Download Lecture 6 (version 2016-03-29)

Lecture 6: Geochemistry, minerals,
high pressure chemistry
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Geochemistry
– The origin and abundance of elements
– Geophysical exploration
– Goldchmidt classification
Minerals
– Systematic classification
– Silicates
– Aluminosilicates
High-pressure chemistry
– Equipment
– Examples
Figures: AJK
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Literature
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The origin of chemical elements
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Big Bang nucleosynthesis (H, He, some Li)
Stellar nucleosynthesis
– Elements up to iron are created by fusion reactions
– Some heavier elements are created by neutron capture processes
Supernova nucleosynthesis (elements heavier than iron)
Ref: Treatise on Geochemistry, Vol 1, p. 9
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Main origin of elements
found on Earth
Figure: Wikipedia
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Abundances of the elements
in the Solar system
Ordinary (baryonic) matter in the universe
is estimated to be 74% H, 24% He, and 2%
heavier elements
Li, Be, B produced by nuclear fusion,
then destroyed by other reactions
Figure: Wikipedia
Two general trends:
1. An alternation of abundance in elements as they have even or odd atomic numbers (the
Oddo-Harkins rule, arises from the details of the helium burning process)
2. A decrease in abundance as elements become heavier. Iron is especially common because it
represents the minimum energy nuclide that can be made by stellar nucleosynthesis.
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Abundances of the elements
in the Earth’s crust
Volatile light
elements: H, He, Ne,
N, C (hydrocarbons)
Anomalously low abundance in
comparison to solar abundance
Figure: AJK
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Some Fundamentals of Mineralogy and Geochemistry, L. Bruce Railsback, http://www.gly.uga.edu/railsback/FundamentalsIndex.html
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Structure of Earth
Solid
Liquid
Solid
http://backreaction.blogspot.fi/2010/06/diamonds-in-earth-science.html
Example: graphite-diamond phase transition
occurs at ~130 km depth for t = 800°C
http://www.spring8.or.jp/en/news_publications/research_highlights/no_57/
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Geophysical exploration
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Seismic methods
– Example: Reflection seismology
– When a seismic wave travelling through the Earth encounters an interface
between two materials with different acoustic impedances, some of the wave
energy will reflect off the interface and some will refract through the interface
– Seismic source: dynamite, seismic vibrator (“thumper truck”)
Geodesy and gravity techniques
Magnetic techniques (e.g. aeromagnetic surveys)
Electrical and electromagnetic techniques
– Magnetotellurics
– Electrical resistivity tomography
– Ground-penetrating radar
The brute-force approach: drill a hole and explore what comes out (next slide)
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Kola Superdeep Borehole
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Drilling began in 1970, reached 12 262 meters in 1989
– Abandoned in 2006
– Mariana Trench: 10 994 m
Site of fascinating geophysical discoveries, but actually
reached only 1/3 of the estimated thickness of the Baltic
continental crust (35 km)
The borehole,
welded shut
Ref: https://en.wikipedia.org/wiki/Kola_Superdeep_Borehole
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Goldschmidt classification (1)
Some elements have affinities to
more than one phase. The main
affinity is given in the table.
