1. 1 Natural Geochemical Enrichments of Elements GLY 4241 - Lecture 3 Fall, 2014

2. 2 Important Low-Abundance Elements Elements used in steel alloys  Vanadium, chromium, nickel, niobium Elements used in rechargeable batteries  Nickel, cadmium, lithium Jewelery  Gold, silver, platinum, palladium Nuclear fuels  Uranium, thorium Miscellaneous  Copper, used in wiring, plumbing, alloys  Mercury, used in electrical switches, pesticides, fluorescent bulbs, etc.

3. Need for Concentration Most of these elements could not be mined, processed, and formulated into useful products at reasonable costs if they occurred everywhere at their average abundances in the crust For example, millions of tons of gold exist in seawater, but the cost of obtaining pure gold from seawater is many times the value of the gold 3

4. Enrichment We need to answer two questions:  1. For any given element, how much enrichment above natural abundance values is needed to produce a mineable ore?  2. What geochemical processes are responsible for producing these natural elemental enrichments? 4

5. 5 Definition of Ore The naturally occurring material from which a mineral or minerals of economic value can be extracted at a reasonable profit (from the Glossary of Geology, 3 rd edition)

6. 6 Factors Influencing Cost of Metals Exploration Mining Rights Acquisition Cost of mining,  Includes cost of compliance with existing environmental regulations Ore separation and processing Transportation of ore to consumer

7. 7 Frank Wigglesworth Clarke, 1847-1931 Early career involved teaching, including a year at Howard University, and 9 years at the Univ. of Cincinnati Later, Chief Chemist, USGS, 1883-1924

8. 8 Definitions Clarke = the average abundance of an element in the crust of the earth Clarke of concentration = the concentration of an element in a rock compared with its average concentration in the earth's crust, or of an element within a particular mineral

9. 9 Clarke of Concentration Clarke of copper is about 55 ppm, or 0.006% In the mineral chalcocite, Cu 2 S, the Cu concentration is 79.8% Thus, the clarke of concentration within this mineral is 79.8/0.006, or 13,300

10. 10 Clarke Values

11. Early Earth The early earth was probably bombarded by solid planetesimals, primarily chondritic meteorites Chondritic meteorites may be left over from the protoplanet stage of the solar system Chondritic meteorites are composed of three different phases, or combinations of these phases 11

12. Chondrite Phases Nickel-iron metal Iron sulfide Silicates, largely olivine or pyroxene 12

13. 13 Exchange Reactions M + Fe silicate ↔ M silicate + Fe M + Fe sulfide ↔ M sulfide + Fe

14. 14 Goldschmidt Element Affinities Siderophile: Elements concentrated in the metallic phase, along with metallic iron Chalcophile: Elements concentrated in the sulfide phase Lithophile: Elements concentrated in the silicate phase Atmophile: Elements concentrated in the atmosphere

15. 15 Meteorite Phases Iron-nickel metal Troilite (sulfide) Silicate

16. 16 Geochemical Classification of Elements

17. Trace Elements Working definition: element whose concentration is less than 0.1% May form their own minerals, but typically are too scarce to do so Typically, trace elements will follow a major element into another mineral, where they replace part of the major element 17

18. 18 Siderophile Characteristics Elements whose valence electrons are not readily available for combination with other elements Positive charge on the nucleus, at least under certain conditions, exerts a strong attraction on the outer electrons, preventing combination These elements usually occur in the native state

19. 19 Chalcophile Characteristics Elements whose valence electrons may be shared, but are not electropositive enough to donate electrons or electronegative enough to accept electrons Thus, the bonds formed are predominantly covalent Since sulfur is much less electronegative than oxygen, sulfur is prone to form covalent bonds with these elements Generally the chalcophile elements have their valence electrons outside a shell of 18 electrons

20. 20 Lithophile Characteristics Elements that are strongly electropositive or electronegative and thus typically donate or accept electrons, forming ionic bonds Most silicate minerals have oxygen ions that can form ionic bonds to metal cations Generally the lithophile elements have their valence electrons outside a shell of eight electrons

21. 21 Atmophile Characteristics Elements that do not readily combine with other elements, or which form diatomic molecules held together in the solid or liquid states only by very weak Van der Waal forces All of the inert gases, with completed shells or subshells, fall into this category

22. 22 Oxygen Secondary atmophile element would not occur in the atmosphere of the earth if the earth were at chemical equilibrium Oxygen is maintained in the atmosphere only by the continual photosynthesis within the biosphere Indeed, the presence of oxygen in an atmosphere is often regarded as an indicator of life on the planet

23. 23 Atomic Volume vs. Atomic Number Vertical scale should be atomic volume