Elemental Distributions in the Universe and the Solid Earth GLY 4241 - Lecture 2 Fall, 2014 Goldschmidt said that an aim of geochemistry was the study of the distribution and amounts of the chemical elements in minerals, ores, rocks, soil, water, and the atmosphere. A study of this information will provide a background for all environmental geochemical studies involving elements.

Elemental Abundance Expression Number of atoms per million atoms Weight percent Volume percent Relative to abundance of silicon Elemental abundances may be expressed in several ways. One is on a number of atoms basis. This assumes that we have, say, one million atoms. Of these one million atoms, so many atoms will be oxygen, so many will be silicon, etc. This is purely a numerical counting system. Another method allows for the weight of each atom. Figures will be reported in weight percent or weight per million grams. If we have a million gram sample, so many grams are oxygen, so many are silicon, etc. This method increases the numbers for heavy elements, while decreasing the numbers for lighter elements. Another method is based on volume. Large atoms (anions and large cations) take up much more volume than small atoms. Another method is used in discussing "cosmic" abundances. The cosmic abundance figures are an attempt to estimate the elemental composition of the entire universe from data gathered by earth-based astronomical and satellite data. They are generally reported relative to the abundance of silicon. Data for all elements are converted to their abundance relative to 1,000,000 atoms of silicon. Although cosmic abundances are not usually useful to an environmental scientist, examining the figures to see just how different the earth really is from the average abundances seen in the universe is useful.

Abundance versus Atomic Number 1. The light elements are much more abundant than the heavy elements. 2. Elements higher than nickel vary less in abundance than lighter elements. 3. A large abundance peak at iron (element number 26) exists, and lesser peaks at other atomic numbers.

Light Element Abundance 4. There is a huge drop in abundance for the Lithium-Beryllium-Boron (Li-Be-B) triplet. This results from two factors: 1) At the Big Bang, nuclear processes that could fuse the proper H or He isotopes into Li and/or the other two were statistically very rare and hence inefficient, and 2) Some of the Li-Be-B that formed and survived may be destroyed in processes with stars. 5. Elements of even atomic number are more abundant than elements of odd atomic number on either side, producing a distinct zig-zag (up-down) pattern to the whole curve. For example, between carbon and oxygen there is a decrease (the element is Nitrogen); between neon and magnesium the decrease element is sodium; the largest drop is between oxygen and neon, the element that thus decreases notably is fluorine.

Oddo-Harkins Rule For elements heavier than H, the Oddo-Harkins rule holds that elements with an even atomic number (such as carbon) are more common than elements with an odd atomic number (such as nitrogen) The effect was first reported by Giuseppe Oddo in 1914 and William Draper Harkins in 1917 Elements with odd atomic numbers have one unpaired proton During nucleosynthesis, these elements are more likely to capture another proton, thus increasing their atomic number In elements with even atomic numbers, protons are paired, with each member of the pair offsetting the spin of the other, enhancing stability The reason for this fluctuating pattern is that elements with odd numbers of nucleons (protons and neutrons) are less stable, resulting in one unpaired (odd) proton or neutron - those that pair these particles result in offsetting spins in opposite directions that enhance stability (all this is part of the quantum theory of nuclear arrangements), and is known as the Oddo-Harkins rule. The least abundant nuclei are the “odd-odd” nuclei, with odd numbers of protons and neutrons. For example:

Odd-Odd Scarcity Li6 7.42 % Li7 92.58% B10 19.80 % B11 80.20% K39 93.20 % K40 0.012% K41 6.73% Results from the Oddo-Harkins Rule

Cosmic Abundances Element Abundance Hydrogen 2.66 x 1010 Nitrogen Helium 1.8 x 109 Magnesium 1.06 x 106 Oxygen 1.84 x 107 Silicon 1.00 x 106 Carbon 1.11 x 107 Iron 9.0 x 105 Neon 2.6 x 106 Sulfur 5.0 x 105 6. The most abundant elements are hydrogen, helium, oxygen, carbon, neon, nitrogen, magnesium, silicon, iron, and sulfur. All these elements have an atomic number less than 27. Cosmic Abundances Normalized to Si = 106

