Kanaga Volcano in Alaska

FAIRLY SIMPLE GEOLOGY EXERCISES

for

STUDENTS AND THEIR TEACHERS

 

Topics

Introduction

Classification

Fairly Simple Mineral Identification

Fairly Simple Rock Identification

Easy (and Cheap) Crystal Growing

Look at What Nature has Done

The Trickle Down Effect

Earthquake!

Locate-a-Quake

Terra Mobilis

Maps and Topography

Rock Crystallization

Oil Exploration

Cookie Mining

Porosity and Permeability

Groundwater Pollution Site Assessment

Bowen's Reaction Series

Gum Drop Crystal Models

Search-a-Word and Word Puzzles

 

FAIRLY SIMPLE ROCK IDENTIFICATION

John J. Thomas

Purpose

       Much to our surprise, rocks fascinate many students. Everyone loves to bring us a rock and ask what kind it is and why they found it where they did. This exercise is not a comprehensive rock identification course. It is not designed to make you a perfect rock identifier, nor will it. The real purpose is to introduce the students (and you) to how to identify rocks and give them (and you) the tools they need to learn more about rock identification. When you complete the exercise, you may not feel secure with rock identification, the exercise covers a lot of material very quickly, but with practice you should feel comfortable with the rocks and their names. Like people, it takes time and effort to turn a stranger into a friend. You will not be able to do this exercise without first learning how to identify minerals.

Comment - You will see several repetitions in this web page. The web page is written exactly like the printed exercise. The printed exercises has several repetitions so that the teacher can copy and distribute only the sheets the students need.

Definitions

Igneous Rock - a rock that crystallized from a melt

Sedimentary Rock - a rock that was deposited from a fluid; water, wind, or ice (yes, ice is a fluid.

Metamorphic Rock - a rock that was recrystallized in the solid.

Mineralogy - the minerals in the rock.

Texture - the way that the grains fit together.

Interlocking - the grains are interlocked. They fit together like the pieces of a jigsaw puzzle.
Clastic - the grains are rounded and fit together like balls in a rack or marbles in a jar.
Grain Size - the size of the grains in the rocks.

Terms used with igneous and metamorphic rocks:

Aphanitic - grains too small to be seen without a magnifying glass
Phaneritic - grains large enough to be seen.
Porphyritic - two noticeably different grain sizes.
Ground Mass - The background material in the rock, usually aphanitic. The material surrounding the minerals you can identify.

Terms used with sedimentary rocks:

Coarse - greater than 2 mm. in diameter. Pebbles, cobbles, boulders.
Sand - 2 mm. to 1/16 mm. in diameter. Sand
Silt - 1/16 mm. to 1/256 mm. in diameter. Dust, gritty.
Clay - less than 1/256 mm. in diameter. Mud or modeling clay.
Cement - the material holding (cementing) the grains together.

For more definitions see the section on each kind of rock.

Equipment

Rock samples

Hardness testers:

Glass plate
Jackknife
Fingernail

Dilute hydrochloric acid (1 acid to 10 water) or vinegar. Vinegar works just as well as hydrochloric acid. Hydrochloric acid will eat through clothes, vinegar will not. Acid is optional. You can identify all the minerals by hardness with out acid. You should check with a supervisor for permission to use acid in the classroom. OSHA has very strict regulations on how acid should be used. The acid used for mineral identification is less acidic than your stomach.

Optional - magnifying glass. You can identify rocks without one, but you can see the minerals and grains much better if you have one. 10X is the best.

Rock Identification Sheet

Teacher Information

       Rock identification may sound scary, but it doesn't have to be. We will give you the clues for rock identification. We do simplify the process, but the techniques that we use are the way a geologist works. So give it your best shot and you should find that you can work with the system.

Identifying Minerals in Rocks:

Test hardness with your hardness testers.

To see cleavage, rotate the rock in the light. If the minerals in the rock have cleavage, they will give a bright, flat reflection of light, like rotating a mirror in the light. Check the number on a single grain and the angle between them.

Look at the luster. Quartz always looks glassy. Feldspar looks shiny, but duller and opaque. Mafic (iron-magnesium) minerals have a metallic luster.

Check color. Quartz is transparent. Feldspar is pink, white, or gray to very dark gray (almost black). Mafic minerals are black or dark green. Calcite looks milky white or gray, dolomite is frequently tan.

Look at the shape of the grains. Calcite is rhombic. Feldspars are rectangular or lath shaped. Mafics are prismatic. Micas are flaky, like students.

