Crystal structure and morphology

What is a lattice?

a periodic repetition of units in three dimension

what is a unit cell?

the smallest group of particles within a crystal that retains the geometric shape of the crystal

General considerations for unit cells

• must be large enough to contain an integral number of formula units

• each corner of the unit cell must be identical, with an identical environmen

• the unit cell must express the symmetry of the atomic relationship

How are unit cells classified?

By their symmetry

Unit cell can be defined by...

• 3 axes (a, b, c) with specific lengths

• 3 angles b/w axes

What is a crystal system?

relates to the structural organisation of the elements on a repeating scale

How many crystal systems are there?

7

what are the crystal systems?

• cubic

• tetragonal

• hexagonal

• orthorhombic

• monoclinic

• triclinic

• trigonal subsytems

What is a cubic crystal system?

• a=b=c

• 3 angles intersect at 90 degrees

• highly symmetrical and structure is consistent in all directions giving an isotropic mineral

• e.g. pyrite

what is a tetragonal crystal system?

• 2 lengths are equal; a=b

• c is longer than a and b; third is arbitrary

• axes intersect at 90 degrees

• unit cell is a rectangular prism

What is a hexagonal crystal system?

• 4 axes; 3 equal in length

• vertical c-axis intersects the smaller axes at 90 degrees

• shorter axes intersect at 120 degrees

• additional fourth axes gives the unit cell 6 sides

• e.g. dolomite

what is a orthorhombic crystal system?

• 3 axes have different lengths

• axes all intersect at 90 degrees

• e.g. olivine

what is a monoclinic crystal system?

• 3 axes of different lengths

• 2 axes, a & c, meet at 90; b doesnt

• e.g. feldspar

what is a triclinic crystal system?

• 3 axes of unequal length

• no angles are 90 degrees

• complex shapes

what is trigonal crystal system?

• 4 axes; 3 equal in length

• vertical c-axis intersects at 90

• short axes intersect at equal angles but not 90

• e.g. quartz, calcite

crystal habit

the general, characteristic external shape of a crystal or cluster of crystals that grew unimpeded

crystal twinning

Occurs when two separate crystals share some of the same crystal lattice points in a symmetrical manner

what forms can crystal twinning take?

• mirror plane

• a rotation axis

• inversion around a point