Meteorites: Types and Pictures of Meteorites Rocks

What is a meteorite

Meteorites are solid objects from comets, asteroids, or meteors that land on the earth.  When recovered, they could look much like random rocks but upon intensive analysis, they are discovered to have originated from outer space.

Although it is agreed that these fragments from space fall annually unto the earth, they are very rare.

Origin of meteorite

They originate in space, travel through the earth’s atmosphere, and eventually land on the ground. Their sizes could range from dust particles to larger rocks, called space rocks because they are solid objects.

Some recovered meteorites have also been traced to originate from the moon and even from other planets.  In fact, a meteorite from Mars has gained recognition as it is believed to contain traces of martian life

Difference between Meteor and Meteorite

When in outer space, they are simply referred to as meteoroids. On entering the atmosphere of a planet or the moon. In earth’s case, they are called Meteors. When these enter Earth’s atmosphere, at very high speed, they burn up. These often appear as fireballs, or shooting stars in the night skies called meteor showers.

Meteor showers are natural phenomena that occur as the earth rotates. When it does, it goes through trails of debris that were created often by a comet.

What are meteorites made of?

For a meteor to be called a meteorite, it has to successfully survive the journey and land on the ground. Meteorites are made of several materials depending on their source. Some are composed entirely of metals,  others a mixture of metals and silicate minerals. Still, others are made entirely of silicates.

Several findings have uncovered different chemistry unique to each meteorite. For instance, some of them were found to contain, liquid water and some organic materials.

With its advent into the earth’s atmosphere, factors can cause the meteorite to heat up and burn up. Such modifying factors include; pressure, frictions, and chemical reactions. Irrespective of what makes up the meteorite, they can also be weathered the same way terrestrial rocks are too. In the same vein, as soon as they land on the ground, they are equally as susceptible to salt, water, oxygen, and acids.

Types of Meteorites

Based on their composition, meteorites have been grouped mainly into three; stony meteorites, iron meteorites, and stony-iron meteorites. This grouping is done based on observable physical characteristics and not necessarily because of chemistry. With this, you can find meteorites with similar physical attributes and different chemical compositions. More studies have created newer groupings of meteorites based on their mineral constituents, chemistry, structure, and isotopes.

1.     Stony meteorites

The commonest meteorites recovered on earth are the stony meteorites. In fact, 94% of all recovered ones belong to this group. Generally, they are so-called because they are composed mostly of silicate minerals. Many stony meteorites also contain iron and nickel in small amounts scattered across them. Their chemistry is similar to the peridotites, and basalts.

Stony meteorites could also be subdivided into two; the chondrites and the achondrites.

• Chondrites

The chondrites are meteorites that contain small-sized spherical bodies (often less than 1mm) called chondrules. These chondrules are remains or dust from before the solar system was formed. At the birth of our solar system, these dust and particles coalesced to form larger rock bodies known as asteroids.

Overall, it is believed that these asteroids that were formed in the early years of the solar system are the parent of the chondrites found in meteorites. Using isotopes of lead, the age of these asteroids found as chondrites have been confirmed to be a pre-solar system.

At the time of the formation of the solar system and the planets, the asteroids were not large enough to undergo melting and planetary differentiation. But were however susceptible to series of metamorphism.

They are then fragmented or broken and included in the formation of other space materials. Due to their inclusions, they are seen as little mineral grains within the ground matrix of the meteorite. The texture, mineralogy, and compositions of these asteroids are very unique. Undoubtedly, they are exclusive to chondritic meteorites and cannot be found in terrestrial rocks.

Asides from the presence of these chondrules, another feature of this group are that they have not experienced any form of change or modification from their parent body.

Also, some chondrites have been observed to contain refractory inclusions as well. These refractory inclusions are essentially calcium and aluminum-rich. Some of the studied chondrites are composed of hydrous minerals, several inclusions, isolated silicate grains, sulfides, as well as metallic inclusions.

The ground matrix of the chondrites is fine-grained and holds together all the components of the meteorite. Many chondrites have been found to contain organic matter especially carbon, hydrogen, and nitrogen-based materials.

