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Rock City
Teacher Background Information
Children love rocks and minerals, yet most third graders will not know the difference
between the two. This unit of study gives you a wonderful opportunity to build on the
child’s natural interest in the topic and to incorporate important habits of mind:
observation and classification.
A material must fit the following four general criteria to be called a mineral:
1. It must be inorganic, meaning minerals typically do not form from the remains of
plants or animals.
2. Minerals are naturally occurring. True minerals are not manmade.
3. Minerals have the same chemical makeup wherever they are found. For example,
the mineral quartz always consists of one part silicon (an element) to two parts
oxygen (another element). Some minerals, like gold, copper, and sulfur, are made
up of only one element. However, most minerals are combinations of several
different elements. Gold found in Georgia is the same as gold found anywhere
else in the world. Gold is gold.
4. Minerals have specific repeating patterns of atoms. This orderly arrangement of
atoms forms the mineral’s characteristic crystal shape. For example, a crystal of
quartz is always hexagonal because of the way the atoms of silicon and oxygen
join together. However, if a quartz crystal does not have much room to grow, it
may not look hexagonal on the outside, even though the atoms on the inside are
arranged in the same orderly pattern.
Minerals are the building blocks of rocks. A rock is made up of one or more minerals.
Rocks can be placed in one of three categories depending on how they form:
1. Igneous rocks form from magma (molten rock) either deep within the Earth (like
granite), or on the Earth’s surface when lava cools and hardens (like pumice).
2. Sedimentary rocks are layered rocks that from primarily from the accumulation
and compaction of sediment which is derived from preexisting rocks by erosion
(weathering by water, wind, or ice - sandstone is an example). Some sedimentary
rocks from by precipitation from solution (like gypsum).
3. Metamorphic rocks form when preexisting rocks - igneous, sedimentary, or
metamorphic- are subjected to extreme temperatures and pressures deep within
the Earth. The intense heat and pressure cause the mineral composition and grain
size to change. For example, limestones become marbles and shales become
slates.
(Ohio Department of Natural Resources; Division of Geological Survey.
http://www.ohiodnr.com/geosurvey/edu/hands11.htm)
To tell the difference between rocks and mineral you must use observation and
classification. Minerals are the same throughout (homogeneous). A mineral will
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generally have the same appearance inside and out. The properties of color and texture
generally do not vary sharply due to this homogeneity. Color and texture generally do
vary sharply in rocks because rocks are made up of a variety of different minerals.
Geologists generally break open rocks and minerals to help them identify or classify a
rock or mineral sample.
Common minerals are identified using physical properties. The most useful mineral
properties include color, luster, crystal form, hardness, cleavage and fracture, and density.
Your students are expected to recognize the following physical attributes: color, shape,
texture, and hardness.
Mineral color results from the absorption and reflection of the different components of
visible light. Humans use color as one means of differentiating everyday items. Only a
few minerals have characteristic colors, making it difficult to identify a mineral solely on
color. Streak color is a different property than just color because the color of the mineral
may be different than the color of the streak (made by rubbing a mineral on a streak
plate/unglazed tile).
Crystal form is the shape of a mineral when covered by smooth crystal faces. Crystal
faces are flat surface that form on a mineral’s outer surface at fixed angles under specific
conditions. The crystal faces are external expressions of the internal atomic structure of a
mineral. Minerals are subdivided into six basic atomic structural groups, depending on
the arrangement of the crystal faces: cubic, tetragonal, hexagonal, orthorhombic,
monoclinic, triclinic. Some minerals do not display a crystal form, but for the ones that
commonly form distinct faces, their geometric shape is very useful in identification.
Third grade students should be able to make some observations about basic geometric
shapes observed. Young children will also make more general observations about shape
(round, etc.).
Hardness is the ability of a mineral to resist being scratched. Differences in the hardness
of minerals determine how and if they can be used in construction and industry. Minerals
with high hardness are used not only as gemstones but also as abrasives for grinding and
polishing other materials. Soft minerals are easily crushed and used in lubricants and
powders to prevent abrasion. Unknown mineral hardness is determined by rubbing one
mineral against another. A mineral will always be able to scratch itself and any softer
mineral or material. Mohs Hardness Scale is a list of ten standard minerals with
increasing hardness.
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Mohs Hardness Scale
Mineral or Other
Material
Talc
Gypsum (chalk)
Human Fingernail
Calcite (limestone)
Fluorite
Apatite
Glass
Feldspar (granite)
Steel File
Quartz
Hardness
Quick Test
1(soft)
2
2.5
3
4
5
5.5
6
6.5
7
Scratched easily by fingernail
Scratched by fingernail
Topaz
Corundum
Diamond
8
9
10(hard)
Barely scratched by copper penny
Scratched easily by steel
Scratched by steel
Scratches glass easily
Scratches both glass and steel
easily
Scratches quartz
No simple test
No simple test
For third grade students, you will want to use common objects from this from Mohs
Hardness Scale (fingernail or a penny, a metal object like a nail).
The penny has long been the standard for a 3, but recently we found this on line
http://geology.about.com/od/mineral_ident/a/coinmohs.htm and thought you might want
to read it as well. It would be fun to have your students copy this experiment to see if
only Indian Head pennies scratch calcite.
For simplicity sake, if you can scratch the surface of the mineral with your fingernail, it is
a soft rock (1-2), if you can scratch the mineral with a penny it is considered medium soft
(3-4), and if you can scratch a mineral with a metal object it is a hard rock (5-6). A
mineral that is not scratched by any of these is a hard mineral (7+).
The essential distinction between rocks and minerals that you want your third grade
students to understand is that minerals are composed of a single substance, while rocks
are composed of numerous distinct grains of different substances.
Questions students can ask themselves when determining if material is a rock or mineral:
• Is it uniform in color?
• Is it composed of individual grains?
• Do you see regular geometric crystal shapes?
Make sure your young students are called geologists as they participate in these lessons.
Give them many opportunities to simply hold rocks and minerals and to make
comparisons. Fill your bookshelves with identification guides and other non-fiction
books so that students can research their questions. Label all the items in your classroom
that are made from rocks and minerals. And remember….science rocks! Enjoy.
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