rock is formed. If the source materials are solid particles the rock is known as clastic (made from clasts), while if the source material is dissolved ions precipitating from a solution the rock is known as chemical or biochemical. Sedimentary rocks make up only about 8% of the crust of the Earth, but cover 75% of the Earth s surface. They are important to us as the source of petroleum and coal, and as our primary source of water (groundwater). They are important to geologists because sedimentary rocks are indicators of the environments in which they form. Table Three: Sedimentary Rocks Siliclastic Rocks Size Class > 256 mm Boulder > 64 mm Cobble Gravel > 2 mm Pebble quartz > 1/16 mm Sand quartz + clay quartz + feldspar > 1/256 mm Silt < mm Clay Rock name Conglomerate Breccia Sandstone Graywacke Arkose Siltstone Shale Chemical and Biochemical Rocks Chemical Biochemical Carbonate Rocks Oolitic Limestone Micrite Dolostone Chalk Coquina Other Chemical Rocks Diatomite Chert Rock Salt or Rock Gypsum Other Biochemical rocks Peat and Coal Procedures for classifying sedimentary rocks: 1. Determine the general composition of the rock (i.e., clastic or chemical/ biochemical). Revised on 4/4/2005 Page 12 of 14

13 Clastic: The rock appears to be made of visible fragments, is porous, or it appears to have distinctive layering. Chemical or Biochemical: The rock is crystalline or microcrystalline, or has identifiable fragments are all shell or skeletal fragments. 2. Classify the rock using the sedimentary rock classification chart. SEDIMENTARY ROCK ANALYSIS AND CLASSIFICATION Composition of rock Textural and other distinctive properties Name of rock Rounded clasts; pebbles/cobbles in a sandy or finer matrix Conglomerate Clastic or detrital Chemical or biochemical Mainly quartz grains, feldspar grains, rock fragments and/or clay minerals Mainly calcium carbonate (CaCO 3 ); so rock effervesces in dilute HCl Mainly varieties of quartz (SiO 2 ); chalcedony, flint, chert, opal, jasper, etc ; does not effervesce in HCl Gravel (>2 mm) generally poorly sorted fragments Angular clasts; pebbles/cobbles in a sandy or finer matrix Mostly quartz; wellsorted sand-size grains Sand (1/16 mm 2 mm) Mixture of quartz and feldspars; poorly sorted, angular grains Mostly silt (1/256 mm 1/16 mm) Mostly clay minerals (<1/256 mm) Well-cemented, dense rock Poorly cemented; porous, earthy rock; usually white to light brown Poorly cemented; porous, earthy rock; usually white to light brown Microcrystalline, conchoidal fracture Chalky texture, feels gritty between teeth; massive, layering not obvious Feels smooth between teeth; May be fissile (easily splits along layers) Massive, no obvious structure; Dull to waxy luster, may display conchoidal fracture Fossils present Very fine, like silt or clay Very fine, like silt or clay Scratches glass Breccia Quartz sandstone Arkose (feldspar sandstone) Siltstone Shale (mudstone) Limestone (microcrystalline) Fossiliferous limestone Chalk Diatomite Chert Revised on 4/4/2005 Page 13 of 14

14 Metamorphic Rocks METAMORPHIC ROCK ANALYSIS AND CLASSIFICATION Appearance, colors, properties Rock name Mineralogy Parent rock Opaque (dark green to black) Harder than glass Greenstone (chlorite, epidote, feldspar) Hydrothermally altered basalt or gabbro Opaque (light Softer than glass Serpentinite (serpentine) surface Greenstone (basalt) green to black) appears waxy or polished Black, may be Soft, low density Bituminous coal carbon Peat or lignite layered or jointed Black, shiny, often shows conchoidal fracture Soft, low density Anthracite coal carbon Bituminous coal Fine grained: no visible crystals, dull surface Coarse-grained: shiny, glittery surface Coarse-grained: distinctive light and dark layers Slaty foliation Slate (clay, mica: biotite, muscovite, chlorite) Schistose foliation Schist (micas, minor feldspars, hornblende, quartz Banded foliation Gneiss (minerals vary but commonly quartz and feldspars in light layers, hornblende and biotite in dark layers) Shale or siltstone Phyllite, basalt, or granite Schist or granite Revised on 4/4/2005 Page 14 of 14