Lecture 6 (9/27/2006)
Crystal Chemistry
Part 5:
Mineral Reactions
Phase Equilibrium/Stability
Intro to Physical Chemistry
Mineral Reactions in Igneous
Environments
Mineral Reactions in Metamorphic Environments
TEMPERATURE (centigrade )
0
400
200
600
800
0
2
0
BURIAL
METAMORPHISM
4
M
HIS
16
18
25
RP
MO
14
Conditions
not found
in crust
6
12
HIGH-GRADE
REGIONAL
METAMORPHISM
TA
ME
RE
SU
10
12
LOW-GRADE
REGIONAL
METAMORPHISM
CRUSTAL
GEOTHERM
S
RE
8
HP
HIG
6
THERMAL (CONTACT)
METAMORPHISM
31
ZONE OF
PARTIAL
MELTING
37
43
50
Role of Volatiles
(H2O & CO2)




Catalyzes reactions
Mobility during
metamorphism leads
to non-isochemical
reactions
Dehydration and
decarbonation during
prograde reactions
Lack of volatiles slows
retrograde reactions
Mineral Reactions in Near-surface
Environments

Chemical Weathering



conversion of minerals into simple layered
silicates (montmorillonite and kaolinite)
de-silicification
dissolution of cations (Na+, K+, Ca++, Mg++)
e.g. K-felspar + acidic water
Muscovite + acidic water
Muscovite + silica + K+
Kaolinite + K+
Mineral Reactions in High Pressure
Environments


Conversion to high
density polymorphs
Increase in
Coordination Numbers
of cation sites
Mineral Stability/Equilibrium



Phase Stability defined by the state (solid, liquid,
gas or vapor) and internal structure of a
compositionally homogeneous substance under
particular external conditions of pressure and
temperature
A Mineral of constant composition is considered
a solid phase
Phase (or mineral) stability is commonly
portrayed on a Pressure-Temperature Phase
Diagram
Phase Diagrams
One Component
Multi-component
Stability, Activation Energy and
Equilibrium




Stability of a phase (or mineral) is related to its internal
energy, which strives to be as low as possible under the
external conditions.
Metastability exists in a phase when its energy is higher
than P-T conditions indicate it should be.
Activation Energy is the energy necessary to push a
phase from its metastable state to its stable state.
Equilibrium exists when the phase is at its lowest energy
level for the current P-T conditions. (Two minerals that
are reactive with one another, may be found to be in
equilibrium at particular P-T conditions which on phase
diagrams are recognized as phase boundaries)
Recognize that by these definitions, most metamorphic
and igneous minerals at the earth’s surface are
metastable and out of equilibrium with their
environment!
Phase Component

Components are the chemical entities
necessary to define all the potential
phases in a system of interest
Thermodynamics (P Chem)


Theoretical basis of phase equilibrium
Three Laws of Thermodynamics
1. Internal Energy (E)
dE = dQ – dW
Q – heat energy
W – work = F * dist = P * area *dist = P * V
at constant pressure - dW = PdV
So, dE = dQ – PdV
dV – thermal expansion
Second and Third Laws of
Thermodynamics
2. All substances strive to be at the greatest
state of disorder (highest Entropy-S) for
a particular T and P.
dQ/T = dS
3. At absolute zero (0ºK), Entropy is zero
Gibbs Free Energy

G – the energy of a system in excess of its internal
energy. (This is the energy necessary for a reaction to
proceed)
G = E + PV - TS
dG = VdP – SdT
at constant T (δG/δP)T = V
at constant P (δG/δT)P = -S
Stable phases strive to have the lowest G
Therefore, the phase with the highest density at a given
pressure and the highest entropy at a given temperature
will be preferred
Relationship of Gibbs Free Energy to Phase
Equilibrium
Clapeyron Equation

Defines the state of equilibrium between
reactants and product in terms of S and V
dGr = VrdP – SrdT
dGp = VpdP – SpdT
at equilibrium: VrdP – SrdT = VpdP – SpdT
or: (Vp –Vr) dP = (Sp –Sr) dT
or: dP/dT = ΔS / ΔV
The slope of the equilibrium curve will be positive
if S and V both decrease or increase with
increased T and P