Geology 103 Planet Earth (QR II), Laboratory Exercises 1. Minerals

Transcription

1 Geology 103 Planet Earth (QR II), Laboratory Exercises 1 Student Name: Section: Minerals Minerals are naturally occurring, inorganic, crystalline solids with a characteristic chemical composition. Most minerals are chemical compounds (combinations of more than one chemical element); exceptions include sulfur (S), diamond and graphite (both forms of pure carbon, C), and native metals, such as gold (Au) and silver (Ag). The chemical composition of a mineral may be a fixed formula, such as that of calcite (calcium carbonate, CaCO 3 ), or may consist of a range of values, as in olivine, which varies from pure magnesium silicate (Mg 2 SiO 4 ) to pure iron silicate (Fe 2 SiO 4 ). A sample of olivine in nature could consist of any fractional combination of iron/magnesium silicate between these two end member compositions. Even minerals that are relatively chemically pure typically have some impurities. Calcite, for example, typically contains a small amount of magnesium (Mg 2+ ) or ferrous iron (Fe 2+ ) substituted for calcium (Ca 2+ ). Most Earth materials are inorganic. Exceptions (organic matter) include proteins and carbohydrates produced by living organisms, as well as hydrocarbons (such as oil and natural gas), which result when dead organic matter is altered by heat and pressure within the Earth. Crystalline materials are those that possess a fixed and ordered arrangement of the atoms that comprise them. For example, quartz, an exceptionally abundant mineral that makes up most of the sand in riverbeds and beaches, is composed mostly of SiO 4 (silica) and a crystalline structure in which each atom of silicon (Si) is surrounded by four atoms of oxygen forming a tetrahedron (triangular pyramid) as shown below. The numerous silica tetrahedra are linked together to form a repetitive framework (neighboring tetrahedra are bound together because they share oxygen atoms). Glass used to make windows and glassware is also composed of SiO 4, with silicon and oxygen combined to form tetrahedra, but the tetrahedra have no precise geometric arrangement with respect to one another. As a result, glass is amorphous (shapeless, lacking a characteristic crystal form), a trait utilized by glass blowers who mold hot SiO 4 and then rapidly cool it (by quenching it in water, disallowing the formation of new quartz crystals) to produce the distinctive shapes of pilsner glasses, punch bowls, and gravy boats. oxygen atom silicon atom The silica tetrahedron. Occasionally Al 3+ may take the place of Si 4+ in the structure. All rocks are solid aggregates of one or more minerals, with a few exceptions. Organic rocks, such as coal, are composed of compressed, altered organic matter. Some volcanic rocks cool so quickly that there is no time for coordinated crystals to organize before the lava solidifies. Such rapid cooling produces volcanic glass. Opal is a largely amorphous mixture of silica and water.

2 Geology 103 Planet Earth (QR II), Laboratory Exercises 2 Physical properties About 3,000 minerals have been discovered, but most of these are rare. Only 30 of these are important constituents within rocks (cohesive aggregates of minerals). Minerals are identified and distinguished from one another by a number of physical properties. The most important and useful of these properties are described below. Crystal form is the regular geometric pattern of planar faces that occur on a crystal specimen that grows in an unrestricted setting, influenced by the mineral s underlying crystalline structure. Crystal habit describes the shape of the crystals of a particular mineral as they occur in nature as well as patterns produced by aggregates of crystals. Because of constraints on growth (such as a lack of growing space) crystal habit may differ from ideal crystal form. Cleavage is the presence of planes of weakness within a crystal, along which the crystal will preferentially break. Cleavage planes do not occur in isolation but rather in parallel sets as a result of weak chemical bonds between atoms in the direction perpendicular to these planes of weakness. The orientation of a parallel set of planes is termed the direction. A mineral may have multiple directions (hence multiple sets of planes). Some minerals lack, breaking only on curved or rough, irregular surfaces. Such breakage (that does not occur along planes) is termed fracture. Conchoidal fracture (characteristic of the mineral quartz) occurs along a smooth, curved surface and is marked by concentric elliptical grooves (as occurs when a glass window is shot by a bb). A graphic illustration of the different type of is given in Table A. Hardness is the ability of a mineral to resist being scratched (and, conversely, to scratch substances that are softer than the mineral). With regard to minerals, hardness does not mean resistance to breakage. The hardness of a mineral is typically reported as a number (1-10, symbolized by the letter H ) based upon a relative scale termed the Mohs hardness scale. Diamond, the hardest known substance, has a hardness value of 10, and talc, the softest mineral, has a hardness of 1. The full scale is given in the following table: number mineral or object number mineral or object 10 diamond 5 apatite 9 corundum 4 fluorite 8 topaz 3.5 copper penny 7 quartz 3 calcite 6 feldspar 2.5 fingernail 5.5 knife; glass 2 gypsum 1 talc The relative nature of the Mohs scale means that diamond is not necessarily twice as hard as apatite (in fact it is more than ten times harder), but rather that it is merely harder than corundum, which is harder than topaz, etc. Hardness can be tested by directly scratching an edge of a mineral against a substance, such as these minerals, for which the value of