Look for special properties. Plagioclase has striations on one cleavage direction. Calcite reacts to acid. Magnetite is magnetic.

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IGNEOUS ROCK IDENTIFICATION

Textures

Interlocking - minerals fit together like a jig-saw puzzle
Aphanitic - grains too small to be seen without a magnifying glass
Phaneritic - grains large enough to be seen.
Porphyritic - two noticeably different grain sizes.

Mineral Identification

Quartz:  hardness of 7
              usually clear, may be a little grayish
              glassy
              always irregular grains, they fit between everything else
              fractured
              looks like broken bits of glass

Potassium Feldspar:  hardness of 5 1/2 - 6 1/2
                                  opaque
                                  pink, white to light gray
                                  two good cleavages at right angles
                                  rectangular grains (lath shaped)
                                  opaque, rectangular minerals with good cleavage

Plagioclase:  hardness of 5 1/2 - 6 1/2
                     opaque
                     white, light gray to very dark gray
                     two good cleavages at right angles
                     rectangular grains (lath shaped)
                     striations on one cleavage
                     opaque, rectangular minerals with good cleavage and striations

Others Minerals: normally not used for identifying igneous rocks.

Muscovite - clear mica

Mafic (iron-magnesium) minerals:

Biotite - black mica
Hornblende - needle-like prismatic black crystals
Pyroxene - dark green or black stubby prismatic crystals
Olivine - yellowish-green rounded and transparent crystals. Usually the size of sand grains.

 

Igneous Rock Identification

Phaneritic
Minerals
Aphanitic
Granite
Quartz more than 10% of the rock.
Potassium feldspar noticeably greater than plagioclase.
Not many other minerals.
Light color, may look pink
Rhyolite
Diorite
Usually no quartz, always less than 10%.
Potassium feldspar and plagioclase approximately equal.
Noticeable other dark minerals.
Intermediate color, salt and pepper rock, may look gray
Andesite
Gabbro

No quartz.
Usually the only feldspar is plagioclase. There can be some
    potassium feldspar.
Very dark gray, may look black.

Basalt
Ultramafic
No quartz, no potassium feldspar, little plagioclase, all dark     mafic minerals.
Dark green to black
 
  Volcanic glass, solid, black.
Obsidian
  Volcanic glass, frothy, light.
Pumice

 

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SEDIMENTARY ROCK IDENTIFICATION

Textures

Interlocking - the grains are interlocked. They fit together like the pieces of a jigsaw puzzle.

Clastic - the grains are rounded and fit together like billiard balls in a rack or marbles in a jar.

Clastic Grain Size - the size of the grains in the rocks.

Coarse - greater than 2 mm. in diameter. Pebbles, cobbles, boulders.
Sand - 2 mm. to 1/16 mm. in diameter. Sand
Silt - 1/16 mm. to 1/256 mm. in diameter. Dust, gritty.
Clay - less than 1/256 mm. in diameter. Mud or modeling clay.
Cement - the material holding (cementing) the grains together.
Sorting - the rock may be all one grain size (well sorted) or more than one grain size (poorly sorted).

Mineral Identification

Quartz: hardness of 7, usually clear, may be a little grayish, glassy looks like grains of glass.

Potassium Feldspar: hardness of 5 1/2 - 6 1/2, pink to light gray, two good cleavages at right angles, opaque, rectangular minerals with good cleavage.

Plagioclase: hardness of 5 1/2 - 6 1/2, gray, two good cleavages at right angles, striations on one cleavage, opaque, rectangular minerals with good cleavage and striations.

Clay: aphanitic, soft, usually gray, but can be red or green, looks muddy.

Calcite: hardness of 3, fizzes in acid.

Dolomite: hardness of 3, fizzes in acid when it is powdered. To powder the mineral, scratch it with a knife, steel nail, or other metal object.

Halite: hardness of 2, tastes salty.

Gypsum: hardness of 2, does not taste salty.

 

Sedimentary Rock Identification


Clastic Sedimentary Rocks (particles or granular texture)

Texture
Rock
Identifying Characteristic
Coarse Conglomerate Rounded particles. Contains rock fragments.
  Breccia Angular particles. Contains rock fragments.
Sand Quartzose Sandstone Mostly quartz
  Arkose Usually red to pink in color, noticeable amount of feldspar, may contain rock fragments.
  Graywacke Poorly sorted, lots of clay and unstable minerals, dark color, may contain rock fragments.
Silt Siltstone Foliated (layered in sheets) or massive (no sheets), gritty, gray to tan.
Clay Shale Soft, foliated (layered in sheets), gray, red, green, sometimes tan.

