Crystal Growth Flashcards
Nucleation
First stage of growth, can only commence after a nucleus/seed has formed.
Supersaturation
Usually required in solution for nucleation. Achieve by changing a mechanism (concentration, T and/or P).
Competing Tendencies for Nucleation and Growth in Cooling Magma
Thermal vibrations and attractive forces.
Crystal Habit
External appearance of crystal and it’s combination of crystal forms. example: equant, fibrous, scaly, tabular.
1) Euhedral
2) Anhedral
3) Subhedral
1) crystals display perfect polyhedral surfaces with regular interfacial angles.
2) Crystals with irregular surfaces.
3) Mixture of above two.
Twinning
Symmetrical intergrowth of two or more crystals of the same substance. Considered a type of planar defect.
Composition Surface
Surface that twinning are united.
Twin Plane
Reflection by a mirror plane
Twin Axis
Angular 180 degree rotation axis common to twinning crystals.
Twin Center
Inversion about a point
Contact Twin
Have regular composition separating two individuals. Twins produced by either reflection or rotation.
Penetration Twin
Made up of interpenetrating individuals having an irregular composition surface. Usually defined by a twin axis direction.
Polysynthetic Twin
Multiple twins that have three or more crystals twinned AND all composition surfaces are parallel.
Cause of Twinning -Growth Twins
Error in stacking of unit cells when crystal first starts to grow leads to twinning.
Causes of Twinning -Transformation Twin
Variations in T and P stresses lattice, warping to relieve stress leads to twinning.
Causes of Twinning -Deformation Twin
Stress put on crystal structure causes crystal to relieve pressure by twinning.
Crystal Defects
As crystal grows, structural defects are incorporated into mineral. Affect growth rate, morphology, strength, conductivity and color.
Point Crystal Defect
Represent holes in atomic structure.
Edge Dislocation
Plane of atoms/ions terminate in a line instead of continuing.
Screw Dislocation
Structural defect arranged along a screw axis.
Radiation Defects
Minerals containing radioactive elements (U, Th) undergo decay. Alpha/beta particles and gamma radiation released from minerals.
Polymorphism
Ability of chemical compound to crystallize with more than one structure. types: Reconstructive, displacive, order-disorder.
Reconstructive Polymorphism
Major reorganization of the crystal structure. Atoms/ions rearranged and bonded into new structure/
Displacive Polymorphism
Do not involve breaking atomic bonds, just slight displacement of atoms/ions and readjustment of bond angles.
Order-Disorder Polymorphism
Different atomic species can occupy the same lattice site. No definite transition point between perfect order and complete disorder.
Pseudomorphism
Replacement in which a mineral retains outward crystal form of another mineral species. Types: substitution, encrustation, alteration.
