Lecture 12: Crystallization & Mineral Reactions Read Chpt 2

Lecture 12:
Crystallization & Mineral Reactions
Crystallization: formation of minerals from solutions, melts or vapors;
go from random to more ordered (crystalline state)
What causes crystallization from a solution?
• Change in composition
• Change in temperature
• Change in pressure
Crystallization from a melt occurs with two competing tendencies:
1. Thermal vibrations that tend to destroy the nuclei of potential minerals
2. Attractive forces that tend to aggregate atoms into crystal structures
Read Chpt 2

Crystal Growth: : How do they form??
Nucleation: a “seed” crystal forms (aka(
“nucleus”); ions constantly come
together, but most redissolve. . Why? Atoms at the surface have
unsatistified bonds (incomplete polyhedra). More surface atoms per unit
solid = less stability (high surface energy)
Critical Size: : must be reached to grow a crystal; if seed grows rapidly, it
will reach a point where the surface energy is lowered enough that
crystallite can keep growing and not be redissolved; ; ions stick best at
steps and kinks, where they can satisfy more unsatisfied bonds at once

MINERAL REACTIONS
Igneous Reactions:
95% of the earth’s s crust is composed of igneous rocks that form from
magmas
Magmas are principally composed of O, Si, , Al, Fe, Ca, Mg, Na and K
(with accessory H 2 O, CO 2 and volatiles like H 2 S, HCl, , CH 4 , CO)
As magmas cool, minerals begin to precipitate in a distinct order:
‘Bowen’s s Reaction Series’

Bowen wanted to answer the question:
How do different types of igneous rocks form? Can you get basalts and
granites from the same “parent” magma?
So, he did a series of lab experiments:
1. Make artificial magma of ‘basaltic’ composition
2. Cool very slowly to a particular temperature
3. Quickly quench
Results? Two different types of “reaction series”
Continuous Reaction Series: : a solid solution of continuously changing
composition forms
Ex. Plagioclase feldspars (Ca rich => Na rich at lower T)
Discontinuous Reaction Series: reactions occur between melt and
previously precipitated crystals; the old crystals dissolve and new
minerals of different composition and structure form
Ex. Olivine => Pyroxene => Amphibole => Biotite

Bowen’s s idea:
Magmatic Differentiation: a single
single homogeneous magma
produces a variety of chemically distinct igneous rocks because of…
Frractional Crystallization: crystals are
crystals are physically separated from
the cooling magma (e.g., by gravity settling or rimming) so that liquid and
crystal cannot react further; ; changes the bulk composition of the magma
(in fact, makes it more SiO 2 -rich, because mafic minerals form first and
settle out)
We will discuss this more in the context of phase diagrams…Bowen was
partially correct, but there are other processes at work as well…

MINERAL REACTIONS
Metamorphic Reactions:
Reaction that occur in the solid state
-typically isochemical, , that is, the bulk chemistry is unchanged (except often
a change in H 2 O)
Examples:
Dehydration reactions: higher T, volatile H 2 O is formed and lost
Decarbonation reactions: : carbonate-rich sedimentary rocks lose
CO 2 with increasing T
CaCO 3 (calcite) + SiO 2 (silica) = CaSiO 3 (wollastonite)) + CO 2 (gas)
Metasomatic: : additional elements are gained or lost by circulating fluids

MINERAL REACTIONS
Weathering Reactions (Physical vs. . Chemical):
Chemical Weathering: thermodynamically driven alteration of minerals to
form new minerals; changes composition and structure; occurs because of
changes in T, P, composition
Primary Minerals: : Quartz, Olivine, Albite, Enstatite, , Muscovite
tend to weather and form
Secondary Minerals: Kaolinite, , Goethite, Gibbsite, Hematite
“Goldich” Weathering Series: : susceptibility to chemical weathering
Quartz

MINERAL REACTIONS
“Goldich” Weathering Series:
Quartz

So what is the answer?
Differences in structure, bonding determine weathering kinetics
More polymerized structures break apart more slowly (more strong bonds
to break!)
Sometimes the reaction product is not solid phase, e.g. secondary layer
silicates, but rather dissolved SiO 2 , Na + , K + , Ca +2 , Mg +2
Examples:
Al 2 SiO 5 (sillimanite)) + SiO 2 (quartz) + H 2 O => Al 2 Si 2 O 5 (OH) 4 (kaolinite)
3KAlSi 3 O 8 (orthoclase) + 2H + => KAl 3 Si 3 O 10 (OH) 2 (muscovite) + 2K + + 6SiO 2

MINERAL REACTIONS
Ultra High Pressure Reactions:
Below the crust at high temperatures and pressures, minerals undergo
transformation reactions into structures more suited to those P,T
conditions
How do we know?
Specimens at the surface, e.g., from kimberlite pipes
Laboratory experiments with diamond anvil cells
* Below 6km, Si probably exists in 6-fold coordination with O, rather than
4-fold
* Increased densities of materials, denser packing of elements
* Perovskite and rutile type structures become important*

Rutile Structure (TiO 2 )
Based on hexagonal closest packing (HCP): ABABABAB layers of HP
Ti atoms fill half of the possible octahedral positions
The cations are always in 6-fold (octahedral) coordination.
The anions are coordinated by 3 cations (trigonal
coordination)
Octahedra link along edges

Perovskite Structure (ABO 3 )
Cubic closest packing (CCP) of oxygen, e.g., ABCABC layers of HP
1/4 of oxygens are replaced by a large A cation
A cation is in 12-fold coordination with surrounding O
B cation is in 6-fold (octahedral) coordination with O
Octahedra share apices only
Very dense structure