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Meteorite Buyer's Guide: Types Of Meteorites
Three main types of meteorites exist, and each type is subdivided into many more classes depending on minerals, structure, chemistry and origin.
Iron Meteorites
While most meteorites are composed primarily of rock and minerals, iron meteorites are distinct for their high metal content, primarily iron and nickel. These meteorites are thought to originate from the cores of ancient, long-vanished planetary bodies, which were once part of our early solar system!
A fragment of the Canyon Diablo meteorite that impacted the Arizona desert 50,000 years ago.
Iron meteorites' formation can be traced back to the chaotic processes that shaped our solar system in its youth, more than 4.6 billion years ago. During this time, collisions and accretion events led to the formation of numerous planetary embryos and protoplanets. These embryonic worlds experienced intense heating and differentiation, where denser materials such as iron and nickel sank to their cores, leaving behind metal-rich remnants that would later become iron meteorites.
On occasion, iron meteorites exhibit a structural composition comprising an inner nickel-iron core enveloped by concentric layers, encompassed by a silicate mantle and outer crust. This structure bears a striking resemblance to terrestrial planets within our solar system, such as Mercury, Venus, Mars, and Earth, all of which possess nickel-iron metallic cores. The study of iron meteorites provides valuable insights into the processes involved in the formation of planetary metallic cores.
Some famous iron meteorites include the Canyon Diablo Meteorite from Arizona, the Sikhote-Alin Meteorite from eastern Russia, the Gibeon Meteorite from Namibia, and the Campo del Cielo Meteorite from Argentina.
Stony-Iron Meteorites
Stony-iron meteorites, known for their remarkable composition, comprise nearly equal proportions of iron-nickel metal and silicate minerals, some of which include semi-precious gemstones. This unique blend of rock and metal renders stony-iron meteorites as some of the most aesthetically captivating meteorites in existence. Among stony-iron meteorites, two prominent classifications are recognized: pallasites and mesosiderites. Each of these categories boasts its distinct characteristics and significance in the study of celestial relics.
Pallasites
The most captivating feature of pallasite meteorites is their composition. These meteorites are a striking amalgamation of olivine crystals, a semi-precious gemstone with a distinct greenish-yellow hue, embedded within a matrix of iron and nickel. This unique combination creates a mesmerizing contrast between the sparkling olivine gems and the metallic matrix, resulting in an otherworldly appearance.
A slice of Sericho pallasite showing large olivine crystals embedded in iron-nickel metal.
The olivine crystals found in pallasites are often of gem quality, coveted by collectors for their vibrant colors and transparency. Some pallasites even contain exceptionally large olivine crystals, known as "peridot" when used in jewelry, which adds to their allure and commercial value.
Exactly how pallasites are formed is still debated. Some scientists believe that they formed in melting asteroids similar to iron meteorites, and represent the boundary between the metal-rich core and the silicate-rich mantle surrounding it. Other scientists believe that they may have formed during asteroid impacts.
Pallasites are a relatively rare meteorite type: less than a hundred are known. Some famous pallasites include the Sericho Pallasite from Kenya and the Admire Pallasite from Kansas.
Mesosiderites
The most distinctive feature of mesosiderite meteorites is their complex and heterogeneous composition. Unlike pallasites, which contain gem-quality olivine crystals, mesosiderites are characterized by a mixture of metal, silicate minerals, and basaltic rock. This combination gives mesosiderites a mottled appearance, with metallic flecks embedded within a rocky matrix. Mesosiderites are further classified based on the ratio of metal-to-silicate content and the presence of certain minerals. These classifications include main group mesosiderites, silicate-poor mesosiderites, and reduced mesosiderites, each with their unique characteristics and history.
Vaca Muerta stony-iron Mesosiderites from Chile.
Mesosiderites are believed to be the remnants of violent cosmic collisions that occurred in the early stages of our solar system's formation, roughly 4.6 billion years ago.
