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Energy & Resources
Heavy-metal sponge
Remotely sensing rock types
Mineral of the Month: Indium


Heavy-metal sponge

In the late 1800s, the copper mining industry in Butte and Anaconda, Mont., left large piles of waste riddled with arsenic, lead and other heavy metals. Now the largest Superfund site in the United States, with more than 300 square miles of contaminated soil and water, Anaconda may also hold what could be a promising solution for cleaning up similar areas: a newly discovered mineral that soaks up the harmful pollutants.

A University of Montana graduate student stands above Silver Bow Creek, Mont. The highly acidic banks consist of 2-meter-thick tailing deposits — waste from the mining industry — that contain high concentrations of metals and arsenic. Courtesy of John Moore.


At the meeting of the Geological Society of America in Denver, Colo., in November, Virginia Tech professor Michael Hochella described a new mineral found in Anaconda — manganese oxide hydrate — that absorbs heavy metals, including zinc, copper and lead, in a fast reaction. Sending a sample of polluted groundwater through a sediment sample containing the new mineral, Hochella says, results in clean water. Toxic metals "ignore" everything but iron and manganese oxides, and the contaminant finds a spot within the mineral structure of the manganese oxide. "It's like pulling iron filings out of a pile with a magnet," Hochella says.

Compared to other chemical compounds that react with heavy metals, manganese oxides "are much more reactive because they have internal sites that can take up heavy metals," Hochella says. The compound's chemical structure is similar to clay minerals' sheet-like structure, he says.

Injecting an area such as Anaconda with the manganese oxide compound could create a reactive barrier to prevent the spreading of the contamination. "The reaction is very rapid — on the order of minutes," Hochella says.

Hochella and his colleagues discovered the mineral in samples taken from the Anaconda Superfund site, using transmission electron microscope (TEM) imaging. This differs from scanning electron microscopes in that the electron beam, which is of much higher energy in TEM, passes directly through the sample. "TEM allows chemistry to be done on much smaller particles," Hochella says.

Although researchers have yet to field-test the application of the new cleanup method, John Moore of the University of Montana says that understanding how manganese oxide hydrate reacts with heavy metals is a step in the right direction for cleaning contaminated mine sites. "The point is, we have to make sure what we're doing is really going to work," Moore says, noting that the method has promise. "It took over 100 years to create this site. You can't just dig it up and take it somewhere; you have to manage a site in situ."

Laura Stafford

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Remotely sensing rock types

One of the routine parts of the mining process involves a geologist going belowground to assess rock walls where rich veins of minerals may occur. As any underground activity is inherently dangerous, some researchers are working on technologies to automate the mapping process — already successfully testing a new remote sensing and infrared technology.

Working with mining companies and using thermal infrared reflectance spectroscopy, researcher Benoit Rivard of the University of Alberta in Edmonton and colleagues tested 37 samples of 10 different rock types — some containing mineable amounts of sulfides (the minerals being mined in the area) and some containing nominal amounts of sulfide — from eight different mines in the Sudbury Basin in Ontario. The researchers found that overall, they were able to correctly classify the rock type and mineral load nearly 80 percent of the time, as reported in the November GSA Bulletin.

The long-term goal, Rivard says, is to put instruments in mines to log the rock types and identify rich mineral seams to help guide the mining process. Right now, he says, mining companies frequently employ one geologist to log cores from multiple mines. "If we can establish an automated approach using instruments that systematically obtain objective results," Rivard says, it can improve efficiency, save money, remove humans from a risky underground environment and remove some of the subjectivity that comes with human analysis of a rock core.

The approximately 80-percent accuracy rate that the researchers obtained for their rock-type classification "is a very good figure," says Alvaro Crósta, a geologist with the University of Campinas in Brazil. Their approach represents a novel use of the technology and should offer a "major advantage for operational applications in an underground mining environment," he says.

The biggest advantage the thermal infrared reflectance spectroscopy offers over other visible infrared spectroscopy, Crósta says, is that the presence of water on the rocks — frequently found in underground environments — does not interfere with the rock-type analysis.

The researchers have not, however, been able to work around the presence of dust or drilling lubricant yet, Rivard says. "But we're working on the technology and instrument development, and we will get there in the next few years," he says. So far, the researchers have focused on hard-rock mines such as those in Sudbury, but they have recently begun working with some companies mining tar sands to see how the technology might apply there.

Megan Sever

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Mineral of the Month: Chromium

John F. Papp, the U.S. Geological Survey chromium commodity specialist, has compiled the following information on chromium, an important metal essential to health and property.

Chromium is one of the most indispensable industrial metals and it plays an essential but hidden role in daily life. Chromium is used in many consumer and building products, and it contributes to a clean, efficient and healthy environment.

As a trace mineral, chromium is also vital for good health. Insufficient amounts result in glucose intolerance. Organ meats, mushrooms, wheat germ and broccoli are all good dietary sources of chromium.

Chromium is also critical in the manufacturing of stainless steel. Chromium makes stainless steel "stainless" by providing a protective coating, which prevents rust and is easily sterilized. It is popular in kitchen fixtures such as countertops and sinks, and also in flatware, cooking ware and utensils. In factories, food-processing equipment parts that come in contact with food are composed of stainless steel. And because of its hygienic properties, medical and dental tools and equipment are also made of stainless steel.

Another one of chromium's critical uses is in transportation. In automobiles, outside of chrome decorations such as ornaments, trim and hubcaps, a more important use of chromium is in the engineering alloys, which are useful in applications where temperatures are high. Exhaust pipes are commonly made of stainless steel, and the catalytic converter, used in most parts of the world to reduce exhaust emissions, is housed in stainless steel. Buses and passenger trains also use stainless steel to reduce vehicle weight and maintenance costs. Chromium in superalloys permits jet engines to operate in a high-temperature, high-stress, chemically oxidizing environment.

On U.S. roadways, chromium pigments are used to make the yellow lines indicating traffic lanes. Chromium-containing pigments also find their way into beauty products.

Fortunately, chromium is in abundant supply, produced from the mineral and ore chromite. In nature, the deposits are of two major geologic types: stratiform and pod-shaped (podiform).

Stratiform deposits occur in South Africa and India, with one of the largest found in the Bushveld Complex of South Africa, a layered intrusion containing more than 11 billion metric tons of chromite resources. One layer alone, informally named the Steelport Seam, contains 1.5 billion metric tons of chromite resources. Podiform deposits are found in Kazakhstan where parts of the ocean floor were pushed over continental rocks by tectonic forces. Together, South Africa, Kazakhstan and India (in order, by tonnage produced) accounted for 79 percent of global production of chromite in 2003.

Since 1965, there has not been chromite ore production in the United States. Small, marginally economic resources are available and could be exploited in the event of a national emergency. U.S. import dependence is only about 75 percent because substantial amounts of chromium are recycled as part of the incorporation of stainless steel scrap in the stainless steel production process, reducing energy consumption and conserving natural resources. The United States produces about 2 million metric tons of stainless steel per year that contains, on average, about 17 percent chromium.

For more information about chromium, visit minerals.usgs.gov/minerals.

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