December 26, 2010
I have often thought to myself that I would have surely studied geology had not I first fallen in love with chemistry. It is in geology that chemistry, perhaps, is at work in the most profound sense. Over the years, I have slowly been collecting rocks when traveling through the American West. I’ll share with you some of the rocks I have gathered and do my best to identify them with my very poor knowledge of geology. Please note that my collection is quite modest; most of these samples were collected at sites not for from the sides of roads.
Here is a rather large piece of quartzite collected near Mt. Wheeler, the highest mountain in Nevada. Quartzite is a metamorphic rock that was originally formed from the quartz in sandstone. The red patch of weathering on the top is probably due to iron oxides.
Here is some pink granite collected near Red Rock Canyon in California. Granite is an intrusive igneous rock which means it cooled slowly from magma or lava. It is also felsic which denotes that it has a high percentage of minerals such as quartz and feldspars which contain mostly silicon, oxygen, aluminum, and magnesium. The pink color is mostly due to potassium feldspar, which is idealized by the formula KAlSi3O8.
Below are two extrusive igneous rocks found off of Green Valley Road near Baker, California. While cooling rapidly from their magma mother liquor, these rocks formed around gas bubbles in the solution. The rock on the right is pumice, and I am guessing the one on the left is vesicular basalt.
This an extremely beautiful mineral composed almost entirely of calcium carbonate. It was probably formed slowly from layers of calcite deposited underwater. It is similar to the tufa formations of the Trona Pinnacles in Death Valley.
Here is a piece of shale from Southern Utah. This is what makes that part of the state so beautiful and Canyonlands National Park one of my favorite places on Earth. When it rains in this rock country, the rivers run red with hematite.
These four specimens are evaporates collected near Badwater, California. Evaporites are water soluble minerals that form after a body of water evaporates. The ones here are mostly sodium chloride atop some mud. As a result, they taste quite yummy.
This rock found near the Grand Canyon is probably my favorite I have collected so far. It is largely comprised of the green mineral malachite, Cu2CO3(OH)2, and the blue mineral azurite, Cu3(OH)(CO3)2. Although the crystals are small, possessing a copper ore is quite neat. Before Grand Canyon was turned into a national park, rocks like this one were mined for copper.
I need help identifying the main mineral in this rock. I found it near Zabriski Point, California. I am guessing it is a mica since it is clear, shiny, fragile, and breaks into long lateral sheets. Any help identifying this sample would be appreciated.
August 21, 2010
After their discovery in the nineteenth century, the rare earth elements (REEs) have become an increasingly significant economic resource. They have many applications in technologically advanced devices such as catalytic converters, petroleum-cracking catalysts, batteries, lasers, television phosphors, and strong permanent magnets (used in computers, superconductors, and speakers, among other things).1 Since over 90% of global REE production comes from China, the exploration for, and exploitation of, REE ore deposits has many political consequences as well.2
The REEs occupy a large swath of the periodic table, and despite their increasing importance, they have remained largely obscure to the general public. In a geochemical context, the term "rare earths" refers to the elements yttrium and the fifteen lanthanides.3 The term "earth" is an old name borrowed from Plato’s four elements by early chemists that refers to insoluble oxides that could not be smelted.4 They were called "rare" because their oxides were not encountered as frequently as the alkaline earth oxides, which produce basic solutions in water. However, their description as "rare" is a bit of a misnomer since most REEs are comparable in crustal abundance to elements such as arsenic, tin, and antimony, elements that are not usually considered rare. Even the rarest REE, lutetium, is 10-100 times more abundant than rare elements like mercury, platinum, and gold.5 The oscillation of the total abundances of the REEs in the Earth’s crust (figure 1) is a reflection of the Oddo-Harkins rule, which states that elements with even atomic numbers should be more stable than those with odd atomic numbers on the basis of nuclear stability. Elements with an odd number of protons have one spin-unpaired proton and are likely to capture another proton in nucleosynthetic pathways.6 
Figure 1: Estimated crustal abundances of the REEs.5 A value for promethium is not given since it has no stable isotopes and is virtually nonexistent in the Earth’s crust.
