Rock Identification Lab

Transcription

1 Rock Identification Lab Name Mineral Hardness Objective: You will determine the relative hardness of common objects relative to the known hardness of index minerals. We will use fingernail, copper, glass, and ceramic. Determine the hardness of each object by finding which minerals will scratch it, or alternatively, whether or not the object can scratch the mineral. Fingernail Glass Copper Ceramic Softer Harder Approximate Hardness 2. Name: a. two minerals that have a hardness greater than glass () (2) b. one mineral that would scratch copper, but not ceramic () c. two minerals that you could scratch with your fingernail () (2) 3. Name a mineral, besides diamond, which could be used to cut glass. 4. How would knowing the relative hardness of a mineral be useful to a geologist?

2 Minerals A mineral is a naturally occurring inorganic substance with a definite chemical composition and a regular internal structure. Most minerals are crystalline. Rocks are composed of one or more minerals. Property Definition How to Evaluate Hardness Resistance to scratching or abrasion Use minerals of known hardness from the Mohs Hardness Kits. Scratch the unknown mineral with a known hardness to determine which mineral is harder. We only need an approximate hardness value. Cleavage or Fracture Breakage of a mineral along planes of weakness or in rough pattern Examine the mineral for areas where the mineral is broken. Look for areas where the light reflects from planar surfaces (cleavage) or the breakage is either irregular or has the appearance of broken glass (fracture). Look at the sample to determine if the mineral is metallic in appearance (opaque and shiny) or non-metallic. Luster Character of the light reflected by a mineral Color Reflected visible light Estimate the color - white, black, green, clear, etc. Minerals to Identify in this Lab Mineral Luster Hardness Clev/Frac Common Colors Other Pyrite metallic 6.5 fracture brassy yellow fool s gold, glassy Kyanite non-metal 4 Blue,green,grey,white long crystals Gypsum non-metal 2 cleavage milky, pink to grey scratch w/fingernail Magnetite metallic fracture black to silver magnetic Quartz non-metal 7 fracture clear, pearl, variable glassy fracture Muscovite mica non-metal cleavage clear to yellow thin sheets Talc non-metal cleavage white to green greasy feel Feldspar non-metal 6 cleavage clear, cloudy, pink Galena metallic 2.5 cleavage metallic silver high density Fill in the table using numbered samples from the box marked Luster Sample Luster Hardness Clev. or Frac? Other Name

3 Igneous Rocks Igneous rocks solidify from molten magma and lava. Cooling and crystallization can occur beneath the surface (plutonic) or on the surface (volcanic). Property Definition How to Evaluate Texture For igneous rocks, refers to the crystal size Glassy (non-crystalline), Coarse(can see mineral grains), Fine (cannot see grains) Color Reflected visible light Note if color is light, medium or dark Density Weight per volume Compare equal size rocks, heavier is more dense Vesicles Voids from air bubbles Open voids permeate rock Dark Minerals Mafic has more than 40% Estimate % of blackish crystals in rock 3

4 Igneous Rocks to Identify in this Lab Scoria (Basaltic) volcanic, black, porous texture, tiny grain size, variable density depending on porosity. Look for vesicles (air pockets) in dark rock. Granite plutonic, generally light colored, lower density, hard, medium to coarse grained, no vesicles. Look for quartz (white, pink), less than 40% dark minerals, lighter color, Gabbro - plutonic equivalent of basalt, dark and may have greenish hues or somewhat shiny, high density, medium grain. Look for dense rock and more than 40% dark minerals. Ryolite volcanic equivalent of granite, light colored, low density, fine grains often 2 sizes. Look for light color, fine grain often with embedded larger crystals, gritty feel. Basalt volcanic, dark, more than 40% dark crystals, fine grain (smaller than gabbro), dense, most abundant rock at the surface of the Earth and Moon. Obsidian volcanic, medium density, black, smooth glassy texture, no vesicles. Look for black, glassy, sharp edges. Andesite volcanic, usually blueish-grey or grey, large (visible) and small (not visible) grains, medium density. Look for lighter color, medium density, and smooth texture w/ larger crystal inclusions. Fill in the table using numbered samples from the box marked Igneous Sample Texture (coarse, fine, glassy) Vesicles? yes/no Light, medium, or dark color More than 40% dark crystals? Density (light, med, heavy) Name 4

