Crystallography

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Crystal

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Any solid with an ordered structure, regardless of whether or not this is evident in its external form

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Crystalline

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Internal arrangement of atoms in a solid that is very ordered

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Unit Cell

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Smallest unit of this structure which possesses same symmetry and properties as entire mineral

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Crystal systems

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Isometric

Tetragonal

Hexagonal

Orthorhombic

Monoclinic

Triclinic

Acronym: Ingrid Takes Heroin Or Meth Too

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Isomorphs

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Minerals with similar crystal structure and slightly different chemical compositions

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Polymorphs

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Minerals with same chemical composition and different structure i.e. graphite and diamond

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Miller Indices

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3-4 digits written inside brackets to represent intersection of a crystal face with x,y,z axes

(x y z)

(x y z w)

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Crystal defects

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Point: absence of an atom or ion in the lattice

Line: dislocations

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Solid solution

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Introduction of atoms/ions into homogeneous structure

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Exsolution

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Initially homogeneous solid solution separates into two distinct parts without splitting

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Crystal twinning

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Two separate crystals share some of the same crystal lattice points in a structure

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Visible light/white light

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Electromagnetic radiation with wavelengths between 390 nm and 770 nm (purple and red)

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Refraction

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n (index of refraction) = V_a (velocity of light in air) / V_m (velocity of light in material)

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Isotropic minerals

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Velocity of light doesn't change inside these minerals

Isometric or amorphous

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Anisotropic minerals

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Velocity of light changes inside minerals

Hexagonal, tetragonal, orthorhombic, monoclinic, and triclinic

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Plane-polarized light

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White light passes through polarizing filter (Polaroid); light becomes plane-polarized: oscillations are constrained to one direction (at right angles to the beam)

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Pleochroism

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Mineral appears to be different colours when observed at different angles

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Relief

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Caused by difference in refractive indices of the mineral and medium

Mineral refractive index is higher than medium's then high relief is observed

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Becke Lines

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Bright lines at the boundaries of minerals with different refractive indices

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Extinction angle

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Angle between cleavage direction and the position where the mineral is "extinguished"

Measured from 0-89 degrees. 90 = parallel extinction (considered zero degrees)

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Birefringence

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Light waves emerging from anisotropic mineral have different phases and interfere, showing interference colours. These can be cross-referenced with a chart to determine birefringence