1. Crystallography Basics
By Cheryl Sill
Science Teacher
MMEW 2014

2. Crystallography The study of atomic and molecular structure
Began with the study of minerals: such as quartz, diamond and graphite
Today, includes the study of crystalline solids such as minerals, viruses, proteins
Use of x-rays, neutron diffraction, high and low-temperature diffraction, microgravity and molecular modeling

3. Why study crystallography at the elementary and secondary level?
Crystals are WAY COOL!
Crystals are good way to introduce chemistry, geology, physics, symmetry, three-dimensional thinking and modeling
Crystals introduce how the particles in a substance are physically arranged.
Crystals are a concrete way to help students understand the relationship between atomic structure and properties of these materials.
Awesome way to reinforce the atomic theory.
Besides, 2014 has been named the Year of Crystallography by the International Union of Crystallography

4. Crystal Definition Regular polyhedral form (solid)
Bounded by smooth faces
Defined chemical compound
interatomic forces of the anions and cations will form the most stable configuration based upon their electron configurations
Usually formed while it is changing from a liquid, gas or in a solution, to a solid

5. Crystal Chemistry
Minerals are classified according to their chemical composition.
Classified according to their dominant anion (negatively charged ion).
Oxides: Hematite, Fe2O3 or Corundum, Al2O3
Silicates: Quartz SiO2 or Microcline KAl (Si3O8)
Carbonates: Calcite, CaCO3 or Strontianite, SrCO3
The chemical composition determines the crystalline shape, or morphology.

6. Crystallography History
1669, Nicholas Steno, a Danish physician and natural scientist, discovered through analysis of numerous samples of the same mineral, when measured at the same temperature, the angles between similar crystal faces remain constant regardless of the size or the shape of the crystal.
Steno's law is called the CONSTANCY OF INTERFACIAL ANGLES
Crystal Face
Crystallographic Axes
Axial Cross

7. Seven Major Crystal Systems
Isometric
Triclinic
Tetragonal
Trigonal
Orthorhombic
Hexagonal
Monoclinic
There are actually 32 classes of symmetry, 230 space groups that are observable through x-ray analysis, but for most secondary school settings I discuss only 6 major classes,. I have not discussed trigonal separately.

8. Crystal Morphology Isometric orthorhombic tetragonal
Notice the angles between all axes are 90o, yet the lengths of the axes vary.
Isometric: all three axes are the same length. Tetragonal has only one axes longer than the other two axes and orthorhombic axes all differ in length.

9. Crystal Morphology Monoclinic Triclinic Trigonal Hexagonal
Monoclinic shape: one axes varies from 90o.
The Hexagonal and Trigonal shapes have 4 axes.

10. Isometric (cubic)
Here is a photo of some excellent sample of pyrite from Peru. Notice that each large crystal looks different, yet the faces are symmetrical in all three axes.

11. Tetragonal
Two good samples of cassiterite to exemplify the 90o angles, but only one axes measurable longer than the other two axes. Both samples were found in China.

12. Orthorhombic
Two beautiful examples of orthorhombic crystals. Above left is Caledonite from Mammoth Mine in Tiger, Arizona and above left is a beautiful Barite sample from Peru.
Notice the axes are still at 90o angles, but the length of all three axes are unequal.

13. Monoclinic
A beautiful transparent sample of Gypsum from Romania is in the photo below, left.
This classic example of orthoclase, or K-Spar as some call it, depicts a perfect example of variation of crystal shape. This sample was found in Portugal.

14. Triclinic Two bright blue crystals of kyanite on a white quartz matrix
A colorless, blocky sample of albite (plagioclase) with a thick dusting of green micro chlorite.

15. Hexagonal
This gemstone quality Beryl, below left, was found in the Northern areas of Pakistan.
Below center is a beautiful sample of Milarite from the Osumilite Group found in Switzerland.
Our common snowflake exemplifies the hexagonal crystalline form in ice.

16. How & where does Crystallography fit into our curriculum?
I place it at the very start of the mineral identification unit.
This is one of the characteristic properties of minerals.
Class 1: Introduce 6 Crystal classification, 3-D Paper Cut-outs
Class 2: Go over the 6 cut-out shapes & the NOVA video of diamonds or National Geographics “splendid Stones”
exploration/splendid-stones-dvd-exclusive
Students set their shapes on top of the name that corresponds with the shape
As they are watching the video, I move around the room & record their construction.
Points (12), participation or criterion-based grading, formative.
Class 3: Day 1 of Crystal Growing Activity: Salt, CuSO4, Alum
Last day of school week, so water evaporates with no disturbance
Class 4: Day 2 of Crystal Growing Activity

17. Mineral Groups
The names of these groups are based upon their chemical composition.
Halides
Oxides
Carbonates
Native Elements
Sulfides
Silicates
Sulfates

18. Activities Paper 3-D shapes of six major crystal shapes
Crystal Growing Activity (need a weekend)
Alum, KAl(SO4)2
Table Salt, NaCl
CuSO4

19. Useful Web-sites http://www.mineralogy4kids.org
Webmineral
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