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

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Presentation on theme: "GE 11a, 2014, Lecture 2 Minerals and rocks; the composition and materials of the earth."— Presentation transcript:

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

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

3 Some symmetry elements are permitted (and common)… Others are (usually) forbidden… They key distinction can be understood as coming down to success or failure to ‘tesselate’ without leaving gaps

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 crystal monomer

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

7 Hexamer Monomer Crystal

8 X-rays have wavelengths similar to inter-atomic spacing in crystals, so passing them through crystals leads to diffraction with a pattern that reveals crystallographic symmetry Manganese Silicide

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10 The magic ratios for ‘packing’ of cations and anions Si, Al Fe, Ca, Mg B, C, N Na, K C Common oxide cations

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

13 Evidence suggesting I’m not lying to you

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

15 The framework of silicate minerals are regular polymers of SiO 4 -4 tetrahedra

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

17 The difference between silicate minerals and glasses

18 Quasi-crystals — the wrong side of the symmetry tracks Diffraction pattern of Icosohedrite

19 Quasi-crystals — the wrong side of the symmetry tracks Solve the ‘space filling’ problem in a fashion analogous to a penrose tiling: ‘Ordered but not periodic’ — local centers with 5, 7, 11 or other forbidden rotational symmetries – But no translational symmetry

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

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

22 A start to understanding the composition of the earth is to look at the sun, which contains most of the atoms in the solar system

23 ‘Fraunhofer lines

24 The solar wind

25 The Genesis mission

26 Genesis: an experiment to directly collect the solar wind

27 The Genesis sample collection module after ‘landing’

28 Picking through the pieces

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30 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 A consensus view of the chemistry of the sun

31 “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 photons D +H = 3 He + … 3 He + 3 He = 4 He + 2H + … 3 He + 4 He = 7 Be + … (and similar reactions to make Li and B) Products quickly decay: 7 Be + e - = 7 Li 7 Li + P = 8 Be 8 Be = 2.4 He Timescale ~ s { Stuck; no way to elements heavier than B (rxn. discovered by H. Bethe, 1939)

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

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

34 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

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

36 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.

37 The next clue comes from primitive meteorites FOV ~ 2 mm FOV ~ 2 cm

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

39 The separation of volatile from other elements can be easily understood as a result of condensation from an initially hot gas

40 The earth like planets formed in the inner solar system, which didn’t cool enough to accrete thick envelopes of ice and gas before the nebula dissipated

41 How did the earth get any volatiles at all?

42 A conceptual model for the secondary origins of volatiles in asteroidal bodies

43 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 which is most like the earth?

44 Much of the diversity in meteorite composition reflects variations in oxidation state of solar nebula (H 2 O/CO ratio)

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

46 Broad groupings of elements in geochemical processes

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

48 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


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