The identification and isolation of the REEs posed a particularly difficult problem for nineteenth century chemists. William Crookes, inventor of the Crookes tube and discoverer of the element thallium, lamented that "the rare earths perplex us in our researches, baffle us in our speculations, and haunt us in our very dreams. They stretch like an unknown sea before us, mocking, mystifying, and murmuring strange revelations and possibilities."7 Cerium and yttrium were the first elements to be discovered in the early 1800’s because they could be isolated from REE minerals in relatively large quantities. In the 1850’s, most of the less abundant REEs were first identified by their unique spectral lines shortly after the invention of optical flame spectroscopy.8 However, the isolation of the individual REEs as pure metals was not accomplished until the 1950’s.9 The element promethium, which has no stable isotopes and does not occur naturally, was also discovered around this time in the by-products of nuclear fission. The troubles that early chemists encountered in distinguishing between the REEs were due to the striking similarities in the chemical reactivity of the REEs. The REEs have similar electronic configurations and generally, only differ in the filling of their 4f shell. In their oxidized states, all of the lanthanides have empty outer 6s and 5d orbitals. The similarity in their chemistry is a direct result of the fact that the change in their electronic configuration is restricted only to the inner 4f orbital.10
All of the REEs form stable trivalent cations (+3) upon removal of two 6s electrons and one 4f (or 5d) electron. In natural environments, europium and cerium are the only redox active REEs. However, divalent samarium, thulium, ytterbium, and tetravalent praseodymium and terbium compounds can be prepared synthetically.11 Under geologic conditions, cerium can be further oxidized to Ce+4, which results in a stable noble gas electron configuration. Ce+4 is a powerful oxidizing agent that is important mainly in aqueous geochemistry.5 Europium(III), on the other hand, can be reduced to form Eu+2 because the resulting [Xe]4f7 electron configuration has a stable half-filled shell. Rocks are often found to have concentrations of cerium and europium that deviate from the trends found in average chondrite meteorites. If aqueous Ce+3 is oxidized to Ce+4, it will likely precipitate out as insoluble CeO2. Thus, in sea water, the concentration of cerium is usually depleted compared to the other REEs.12 The relative concentration of cerium will be greater, however, in a mineral like zircon where Ce+4 (ionic radius of 0.94 Å) can substitute for Zr+4 (0.84 Å).13 While cerium anomalies can sometimes give information about the ways in which surface water interacts with sediments, europium anomalies are, in general, more common and are especially usefully in gleaning about igneous processes.
The six-coordinate ionic radii of the trivalent REE cations range from 0.86 Å (lutetium) to 1.03 Å (lanthanum). The smooth decrease in ionic radius of the lanthanides with increasing atomic number due to increased effective nuclear charge is known as the lanthanide contraction. As a result of their ionic size, REE concentrations are, for example, typically higher in granites than in basalts. This is because the REEs are incompatible with the major elements of mafic minerals like Mg+2 and Fe+2, whose ionic radii are too small. Europium, however, shows the opposite trend and usually has a higher concentration, a so-called positive anomaly, in basalt than in granite because in the reduced form, it can readily substitute for Ca+2. Therefore, a large share of the europium present crystallizes along with the calcium-containing plagioclases in basalt magmas. Due to the lanthanide contraction, the lighter REEs (lanthanum through samarium) have greater ionic sizes and thus are less compatible with Mg+2 and Fe+2 than the heavier REEs (europium through lutetium). As a result, the tendency for REEs to be more concentrated in felsic rocks as compared to mafic rocks is more pronounced for lighter REEs than for heavier REEs.14
One area of geology in which REE distributions have been extensively utilized is in the determination of the history of the Moon. The fact that the crust of the Moon is relatively enriched in REEs supports the hypothesis that the Moon was created from the accretion of debris that formed after a large body collided with the Earth.15 As a result of the great amount of energy released from such an impact, a large portion of the Moon would have become molten. Upon cooling, minerals with high melting points such as olivine and pyroxene would have crystallized out of the lunar magma ocean first and sunk to the mantle. Basalt and anorthositic plagioclase, the two major components of the lunar crust, would have then crystallized last. The basalt is the main component of the dark-colored lunar maria while the lighter highlands of the Moon are composed mostly of anorthosite. Both of these crustal rocks are heavily enriched in REE’s compared to the Moon’s interior. The lunar maria contain high concentrations of incompatible elements that do not crystallize readily. This material is known as KREEP, which contains the elements in its acronym- potassium, the REEs, and phosphorous- but also significant quantities of additional incompatible elements such as uranium and thorium. The radioactivity of these latter two elements is thought to supply the heat that drove the ancient long-lived volcanism that formed the basaltic "seas" of the lunar maria. The KREEP material shows a negative europium anomaly while the anorthosite-containing highlands show a positive europium anomaly because Eu+2 can readily substitute for the Ca+2 in anorthosite.
