Many important discoveries, inventions, and modern technological devices depend upon principles learned from mineral specimens. According to William D. Callister, Adjunct Professor at the University of Utah, “Historically, much of understanding regarding the atomic and molecular arrangements in solids has resulted from x-ray diffraction investigations; furthermore, x-rays are still very important in developing new materials.”1 Some of the first x-ray diffraction experiments were performed by William Lawrence Bragg (1890-1971). In 1914 Bragg published the earliest information about crystal structures determined by x-ray diffraction.2 Bragg’s work with mineral structures was later used by physicists, chemists, and engineers. Physicists used Bragg’s work in the study of optics (diffraction patterns) and the “light as a wave” model. Chemists studied the atomic structure of materials based on information provided by Bragg. And material scientists and engineers use atomic structure information provided by Bragg in the study of material properties and new materials development.
Despite the historical importance of mineral specimens for the promotion of many sciences, government agencies are eliminating public access to known and new mineral species. Mining operations are closed because they are no longer commercially viable. But many of the mines still have scientific value in the form of mineral specimen deposits. In the past, a mining operation closed, but the mine itself was left accessible. The U.S. Department of the Interior summarizes the collective governmental position on mine closure in its 1998 Utah Abandoned Mine Land Oversight Report. It says, “The primary goal of the national Abandoned Mine Land (AML) program is to mitigate the effects of past mining by reclaiming abandoned mines, with a primary emphasis on correcting the most serious problems endangering public health, safety, general welfare, and property.” 3 Public safety is a reasonable concern. However, government agencies are not simply “protecting” the general public from accidentally stumbling upon an old mining operation. They are bulldozing, cement capping, and rebar gating access to old mines still rich in mineral specimen material. It is arguable that if governmental concern is public safety, the government could do more good (in terms of injuries and deaths per year) by closing ski resorts and other generally accepted hazardous activities.
Amateur mineralogists and mineral collectors regularly collect, preserve, and even discover mineral species. Efforts of these collectors are shared in publications such as Mineralogical Record, www.mindat.org, Rock & Gem, and in rock clubs and rock shops around the country. Allowing government agencies to destroy access to scientifically rich material demonstrates overconfidence in current knowledge, and limits future knowledge. After reviewing significant discoveries made using mineral specimens, scientists and engineers should be alarmed by “abandoned” mine closures and the increasing difficulties in starting a mining operation in the United States.
A major development in atomic theory utilized gold, a mineral. In 1909 Hans Geiger and Ernest Mardsen measured the deflection of alpha particles directed at a sheet of thin gold foil. The lab run by Ernest Rutherford was able to prove that atoms contain a central nucleus. Their work led to the Bohr atomic model. This step was critical to the development of modern atomic models.4
In 1904, Paul Heinrich von Groth (1843-1927) theorized about internal crystal structures. He said, "A crystal consists (1) of a number of interpenetrating point systems, each made up of similar atoms; (2) each point system consists of a number of interpenetrating space lattices made up of similar atoms arranged in a parallel manner; and (3) all space lattices of the resulting structure are congruent, that is, possess the same elementary parallelpiped."5 By creating a crystal structure model, Groth was theorizing about how atoms connect to create solids. In his 1914 work, Bragg confirmed early theories about how atoms form crystal structures and thereby allowed scientist to theorize further about atomic connecting mechanisms and even atomic structure. Scientists and engineers still use Bragg's equation to measure the space between lattice planes in a crystal. Bragg's equation is mλ = 2dsinθ, where m is a positive integer, λ is the wavelength of the x-ray, θ is the angle at which the x-ray strikes the crystal face as measured from the face, and d is the distance between lattice planes.
Bragg's diffraction experiments also proved that light acts like a wave. Throughout Albert Einstein's (1879-1955) life, scientists debated about the nature of light and quantum mechanics. Some scientists claimed that light was made up of photon particles. Others believed light was a wave. These debates wouldn't have been possible without Bragg's proof that light diffracts like a wave. Most scientists today accept both the wave model and the particle model of light. No one has yet created a cohesive model that explains all of light's properties and behaviors.
