Crystal Structure Lab

Problem/Question

How do crystals form?

Background:

Crystal structure is one of the basic properties of mineral identification. A crystal is a solid body bounded by natural plane faces that are the external expression of a regular internal arrangement of constituent atoms, molecules, or ions. The formation of a crystal by a substance passing from a gas or liquid to a solid state, or going out of solution (by precipitation or evaporation), is called crystallization.

Classification of Crystals

The particles in a crystal occupy positions with definite geometrical relationships to each other. The positions form a kind of scaffolding, called a crystalline lattice; the atomic occupancies of lattice positions are determined by the chemical composition of the substance. A crystalline substance is uniquely defined by the combination of its chemistry and the structural arrangement of its atoms. In all crystals of any specific substance the angles between corresponding faces are constant (Steno’s Law, or the First Law of Crystallography, published by the Danish geologist Nicolaus Steno in 1669). Crystalline substances are grouped, according to the type of symmetry they display, into 32 classes. These in turn are grouped into seven systems on the basis of the relationships of their axes, i.e., imaginary straight lines passing through the ideal centers of the crystals.

Crystals may be symmetrical with relation to planes, axes, and centers of symmetry. Planes of symmetry divide crystals into equal parts (mirror images) that correspond point for point, angle for angle, and face for face. Axes of symmetry are imaginary lines about which the crystal may be considered to rotate, assuming, after passing through a rotation of 60°, 90°, 120°, or 180°, the identical position in space that it originally had. Centers of symmetry are points from which imaginary straight lines may be drawn to intersect identical points equidistant from the center on opposite sides.

The crystalline systems are cubic, or isometric (three equal axes, intersecting at right angles); hexagonal (three equal axes, intersecting at 60° angles in a horizontal plane, and a fourth, longer or shorter, axis, perpendicular to the plane of the other three); tetragonal (two equal, horizontal axes at right angles and one axis longer or shorter than the other two and perpendicular to their plane); orthorhombic (three unequal axes intersecting at right angles); monoclinic (three unequal axes, two intersecting at right angles and the third at an oblique angle to the plane of the other two); trigonal, or rhombohedral (three equal axes intersecting at oblique angles); and triclinic (three unequal axes intersecting at oblique angles). In all systems in which the axes are unequal there is a definite axial ratio for each crystal substance.

Crystals differ in physical properties, i.e., in hardness, cleavage, optical properties, heat conductivity, and electrical conductivity. These properties are important because they can help geologists determine the mineral or rock type. In this lab we will grow our own crystals and view the process of crystal formation and apply that to how minerals are formed.


Part 1. Crystal Creation—Grow spikes of crystals in the sun.

Materials

*      Black construction paper

*      Scissors

*      A pie pan, cake pan, or shallow bowl

*      Warm water

*      Epsom salt

 

Procedure

1.      Use your scissors to cut the black paper so it will fit in the bottom of your pie pan.

2.      Add 1 tablespoon of Epsom salt to 1/4 cup of warm water. Stir until the salt is dissolved.

3.      Pour the salty water onto the black paper in the pie pan.

4.      Put the pie pan out into the sun. When the water evaporates, you'll see lots of crystal spikes on the black paper!

Tip: This activity works best on a sunny day.

 

Why does Epsom salt make crystal spikes?

When you add Epsom salt to water, the salt dissolves. When you leave the pan in the sun, the water evaporates and the salt forms crystals shaped like long needles.

If you tried this experiment with table salt instead of Epsom salt, you wouldn't get crystal spikes. That's because table salt and Epsom salt are chemically different, so the crystals that they form are very different.Photo of salt crystal art

Part 2. Rock Candy

Materials

1 Glass jar or drinking glass
1 Piece of cotton string
1 Pencil or stick
1 Paper clip
1 Food coloring (optional)
1 c Water
2 c Sugar
Additional sugar

Procedure:

1.      Tie a short piece of cotton string to the middle of the pencil or stick. Attach a paper clip to the end of the string for a weight. Moisten the string very lightly, and roll in a bit of sugar (this will "attract" the sugar crystals from the syrup to the string). Place the pencil or stick over the top of the glass or jar with the string hanging down inside.

2.      Heat the water to boiling, and dissolve the 2 cups of sugar into it. For the biggest crystals FAST, heat the sugar-water solution a SECOND time, and dissolve as much additional sugar as you can into it. Add a few drops of food coloring to the solution if desired.

3.      Pour the solution into the prepared glass or jar and leave undisturbed for a couple of days. Depending on how much sugar you were able to dissolve into the water, you should start to see crystals growing in a few hours to a few days.

 Part 3. Crystal Gardens

Materials:

6 tb Salt
6 tb Liquid bluing
6 tb Water
1 tb Ammonia

Procedure:

Combine salt, bluing, water and ammonia. Pour over small pieces of rock or coal in a shallow GLASS or CHINA bowl. Drip food coloring on top if you desire. Crystals will begin to grow soon. Add water occasionally to keep crystals growing. You'll probably want to place dish on tray or wooden board as crystals grow over the sides of the bowl.

 

Analysis/Conclusions

 

Directions: Answer each of the following on a separate sheet of paper.

  1. Minerals are classified as naturally occurring inorganic solids, which possess an orderly internal structure and a definite chemical composition. With that in mind, which of the crystals you made in this lab would be considered a mineral? Explain your reasoning for each.
  2. Define crystallization.
  3. Classify each of the crystalline systems formed in this experiment. Refer back to page 1 if necessary.
  4. Based on this experiment, explain how you think a quartz type crystal such as Amethyst might form?
  5. In all crystals of any specific substance the angles between corresponding faces are constant (a.k.a. Steno’s Law). Explain that statement in your own words.
  6. Why might it be difficult to identify a mineral by its color? Cite at least one specific example from this experiment.