Crystallography BasicsBy Cheryl SillScience TeacherMMEW 2014

Crystallography

The study of atomic and molecular structureBegan with the study of minerals: such as

quartz, diamond and graphiteToday, includes the study of crystalline

solids such as minerals, viruses, proteinsUse of x-rays, neutron diffraction, high and

low-temperature diffraction, microgravity and molecular modeling

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 https://www.youtube.com/watch?v=AlBPajICFIU&feature=youtu.be

Crystal Definition

Regular polyhedral form (solid) Bounded by smooth facesDefined 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

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.

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

Seven Major Crystal Systems

IsometricTetragonalOrthorhombicMonoclinic

TriclinicTrigonalHexagonal

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.

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.

Crystal MorphologyMonoclinic Triclinic Trigonal Hexagonal

Monoclinic shape: one axes varies from 90o. The Hexagonal and Trigonal shapes have 4 axes.http://webmineral.com/crystall.shtml#.U5Xk88afG8o

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.

TetragonalTwo 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.

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.

MonoclinicA 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.

TriclinicTwo 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.

HexagonalThis 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.

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” http://www.pbs.org/wgbh/nova/tech/artificial-diamonds.html

http://shop.nationalgeographic.com/ngs/product/dvds/adventure-and-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

Mineral Groups

The names of these groups are based upon their chemical composition.

Native ElementsSilicates

HalidesOxidesCarbonatesSulfidesSulfates

ActivitiesPaper 3-D shapes of six major crystal

shapesCrystal Growing Activity (need a weekend)

Alum, KAl(SO4)2 Table Salt, NaCl CuSO4

Useful Web-sites

http://www.mineralogy4kids.orghttp://www.amercrystalassn.orgWebmineralhttp://www.nature.com/news/specials/crystall

ography-1.14540http://www.rockhounds.com/rockshop/xtal/par

t1.shtmlhttp://www.mindat.org