3 Geology 103 Planet Earth (QR II), Laboratory Exercises 3 hardness is known. It is convenient to use fingernails (H = 2.5, between that of gypsum and calcite), copper pennies (H = 3), or glass plates (H = 5.5) to test for hardness. Specific gravity refers to the density of a mineral, reported without units, in relation to the density of water (which has a density of 1000 kg/m 3 or 1 g/cm 3 ). For example, the mineral galena (lead sulfide, PbS) has a density of 7500 kg/m 3 (or 7.5 g/cm 3 ), so its specific gravity (symbolized by G ) is 7.5. A few minerals, such as galena and pyrite (iron sulfide, FeS 2, G = 5) are so dense that their great specific gravity can be felt simply by lifting them in hand sample. Most important rock forming minerals have a specific gravity between 2.5 and 3.5. Color is an obvious characteristic of minerals, but unfortunately it is rarely useful in making a mineral identification. Many minerals can vary greatly in color, depending upon trace elements that are present. Rubies are red, and sapphires are blue, but both are examples of the mineral corundum (Al 2 O 3 ). Substitution of trace amounts of chromium for aluminum produces the red color of rubies. Sapphires are blue because of the presence of trace amounts of iron and titanium substituted for the aluminum. With that said, however, some minerals still possess their own characteristic colors that can be used to aid their identification. Luster is a qualitative description of the shininess of a mineral, i.e., the manner in which the substance reflects light. Highly reflective opaque materials such as silver mirrors, gold bricks, and the minerals pyrite and galena are said to possess a metallic luster. Other types of luster include adamantine (brilliant, literally diamond-like ), vitreous (glassy), pearly (having a colorful, iridescent sheen), greasy, and earthy (dull) Streak is the color of a mineral in powdered form. The name is derived from the practice of scratching a relatively softer (H < 7, the hardness of porcelain) mineral against a porcelain plate and observing the color of the powder that forms. Streak is white for most minerals that do not possess metallic luster. It is more colorful and diagnostic for metallic minerals, and can differ markedly from the color of the specimen in hand sample. In addition to the aforementioned properties, a few minerals have unique attributes that allow them to be easily identified. Calcite fizzes if it contacts dilute hydrochloric acid. Bubbles form as the acid dissolves the calcium carbonate to produce carbon dioxide gas. Magnetite (Fe 3 O 4 ) will attract a bar magnet. Halite (NaCl, the mineral used for table salt) has a characteristic salty taste.

4 Geology 103 Planet Earth (QR II), Laboratory Exercises 4 Table A An Illustration of Cleavage Directions # of Directions Illustration and Example 1 muscovite (mica) 2 feldspar (at right angles) amphibole (not at right angles) 3 halite (at right angles) calcite (not at right angles) 4 fluorite 6 sphalerite