• Achondrites

Achondrites are rarer than chondrites. This group of meteorites is so-called because they do not contain chondrules. This class encompasses all stony meteorites that do not contain chondrules. About 8% of all recovered meteorites belong to this group.

Many of the recovered achondrites like other meteorites have been confirmed to have originated from asteroids. However, some were confirmed to have originated from other planets such as Mars. Achondrites from Mars are called shergottites, nakhlites, and chassignites. Some recovered achondrites have also originated from the moon.

Achondrites are mainly silicates and bear various geologic signatures. They undoubtedly have a similar appearance to the terrestrial igneous rocks and if not well examined, could pass as silica deficient rocks. They show signs of heat alterations and signs of geochemical differentiation making them very similar to terrestrial volcanic rocks.

The various observable textures and patterns on these rocks show that they were formed and have been modified by igneous activities. Thus the chondrules within them have been destroyed. It is also believed that the terrestrial planets were formed in the same way these meteorites were formed.

Achondrites like the chondrites still contain little percentages of iron. They have been grouped into three groups Howardites, Eucrites, and Diogenites based on their specific compositions. Although the Howardites and Eucrites are composed mainly of plagioclase and pyroxene, the percentages in each group are slightly different. The composition makes them appear more like basaltic rocks.

2.     Iron meteorites

Iron meteorites are believed to be the cores of asteroids. They are were formed during the decay of radioactive elements in many asteroids. This resulted in the differentiation of its components and the iron, being denser sank to the center. The remaining constituents formed concentric regions around this core.

As their name implies, they are composed mainly of iron-nickel metal. These cores are very dense and are surrounded by concentric layers as seen in Earth, Mars, Venus, and Mercury. Although they are the easiest type to identify, this group is much rarer than their stony counterparts making up 5% of all witnessed meteorite falls.

Iron meteorites are also called siderites and are composed of iron-nickel metals; kamacite and taenite. Asides from the iron-nickel metal found in them, other minerals such as graphite, troilite, schreibersite could occur in minor amounts or as minor inclusions.

To easily differentiate them from man-made iron products, they display a distinct crystal structure when etched with acid. Also, the amount of nickel found with the iron is higher than for man-made iron products.

Sub-classifications of iron meteorites are very wide. The older classifications of the iron meteorites are based on the structure as observed in the displayed patterns when etched with acids. This pattern is obviously formed as a result of the nickel present in the system.

The newer system however is based on the composition of each recovered iron meteorite. Here, the amount of germanium, gallium, and iridium present in each meteorite is considered.

These are very resistant to atmospheric changes, ablation, and weathering making them easily recoverable.

3.     Stony iron meteorites

Stony iron meteorites are the third type and the rarest of the meteorites accounting for the remaining 1% of all recovered meteorites. These have an almost equal amount of silicate minerals and iron-nickel metal. Because of the way they are mixed in each meteorite, they create a beautiful piece.

All stony-iron meteorites have undergone alterations thus you might not find chondrules in them. In a way, they are achondrites as well. The iron-nickel metals found here are similar to those of the iron meteorites (kamacite and taenite).

Olivine is the predominant silicate mineral here giving them a dark color as well. Other accessory minerals similar to those in the other meteorite types could also be seen. The proportion of each in specific meteorites causes the division into two subclasses. These are pallasite and mesosiderites.

• Pallasite

Several beliefs surrounding the origins of pallasite exist, suggesting they remain from the core-mantle boundaries of asteroids. Another school of thought presumes they were formed due to the mixture of materials from both the core and mantle of these differentiated asteroids.

Pallasites were named after the German scientist Peter Pallas and make up less than 2% of known meteorites on earth. They are seen to consist of crystals of olivine sometimes peridot scattered within a ground matrix of iron-nickel. In some pallasites, the olivine occurs in clusters, patterns, or veins through the metal.

Many times, if the purity of the peridot is high enough it becomes very precious as a gem. It should be noted that in 1902, the 45kg Marjalahti meteorite that fell in Russia had very high purity. This meteorite set the standard for what gem-quality peridot should be.

