Crystallography Basics
By Cheryl Sill
Science Teacher
MMEW 2014
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
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 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
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
 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.
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 Morphology
 Monoclinic
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.
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.
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.
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.
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.
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.
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-andexploration/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
 Halides
groups are based upon
 Oxides
their chemical
composition.
 Carbonates
 Native Elements
 Sulfides
 Silicates
 Sulfates
Activities
 Paper 3-D shapes of six major crystal shapes
 Crystal Growing Activity (need a weekend)
 Alum, KAl(SO4)2
 Table Salt, NaCl
 CuSO4
Useful Web-sites
 http://www.mineralogy4kids.org
 http://www.amercrystalassn.org
 Webmineral
 http://www.nature.com/news/specials/crystallog
raphy-1.14540
 http://www.rockhounds.com/rockshop/xtal/part
1.shtml
 http://www.mindat.org