Chapter 3
Chapter 3: Patterns in Nature: Minerals
Feature Articles
The Human Angle: Asbestos Woes
by Stephen Marshak
“Asbestos” is a generic name for a variety of fibrous minerals, meaning minerals that look like clusters of fine threads. Asbestos fibers are strong and nonflammable so they have been used in a variety of products like firefighting suits, building and pipe insulation, ceiling plaster, floor tiles, theater curtains, and brake pads.
Three distinct minerals comprise most asbestos products—chrysotile, amosite, and crocidolite, known as white, brown, and blue, respectively. White (chrysotile) asbestos is the most commonly used variety, occurring in about 95% of asbestos products. It is a member of the large class of minerals collectively known as serpentine. Serpentine deposits are found in remnants of oceanic lithosphere that were shoved up onto the edge of continents during mountain building, such as occurs during collisions.
Environmental concerns about asbestos came to the attention of the public after some highly publicized lawsuits claiming that exposure to asbestos fibers caused mesothelioma, a rare form of cancer that attacks the lining of the chest and abdomen, and asbestosis, a disease in which the lungs become clogged with asbestos fibers, harden, and stop functioning.
Doctors think that asbestosis develops because some asbestos minerals break into 5-to-50-micron-long (1 micron = 10-8 cm) fibers and are inhaled along with other dust; but unlike other dust, these fibers fit snugly into pores in the lung and stay put. The exact process by which asbestos causes cancer isn’t really known. According to one hypothesis, fibers may enter the bloodstream by piercing the lung and entering capillaries. Perhaps the fibers enter cells and somehow interfere with DNA, causing the cells to become malignant.
No one is certain whether all asbestos fibers are dangerous, or if only certain dimensions or compositions of the fibers are. Some studies suggest that white asbestos is less dangerous than brown or blue because its fibers are curly and cannot pierce the lung tissue. In any case, asbestos is most dangerous when it occurs as dust in the air, such as may happen in asbestos factories or mines, or when an asbestos-containing building is renovated. Asbestos that is sealed behind paint, tape, or drywall might not be much of a hazard. In many cases, the dust created by ripping asbestos out of a building may present more of a risk than the intact asbestos.
Because of the possibility that not all asbestos poses an extreme danger, some experts think that the immense effort to remove asbestos from buildings in the United States has actually increased the risk to occupants, because such work may cause asbestos to enter the ventilation system. Current regulations do not rigorously distinguish between the more dangerous and less dangerous types. Some geologists argue that it may be worthwhile to study the issue further before going ahead with universal removal of asbestos.
The Human Angle: Keeping Time with Quartz
by Stephen Marshak
Crystals of quartz have a property known as piezo-electricity. This means that when you apply pressure to a quartz crystal, silicon and oxygen ions shift and cause positive and negative charges to appear on opposite crystal faces. The reverse of this effect also occurs, so that if positive and negative charges are applied, the crystal contracts slightly. Because of this property, a quartz crystal can be made to alternately expand and contract (that is, oscillate) by passing an alternating electrical current (one in which the flow direction of electrons rapidly reverses back and forth) through it. Every crystal has a characteristic frequency, measured in oscillations per second, at which it likes to vibrate. The frequency depends on the size of the crystal, just as the frequency (pitch) of a vibrating bell depends on the size of the bell. When the frequency of the alternating electrical current applied to a crystal comes close to the crystal’s characteristic frequency, the crystal "locks" the electrical current to its frequency. The regularity of this oscillation provides a basis for keeping time. Oscillating quartz crystals thus provide the heart of a quartz watch.
How is a quartz watch made? First, the watchmaker obtains tiny quartz fragments, made by fracturing larger crystals. The crystals are so small that they oscillate at frequencies of between 100,000 and 2.5 million times per second when a current from a small battery passes through them. Devices called "frequency dividers" reduce the vibration to a few beats per second, and these beats either drive gears that then turn the hands of the watch, or control the digital image that appears on the watch face
The Human Angle: Geologists Take A Closer Look at the Holy Crown of France
by Elizabeth Lane Mason
Emeralds are more than just precious gems. Geologists have found that these verdant minerals have an atomic signature that can reveal where on earth they were formed. This information might help historians to sort out ancient trade routes and reveal the murky origins of some famous Old World emeralds. Dealers may be able to use the technique to authenticate the quality of their gems. And, perhaps geologists could discover clues to a long-lost emerald mine.
Emeralds are a type of beryl (Be3Al2(Si6O18)), usually formed as magma cools to become granite. Beryl is typically white or pale green or blue, but if the granite happens to encounter rocks rich in chromium or vanadium as it is cooling, emeralds can form. However, Colombian emeralds, sought after for the intense color and exceptional clarity that sets them apart from most other emeralds, have a peculiar and unique history.
Hundreds of millions of years ago, South American black shales containing chromium and vanadium were washed off the west coast of the continent and collected on the sea floor. Subsequently, the eastward-moving Caribbean Plate collided with Brazilian Plate, forcing the sea floor and its blanket of black shales onto the South American continent. As the plates continued to smash together, folding and faulting provided conduits for hot fluids to rise through the black shales, picking up vanadium, chromium and the other ingredients of emeralds along the way. The mixture collected beneath impermeable shale layers until the pressure became great enough to break the rock apart, squirting the mineral-laden fluid into the cracks where it cooled, crystallizing the dazzling emeralds.
Geologists working to piece together the history of the Colombian emeralds discovered that the minerals have oxygen isotope ratios that are specific to the mine that each mineral came from. Researchers from the Petrographical and Geochemical Research Center in Vandouevre-les-Nancy, France, took this discovery to museums to see if they could use these unique oxygen isotope footprints to uncover the origins of emeralds in ancient artifacts. The results of their work, published in the January 28, 2000 issue of Science, shed light on the origins and history of precious specimens such as a 13th century French crown and the treasure from a sunken Spanish galleon. The scientists are currently working to see if other gems such as rubies and sapphires may have similar oxygen isotope tags.
REFERENCES
Giuliani, G., M. Chaussidon, H-J. Schubnel, et al. 2000. Oxygen isotopes and emerald trade routes since antiquity. Science287: 631-633.