Meteorites

Meteorites Meteorites are classified into three main categories: stones, stony-irons and irons, depending on their dominant composition. Stones are similar to common terrestrial rocks in that their mineral composition is dominated by silicates, by far the most prevalent rock forming minerals on our planet. Irons are mostly metallic in composition; they consist of alloys of iron (Fe) and nickel (Ni), in varying proportions. Stony-irons are combinations of both; they contain silicate and metallic phases in approximately equal amounts. Stones are subdivided into two classes: chondrites and achondrites. Chondrites get their name from the fact that they all (with some exceptions) contain chondrules, tiny mineral spherules made mostly of silicates. Although some may be as large as a few millimeters in diameter, most chondrules are less than 1mm across. In chondrites, chondrules are bound within a consolidated and fine-grained background matrix. Chondrites are the most primitive meteorites known. That is, they are the most ancient ones in terms of when their constituents came together to form a rock, and the most unprocessed ones in terms of how little their materials have been altered since this rock formed. Achondrites, on the other hand, lack chondrules and represent more processed materials. Earth's surface rocks would be anchondrites were they meteorites; they lack chondrules and are the result of extensive geological processing (melting, for instance) Stone Meteorites Stone meteorites are by far the most common making up nearly 94% of all that fall. Stone meteorites are generally believed to be material from the mantle and crustal areas (surface) of an asteroid. 1) Chondrites: Chondrites make up 86.1% of all that fall. They are composed of small round crystals of olivine, pyroxene and plagioclase called "chondrules" packed together with varying amounts of metal as small flakes. These chondrules have been partly destroyed through impact and heating in most meteorites. The number associated with a chondrite's classification indicates how much the meteorite has been altered. The lower the number, the less altered the meteorite is since its formation. Higher numbers indicate the meteorite has seen more metamorphism - generally fewer chondrules are visible H group: Olivine-bronzite chondrites. Higher in iron, generally show more visible iron flakes than other stone meteorite classes, bronze is the predominant pyroxene mineral in these meteorites. L group: Olivine-hypersthene chondrites. Lower in iron, less visible metal than the H group stones, hyper sthene is the primary pyroxene mineral. LL group: Amphoterites. Very little iron visible and low iron in the minerals. These meteorites are generally more highly brecciated (composed of fragmented rock) than other meteorites. Enstatite chondrites: Composed of enstatite (iron free pyroxene) as the silicate. All of this meteorite's iron is visible as free metal. Carbonaceous chondrites: This rare group of meteorites contain many unusual features such as organic compounds (including amino acids - the building blocks of life), interstellar materials, microscopic diamonds, and inclusions of material formed before and from out side of our solar system (the only such material we have). Carbonaceous chondrites are generally gray to black in color and show well defined chondrules. Carbonaceous chondrites are probably the most studied group of all meteorites. 2) Achondrites: These rare meteorites make up a mere .7% of our collections but compose 7.8% of all that fall. They do not have the chondrules characteristic of chondrites, nor do they show the tell tale flakes of nickel-iron. These meteorites tend to match certain earth rocks so they generally have to be seen to fall or still show some of the fusion crust (the burnt exterior formed by its flight through the atmosphere) to be identified as a meteorite. Most achondrites are believed to be lavas or impact breccias off the surface of asteroids. One small group of achondrites, the SNC group, are believed to be from the surface of Mars! Iron Meteorites Iron meteorites are composed all most entirely of nickel-iron with trace amounts of carbon, phosphorous and sulfur. They are believed to be from the core of a large asteroid. iron meteorites show varying textures depending upon their nickel content. Though they are quite common in our collections, iron meteorites make up only about 4.6% of all that fall. Hexahedrites: Lowest in nickel content (6% or less) show weak lines on acid etching called "Neumann lines" which form in a hexahedral structure. Octahedrites: About 7-10% in nickel content; show "Widmanstatten figures" an octahedral crystallization pattern of the nickel-iron unique to meteorites, when etched. They are broken down into the classifications of fine, medium or coarse depending upon the width of the bands in the etch pattern (higher nickel content = narrower bands). Ataxites: Highest in nickel content (16% or more), these meteorites show no structure when etched with acid. Stony Iron Meteorites Stony-iron meteorites are generally about 50% nickel-iron and 50% silicate material. As a group they are the rarest of the meteorites, making up only about 1.5% of all that fall. Pallasites: Pallasites are easily the most beautiful of all meteorites being composed of olivine crystals set in a nickel-iron matrix. They are believed to be formed at the core-mantle boundary of an asteroid where the iron from the core would mix with olivine form the mantle. Mesosiderites: These are a mixture of non-continuous metal grains, pyroxene, plagioclase and olivine. From the shattered appearance of the components of these meteorites, it has been suggested they formed by the violent impact of two asteroids; one metal rich and one silicate rich. Tektites It is not known exactly how tektites form. They are composed almost entirely of glass. Many theories have been suggested for their origin, ranging form glass meteorites to melted soil blasted off the moon in a large impact. Current theory holds that they are most likely melted soil that was blasted high into the atmosphere by a large impact here on earth. Many tektites show the pitting, stretching and flow lines formed from their high speed flights through the atmosphere. Falls are meteorites whose arrival on Earth was witnessed and recorded. Their time of fall is thus relatively precisely known. These meteorites were usually recovered shortly after their arrival, although often enough in the case of showers, additional fragments from a given fall may be recovered a long time after the fall occurred. When all falls exclusively are considered, a reasonably good estimate of the general population of meteorites reaching the Earth may be made. The vast majority of falls are stones (9d2.8%), most of which turns out to be chondrites (85.7% of all falls). Irons are rare (5.7% of falls); stony-irons rarer still (1.5%). In other words, most meteorites falling on Earth are by far chondrites. Finds are meteorites that were not seen to fall but were subsequently discovered on the ground, often long after they landed. Their arrival on Earth (time, circumstances) is thus not well documented. The vast majority of meteorites in museum and private collections around the world are finds, not falls. Because stones tend to look like ordinary terrestrial rocks, especially if they were subjected to weathering, they are easily overlooked. Stone finds are therefore rare in spite of the commonness of stones among falls. Meteorite collections are instead dominated by irons, which not only have a distinctive appearance and are therefore easier to spot, but they resist particularly amenable to being found by metal detectors. Stony-irons would also be common among finds if it weren't for their lesser resistance to weathering compared to irons and, more importantly, for their extreme rarity among falls in the first place. Meteorites, whether falls or finds, are usually given the name of the locality (post office, if any) nearest the site where they were recovered. In cases where many meteorites representing several falls are found within a relative small area (individual blue ice fields in Antarctica for instance), the meteorites are dexignated by an abbreviated locality name (the same name for all meteorites form that area) followed by a number giving the year of recovery and a serial number. ALH81005, for instance, is meteorite number 5 among those recovered in the Allan Hills area of Antarctica during the 1981-1982 field season (Note: number 5 does not necessarily mean that this meteorite was the fifth one recovered). For personal service, product details and pricing, email us at dinostrinc@aol.com with your request.