5 Geology 103 Planet Earth (QR II), Laboratory Exercises 5 Table B Metallic or Dark-colored, Nonmetallic Minerals mineral characteristics mineral name other features black or dark/gray Magnetite ( lodestone ; used as a compass; type of iron ore) [high specific gravity] metallic luster magnetic non-magnetic cubic shape gold/bronze colored silvery/lead colored Pyrite ( fool s gold ; iron sulfide) Galena (lead ore; PbS) [cubic crystals; no ; black streak; striations] [cubic crystals and ; very high specific gravity] not cubic no soft hard red/brown streak crystal faces may be visible; purple, green, or gray no crystal faces; olive green; brownish or grayish green Graphite (pencil lead; C) Hematite (iron oxide; type of iron ore) Quartz (SiO 2 ; conchoidal fracture) Olivine (occurs in mafic or ultra-mafic rocks) [dark streak on paper] [specular: metallic luster, steel color, red/brown streak; may have foliation; earthy: red/brown color and streak] [gray variety is smoky; can also be clear, white, or pink] [light variety is glassy and light green] hard crystal faces may be visible; generally dark brown/maroon Garnet (can be a gem or abrasive) [can be opaque or glassy; crystals are generally equant or 12 sided] non-metallic luster; dark color good two directions, not at right angles two directions; approx. at right angles Amphibole (occurs in igneous and metam. rocks) Pyroxene (occurs in igneous and metam. rocks) [may occur in bladelike or needle-like crystals] [may occur in small blade-like or needlelike crystals] soft or medium hardness good one direction; splits into very thin sheets 4 directions Biotite (Mica) (occurs in igneous and metam. rocks) Fluorite (state mineral of Illinois) [usually very shiny; sheets are somewhat transparent & flexible] [dark type is purple; may also be clear, green or yellow; cubic or trapezoidal]

6 Geology 103 Planet Earth (QR II), Laboratory Exercises 6 Table C Light-colored, Nonmetallic Minerals mineral characteristics mineral name other features Talc [usually milky white, greasy feel; no (pulverized to yellowish, or makes distinct make talcum greenish] mark on cloth powder) soft medium hardness good good cubic Good ; non-cubic may split into thick sheets one direction; splits into very thin sheets reacts with HCl only powder reacts with HCl no reaction with HCl Gypsum (used in drywall) Muscovite (mica) (occurs in ign. and metam. rocks) Halite (common table salt) Calcite (CaCO 3 ; found in limestone; shells) Dolomite (CaMg(CO 3 ) 2 ) Fluorite (state mineral of Illinois) [usually clear or gray] [usually light tan or brown; sheets are transparent and flexible; some varieties are harder then fingernail] [tastes salty; generally clear or light gray; transparent or translucent] [breaks into white to clear blocks that resemble a pushed over cube] [may occur as rhombshaped crystals] [clear, green or yellow crystals; some varieties may be dark; may be cubic or trapezoidal; in 4 directions] hard good no white or gray pink or off-white clear, white, pink; may have crystals no crystal faces; light green, glassy Plagioclase (variety of feldspar) Orthoclase (a.k.a. K-feldspar) Quartz (SiO 2 ; common in sandstone) Olivine (gem version is peridotite; occurs in igneous and metam. rocks.) [brick-shaped crystals with a milky luster; may have striations] [milky luster, brickshaped crystals; no striations] [can be clear, white, pink, smoky, or purple; some variety may be dark] [moderately high sp. gr.; may occur in a cluster; some may be dark]

7 Geology 103 Planet Earth (QR II), Laboratory Exercises 7 Assignment: Mineral Properties 1. You will be provided with 5 geologically important minerals on a tray. They are: biotite, calcite, plagioclase, amphibole, and quartz. Examine these samples carefully. For each sample, list: a) the mineral name, b) hardness (Use soft if it is softer than a fingernail, medium if it is harder than fingernails but softer than glass, and hard if it is harder than glass.), c) color or colors of the hand specimen (not the streaks), and d) number of directions (i.e., 0, 1, 2, 3, or 4). Sample # Mineral Names Color(s) Hardness # of Cleavage Directions Describe at least one physical property that you could use to tell the difference between the minerals in the following pairs (e.g., one vs. three directions of ): Mineral Pairs Diagnostic Physical Property Halite vs. Quartz Galena vs. Biotite Calcite vs. Quartz

8 Geology 103 Planet Earth (QR II), Laboratory Exercises 8 Mineral Identification 3. Use the mineral identification flow chart given in Tables B & C to identify the unknown minerals. Describe the specific properties that you use in your identification (e.g., 1 direction and/or hardness > 5.5 etc.). You should list enough properties to distinguish each unknown sample from all other minerals in the tables. Sample Number: Mineral Name: Properties used for identification: Sample Number: Mineral Name: Properties used for identification: Sample Number: Mineral Name: Properties used for identification: