Crystallography Basics By Cheryl Sill Science Teacher MMEW 2014
Feb 23, 2016
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