Stony Meteorites
Most meteorite finds are stony meteorites. These consist almost entirely of silicate minerals with only minor amounts of metal. The two subgroups of stony meteorites chondrites and achondrites.
Chondrites
Chondrites are some of the most primitive rocks in our solar system at over 4.5 billion years old. Chondrites possess a distinct appearance characterized by the presence of tiny silicate mineral droplets intermingled with small grains of sulfides and iron-nickel metal. These millimeter- to centimeter-sized granules evoke the imagery of sand grains, lending chondrites their name derived from the Greek word 'chondres', meaning grain.
Chondrites serve as invaluable cosmic time capsules, preserving material from which the entire solar system coalesced. Unlike larger planetary rocks, which have undergone extensive geological transformations, chondrites have experienced minimal alteration. As a result, they offer a unique window into the primordial conditions of our solar system's formation. The study of chondrites provides crucial insights into the intricate processes that gave birth to the celestial bodies in our cosmic neighborhood.
Achondrites
Achondrites encompass meteorites originating from parent bodies, including asteroids, Mars, and the Moon. These meteorites bear the mark of their often igneous nature, signifying that at some point in their history they underwent a transformation into molten magma. As magma gradually cools and solidifies, it takes on a characteristic concentric layered structure—a process scientifically termed igneous differentiation.
This process of igneous differentiation also played a pivotal role in the formation of the rocky planets within our solar system, namely Mercury, Venus, Earth, and Mars. It was responsible for creating their distinct planetary crusts, mantles, and cores. Consequently, the study of achondrites offers profound insights into the internal structures and formative processes that shaped these planets, providing valuable knowledge about our own home planet and its planetary counterparts.
About Meteorites
Meteorites are defined as chunks of space rock of any size that have breached the Earth’s atmosphere and collided with the surface. Space rock constantly falls from the sky and has the potential to become meteorite, though unless the rock is larger than the size of an American football field then it will disintegrate as it hits the atmosphere and is subjected to different pressures. This breakup is why most meteorites are smaller than the size of a fist. Although the average meteorite is somewhere between the size of a coin and a baseball, some meteorites have kept a portion of their significant mass as they crashed down to Earth. Two 30+ ton meteorites in Greenland (Cape York) and Argentina (Campo del Cielo), and a 60+ ton chunk lodged in the ground in Namibia (Hoba) are some of the largest single masses known!
A 60-ton meteorite is a staggering size, considering that only about 5 percent of the original body makes it through the atmosphere when all is said and done. This means that the meteoroid responsible for the Hoba meteorite had to be, at the very least, ~1,300 tons in mass!
Many meteorites are rather unassuming and look a lot like any other Earth rock, but may be slightly shinier. This shine is the product of the meteorite's outer surface melting from the extreme heat of atmospheric entry and is referred to as fusion crust. These rocks are often a dark red-brown, are pocked or covered in thumbprint-like marks called regmaglypts, and are quite hefty for their size. Meteorites are a lot more rock-like in appearance than the impact glasses associated with the meteorite impact, which are often much more colorful.
Meteorites' overall smoothness may be one of their dead giveaways. Granted, many other stones may be smoothed by erosion, either from sand abrasion, hydrological forces, or wind exposure. However, meteorites' telltale signs of fusion crusts and regmaglypts cna often make then stand out. It’s a subtle difference, but it is observable. When all is said and done, most agree that meteorites' densities are the most remarkable and what most use as a physical indicator without intrusive chemical testing.
Where Do Meteorites Come From?
Put simply: space! More specifically, 99.8% of the meteorites on Earth originated in the asteroid belt between Mars and Jupiter, or from random asteroids and meteoroids floating around out there. However, the remaining 0.2% of the meteorites are consistent with material found on Mars and the moon. Meteorites from the moon and Mars are important to scientists' understanding of how interstellar collisions happen between planets and celestial bodies within our own solar system and the greater Milky Way.
