Earth Science Today
Russ Colson
Minnesota State University Moorhead

Physical Geology:

     Topic 2: Natural resources, chemical processes, What is the Earth made of?

Some Philosophical opening remarks:
    Who we are as a society reflects in part the fact that natural resources are not evenly distributed.  This heterogeneous distribution encouraged ancient societies to learn to communicate and get along.  Trade in resources further encouraged the trade of ideas and cultures as well.  Uneven distribution of resources encouraged the occupation of territories that might have otherwise been avoided.  Many societies became influential in the world of ideas in part because they were at crossroads of trade where they absorbed ideas from many places, digested them, and spread them to the world in a new form because of their position at a center of trade.  Ancient Palestine was at such a crossroads of trade for many centuries.  Much of our culture, writing, and religion can be traced to this region of the world.  Thus, to some extent, we can think of the distribution of resources on Earth as a nursemaid beckoning us to explore, think, share ideas, and learn to get along with other people.

Chemical differentiation is the process by which a single chemical substance is divided into two or more new substances each with a different chemical composition than the original (thus the words "chemical" and "different" in "chemical differentiation").  The number of processes by which earth (and other planets) can be differentiated is almost infinite.  However, every process can be described as having two steps: 1)  Chemical separation:  a process whereby a single substance becomes two different substances.  For example, if we partly evaporate salt water, the salt stays with the liquid part (making it saltier) and the vapor part is mostly salt free.  Thus we have two new compositions.

2)  Physical separation:  Once we have two new substances, we need some process for physically separating them.  For example, when water is partly evaporated, the vapor part is less dense and rises up out of the liquid into the air, thereby separating the two.
 

 The key concepts in understanding chemical separation are phase change and partitioning. 1)  A particular collection of atoms can exist in a number of different phases, depending on the temperature, pressure, concentrations of various elements, pH, oxygen activity, and other properties of the system (e.g. water exists as solid at low temperature, liquid at higher temperature, and gas at higher temperature still).  As seen in the example of water, the stable phase can change as conditions change.  Phase changes occur in many situations.  Mollusks crystallize solid CaCO3 out of the sea to make shells, olivine and other minerals crytallizes from molten rock (magma) as it cools within the earth or at the earth's surface, ore minerals can crystallize from hot water within cracks in the rock of the earth, grease coagulates out of your greasy soup as it cools, raindrops or snowflakes condense out of air, and so on.

2)  Partitioning is a measure of how a particular element partitions, or distributes itself, between two phases.  Every element of the periodic table will have a natural, or equilibrium, distribution between any two phases.  The distribution will depend on temperature, pressure, composition and other properties of the system.  Partitioning can be defined as the concentration of an element in one phase divided by the concentration in another phase.  For example, the concentration of salt in liquid water (in the thought experiment above) is much higher than the salt concentration in the water vapor.  Thus the partition coefficient (D) for salt in liquid water/salt in vapor is very large.  The partition coefficient is therefore a measure of the tendency of an element to go into one phase rather than another.  If D>>1, most of the element will go into the phase put on top of the ratio.  If D<<1, most of the element will go into the phase put on bottom of the ratio.  If D is nearly equal to 1, then the element "likes" each phase about equally.

 Example calculation of a partition coefficient:  Partition coefficients are measured experimentally in a laboratory by measuring the concentrations of an element in phases that are held in chemical equilibrium.  Suppose that you do an experiment to measure the partitioning of a pollutant (we'll call it Death-X) between liquid water and clay.  You add 1 gram of Death-X to 10 grams of water and 5 grams of clay.  Concentration is defined as grams of Death-X for each gram of water or clay.  You stir the clay into the water, allowing the Death-X to reach equilibrium between clay and water.  You measure the amount of Death-X in the water (defined as grams Death-X) and find it to be 0.1 gram.   Question 1:  Is most of the Death-X in the water or the clay?
  Question 2:  What is the partition coefficient for Death-X in Clay/Death-X in water?
  Question 3:  If you spilled the Death-X on a clay-rich layer of sediment above your neighbors well (oops), do you think it would be likely to reach the water table and get into your neighbors wellwater?  (answer, est1b1.html) More involved puzzles in partitioning and chemical differentiation are found at
petropuzzl/cooking.htm The key concepts in understanding chemical separation are property and process.
 