Minerals have been used to learn about polarized light. Polarized light's wave motion is confined to one plane. Light can be polarized by double refraction, absorption, or reflection. All three types of polarized light can be demonstrated with crystals. William Nicol (1770-1851) invented the first polarizing prism.6 The Nicol prism uses a rhombohedral crystal of calcite, cut at 68°, split diagonally, and then joined again with Canada balsam (a turpentine that is used as a glue and is almost invisible when dried). The prism refracts one plane of light with an index of refraction of n = 1.658, and another plane of light with an index of refraction of 1.486. This prism was once used extensively.
Tourmaline is used to demonstrate polarized light by absorption.7 Tourmaline crystals are anisotropic (they have different material properties in different directions). They absorb light in some directions, and reflect it in others. Polarizing sheets are made with an acetate base using this same principle. Polarized lenses are used in consumer products such as sunglasses and scientific instruments like microscopes.
Light is polarized when it reflects from a smooth, nonmetallic surface. The extent of polarization depends on the angle of reflection and the index of refraction of the reflecting material. The angle at which light with one particular polarization cannot be reflected is called Brewster's angle after Sir David Brewster (1781-1868). While Brewster wasn't the first to discover this phenomenon, he does get credit for independent discovery. He also discovered crystals with two axes of double refraction and made connections between optical structure and crystalline forms.8 Brewster's angle is calculated with Snell's law, n1sinθ1 = n2sinθ2, where n1 is the index of refraction of the material in which the light is coming from, θ1 is the angle from which the light enters the second material measured from the normal to the plane, n2 is the index of refraction of the material being entered, and θ2 is the direction of travel in the second material measured from the normal to the plane. Brewster's Law is calculated as θ1 = θB = arctan(n2/n1).
There are many more examples of how rocks, minerals, and crystals have demonstrated general scientific principles. For instance, the element phosphorous phosphorescences. Phosphorescence is used in many electronic devices, including televisions. It is shortsighted to close access to rocks and minerals which are scientifically valuable natural resources. It may be argued that only trained scientists should maintain access to abandoned mines since it was trained scientists making these example discoveries. There are two arguments against that course of action. First, mineral collectors regularly provide museums with quality mineral specimens. Second, as Adam Smith suggested in The Wealth of Nations, the layman may be in the best position to invent things to improve his condition. For the advancement of scientific knowledge and technology, all levels of government in the United States should allow access to abandoned mines and facilitate the distribution of new mining permits.
Anita Dalrymple
Physics 123, Spring 2007
(modified for Rockpick Legend Co.)
Comments about this article should be directed to rockshop@rocks4u.com.
References
1 Callister, William D., 2007, Materials Science and Engineering, An Introduction, 7th ed., John Wiley & Sons, Inc., New York, NY, p.67.
2 Klein, Cornelis, 1985, Manual of Mineralogy, 20th ed., John Wiley & Sons, Inc., New York, NY, p. 7.
3 1998 Utah Abandoned Mine Land Oversight Report, U S Department of the Interior, INTERNET source available at http://www.osmre.gov/utahaml98.htm, accessed June 9, 2007.
4 Geiger-Mardsen Experiment, Wikipedia, INTERNET source available at http://en.wikipedia.org/wiki/Geiger-Marsden_experiment, accessed June 12, 2007.
5 Kraus, Edward Henry, 1936, Mineralogy: An Introduction to the Study of Minerals and Crystals, 3rd ed., McGraw-Hill Book Company, Inc., New York, NY, p. 146.
6 Nicol Prism, Wikipedia, INTERNET source available at http://en.wikipedia.org/wiki/Nicol_prism, accessed June 12, 2007.
7 Klein, Cornelis, 1985, Manual of Mineralogy, 20th ed., John Wiley & Sons, Inc., New York, NY, p. 236.
8 David Brewster, Wikipedia, INTERNET source available at http://en.wikipedia.org/wiki/David_Brewster, accessed June 12, 2007.
No comments:
Post a Comment