Slide 1
A crystal’s atomic order shapes its properties
A crystal contains an ordered atomic arrangement. Its structure helps determine a mineral’s properties as well as its external form.
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A visible geometric crystal face requires suitable growth space. A grain with an irregular outline can still have an ordered interior, so outward shape alone does not decide whether the material is crystalline.
Slide 2
The same elements can form different structures
The same elements can form different structures. Changing how atoms connect can produce strikingly different material properties.
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Diamond and graphite both consist of carbon, but their bonding networks differ. Diamond’s three-dimensional network and graphite’s layered structure produce different hardness and electrical behaviour without changing the element involved.
Slide 3
The response can depend on direction through the crystal
Crystal properties can depend on direction. Bonds and atomic spacing are not necessarily equivalent along every direction through a crystal.
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Cleavage occurs when a mineral tends to break along particular planes. It differs from a growth face: one records how a crystal formed, while the other reveals a preferred route of fracture.
Slide 4
Defects change colour, conductivity or strength
Vacancies, substitutions and other defects interrupt ideal order. Small departures from a perfect lattice can influence colour, conductivity and strength.
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A trace element can replace some atoms without destroying the overall structure. Defect concentration and type therefore help explain why two specimens assigned to the same mineral species can look or behave differently.
Slide 5
Diffraction patterns test the internal arrangement
Waves scattered by a crystal interfere according to atomic spacing. Diffraction patterns provide evidence about the ordered structure inside.
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X-ray diffraction can distinguish phases that look similar to the eye. Interpreting a powder pattern requires reference data and attention to mixtures, because a specimen can contain several crystalline materials together.
Sources
- Lattice Structures in Crystalline Solids
OpenStax, Rice University
- Diamond
Gemological Institute of America
- Optical Defects in Diamond: A Quick Reference Chart
Gemological Institute of America