Week 2 - Crystallography Flashcards
What is a crystal?
A regular polyhedral object with flat faces, bounded by smooth crystal faces.
What is a crystal lattice?
A 3D arrangement of atoms inside a crystal.
What are crystallographic axes?
Imaginary lines that coincide with axes of symmetry and help orient a crystal.
What are the three factors that define crystal systems?
1) The number of axes
2) the length of the axes
3) the angles at which the axes meet.
Which crystal system has all axes equal and intersecting at 90°?
Cubic (Isometric).
Give examples of minerals in the Cubic system.
Pyrite, fluorite, garnet.
What is unique about the Tetragonal system?
It has two equal axes (a and b) and one different axis (c), all intersecting at 90°.
Name minerals in the Tetragonal system.
Zircon, rutile.
Describe the Orthorhombic system.
All axes are of different lengths and intersect at 90°.
Which minerals belong to the Orthorhombic system?
Topaz, olivine.
What is the defining feature of the Hexagonal system?
It has four axes: three in one plane and one perpendicular.
What is the Trigonal system a subdivision of?
The Hexagonal system.
Which crystal system has one oblique angle with all different axis lengths?
Monoclinic.
What are examples of Monoclinic system minerals?
Gypsum.
Describe the Triclinic system.
All axes have different lengths and angles, with no symmetry.
Which mineral belongs to the Triclinic system?
Feldspar
What happens when a crystal is rotated on an axis of symmetry?
It appears unchanged after specific angular rotations.
How many degrees are between repetitions in 6-fold symmetry?
60 degrees.
What type of symmetry repeats every 120 degrees?
Trigonal symmetry
Define a mirror plane in crystals.
A plane that divides a crystal into mirrored halves.
What is the center of symmetry?
A point in a crystal where every point is mirrored by another.
What is the term for a crystal with well-formed faces?
Euhedral.
What do you call a crystal with poorly formed faces?
Subhedral.
What is anhedral crystal habit?
Crystals with no faces.
What are the common forms of crystal habits?
Equant, elongate, columnar, fibrous.
Name special crystal habits.
Radiating, parallel, foliated, banded.
