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

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

2. Earth contains a great diversity of mineral and rock types — at least 10x that known from other planets and early solar system bodies Silicates Sulfides Halides Clays Minerals Oxides and Hydroxides Carbonates, Phosphates Sulfates, Nitrates, Borates Steno Clastic sediments (sands, silts, clays) Chemical sediments (salts, some clays) Rocks Leibniz Metamorphic rocks Igneous (silicate melts)

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

4. An early reasonable-seeming (but wrong) idea Macroscopic cubes (and so forth) are made of microscopic cubes

5. But we know that the chemical ‘entities’ that make up crystals are actually molecular structures that are not symmetric shapes like cubes or hexagons. How do they make such regular shapes? e.g., crystals and molecules of insulin monomer crystal

6. The answer to this mystery is that low-symmetry objects can ‘fit’ together into high-symmetry arrangements

7. Monomer Hexamer Crystal

9. The magic ratios for ‘packing’ of cations and anions

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

11. Evidence suggesting I’m not lying to you

12. These cation/anion units can share anion corners, edges or faces to make larger ‘superstructures’

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

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

15. The difference between silicate minerals and glasses

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

18. Interior of the Genesis sample collection module

19. The Genesis sample collection module after ‘landing’

20. Picking through the pieces

22. 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

23. “Hydrogen as food’ hypothesis: Burbidge et al., 1957 (built on ideas of Gamow re. nucleosynthesis in big bang) I. H burning neutrino positron (rxn. discovered by H. Bethe, 1939) H + H = D + + +  +  photons D +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 = 8Be 8Be = 2.4He Stuck; no way to elements heavier than B Timescale ~ 10-16 s

24. “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 Willie Fowler, Salpeter and Hoyle Show the solution is the following reaction in red giant stars: 4He + 4He + 4He = 12C Opens possibility of many similar reactions: 12C + 4He = 16O 16O + 4He = 20Ne 20Ne + 4He = 24Mg Collectively referred to as ‘He burning’ “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

25. Advanced burning: origin of the 2nd quartile of the mass range 12C + 12C = 23Na + H 16O + 16O = 28Si + 4He CNO cycle 12C + P = 13N = 13C 13C + P = 14N 14N + P = 15O = 15N 15N + P = 12C + 4He

26. The E process (for ‘Equilibrium’): why the cores of planets are Fe-rich A quasi-equilibrium between proton+neutron addition + photo-degradation Promotes nuclei with high binding energy per nucleon

27. Neutron capture as a mode of synthesizing heavy elements Occurs in environments rich in high-energy neutrons, such as super-novae

28. 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 abundance Consumed in He burning • Overall ~ exponential drop in abundance with increasing Z Drop in bonding energy per nucleon w/ increasing Z • Even Z > odd Z Memory of He burning • Fe and neighbors are anomalously abundant Maximum 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.

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

32. II. Accretion of the Earth (and inheritance of interstellar dust)

33. But primitive meteorites are diverse; how are we to know which is most like the earth? letters indicate compositional fields of various types of primitive meteorites Earth is somewhere near here

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

35. How do we guess the composition of the bulk earth if both terrestrial rocks and meteorites are so variable?

36. Broad groupings of elements in geochemical processes

37. The earth’s mantle is mostly chondritic, but depleted in moderately volatile elements (K, Na) 1 Silicate earth CI chondrites Are they simply missing, or hiding somewhere in the earth? We’ll revisit this question later

38. The earth’s mantle is also depleted in siderophile elements (Ni, Cu, Au) Silicate earth CI chondrites 0.1 Are they simply missing, or hiding somewhere in the earth? We’ll revisit this question later too