Not only do the concentrations of REEs provide evidence for the existence of an ancient lunar magma ocean, they also have been used to date the formation of the magma at 70 million years after the formation of the solar system.16 This calculation is done through the use of 147samarium-143neodymium dating. Because the chemical behavior of these two REE’s are so similar, they are not likely to be differentiated to any significant degree over the history of the rock. Thus, where other isotopic dating techniques may be influenced by weathering and other alterations, the Sm-Nd technique can give an accurate age for the initial crystallization of many rocks.14 138Lanthanum-138cerium and 176lutetium-176hafnium are two other REE isotope pairs that are often used in geochronology studies.17The same principles of the partitioning of REEs on the Moon, of course, can be applied equally well to igneous processes on Earth. Due to their chemical stability, in general, geochemical processes have only separated the REEs into the two broad categories of the light and heavy REEs. As on the Moon, the smaller-sized heavy lanthanides and Y+3 are more soluble in the mafic materials of the mantle and so are depleted relative to the lighter REEs in the crust. In fact, in most REE ores, the four lightest REEs, lanthanum, cerium, praseodymium, and neodymium make up more than 80% of the REEs present.5
The majority of REEs mined come from pegmatites, very coarse-grained igneous rocks, or carbonatites, rocks with high percentages of carbonate minerals. The most economically important REE minerals are bastnäsite ((Ce, La)CO3F), monazite ((Ce,La)PO4), and xenotime (YPO4). Bastnäsite and monazite are both light REE ores while xenotime is the most prevalent heavy REE ore.18 The heavy REEs frequently substitute for Zr+4, and as such are often found in significant quantities in zirconium minerals such as zircon and eudialyte. In REE minerals, the lattice structure often dictates whether the crystal will accommodate light or heavy REEs preferentially. For instance, monazite and xenotime are both phosphate minerals, but monazite has a monoclinic crystal system that more readily accommodates the larger-sized lighter REEs than does xenotime’s tetragonal crystal system. In terms of crystal structure, wakefieldite ((Ce,La)VO4) is the vanadate analog of xenotime, but contains the light REEs. When the phosphate anion is replaced by the larger vanadate anion, the tetragonal crystal system preferentially accommodates the larger light REEs.19
World production of rare earth oxides is growing rapidly, but at approximately 100,000 metric tons per year its production is small in comparison with more common metal oxides.2 (The current global output of copper is about 15,000,000 metric tons per year.)20 In the 1950’s and 1960’s, large-scale REE production began, and most of the rare earths came from monazite sands in Brazil, India, and South Africa. During this time, however, a large bastnäsite intrusion was discovered in Mountain Pass, California in the Mojave Desert, and the United States dominated the rare earth market for the next forty years with this light REE ore deposit (figure 2, left). Today, China produces almost 90% of the world’s REEs and contains approximately 40% of the world’s estimated reserves.18 The mine in Mountain Pass cannot compete with low Chinese prices and has shut down. In less than a decade, the United States went from being entirely self-sufficient in terms of REE production to now importing almost all of its REEs from China. According to the USGS, "the United States is in danger of losing its longstanding leadership in many areas of REE technology…because of [its] dependence on imports from China."2 The main Chinese REE source is the Bayan Obo deposit in Inner Mongolia. This deposit is the world’s largest known REE reserve and is dominated by the light REE minerals bastnäsite and monazite.18 
Figure 2: Relative concentrations of REEs in the light REE-dominated bastnäsite ore in Mountain Pass, California and the heavy REE-dominated lateritic clay ore in southern China.2
