In order for the reader to gain the most from this book, we must introduce a few necessary concepts, research tools and methods, and a little about minerals and terminology. Words that appear in bold type, with the exceptions of major heading and acronyms, can be found in a glossary below in the menu.
Up to 1960, about 1500 meteorites had been recovered worldwide as falls (those seen to fall to Earth) and finds. They were divided into 3 major categories: stony, stony-irons, and iron meteorites. Stony meteorites were divided into chondrites and achondrites; chondrites contain chondrules that are spheroids of melted rock whereas achondrites do not. Further divisions were made into classes and groups. Today, there are more than 30,000 meteorites classified into as many as 65 classes, groups and grouplets. This huge increase in the number of meteorites can be attributed to:
Meteorites have become somewhat valuable, so greed has also been a big factor in desert recoveries that are mostly done without regard to scientific recovery methods. Fortunately for science, those meteorites recovered in Antarctica are collected by nationally sponsored expeditions (USA and Japan) and are protected as national property.
Meteorites of different classes plot in distinct fields on an oxygen isotope diagram.
Concurrent with an increase in meteorite recoveries was a dramatic increase in technology and analytical instrument development. Meteoriticists (scientists who work on meteorites) continue to make deep inroads into the mysteries of meteorites and to classify meteorites on the basis of many kinds of chemical information. After visual examination and possibly bulk chemical composition, the next best constraining parameter is knowledge of a meteorite's oxygen isotope content. Oxygen from any solar system location consists of three stable isotopes 16O, 17O, and 18O (the superscripts are the mass numbers of the isotope, equal to the sum of its protons and neutrons); 16O is by far the most abundant isotope. However, there are very different abundances in different source example, the isotopic compositions of most major meteorite classes, the Earth, Moon, and Mars, all are distinctly different from one another.
Similar discriminating diagrams can be constructed for various elements and their isotopes and for trace element contents, These low abundance elements occur in parts per million (ppm) to parts per billion (ppb) amounts. Additional analytical results can give us radiogenic ages (e. g., rubidium to strontium or 87Rb to 87Sr), temperatures of formation, and thermal histories since formation, among much other important information.
Henry Clifton Sorby. Source: N. Higham (1963) A Very Scientific Gentleman - The Major Achievements of Henry Clifton Sorby. Oxford: Pergamon Press. 160 pp.
As we mentioned earlier, the petrographic microscope is a wonderful analytical tool and has been around for 140 years. An innovative, wealthy English gentleman by the name of Henry Sorby (above), invented this microscope and spent much of his leisure time, of which he apparently had an abundance, looking at paper-thin slices of mostly chondritic meteorites glued to glass slides (rock thin sections). Naturally, he is also known as the "Father of Petrography," the study of rocks in thin section. A short biography of this innovative scientist can be found at the The Sorby Natural History Society site.
Because many of the photographs shown in this web site are photomicrographs, taken with a polarizing microscope, we should briefly discuss how it works. The fundamental components are a white light source (lamp), a lower polarizer, a rotating stage to hold the rock thin section, a lens, an upper polarizer and a viewing eyepiece. As an analogy, the polarizers work like Polaroid sunglasses. Light actually vibrates in all directions perpendicular to its pathway. Light passing through the Polaroid lens is allowed to vibrate in only one direction (is plane-polarized), which reduces the sunlight glare. If you take the lenses out of the frames and place one lens over the other and rotate one lens 90°, no light will pass through and everything is dark. This is because the plane-polarized light from the first lens is blocked out by the polarizing direction of the second lens, which is offset by 90°.
Principle of petrographic microscope cross-polarized light. Source: http://epswww.unm.edu/facstaff/selver/EPS%20303/optics.html.
In the case of the microscope, plane polarized light from the first lens is passed through the crystals in a very thin section of rock (25-30 micrometers). As it does so, the plane-polarized light is twisted a bit and breaks up into two directional components before it enters the upper polarizer. One of the directional components is slower than the other one and they become "out of phase" when they are resolved in the upper polarizer. This difference in velocity is called retardation and results in the production of colors as shown in the photomicrographs labeled "crossed polarized light or XPL." In these photographs, some areas appear black, which is due to either the presence of an opaque mineral (light will not pass through) or glass (plane polarized light will not function properly because glass lacks crystal structure) or, light is completely blocked out. This is because critical elements of the crystal structure are oriented in an unfavorable position to the polarizers (remember the polarizing sunglasses). There are many other useful petrographic methods, although we probably have explored enough optical physics for a while.