Crystal Chem Crystallography • Chemistry behind minerals and how they are assembled – Bonding properties and ideas governing how atoms go together – Mineral assembly – precipitation/ crystallization and defects from that • Now we will start to look at how to look at, and work with, the repeatable structures which define minerals. – This describes how the mineral is assembled on a larger scale
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Crystal Chem Crystallography - University of Vermontgdrusche/Classes/GEOL 110 - Earth Materials/2010 fi… · Crystal Chem Crystallography •Chemistry behind minerals and how they
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Crystal Chem Crystallography• Chemistry behind minerals and how they are
assembled
– Bonding properties and ideas governing how atoms go together
– Mineral assembly – precipitation/ crystallization and defects from that
• Now we will start to look at how to look at, and work with, the repeatable structures which define minerals.
– This describes how the mineral is assembled on a larger scale
Symmetry
Symmetry Introduction
• Symmetry defines the order resulting from how
atoms are arranged and oriented in a crystal
• Study the 2-D and 3-D order of minerals
• Do this by defining symmetry operators (there are
13 total) actions which result in no change to the
order of atoms in the crystal structure
• Combining different operators gives point groups –
which are geometrically unique units.
• Every crystal falls into some point group, which are
segregated into 6 major crystal systems
2-D Symmetry Operators
• Mirror Planes (m) – reflection along a plane
A line denotes
mirror planes
2-D Symmetry Operators
• Rotation Axes (1, 2, 3, 4, or 6) – rotation of 360,
180, 120, 90, or 60º around a rotation axis yields
Table 5.3 of Klein (2002) Manual of Mineral Science, John Wiley and Sons
Hexagonal class
Rhombohedral
form Hexagonal
form
Crystal Morphology (habit)
Nicholas Steno (1669): Law of Constancy of
Interfacial Angles
Quartz
120o
120o
120o 120o 120o
120o
120o
Crystal Morphology
Diff planes have diff atomic environments
Crystal
MorphologyGrowth of crystal is affected by the conditions and matrix from which they grow. That one face grows quicker than another is generally determined by differences in atomic density along a crystal face
Note that the internal order of the atoms can be the same but the crystal habit can be different!
Crystal Morphology
How do we keep track of the faces of a crystal?
Face sizes may vary, but angles can't
Thus it's the orientation & angles that are the best
source of our indexing
Miller Index is the accepted indexing method
It uses the relative intercepts of the face in question