China’s dominance of the REE market is particularly strong because it also has large reserves of heavy REEs (figure 2, right). The less-abundant heavy REEs are becoming increasingly important in many burgeoning high technology industries. For example, erbium-doped fiber amplifiers are now an essential component of fiber optics, and dysprosium is used in the magnets of hybrid electric vehicles.21 China’s heavy REEs come primarily from lateritic clay ores in Southeastern China. Laterites are surface deposits that have undergone extensive weathering due to a hot and wet tropical climate. Weathering leaches out the soluble alkali and alkali-earth metals, leaving aluminum rich clays and minerals like kaolinite, gibbsite, and bauxite behind since aluminum is insoluble in normal pH ranges. REEs are also generally insoluble and so are often concentrated in aluminum clays. If the REE concentrations become high enough, REE-dominated minerals such as lanthanite ((La,Ce)2(CO3)3•8(H2O)) and florencite ((La,Ce)Al3(PO4)2(OH)6) can form.22 Mafic rocks, which have higher concentrations of the heavy REEs, are more easily weathered than felsic rocks. In addition, the heavier lanthanides tend to hydrolyze with greater ease than the lighter ones, and as a result, they can often become transported to clay ores where they are trapped by adsorbing onto porous clay surfaces.23 Both of these phenomena explain why laterites often have unusually high concentrations of heavy REEs. Another major advantage of the Chinese REE lateritic clay ores over the traditional ores is that they have very low concentrations of thorium and uranium. These elements are often abundant in the monazite and bastnäsite ores, and their radioactivity makes processing more difficult and expensive.18The increasing demand for REEs has led geologists to search for viable REE deposits outside of China. One of these deposits that has just recently begun to be exploited is the Ilimaussaq intrusive complex on the southwestern coast of Greenland. This complex boasts an odd assortment of igneous minerals that contain over fifty elements including considerable amounts of the valuable heavy REEs.24 These reserves are potentially so economically important that their development may provide the extra impetus that Greenland needs in order to gain sovereignty from Denmark. The rocks at Ilimaussaq are peralkaline, which means that they have a large concentration of alkali metals and low concentrations of aluminum. They also tend to have high amounts of the incompatible elements such as zirconium and the REEs, which are found particularly in the mineral eudialyte.25 It has been shown experimentally that melts with typical concentrations of alkali metals can hold no more than 100 ppm zirconium. However, the solubility of zirconium and by extension the REEs in peralkaline melts can exceed 1% by weight. This increased solubility has been attributed to the formation of bridged alkali-silicate and alkali-fluoride complexes that can accommodate typically incompatible elements.26 Despite the extraordinary solubility of REEs in peralkaline liquids, the high concentrations of REEs found in the Ilimaussaq complex cannot be explained by magmatic processes alone. Thus the REE ores in Greenland must have been formed through both magmatic and hydrothermal processes, but the relative importance of the two is still a major unanswered question.27
Once a REE deposit is located, the ore is processed and converted to REE metals through a series of carefully designed chemical reactions.8 Typically, the raw ore is first boiled in a strong solution of sodium hydroxide. The insoluble material is then filtered off and contains mostly rare earth, thorium, uranium, and zirconium hydroxides, oxides, and silicates. Due to their lower electronegativity, the rare earths are more basic than thorium, zirconium, and uranium. To take advantage of this property, the residue is reacted with hydrochloric acid, and the more basic rare earths convert into soluble chloride salts while the other elements remain unreacted. Radium, a decay product of radioactive thorium, is then removed by adding barium sulfate to the mixture since radium sulfate is more insoluble than barium sulfate. The resulting lanthanide chlorides are then dried in an atmosphere of hydrogen chloride gas and then electrolyzed in the molten state to give mischmetal, a mixture of rare earth metals that is often used as flint (this was one of the first practical application of the REEs). More complicated procedures that take advantage of small differences in the solubilities of the rare earths can then be used if the individual REEs must be separated, but these processes are expensive. Only about ten metric tons of lutetium metal, for example, are produced each year globally, and it costs about six times as much as gold, making it the rarest and most expensive stable metal on Earth.9
Figure 3: The experimental mineral/melt partition coefficients for REEs in garnet5 (red line) are representative of the smooth progression of the geochemical behavior of the REEs as a function of atomic number. The theoretical sold-gas solar nebula partition coefficients for REEs in a hypothetical mineral (blue line) show no such smooth correlation.28 Both plots have been normalized to the La values.
