METEORITE OR METEORWRONG?density & specific gravityDensityDensity is the term for how heavy an object is for its size. Density is usually expressed in units like grams per cubic centimeter (g/cc or g/cm3), kilograms per cubic meter, pounds per cubic inch (cubic foot or cubic yard), or pounds per gallon. Rocks vary considerably in density, so the density of a rock is often a good identification tool and useful for distinguishing terrestrial (Earth) rocks from meteorites. Iron meteorites are very dense, 7-8 g/cm3. Most meteorites are ordinary chondrites, and ordinary chondrites have a density half as much. Most ordinary chondrites are in the range 3.0 to 3.7 g/cm3, which is denser than most terrestrial rocks. For example, limestone (2.6 g/cm3 or less), quartzite (2.7 g/cm3), and granite (2.7-2.8 g/cm3) are all common low-density rocks. Some meteorites have low densities (<3.0 g/cm3), but such meteorites are rare among meteorites. The density of basalt, one of the most common kinds of terrestrial volcanic rocks, can be as high as 3.0 g/cm3. The only types of terrestrial rocks that are more dense than meteorites are ores - oxides and sulfides of metals like iron, zinc, and lead. For example, rocks composed of hematite or magnetite (iron oxides) are often mistaken for meteorites (see concretions). Such rocks have high densities, 4.5-5g/cm3, which is greater than that of any kind of stony meteorite.
Specific GravityIn order to measure density, it is necessary to measure the volume of a rock. That's hard to do accurately. Just as useful as density, however, is the specific gravity. Specific gravity is the ratio of the mass (weight) of a rock to the mass of the same volume of water. Water has a density of 1.0 g/cm3, so the numeric value of specific gravity for a rock is the same as that for density. Because specific gravity is a ratio, it has no unit. Specific gravity is easier to measure than density. In order to measure specific gravity you need a balance or scale with a hook on the bottom. The technique is described in most high school physics books and most high schools (general science and physics labs) would have a single-beam or triple-beam balance that could be used for measuring specific gravity. It may be difficult to obtain an accurate measure for a small rock, e.g., <10 grams.
References: Britt D. T. and Consolmagno G. J. (2003) Stony meteorite porosities and densities: A review of the data through 2001. Meteoritics and Planetary Science, volume 38, number 8, pages 1161–1180. Grady M. M. (2000) Catalogue of Meteorites, With special reference to those represented in the collection of the Natural History Museum, Fifth Edition, Cambridge University Press, Cambridge, 689 pages and CD-ROM. Warren P. H. (2001) Porosities of lunar meteorites: Strength, porosity, and petrologic screening during the meteorite delivery process. Journal of Geophysical Research - Planets, volume 106, number E5, pages 10,101–10,111. Wilkison S. L., McCoy T. J., McCamant J. E., Robinson M. S., and Britt D. T. (2003) Porosity and density of ordinary chondrites: Clues to the formation of friable and porous ordinary chondrites. Meteoritics and Planetary Science, volume 38, number 10, pages 1533–1546.
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