Stories of the Lithosphere
Gold, Pollution, and Farmland—M-Ore Puzzles
Earth Science Essentials
by Russ Colson
Cr Ore—Crystalline Residue of a Magma Chamber
Black chromite layer in green serpentinized olivine, Webster Addie dunite mine, North Carolina. Orange color is from oxidation of the iron in the sample on weathered surfaces.
--PS. I collected this rock while courting my future wife! Back then, it didn't have the oxidized surface!
One type of Chromium ore forms when a mafic magma chamber cools and begins to crystallize. Often, one of the first crystals to form is olivine, which is denser than the magma and can sink, as we have discussed. Chromite is sometimes another early crystal—it contains the elements iron, chromium, and oxygen, along with smaller amounts of the rest of the periodic table. Like olivine, chromite is denser than the magma and can settle out in layers. After the magma has entirely frozen into a layered intrusive, erosion might expose it at the surface where the ore can be mined.
The great Bushveld of South Africa, a layered intrusive complex exposed at the surface by ages of erosion, formed over 2 billion years ago. The large chromium deposits at Bushveld was once part of a magma chamber as big as West Virginia and over 5 miles deep.
Often the compatible elements, like Cr, occur in or near mafic or ultramafic rocks. These type of rocks are sometimes called 'primitive' meaning that they have not undergone significant chemical differentiation and so still have a significant amount of their compatible elements present in the melt.
Fe Ore—Ocean Salt from 2.6 billion years B.C.
Outcrop of Banded Iron Formation (BIF), Soudan MN. Gray bands are hematite, the iron ore. Orange bands are jasper. Photo courtesy of Dustin Wenzel.
Earth is not making any more banded iron formations. This kind of ore deposit only formed between about 2.6 and 1.8 billion years ago.
What's with that? Why then? Why not before? Why not today?
The answer lies in mass transport and precipitation that we talked about in lecture. In its reduced state (Fe2+) iron is soluble in water. In this state, significant amounts of iron can dissolve in sea water (like the salt in today's oceans). Once dissolved, currents in the ocean can move the iron from one place to another. In its oxidized state (Fe+3), iron precipitates. In this state, so little iron dissolves in sea water that little can be moved from one place to another even over very long periods of time.
Today, there is so much oxygen in Earth's atmosphere, and therefore in its oceans, that iron is present as Fe3+ and can't dissolve in the oceans or be moved around—thus, no mass transport. Before 2.6 billion years ago, there was so little oxygen around that there were few or no environments where the iron could be oxidized—thus, no precipitation. Because of this, we only get BIFs during this ancient period of time when photosynthetic bacteria were conquering the Earth and slowly turning our atmosphere into an oxygen-based one.
Au Ore—Sand from a mountain range
Gold in quartz from Jamestown, CA
Gold is an incompatible element, meaning that it gets concentrated in the more evolved, silica and aluminum rich magmas (called sialic magmas). Because of this, Gold is often found with quartz.
However, when the sialic rocks containing the gold weather away, the gold forms little grains of sand that wash down the rivers toward the sea. As we've learned in previous units, sediments can be deposited where the water slows down.
Because gold is more than 7 times denser than typical rock, it tends to settle more easily that other sand grains of equal size. (Gold bars that are 12 inches long, 5 inches wide, and 3 inches deep—about the size of the gold bars that you see the bad guys stealing in inaccurate movies—would weigh 120 pounds each).
The settling can cause the gold to accumulate at point bars on the inner bends of rivers where the water slows down.
This kind of gold deposit is called a placer deposit.
When the old prospectors panned for gold, they were looking for placer deposits.
Al Ore--Remains of a Tropical Soil
Aluminum is highly insoluble in water, regardless of conditions of acidity or oxygen.
Whereas most ores get concentrated when the element gets dissolved from a large area and precipitated in a small area, aluminum gets concentrated when everything else dissolves away and leaves aluminum oxides and hydroxides behind as the last remaining solid. This occurs in soils of tropical rain forests where there is high rainfall, acidic conditions, and presence of organic molecules called chelating agents that help dissolve otherwise insoluble elements.
The rock that forms from this process is called bauxite, the main ore for aluminum.
The sample on the left side of the picture above is from Guyana--yeah that makes sense, it's tropical.
But the paperweight on the right is made from bauxite found in central North Dakota, brought into North Dakota from Canada by glaciers.
What do you think this says about past climate in northern North America?
Dilithium Crystals--Plot conflict from a TV Franchise
When one considers an imaginary material, it's hard to say how it might have formed, or where it might be found. Star Trek episodes dealing with dilithium are not even consistent with each other--some treat dilithium as an element (which doesn't exist), and others treat it as a rare crystalline compound of lithium (which hasn't been found but might exist somewhere).
Because lithium is not easily made in the hearts of stars where most elements are born, lithium is more rare than other elements of comparable mass. For example, its concentration in the Earth is 100,000 times lower than silicon. Thus, when it's found in significant quantities, it rarely makes up half or even a quarter of the mineral that's present, and it doesn't occur in a form that might be called 'di' lithium--that is, two lithium atoms together in a single unit of the crystal. Most commonly it occurs in minerals in the company of lots of silicon and oxygen (silicates) or minerals with lots of phosphorous and oxygen (phosphates).
Might there be a strange mineral with 'di' lithium that forms on another world? Well, maybe. But there would need to be a way to move lots of Li from a large area and precipitate it in a concentrated small area.
One of the reasons that the concentration of lithium is so low, and why it doesn't form easily in the hearts of stars, is that the nucleus of lithium is less stable (higher energy) than the nuclei of other isotopes of comparable mass. This means that there is more nuclear energy present in lithium than in the elements of comparable mass, like helium or carbon.
More energy in the nucleus means more energy that could be tapped by a nuclear reactor. Who knows? Maybe Lithium will someday be used in fusion reactors to generate energy, and the Star Trek story will become true (sort of).
Last updated June 1, 2015. All text and pictures are the property of Russ Colson.
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