 

Chemical Sedimentary Rocks (interlocking or jigsaw puzzle texture)

Mineralogy
Rock
Identifying Characteristic
Calcite Limestone Hardness of 3, calcite fizzes in acid, frequently has fossils.
Dolomite Dolomite Hardness of 3, dolomite fizzes when you powder it by scratching it with a knife, usually does not have fossils.
Halite Rock Salt Hardness of 2, tastes salty.
Gypsum Gypsum Hardness of 2, does not taste salty.
Silica Chert, Flint Hardness of 7, luster is glassy to waxy, shows concoidal fracture.

 

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METAMORPHIC ROCK IDENTIFICATION

Textures

Interlocking - the grains are interlocked. They fit together like the pieces of a jigsaw puzzle.

Foliated - layered in sheets. The texture looks like a stacking of leaves or papers.

Non-foliated - the texture of the rocks is massive. There is no layering.

Mineral Identification

Quartz: hardness of 7, usually clear, may be a little grayish, glassy looks like grains of glass.

Potassium Feldspar: hardness of 5 1/2 - 6 1/2, pink to light gray, two good cleavages at right angles, opaque, rectangular minerals with good cleavage.

Plagioclase: hardness of 5 1/2 - 6 1/2, gray, two good cleavages at right angles, striations on one cleavage, opaque, rectangular minerals with good cleavage and striations

Muscovite: hardness 2 1/2 - 3, 1 excellent direction of cleavage, clear mica.

Biotite: hardness 2 - 2 1/2, 1 excellent direction of cleavage, black mica.

Chlorite: hardness 4-5, 1 excellent direction of cleavage, green mica.

Calcite: hardness of 3, fizzes in acid.

Dolomite: hardness of 3, fizzes in acid when it is powdered. To powder the mineral, scratch it with a knife, steel nail, or other metal object.

 

METAMORPHIC ROCK IDENTIFICATION

Foliated

Rock
Texture
Identifying Characteristics
Slate Foliated
Dull luster
Clay. Looks like shale, but shale is soft and slate is hard. Red, green, gray, black.
Phyllite Foliated
Silky luster
Tiny micas. The micas are not large enough to see, but do give the rock a silky luster slightly shiny surface.
Schist Foliated
Shiny luster, Metallic
Shines like polished metal. The rock contains coarse grained micas. May have other large crystals, commonly dark colored amphibole, pyroxene, and garnets.
Gneiss Light and dark bands Light and dark Phaneritic minerals in light and dark bands or layers. Light layers are quartz and feldspars. Dark bands are dark micas, hornblende, pyroxene, garnets, and others.

 

Non-foliated

Rock
Texture
Identifying Characteristics
Quartzite Massive, may have color or grain patterns. Mostly quartz what an interlocking texture.
Marble Massive, may have colored patterns. Mostly calcite, may be dolomite. May have may have colored patterns. Commonly white, but colored patterns can be a variety of color including pink, purple, and green.

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DISTINGUISHING CHARACTERISTICS OF THE ROCK TYPES

Rock Type
Unique Features
Igneous

2/3 or more of the rock is feldspars.
Ultramafic rocks.

Sedimentary

Clastic texture.
Rocks with well preserved fossils.
Rocks with 90% or more quartz with a clastic texture.
Rocks with 75-100% calcite.

Metamorphic Foliated texture and mineralogy of phyllite, schist, and gneiss.
Rocks with 90% or more quartz with an interlocking texture.
Rocks with 75-100% calcite.


Crossovers

Ultramafic rocks: can occur as metamorphic rocks. They will usually be foliated.

Shale and slate: have the same grain size and the same texture. Shale is usually soft and slate is usually hard. Slate should have a ring to it when you hit it with metal.

Limestone and marble: have calcite and/or dolomite. Both are interlocking. Limestone tends to be finer grained, marble coarser. Limestone is more likely to have well preserved fossils.

Sandstone and quartzite: both are predominantly quartz. Sandstone has clastic texture. Quartzite has interlocking texture.

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ROCK IDENTIFICATION SHEET


Sample Number ________________

Texture:       Interlocking       Clastic      Foliated

                   Phaneritic (visible)       Aphanitic (not visible)

                  Grain size in mm. __________________

Mineralogy:

 

Special Features:

 

Rock Type (Igneous, Metamorphic, or Sedimentary): ___________________________

Rock Name: ________________________________


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