2 4 crystal growth and phase diagrams

Mineral Stability What controls when and where a particular mineral forms?. Mineral Stabilitymetastable minerals from their environment – activation energy... One component diagrams Fi

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

 Understand engineering hazards, water cycle

 Understand how humans effect the earth: climate…

Fig 5-1

 A system for organizing mineralogical changes

The Rock Cycle

Bowen’s reaction series

3 requirements for mineral stability

Mineral Stability

metastable minerals

from their environment

– activation energy

How can stability be estimated?

component)

fixed

Components and Phases

 H 2 O

 Al 2 SiO 5

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

H2O phase diagram

Only component

is H2O

More complete H2O diagram

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 – CaAl 2 Si 2 O 8

 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?

 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

(1) The crystals are always in equilibrium with the melt

(2) Minerals have homogeneous compositions throughout

100% Albite

2 Si2O8

Lever Rule

%B = qr/qs

Fig 5.5

Fraction of two components relate

to the relative lengths of tie lines

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

(Na- 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

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

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

Rates of growth

 Slowest growing faces are often most

prominent

 Fast growth causes faces to disappear

 This is why minerals have common forms

 {001} faces parallel to

layers of bonded Na

and Cl

 Face is charge neutral

 Weak attraction from

this face to either ion

 {111} faces parallel layers of pure Na and Cl