Crystallisation from meltsCrystallisation from melts Why do crystals form?Why do crystals form? What controls their shape and size?What controls their shape and size? Three aspects to the growth of a crystal Three aspects to the growth of a crystal

areare Nucleation: formation of a stable nucleusNucleation: formation of a stable nucleus Diffusion of material to the nucleusDiffusion of material to the nucleus Growth of the crystal by adding atomsGrowth of the crystal by adding atoms

The slowest of these three aspects generally The slowest of these three aspects generally controls the shape and sizecontrols the shape and size

Different types of crystalsDifferent types of crystals If diffusion is slow, the crystal If diffusion is slow, the crystal

grows spikes to get at the new grows spikes to get at the new material needed & forms a material needed & forms a dendrite or a skeletal crystaldendrite or a skeletal crystal

If nucleation is the restriction (a) If nucleation is the restriction (a) spherulites can form (many spherulites can form (many radiating crystals grow from one radiating crystals grow from one nucleus or (b) a few large nucleus or (b) a few large crystalscrystals

If growth is the restriction (slow If growth is the restriction (slow crystallisation) then well formed crystallisation) then well formed crystals developcrystals develop

PerovskitePerovskite(Ca,Na,Fe,Ce)(Ti,Nb)O(Ca,Na,Fe,Ce)(Ti,Nb)O33

A very dense mineral that A very dense mineral that is cubic with six oxygens is cubic with six oxygens around each Ti/Nb and Ca around each Ti/Nb and Ca between each four between each four octohedra. It occurs as a octohedra. It occurs as a minor mineral in syenitesminor mineral in syenites

Deep in the mantle calcium Deep in the mantle calcium and magnesium silicates and magnesium silicates take on this dense form. take on this dense form. Model is for MgSiOModel is for MgSiO33 (Si (Si =yellow)=yellow)

Mineral - Melt interfaceMineral - Melt interface Computer simulated interface Computer simulated interface

between perovskite (MgSiObetween perovskite (MgSiO33) ) on left and melt. The silicon on left and melt. The silicon atoms (yellow) have six atoms (yellow) have six oxygens around them (green) oxygens around them (green) in the crystal but only four in in the crystal but only four in the melt. There is a very the melt. There is a very complex layer at the interfacecomplex layer at the interface

The shape of grains is The shape of grains is generally an attempt to generally an attempt to minimise the number of minimise the number of unsatisfied bonds associated unsatisfied bonds associated with boundarieswith boundaries

Steno strikes again: the law of Steno strikes again: the law of constant angles between faces.constant angles between faces. Steno of stratigraphy fame Steno of stratigraphy fame

noted that even though noted that even though different crystals of the different crystals of the same mineral look different same mineral look different the angles between the the angles between the faces are constant. This faces are constant. This reflects the symmetry and reflects the symmetry and shape of the building shape of the building blocks. All the quartz blocks. All the quartz crystals have the same crystals have the same faces on a stereo netfaces on a stereo net

NUCLEATIONNUCLEATION A) “artists impression of:A) “artists impression of:• (a) nucleus of a metal (a) nucleus of a metal • (b) nucleus of an organic (b) nucleus of an organic

crystalcrystal• (c) nucleus of a long-chain (c) nucleus of a long-chain

molecular liquidmolecular liquid B) Nucleus must reach a B) Nucleus must reach a

critical size before it is critical size before it is stable. The excess stable. The excess energy associated with energy associated with the surface of the nucleus the surface of the nucleus must be < the energy must be < the energy reduction resulting from reduction resulting from the crystal bondingthe crystal bonding

C) Nucleation is C) Nucleation is commonly on an existing commonly on an existing substrate substrate (heterogeneous) (heterogeneous)

Why a small cluster of atoms Why a small cluster of atoms may not form a stable nucleusmay not form a stable nucleus

Eight atoms have only Eight atoms have only 50% of bonds, the 64 50% of bonds, the 64 atoms have 75% of all atoms have 75% of all the bondsthe bonds

It is a balance between It is a balance between the lower energy of the the lower energy of the crystal bonds and the crystal bonds and the higher energy of the higher energy of the interface. Stable only interface. Stable only when XS surface energy when XS surface energy is loweris lower

DISLOCATIONSDISLOCATIONS Every crystal has Every crystal has imperfections or imperfections or defectsdefects

a) cubic latticea) cubic lattice b) lattice with an b) lattice with an

edge dislocation edge dislocation formed by having a formed by having a half plane of atoms half plane of atoms above the above the dislocationdislocation

c) a screw c) a screw dislocation (left dislocation (left handed in this case)handed in this case)

Growing a crystalGrowing a crystal

Atoms attach better at Atoms attach better at a step (Y) than on a a step (Y) than on a planeplane

Screw dislocations Screw dislocations produce steps that produce steps that the crystal uses to the crystal uses to growgrow

Re-entrants along Re-entrants along twins also help twins also help because they are because they are sites of dislocationssites of dislocations

Crystal faces Crystal faces are:are:

Parallel to lattice Parallel to lattice planes in which there planes in which there are strong bonds and are strong bonds and across which there across which there are few or weak are few or weak bondsbonds

Biotite (001) faces Biotite (001) faces lack only the few lack only the few bonds through bonds through potassium ionspotassium ions

3 Types of Twinning 3 Types of Twinning Growth twins that form as the Growth twins that form as the

growing crystal adds on a growing crystal adds on a layer in a different orientationlayer in a different orientation

Deformation twins when shear Deformation twins when shear rotates the lattice into a twin rotates the lattice into a twin orientation (only a very orientation (only a very modest strain involved)modest strain involved)

Transformation twinning as a Transformation twinning as a crystal changes symmetry as crystal changes symmetry as it cools (e.g. microcline and it cools (e.g. microcline and cordierite)cordierite)

Some twins are not seen at all Some twins are not seen at all under the microscope and under the microscope and some only as a re-entrant (e.g some only as a re-entrant (e.g biotite)biotite)

SIMPLE TWINSSIMPLE TWINS Sanidine and orthoclase Sanidine and orthoclase phenocrysts seem to phenocrysts seem to invariably have a simple invariably have a simple twin. Why might this be twin. Why might this be so?so?

Option A: It is groups of Option A: It is groups of atoms in twin orientation atoms in twin orientation that grow large enough that grow large enough to form a stable nucleusto form a stable nucleus

Option B: Trace Option B: Trace elements that are elements that are concentrated in the first concentrated in the first stage of growth (Ba,Sr) stage of growth (Ba,Sr) allow a twin to formallow a twin to form

Option C: ???????Option C: ???????