5 Sedimentary Rocks Sediment is deposited in a number of ways including movement by water, air, gravity or other erosional agents, and by deposition in place through chemical and biological processes. Hardening of this material results in sedimentary rock. Property Definition How to Evaluate Clastic Obvious pieces of rock (clasts) fused into a matrix Observe clast size, Mixture (many sizes), coarse, medium (sand), fine (smooth). Chemical origin Formed in place by chemical processes Chemical deposition usually results in a uniform mineral throughout. Crystalline texture can range from course Organic origin Formed in place by biological processes (easily visible to the eye) to fine. Biotic formation may show fossils (limestone) or black plant material, such as coal or lignite. 5

6 Sedimentary Rocks to Identify in this Lab Travertine (calcareous tufa) - limestone deposited from water in caves, lakes, streams, and percolating groundwater, highly porous, off-white color, non-clastic (chemical deposition). Halite (rock salt) translucent crystalline salt, usually impregnated with colored impurities that can result in many different colors, chemical deposition, soft hardness, look for salty taste Sandstone clastic, sand (barely visible grains), variable color and hardness, gritty feel. Look for gritty feel (like sandpaper) and uniform, barely visible grains. Bituminous Coal non-clastic, organic origin, black color, low density. Look for black, low weight. Fossil Limestone chemical/biotic composition, fairly hard, generally light colored, smooth to rough. Limestone may be smooth or, in fossil limestone, look for fossils of marine creatures. Gypsum chemical deposition, fine grain (gritty or sandy to touch), white color, bubbles under acid. Look for uniform white color, hardness of 2. Breccia - clastic, large clasts easy to see, mixed hardness, mixed color, rough to touch. Look for mixture of clast sizes, including large fragments. Key feature is angular particles. Conglomerate large clasts of gravel and sand cemented together, fragments often rounded, matrix material often light color, very rough to touch. Key feature is rounded particles. Oil Shale clastic, fine grain (cannot see grains) uniform texture, may show layers, fairly smooth to touch. Look for smooth, banded or layered rock. Fill in the table using numbered samples from the box marked Sedimentary Sample Clastic? yes/no Clast shape (round, angular, mixed, sand, silt or clay) Organic origin? (fossils, black) Chemical origin? (uniform) Name 6

7 Metamorphic Rocks Metamorphic rocks have changed from their original form as igneous or sedimentary rocks due to temperature and pressure or through chemical alteration. Property Definition How to Evaluate Texture The relationship between the materials of which a rock is composed Foliated textures show a distinct planar character. This means that the minerals in the rock are all aligned with each other. This planar character can be flat like a piece of slate or folded. Non-foliated textures have minerals that are not Banding Grain Size Foliated rock layering characteristic shape aligned. Essentially, the minerals are randomly oriented. Look for layering in the rock, usually different colors. It can be either linear (straight) or distorted (wavy). Fine to coarse 7

8 Metamorphic Rocks to Identify in this Lab Gneiss foliated, medium to coarse grained, hard, variable colors, rough to the touch. Often key feature is distorted banding. Garnet Schist foliated, fine to medium grains (see by eye), hard, variable color, sometimes shiny, lack banding. Look for quartz, mica, and/or garnet (red crystals) in rock. Serpentine non-foliated, medium grain, mixed hardness, contains dark to light green olivine crystals, medium grain size; green and scaly appearance gives the name Slate foliated, very fine grain (cannot see crystals), hard and brittle, darker color, may contain mica giving a sheen, feels fairly smooth, layering often visible. Marble non-foliated, medium grain includes soft minerals, all colors, gritty to the touch. Check hardness, looking for soft minerals (may not even scratch glass, sometimes). Graphite forms from coal, dark color, dull sheen, fine grain, black residue to touch, scrape black portion across paper to see if leaves a dark line like a pencil. Quartzite non-foliated, medium grain, very hard (6 or 7), pale colors, gritty feel like sand. Check hardness (will scratch glass and ceramic), sandy feel. Soapstone foliated, fine grain, soft, generally grey/blue color, feels smooth and greasy. Can usually scratch with fingernail and feels slippery. Fill in the table using numbered samples from the box marked Metamorphic Sample Texture (foliated, non-foliated) Banding? Grain size (coarse, med, fine) Feel (sandy, (greasy, smooth) Other distinctive feature Name 8