Figure: Wikipedia
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Goldschmidt classification (2)
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Lithophile = rock-loving elements
– Remain on or close to the surface because they combine readily with oxygen,
forming compounds that do not sink into the core
– The strong affinity for oxygen causes them to associate very strongly with
silica, forming relatively low-density minerals that thus float to the crust
– Many lithophile metals are of considerable value as structural metals
(magnesium, aluminium, titanium, vanadium), but the process
of smelting these metals is extremely energy-intensive
Siderophile = iron-loving elements
– High-density transition metals which tend to sink into the core because they
dissolve readily in iron either as solid solutions or in the molten state
– Many siderophile elements have very small affinity for oxygen (e.g. gold)
– Form stronger bonds with carbon or sulfur, but even these are not strong
enough to separate out with the chalcophile elements
– Include technologically highly important precious metals
Ref: https://en.wikipedia.org/wiki/Goldschmidt_classification
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Goldschmidt classification (3)
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Chalcophile = chalcogen-loving / ore-loving elements
– Remain on or close to the surface because they combine readily with sulfur
and/or some other chalcogen other than oxygen, forming compounds which
do not sink into the core,
– Sulfides are much denser than the silicate minerals formed by lithophile
elements and chalcophile elements separated below the lithophiles at the
time of the first crystallisation of the Earth's crust
– Because the minerals they form are nonmetallic, this depletion has not
reached the levels found with siderophile elements
– Chalcophiles can be easily extracted by reduction with coke
Atmophile = gas-loving (volatile) elements
– Remain mostly on or above the surface because they are, or occur in, liquids
and/or gases at temperatures and pressures found on the surface
– Strongly depleted on earth as a whole relative to their solar abundances
owing to losses from the atmosphere during the formation of the Earth
– Carbon: CO, CO2, hydrocarbons
Ref: https://en.wikipedia.org/wiki/Goldschmidt_classification
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Minerals
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1995 definition of a mineral from The International Association of Minerals (IMA):
– "A mineral is an element or chemical compound that is normally crystalline
and that has been formed as a result of geological processes“
More detailed (and controversial) definition (Tasa 2007):
1. Naturally occurring
2. Stable at room temperature
3. Represented by a chemical formula (note: many are solid solutions!)
4. Usually abiogenic (not resulting from the activity of living organisms)
5. Ordered atomic arrangement
IMA has approved over 5000 minerals (2016)
– IMA Database of Mineral Properties
(http://rruff.info/ima/)
The RRUFF™ Project is creating a complete set
of high quality spectral data from well
characterized minerals (http://rruff.info)
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Silicates form 90% of the Earth’s crust
Some Fundamentals of Mineralogy and Geochemistry, L. Bruce Railsback, http://www.gly.uga.edu/railsback/FundamentalsIndex.html
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Some Fundamentals of Mineralogy and Geochemistry, L. Bruce Railsback, http://www.gly.uga.edu/railsback/FundamentalsIndex.html
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Some Fundamentals of Mineralogy and Geochemistry, L. Bruce Railsback, http://www.gly.uga.edu/railsback/FundamentalsIndex.html
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Silicates
Ref: West p. 81
α-quartz SiO2
P3221 (154)
Fosterite Mg2SiO4
Pbnm (62)
Figures: AJK
”Tectosilicates”, Quartz family +
aluminosilicates: 75% of the crust
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Aluminosilicates: Feldspars
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By far the most abundant group of
minerals in the earth's crust, forming
about 60% of terrestrial rocks
Feldspar K(AlSi3O8)
C 1 2/m (12)
Figure: Wikipedia
Figure: AJK
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Aluminosilicates: Zeolites
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Microporous aluminosilicate minerals
Na+
Na+
– Cronstedt 1756: heating stilbite,
NaCa4(Si27Al9)O72·28(H2O), produced steam
Na+
Na+
– Zéō -> to boil
– Líthos -> rock
Can be considered as molecular sieves, with actual
Na+
Na+
industrial applications in:
– Ion-exhange (e.g. water purification and
Na+
Na+
softening)
– Catalysis (e.g. In petrochemical industry)
LTA zeolite (Pm-3m)
http://www.iza-structure.org/databases/
[Na12(H2O)27]8[Al12Si12 O48]8
http://www.hypotheticalzeolites.net/
Figure: AJK
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Ref: https://en.wikipedia.org/wiki/Zeolite
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Some Fundamentals of Mineralogy and Geochemistry, L. Bruce Railsback, http://www.gly.uga.edu/railsback/FundamentalsIndex.html
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Some Fundamentals of Mineralogy and Geochemistry, L. Bruce Railsback, http://www.gly.uga.edu/railsback/FundamentalsIndex.html
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High-pressure chemistry
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High pressures (> 1 GPa = 10 000 atm) enable the synthesis of huge number of
new materials that are unattainable in the atmospheric pressure
In ultra-high pressures (Mbar range, > 100 Gpa), the compression energy rivals or
even exceeds the energy of the chemical bond!