For the most part, the geochemical properties of the REEs vary as a smooth function of atomic number (figure 3, red line) and their behavior, as described above, has important ramifications for where REEs are found, how they are extracted, and how they differentiate in igneous processes on both the Earth and Moon. The cosmochemistry of the REEs, however, differs dramatically from their geochemistry. When discussing the cosmochemical properties of the REEs, the volatilities of REEs must be considered, and these do no correlate in any predictable manner with ionic size. In fact, under the conditions thought to exist at the time of the formation of the solar system, the relative REE volatilities have been shown to vary by a factor of 108 (figure 3, blue line) when calculated using the equilibrium constants for the two reactions below describing the condensation of REEs from nebula gas28:
2M(g) + H2O(g) = 2MO(g) + H2(g)
2MO(g) + H2O(g) = M2O3(s) + H2(g)
where M represents a REE. The variability of REE volatilities stems from differences in the electronic structures of the elements in the gas phase. In the solid and liquid phases, the 4f, 5d, and 6s orbitals are all very close in energy, but in the gas phase, these differences are exaggerated so subtle changes in REE electron configurations become important. As gases, the REEs can exist as monatomic species (M(g)), divalent monoxides (MO(g)), or tetravalent dioxides, and the preferred state depends on the specific REE. However, when the REEs condense to form solids, they form trivalent sesquioxides (M2O3(s)). Thus in geochemical fractionations which involve liquid to solid transitions, the oxidation states of REEs remains constant at +3, but in cosmochemical fractionations which involve gas to solid transitions, the oxidation states of REEs changes, resulting in an irregular distribution of REEs.
Many calcium-aluminum inclusions in meteorites have been discovered to show an irregular fractioning of REEs that matches closely with calculated condensation parameters. In this way, the REE distribution patterns can unambiguously discriminate between those inclusions in a meteorite that were formed from nebular condensation processes and those that were formed from planetary igneous processes. As a result, REEs provide much of the strongest evidence for what is hypothesized about the conditions of the early solar system.
Since their discovery in the nineteenth century, the REEs, despite their relative obscurity, have been increasingly used in a wide range of fields and applications. REEs give invaluable information to both the geochemist and the cosmochemist, although their chemical behaviors differ drastically in these two contexts. The strikingly similar geochemical properties of the REEs are directly related their similar electronic configurations. The REEs are lithophile elements and as such, concentrate in the crusts of both the Earth and the Moon. All of the REEs form stable trivalent cations, but europium and cerium are the only REEs that are redox active under typical conditions. Cerium anomalies are important in aqueous systems while europium anomalies often help explain igneous processes. Understanding REE geochemistry is becoming progressively more important given the recent surge in demand for REEs. China currently dominates the REE market with the majority of the REE production and possesses versatile light and heavy REE reserves. Production from a large and unique REE ore deposit in southwestern Greenland, however, may initiate the demise of the Chinese REE monopoly. The level of REE production in the recent future will be particularly significant and may dictate the success of the burgeoning fiber optic, rechargeable battery, and semiconductor industries.
References
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