2 4 crystal growth and phase diagrams

Mineral Stability


What controls when and where a
particular mineral forms?





Commonly referred to as “Rock cycle”
Rock cycle: Mineralogical changes that occur
because of variations in geologic environment

Knowing answer provides information
about earth history or processes


Mineral formation


Why would you want to know earth
history or processes:





Find: ore deposits, oil and gas, building
materials
Understand engineering hazards, water cycle

Understand how humans effect the earth:
climate…


The Rock Cycle



A system for
organizing
mineralogical
changes

Fig. 5-1


Bowen’s reaction series
Fe, Mg - silicates

Changing composition

Ca, Na - silicates

Ca, Na, Fe,
Mg - silicates

K-spar

Qtz

3 requirements for
mineral stability


Constituents




Available reactants/elements (X)

Correct environmental conditions (energy)



Pressure (P)
Temperature (T)


Mineral Stability




More stable position is one of lower
energy
Minerals may not be stable – e.g.
metastable minerals




Mineral contains more energy than expected
from their environment

Energy required to overcome metastability
– activation energy


Activation Energy:
- energy to shake book off
shelf
- Energy required to change
mineral phases

Fig 5-2


How can stability be estimated?


Algebraically:





Physical chemistry/Thermodynamics
Estimates of ∆G – Gibbs free energy

Graphically – “phase diagrams”:



Essentially figures of solutions to ∆G problems
Many types, common ones:
One component – P & T variable, X fixed (i.e. the
component)
 Two (or more) components – T & X variable, P
fixed



Components and Phases






Component – Chemical entity


H2O



Al2SiO5

Phase – physically separable part of a
system; e.g.


for H20: ice, water, water vapor



for Al2SiO5: Sillimanite, Kyanite, Andalusite

One and two component phase diagrams


Several types of 2-component diagrams


One component diagrams






Fields – where only one phase (mineral) is
stable
Lines – where two phases are stable
simultaneously
Points – where three phases are stable

One component diagrams


If P and/or T changes



One phase converts to another
Examples:
H2O – component; ice, water, and vapor are
phases
 Al SiO – component; Kyanite, Andalusite,
2
5
Sillimanite are phases



Al2SiO5 Phase
diagram
∆G = f(P,T)
Phase with lowest ∆G is
stable
Lines mark boundaries of
regions with the lowest
∆G
Very useful to remember
for metamorphic
reactions
Fig. 5.3

H2O phase diagram
Only component
is H2O


More complete H2O diagram
There are 15
polymorphs of ice
Ice IX stability:
T < 140 K
2 kbar < P < 4 kbar

Commonly shown P
& T conditions

tetragonal
Ice 9: Kurt Vonnegut, Cat’s Cradle, melting T = 45.8ºC at P = 1 Atm


Two component phase diagrams




What happens if there are two
components in a system?
Example: Plagioclase feldspars – two
components with complete solid solution

(at high T, otherwise “exsolution”)


Albite– NaAlSi3O8



Anorthite – CaAl2Si2O8



Any composition in between the two end
member compositions









How does solid (and melt) composition
vary during crystallization?
How does composition vary as solids melt
melt to form magma?
OR…
If you know the composition of a
plagioclase feldspar, can you determine T
and P of crystallization?

Two component phase diagram with
complete solid solution
= Na, Ca, Al, SiO2

= (Na,Ca)xAlySizO8
100% Albite
– NaAlSi3O8

Mole % Anorthite

100% Anorthite
– CaAl2Si2O8


Equilibrium Crystallization
Start
An77
An68
End

100% Albite
– NaAlSi3O8

Mole % Anorthite

An55

100% Anorthite

– CaAl2Si2O8

(1) The crystals are always in equilibrium with the melt
(2) Minerals have homogeneous compositions throughout
Fig. 5-14a


Lever Rule

%B = qr/qs

%A = rs/qs

Fraction of two components relate
to the relative lengths of tie lines

Fig. 5.5


Non-equilibrium crystallization


Results in “zoning”






Individual mineral grains may vary in

composition from center to edge
Easily observed petrographically

Very common in plagioclase feldspars


Zoned Plagioclase crystal
Oscillatory zoning

Fig. 12-12

Other types of zoning
include:
(1)Normal zoning (Carich centers)
(2)Reverse zoning (Narich centers)




Zoning reflects change in P and T when
mineral crystallizes




Crystallizing mineral in disequilibrium with
composition of melt

Can be explained by non-equilibrium
crystallization using phase diagram

Non-Equilibrium Crystallization
Start

Normal Zoning
An77
An77
An77

An68
An55

Mole % Anorthite

Minerals show zoning –
heterogeneous compositions

Fig. 5-14b


Controls on zoned crystals





Diffusion rate through solid crystal
Time allowed for diffusion to occur
Diffusion is rapid in olivine – few zoned

crystals




Mostly equilibrium

Diffusion slow in plagioclase


Commonly zoned


Two component phase diagram - No solid solution
Ca, Mg, Al, SiO2 =

At me diopside, anorthite, and melt present

Fig. 5.4

At me, diopside begins xtll, anorthite continues xtll NO HEAT LOST –
remains 1237º C – until all solid. Composition is 75% An, 25% Di. When
first reach 1237º C, system is 48% anorthite, 52% melt