Crystallisation from melts
 Why
do crystals form?
 What controls their shape and size?
 Three aspects to the growth of a crystal
are



Nucleation: formation of a stable nucleus
Diffusion of material to the nucleus
Growth of the crystal by adding atoms
The slowest of these three aspects generally
controls the shape and size
Different types of crystals



If diffusion is slow, the crystal
grows spikes to get at the new
material needed & forms a
dendrite or a skeletal crystal
If nucleation is the restriction (a)
spherulites can form (many
radiating crystals grow from one
nucleus or (b) a few large
crystals
If growth is the restriction (slow
crystallisation) then well formed
crystals develop
Perovskite
(Ca,Na,Fe,Ce)(Ti,Nb)O3


A very dense mineral that
is cubic with six oxygens
around each Ti/Nb and Ca
between each four
octohedra. It occurs as a
minor mineral in syenites
Deep in the mantle calcium
and magnesium silicates
take on this dense form.
Model is for MgSiO3 (Si
=yellow)
Mineral - Melt interface


Computer simulated interface
between perovskite (MgSiO3)
on left and melt. The silicon
atoms (yellow) have six
oxygens around them (green)
in the crystal but only four in
the melt. There is a very
complex layer at the interface
The shape of grains is
generally an attempt to
minimise the number of
unsatisfied bonds associated
with boundaries
Steno strikes again: the law of
constant angles between faces.

Steno of stratigraphy fame
noted that even though
different crystals of the
same mineral look different
the angles between the
faces are constant. This
reflects the symmetry and
shape of the building
blocks. All the quartz
crystals have the same
faces on a stereo net

A) “artists impression of:
• (a) nucleus of a metal
• (b) nucleus of an organic
crystal
• (c) nucleus of a long-chain
molecular liquid


B) Nucleus must reach a
critical size before it is
stable. The excess
energy associated with
the surface of the nucleus
must be < the energy
reduction resulting from
the crystal bonding
C) Nucleation is
commonly on an existing
substrate
(heterogeneous)
NUCLEATION
Why a small cluster of atoms
may not form a stable nucleus


Eight atoms have only
50% of bonds, the 64
atoms have 75% of all
the bonds
It is a balance between
the lower energy of the
crystal bonds and the
higher energy of the
interface. Stable only
when XS surface energy
is lower

Every crystal has
imperfections or
defects
 a) cubic lattice
 b) lattice with an
edge dislocation
formed by having a
half plane of atoms
above the
dislocation
 c) a screw
dislocation (left
handed in this case)
DISLOCATIONS
Growing a crystal

Atoms attach better at
a step (Y) than on a
plane
 Screw dislocations
produce steps that
the crystal uses to
grow
 Re-entrants along
twins also help
because they are
sites of dislocations
Crystal faces
are:

Parallel to lattice
planes in which there
are strong bonds and
across which there
are few or weak
bonds
 Biotite (001) faces
lack only the few
bonds through
potassium ions
3 Types of Twinning

Growth twins that form as the
growing crystal adds on a
layer in a different orientation
 Deformation twins when shear
rotates the lattice into a twin
orientation (only a very
modest strain involved)
 Transformation twinning as a
crystal changes symmetry as
it cools (e.g. microcline and
cordierite)
 Some twins are not seen at all
under the microscope and
some only as a re-entrant (e.g
biotite)




Sanidine and orthoclase
phenocrysts seem to
invariably have a simple
twin. Why might this be
so?
Option A: It is groups of
atoms in twin orientation
that grow large enough
to form a stable nucleus
Option B: Trace
elements that are
concentrated in the first
stage of growth (Ba,Sr)
allow a twin to form
Option C: ???????
SIMPLE TWINS