GE 11a, 2014, Lecture 2Minerals and rocks; the composition and materials of the earth

Earth contains a great diversity of mineral and rock types — at least 10x that known from other planets and early solar system bodies

Silicates

Oxidesand Hydroxides

Carbonates, PhosphatesSulfates, Nitrates, Borates

Halides

Sulfides

Clays

Igneous (silicate melts)

Clastic sediments(sands, silts, clays)

Chemical sediments(salts, some clays)

Metamorphicrocks

Minerals

Rocks

Leibniz

Steno

Symmetry is key to understanding mineral structure, but needs to beunderstood as something different from ‘shape’.

An early reasonable-seeming (but wrong) idea

Macroscopic cubes (and so forth)are made of microscopic cubes

But we know that the chemical ‘entities’ that make up crystals are actuallymolecular structures that are not symmetric shapes like cubes or hexagons. How do

they make such regular shapes?

e.g., crystals and molecules of insulin

crystal monomer

The answer to this mystery is that low-symmetry objects can ‘fit’ togetherinto high-symmetry arrangements

Hexamer

Monomer

Crystal

The magic ratios for ‘packing’ of cations and anions

“Closest packing” arrangements—a good starting concept for most oxide and sulfide minerals

Evidence suggesting I’m not lying to you

These cation/anion units can share anion corners, edges or faces to makelarger ‘superstructures’

The framework of silicate minerals are regular polymers of SiO4-4 tetrahedra

Combinations of silicate ‘polymer’ structures and metal-oxide octahedra can creatediverse structures. E.g., sheet-like micas:

The difference between silicate minerals and glasses

How did we end up at this mix of elements as the ingredients for the earth?

Interior of the Genesis sample collection module

The Genesis sample collection module after ‘landing’

Picking through the pieces

Features that demand an explanation:• H and He are by far most abundant elements• Li, Be and B are anomalously low in abundance• Overall ~ exponential drop in abundance with increasing Z• Even Z > odd Z• Fe and neighbors are anomalously abundant

“Hydrogen as food’ hypothesis: Burbidge et al., 1957(built on ideas of Gamow re. nucleosynthesis in big bang)

I. H burning

H + H = D + + + +

positron neutrino

photonsD +H = 3He + …

3He + 3He = 4He + 2H + …

3He + 4He = 7Be + … (and similar reactions to make Li and B)

Products quickly decay:7Be + e- = 7Li

7Li + P = 8Be8Be = 2.4He

Timescale ~ 10-16 s { Stuck; no way to elements heavier than B

(rxn. discovered by H. Bethe, 1939)

Willie Fowler, Salpeter and Hoyle

“Would you not say to yourself, 'Some super- calculating intellect must have designed the properties of the carbon atom, otherwise the chance of my finding such an atom through the blind forces of nature would be utterly minuscule.' Of course you would . . .. A common sense interpretation of the facts suggests that a superintellect has monkeyed with physics, as well as with chemistry and biology, and that there are no blind forces worth speaking about in nature. The numbers one calculates from the facts seem to me so overwhelming as to put this conclusion almost beyond question.”

F. Hoyle

Show the solution is the following reaction in red giant stars:

4He + 4He + 4He = 12C

Opens possibility of many similar reactions:

12C + 4He = 16O16O + 4He = 20Ne

20Ne + 4He = 24Mg

Collectively referred to as ‘He burning’

“We do not argue with the critic who urges that stars are not hot enough for this process; we tell him to go and find a hotter place.”A. Eddington

Advanced burning:

origin of the 2nd quartile of the mass range

12C + 12C = 23Na + H

16O + 16O = 28Si + 4He

CNO cycle

12C + P = 13N = 13C13C + P = 14N14N + P = 15O = 15N15N + P = 12C + 4He

The E process (for ‘Equilibrium’): why the cores of planets are Fe-richA quasi-equilibrium between proton+neutron addition + photo-degradation

Promotes nuclei with high binding energy per nucleon

Neutron capture as a mode of synthesizing heavy elements

Occurs in environments rich in high-energy neutrons, such as super-novae

Features that demand an explanation:• H and He are by far most abundant elements

H primordial; He consequence of 1˚ generation H burning

• Li, Be and B are anomalously low in abundanceConsumed in He burning

• Overall ~ exponential drop in abundance with increasing ZDrop in bonding energy per nucleon w/ increasing Z

• Even Z > odd ZMemory of He burning

• Fe and neighbors are anomalously abundantMaximum in bonding energy per nucleon at Fe

These factors are directly responsible for the fact that terrestrial planets are made of silicates and oxides (‘rocks’) with magnetic Fe cores.

N

Primitive meteorites look a lot like the sun (minus the gas and all the hotness)

II. Accretion of the Earth

(and inheritance of interstellar dust)

letters indicate compositional fields of various

types of primitive meteorites

Earth is somewhere near here

But primitive meteorites are diverse; how are we to know whichis most like the earth?

Much of the diversity in meteorite composition reflects variations in oxidation state of solar nebula (H2O/CO ratio)

How do we guess the composition of the bulk earth if both terrestrial rocksand meteorites are so variable?

Broad groupings of elements in geochemical processes

The earth’s mantle is mostly chondritic, but depleted in moderately volatile elements (K, Na)

Silicate earthCI chondrites

Are they simply missing, or hiding somewhere in the earth? We’ll revisit this question later

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The earth’s mantle is also depleted in siderophile elements (Ni, Cu, Au)

Silicate earthCI chondrites

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Are they simply missing, or hiding somewhere in the earth? We’ll revisit this question later too