BACKGROUND:
Quartz is a very common mineral. It is composed of
silicon and oxygen atoms, and has the chemical formula SiO2
(silicon dioxide). Quartz is very hard (7 on the Moh’s Hardness Scale). It
grows in an easily recognizable hexagonal crystal habit. It fractures when
broken, i.e., it does not have cleavage.
Quartz occurs in many varieties. Most common is the
clear variety, which the ancient Greeks called "crystallos" or
"clear ice." The word "crystallos" eventually evolved
into the word "crystal." Depending on its color, which is caused
by trace chemical impurities, quartz forms a great variety of gemstones
including agate, amethyst, adventurine, bloodstone,
smoky quartz, carnelian,
citrine, onyx, rose quartz, and tiger's eye.
Quartz is a very important mineral commodity. It has
many uses including applications in the computer, communications, food, and
jewelry industries. Quartz is also the main component of the sand used to
produce concrete and glass, as well as in sandpaper, sandblasting, and
smelting.
Your students may be familiar with two other
substances that are composed of silicon dioxide, obsidian and glass. While
these materials have the same composition as quartz, they lack a crystalline
structure, so they are not minerals. They are classified as amorphous
solids.
PROCEDURE:
- Get a beaker with water. Explain to the students that the water
represents "pure" quartz because it contains no impurities. As
they add food coloring to the water, explain how the resulting color
change is analogous to colored quartz. Make sure they understand that a
tiny, or trace, amount of color gives the whole substance a different
look.
- In exercise 2, have the students examine the following examples of
silicon dioxide and describe their characteristics. They should also
determine whether the substances are minerals, rocks, or amorphous
solids.
ROSE QUARTZ - a mineral, a common gemstone; pink color caused by
trace amounts of titanium
ADVENTURINE - a mineral, a common gemstone that has inclusions of
mica and hematite imparting a green color to the stone
CLEAR CRYSTAL OF QUARTZ - a mineral, 6-sided crystal is the shape
in which a quartz crystal grows
CHERT - a sedimentary rock, made of very fine quartz crystals;
composed of fossil radiolarians (one celled organisms that make a
skeleton of quartz)
CITRINE - a mineral, yellowish-brown color caused by high
temperatures
AMETHYST - a mineral, purple color, caused by ferric iron in
trace amounts
MILKY QUARTZ - a mineral, color caused by trace amounts of water
inside the mineral
QUARTZ GEODE - a rock composed of many visible quartz crystals:
formed in a rock cavity that slowly filled with quartz along its margins
OBSIDIAN - an igneous rock, non-crystalline silicon dioxide
GLASS - man-made, non-crystalline silicon dioxide
QUARTZ SANDSTONE - a sedimentary rock, composed of rounded quartz
grains cemented together
QUARTZITE - a metamorphic rock, probably originally quartz
sandstone, that has been subjected to extreme pressures and temperatures
QUARTZ SAND - rounded quartz grains not cemented together
- The students should conclude that quartz is a very common mineral that
can occur in a wide variety of forms. It sometimes makes up gems, or
entire rocks like quartzite or quartz sandstone. The reason quartz is
found in many materials is that quartz is hard and thus lasts longer
than most minerals.
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