Hydrogen-Helium Dominence 7. Hydrogen and helium are by far the most abundant elements. The regularities observed in this table are the function of nuclear, not chemical, properties. They are in large part due to processes occurring inside stars, known as nucleosynthesis. A discussion of nucleosynthesis is beyond the scope of this course. A good review of element production in stars are found at a site maintained by the Wright Center for Science Education; Tufts University (2005).

Solar System versus Earth If the two major elements in the universe are hydrogen and helium, why are these elements scarce on earth? The earth is a differentiated planet. It formed by condensation of gaseous matter around a proto-sun. The condensate was greatly enriched in elements that condense at higher temperatures than hydrogen and helium, which have the two lowest freezing and melting points of all the elements. Thus, the abundance of elements on earth does not resemble the cosmic abundance scale. During the subsequent evolution of the earth, the planet is believed to have remelted due to meteorite impact. This remelting allowed a density separation of the elements. Heavier elements sank to the middle of the earth, while the lighter elements rose to the outer parts. The earth is traditionally divided into the core, the mantle, and the crust. An iron-nickel metal phase is believed to make up the core. The mantle and the crust are dominated by silicate minerals, with those in the mantle being enriched in iron and magnesium (mafic minerals) compared with those in the crust (felsic minerals). We can only directly sample the crust. Thus, the best information about the abundance of elements on earth that we have is for crustal rocks.

Crustal Abundances in the Earth Element Abundance Oxygen 46.6 Sodium 2.8 Silicon 27.7 Potassium 2.6 Aluminum 8.1 Magnesium 2.1 Iron 5.0 All Others 1.5 Calcium 3.6 Source: http://hyperphysics.phy-astr.gsu.edu/hbase/tables/elabund.html Most of the earth is composed of a handful of elements. Table 2-2 lists values for the nine most common elements, expressed in weight percent. Approximate percentage by weight

Continental versus Total Crust Oxide Cont. Crust Total Crust TotalCrust SiO2 61.0 59.3 MgO 3.1 4.0 TiO2 0.8 0.9 CaO 5.7 7.2 Al2O3 15.6 15.8 Na2O 3.0 Fe2O3 2.6 K2O 2.9 2.4 FeO 3.9 4.4 P2O5 0.3 0.2 MnO 0.1 Total 100.0 Even within the crust, differentiation exists. Table 2-3 shows the relative abundance of major elements in the continental and total crust, expressed as weight percent oxides. Oceanic crustal rocks are enriched in iron, magnesium, and calcium compared with continental crustal rocks. Continental crustal rocks are enriched in silicon, aluminum, and potassium relative to oceanic rocks. Most of these differences can be accounted for by the weight of the elements. Iron, magnesium, and calcium are all heavier than silicon and aluminum. Potassium remains to be explained. Potassium is heavier than silicon or aluminum, so we might expect that it would be concentrated in the mantle. The ion of potassium (K+) is very large. It does not fit well into the crystal sites available in the minerals present in the mantle. Thus, by default, it ends up in the crust. It should also be noted that, to a precision of 0.1%, only eleven elements make up all of the earth's crust. Many other elements are present, but only in small amounts. After Mason and Moore, 1982, p.44

Different Types of Data Table 2-4 shows the effect of different methods of reporting data. The eight most common elements in the earth's crust are tabulated according to atom percent, weight percent, and volume percent. Note that oxygen is more than 90% by volume, more than 60% by number of atoms, but only 46% by weight. Thus the way in which we choose to present data can have a significant impact on our perception of the data. We need to choose carefully how we manipulate data to be sure the point we are making is clear, and that conclusions we draw are real.