Meteorites aren’t necessarily formed as much as they are broken off of extraterrestrial bodies and tossed into space from impacts and collisions. In space, they're known as meteoroids. Often, these meteoroids will drift around in space as debris, asteroids, or comets before they enter the Earth’s atmosphere, explode, and fall to the surface. Meteorites are the results of meteoroids entering the Earth's atmosphere (where they are then known and seen as meteors) and disintegrating from high pressure and superheating from friction with the atmosphere. When that meteor hits the ground, it becomes a meteorite.
Meteorites are found all over the world. That said, some areas seem to experience more impacts than others. Maps that chart the 190 confirmed impacts with Earth show a trend of impact at northern and southern latitudes with little attention to areas nearest the equator. It’s easiest to find meteorites in large, flat, open areas like deserts where they are less difficult to spot. Another important thing to remember is that meteorites aren’t often found after meteor showers. This is simply because the material that makes up the meteors responsible for the showers is quite fragile, and will be too small to have a chance to enter the atmosphere. Only significantly large extraterrestrial bodies of viable entry size can produce meteorites.
How Are Meteorites Classified?
This is a rather complex topic, but here is a brief overview. Depending on who you ask, there are a few different classification schemes. The primary scheme divides meteorites into three types based on their composition: stony, iron, and stony-iron. These classifications are observable by physical differences in each meteorite's texture, composition, and mass, which also gives clues about the meteorite's age and origin. This is the traditional scheme that is further subdivided by the Rubin Catalog (2000) and its progenitor’s alternate schemes, Krot et al. (2003) and Weisberg et al. (2006). These schemes all operate on the basis of the traditional scheme, with numerous subclassification alternatives based upon a variety of factors that change as more information becomes available.
Meteorite Classifications
Stony meteorites are composed of rocky material and silicate minerals. These bodies make up most of the meteorites we know about. The materials that stony meteorites are made of is believed to have once been the outer crusts of planetary remnants that make up the asteroid belt. Stony meteorites can be further subdivided into two types, called chondrites and achondrites. Chondrites are meteorites that contain grainy inclusions called chondrules, which were once a part of the solar nebula that dates prior to the creation of not only the Earth, but our solar system as well! Chondrites provide scientists with data about what the vastness of space was like before the solar system’s existence and contain the oldest material observable on Earth. Achondrites lack chondrules, and are mostly composed of volcanic rock blasted off of the surfaces of other celestial bodies into space by asteroid collisions or meteorite impacts.
Iron-type meteorites are primarily composed of iron and nickel, with a handful of other metals. These metallic meteorites are often the easiest to identify after millions of years post-impact, because they are quite different from the surrounding material, especially when it comes to their mass-to-surface area ratio. They are exceptionally heavy for their size, since iron is a high-density metal. Extraterrestrial iron and nickel are believed to be chunks of cores from the protoplanet remnants and asteroids the asteroid belt: this even holds true with the Earth, since our own planet's core is composed of nickel and iron.
Unsurprisingly, stony-iron types are a mixture of rocky and metallic components. This class of meteorite carries appearances of stony meteorites but the masses of iron meteorites. These meteorites are a catch-all category whose characteristics fall outside strictly metal or stony meteorites.
Additionally, institutions generally well-informed on the happenings in space such as NASA have begun to classify meteorites as either undifferentiated or differentiated. This separation is demarcated by the process of differentiation, which is when individual components of a meteorite separate and gather by likeness within the molten body as it cools. This is why the iron content of stony-iron meteorites is most concentrated in their centers. Undifferentiated meteorites are composed of either a singular component or were too small to differentiate, since they cooled faster than larger chunks during atmospheric entry. As it stands, chondrites are classified as undifferentiated meteorites, and irons, achondrites, and stony-iron types are differentiated meteorites.
Meteorite Classification Chart from Nasa.gov
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