Crystal Growth

Question 1

Nucleation

Answer 1

First stage of growth, can only commence after a nucleus/seed has formed.

Question 2

Supersaturation

Answer 2

Usually required in solution for nucleation. Achieve by changing a mechanism (concentration, T and/or P).

Question 3

Competing Tendencies for Nucleation and Growth in Cooling Magma

Answer 3

Thermal vibrations and attractive forces.

Question 4

Crystal Habit

Answer 4

External appearance of crystal and it’s combination of crystal forms. example: equant, fibrous, scaly, tabular.

Question 5

1) Euhedral
2) Anhedral
3) Subhedral

Answer 5

1) crystals display perfect polyhedral surfaces with regular interfacial angles.
2) Crystals with irregular surfaces.
3) Mixture of above two.

Question 6

Twinning

Answer 6

Symmetrical intergrowth of two or more crystals of the same substance. Considered a type of planar defect.

Question 7

Composition Surface

Answer 7

Surface that twinning are united.

Question 8

Twin Plane

Answer 8

Reflection by a mirror plane

Question 9

Twin Axis

Answer 9

Angular 180 degree rotation axis common to twinning crystals.

Question 10

Twin Center

Answer 10

Inversion about a point

Question 11

Contact Twin

Answer 11

Have regular composition separating two individuals. Twins produced by either reflection or rotation.

Question 12

Penetration Twin

Answer 12

Made up of interpenetrating individuals having an irregular composition surface. Usually defined by a twin axis direction.

Question 13

Polysynthetic Twin

Answer 13

Multiple twins that have three or more crystals twinned AND all composition surfaces are parallel.

Question 14

Cause of Twinning -Growth Twins

Answer 14

Error in stacking of unit cells when crystal first starts to grow leads to twinning.

Question 15

Causes of Twinning -Transformation Twin

Answer 15

Variations in T and P stresses lattice, warping to relieve stress leads to twinning.

Question 16

Causes of Twinning -Deformation Twin

Answer 16

Stress put on crystal structure causes crystal to relieve pressure by twinning.

Question 17

Crystal Defects

Answer 17

As crystal grows, structural defects are incorporated into mineral. Affect growth rate, morphology, strength, conductivity and color.

Question 18

Point Crystal Defect

Answer 18

Represent holes in atomic structure.

Question 19

Edge Dislocation

Answer 19

Plane of atoms/ions terminate in a line instead of continuing.

Question 20

Screw Dislocation

Answer 20

Structural defect arranged along a screw axis.

Question 21

Radiation Defects

Answer 21

Minerals containing radioactive elements (U, Th) undergo decay. Alpha/beta particles and gamma radiation released from minerals.

Question 22

Polymorphism

Answer 22

Ability of chemical compound to crystallize with more than one structure. types: Reconstructive, displacive, order-disorder.

Question 23

Reconstructive Polymorphism

Answer 23

Major reorganization of the crystal structure. Atoms/ions rearranged and bonded into new structure/

Question 24

Displacive Polymorphism

Answer 24

Do not involve breaking atomic bonds, just slight displacement of atoms/ions and readjustment of bond angles.

Question 25

Order-Disorder Polymorphism

Answer 25

Different atomic species can occupy the same lattice site. No definite transition point between perfect order and complete disorder.

Question 26

Pseudomorphism

Answer 26

Replacement in which a mineral retains outward crystal form of another mineral species. Types: substitution, encrustation, alteration.