Rock Cycle - Minerals (6A)

  • Exploring the different varieties of quartz.
  • Discovering the importance of quartz.
  • crystalline
  • mineral
  • Rock Cycle - Minerals (6A)
  • food coloring
  • 250 ml beaker
  • eye dropper

Students examine different forms of quartz.

Many geodes are filled with quartz.


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.

  1. 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.
  2. 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

  3. 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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