– New materials that are completely unintuitive from the ”normal” point of view
8 GPa
Phase diagram of SiO2 (see lecture 4)
Phase diagram of H2O (see lecture 4)
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Chem. Soc. Rev., 2006, 35, 855
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By the time a typical solid or liquid is compressed to above a few hundred
thousand atmospheres, its molar volume is reduced by approximately 50%
Once the megabar range is reached, average interatomic distances can be
decreased by up to a factor of two.
It is to be expected that large changes will occur in the outer electron shells under
extreme densification conditions, and that these will lead to substantial
modifications of the chemical and physical properties
It is known that such large changes in molecular and electronic structure do in fact
occur, and that the very arrangement of the Periodic Table might have to be
modified for high pressure conditions.
As a simple example, we can consider the typical alkaline earth metals such as Ca
and Sr that possess a fully close-packed fcc structure at ambient conditions
However, pressurising Ca to P > 200 kbar (20 GPa) causes it to transform to a less
efficiently packed bcc structure with a lower coordination of the metal atoms
– Pressure-induced mixing occurring between 3d and 4s electronic shells, giving
Ca the character of a transition metal rather than an alkaline earth element
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High-pressure chemistry
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Ultimately, it. is thought that most substances should become metallic at the most
extreme pressures, as the close approach of atoms results in electronic overlap
In moderate pressures (perhaps < 10 GPa), the pressure effects are not as extreme
as in ultra-high pressures and typical phenomena are e.g. the increase of
coordination number and structure type
– In moderate pressures a huge number of new materials that are only
metastable in atmospheric pressure, can be realized
– Often the new materials remain intact in atmospheric pressure (diamond!)
Ref: West p. 226
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Bridgman press (1)
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In 1905, Percy Bridgman was a doctoral student at Harvard, working on a project
that involved the generation of a few hundred up to 1000-2000 atmospheres
Frustrated while waiting for a delivery of replacement parts, he invented his own
approach to pressure-sealing
This new technique immediately permitted controlled experiments up 7000 atm
Incorporating changes in the pumping system, he ramped the pressure up to a
record 20 000 atm (2 GPa)
Here, well-known substances began to behave in unusual ways. For example, solid
H2O transformed into its ice-VI form, which only started to melt above 100 °C
Further technical innovations, including development of the opposed-anvil device
for compressing solids, led to the generation of unprecedented high pressures into
the 5–10 GPa range
– Here, new dense polymorphs of solids are obtained, and the physical and
electronic properties of elements and compounds change dramatically
Bridgman received the 1946 Physics Nobel prize for his pioneering work
Ref: Nature Materials 2005, 4, 715 - 718
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Bridgman press (2)
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Large volume high-pressure devices derived
from those designed by Bridgman are used for
– Materials synthesis
– Crystal growth and phase-equilibrium
studies
– Determination of properties such as
electrical conductivity, melting and
rheological phenomena in materials
This large press is installed at the National
Institute for Materials Science, Tsukuba, Japan;
photo courtesy of T. Taniguchi.
– The scale is indicated by the access stairway
to the sample loading area.
Ref: Nature Materials 2005, 4, 715 - 718
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Diamond anvil cell
Figure: Choong-Shik Yoo, Washington State University
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Crystal
structure of
Na-hP4
(P63/mmc)
Our calculations
reveal that an
insulating
electronic state
emerges
because
compression
causes the 3d
bands to rapidly
drop in energy
relative to the 3p
bands and
increasingly
hybridize with
them
Na-Na interatomic
distance decreases
from 3.72 Å to 1.89 Å
The theoretical calculations performed by Ma and
colleagues elucidated the reason for the dramatic
transformation on sodium. At pressures of more than 2
million atm, sodium is strongly (5-fold) compressed so
that the atoms overlap and force their outer electrons
into the interstitials between the atoms, where
electron density strongly localizes. This is responsible
for the collapse of the metallic state. Sodium thus
transforms to an elemental ionic solid where sodium
atoms play the role of cations while the localized
electrons behave as anions
(http://www.aps.anl.gov/Science/Highlights/Content/A
PS_SCIENCE_20090420.php)
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