1)  In order for them to separate physically, two different materials must differ in some physical property.  Properties may include density, size, shape, whether they are solid, gas, or liquid, hardness, ductility, or any other physical property.  For example, think about separating a beaker of glass and wooden marbles that are all the same size and shape.  How could you do it?  What property does that separation depend on? (answer, est1b2.html)
2)  Some natural process must operate on the different properties in order to separate two materials.  Materials might settle through water at different rates, rise buoyantly or sink through air, water, magma or hot, plastic rock, or be moved differentially by water or wind.  What is the process by which glass and wooden marbles could be separated in the puzzle above? (answer, est1b2.html)
1)  Oil deposits:  Oil forms when microscopic bits of organic matter (dead planktonic creatures mainly) are buried in rock.  Pressure and temperature generate an oily material which is less dense than the water.  The water and oil both exists in the pores of the rock.  Because the oily material is less dense than water, it rises buoyantly until it either escapes to the Earth's surface (and is lost to human use) or it is trapped in the Earth and accumulates.
Thought puzzles: Where will oil accumulate? (puzzles, est1b3.html)
Related Activities: What are porosity and permeability? (activities, est1b4.html, ~2.2 minutes to load at 14.4K baud)

2)  Core and Crust Differentiation:  The most significant event in the chemical history of Earth was the separation of a metallic core from the bulk material of the planet.  This occurred as a metal phase formed from the silicate, rock-like material of a hot earth (possibly partly molten).  The metal phase, being more dense than the silicate, sank to the center of the Earth.  A smaller differentiation, but one with large consequences for the surface of the Earth where we live, was the formation of Earth's crust.  This occurred substantially later than the formation of the core.  Areas of partial melting occurred within the otherwise solid mantle.  The liquid had a different composition and was less dense than the mantle, rising toward the surface.  This less dense material, through many intermediate differentiation steps, became the crust.

 Thought Puzzle:  Core/mantle differentiation.  Suppose that initially the concentration of Fe (iron) in the Earth was about 27%.  The concentration of Mg (magnesium) was about 14%.  From laboratory experiments you measure the partition coefficient for Fe (Fe in metal/Fe in silicate) to be >>1.  You measure the partition coefficient for Mg in metal/Mg in silicate <<1.  Which four of the following do you expect to be true of Earth?
Mg in mantle = 14%,     Mg in core = 14%        Fe in mantle = 27%        Fe in core = 27%
Mg in mantle <14%        Mg in core < 14%        Fe in mantle < 27%        Fe in core < 27%
Mg in mantle >14%        Mg in core > 14%        Fe in mantle > 27%        Fe in core > 27%
Answers, est1b5.html

Au (gold) and Ni (nickel) have partition coefficients metal/silicate that are very high.  Where do we expect most of Earth's gold and nickel to be found?

 3)  Chromium Ore Formation:  Chromite, an ore of chromium, crystallizes from basaltic magma.  Because it is more dense than the magma, it sinks.  This can produce layers of concentrated chromite at the bottom of former magma chambers (former magma chambers because we can not go into magma chambers when they are still molten, and because magma chambers occur deep in the Earth where we can't easily go, we need to allow erosion to excavate these chambers for us after they have cooled).  (sketch, est1b6.html) Here are some general comments on movement of matter on Earth.  Matter moves by convection (e.g. rising and sinking air in the atmosphere, rising and sinking water in the ocean, and rising and sinking magma or silly-putty-like rock) and advection (lateral movements of stuff).  During these movements, phase transitions can occur (for example, rain condensing from air as it rises, crystallization or additional melting in a magma as it rises, crystallization of ore minerals in fractures in the rock as hot mineral-rich water moves to cooler regions, etc).  These phase transition are accompanied by partitioning (for example, a volatile, or easily evaporated, pollutant in a river may be partitioned into the air, electrically charged particles in a thunderstorm may be partitioned into liquid or ice particles and result in lightning, trace elements may be partitioned into ore minerals).  In combination, these processes move elements around on Earth, concentrating them in some places, diluting them in others.
  Lab on measuring partition coefficients (est1b7.html)

          Lab on locating oil deposits and geological mapping and cross-sections (MSWORD file).

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