These beautiful pieces are well sought and earn high prices, especially for their gem content. They can be cut and polished to improve their market price.

• Mesosiderite

Mesosiderites are the other groups of stony-iron meteorites. The name is sourced from Greek, “meso” meaning half and “sideros” meaning iron showing clearly their nature. These unlike the beautiful pallasites appear more like collages. Mesosiderites are amongst the rarest meteorites with less than 150 documented pieces. Many of these have been mistaken for other materials until confirmed otherwise.

Their silicate and metal ratio is almost equal. The main iron minerals here are kamacite, taenite, and tetrataenites. The silicates are mainly calcium-rich plagioclase and pyroxenes. Other minerals present in smaller amounts are sulfides.

One of their features is the smashed-up look of the silicate minerals here that clearly show the effect of igneous activities. This has pointed to the proposal that they were formed when two differentiated asteroids; one rich in silicate and the other rich in metal collided.

Thus, the silicate constituents can be ascertained to be from achondritic sources. The metal on the other hand bears signatures that can be traced to the core of other asteroids.

How do is a meteorite found?

Most of the recovered meteorites however are of considerable sizes with some being large enough to form craters. Many of the meteorites recovered on the earth are of stony types. There are two ways in which meteorites have been recovered; as finds and as observed falls.

Meteorite finds were being observed as early as the twelfth century and have continued today. This group is meteorites recovered by meteorite hunters or by random people accidentally. These could be found buried in the ground or just lying on the ground as weird rocks that capture attention.

Meteorite falls on the other hand are recovered based on eyewitnesses. These are observed from their entrance into the earth’s atmosphere and followed till they land on the ground.

Rare meteorites

The rarest form of meteorites is the stony-iron type. The almost equal portions of silicate to iron makes them very desirable. Some other types are rare because of remarkable qualities associated with them such as their size or manner of fall. Here’s a list of some of these rare meteorites recovered either as falls and finds include;

•  Hoba meteorite found in 1920 in Grootfontein, Namibia is the largest meteorite found on the earth. Weighing about 60,000kg and covers an area of about 6.5 square meters, the weight of this iron meteorite has made it impossible to be moved. Believed to have landed 80,000 years ago, this is the largest piece of iron occurring naturally on earth. Consequently, to date, it still sits at the site where it was found and studies on the meteorite have to be carried out from fragments from it.

• The largest stony-iron meteorite is the Brenham meteorite. This pallasite meteorite was found in Kansas, United State. It weighs about 650 kg and is presently kept in a private collection in Texas.

• The Chelyabinsk Meteor appeared as a brightly burning star on the morning of 15^th February 2013 in Chelyabinsk Russia. The impact of this giant fireball shattered many windows and injured more than 1500 people. However, they landed on the earth as fragments with the largest piece found in Chebarkul Lake.

• ALH 84001: was discovered in December 1984 in Antarctica. It weighed just about 1.93 kg but is special because it is believed to be one of the oldest meteorites to land on the earth. It was speculated to have come from Mars and that it holds traces to life on Mars making it very special.

• Gibeon Meteorite: this did not fall as a large stone. Rather it is famous as the most extensive meteorite shower on earth coving about 390km. This iron meteorite is named after the town Gibeon where it was found and was believed to have arrived on earth during prehistoric times.  Portions of this have been cut to make weapons during those times and as part of jewelry in modern times.

How much are meteorites worth?

Space rocks as they are popularly known are very rare. Despite the large amount that is recorded to get into the earth’s atmosphere yearly, there are very few finds. If recovered, they are sold per gram.

Their prices depend on the types as some types cost more than others. Stony meteorites which are very scarce can be sold for around $2 to $300 per gram. Iron meteorites are sold between $0.5 and $5 per gram. Very scarce ones can sell for as high as $1000 per gram. Prices could also depend on the size, the amount, the preparation, and how well it was stored.

Meteorites that land on personal properties by law belong to the individuals. And yes you could pick them up as soon as they land and if they pass the confirming tests, can be sold. They are not radioactive would have cooled off before landing so there is no fear of getting burnt. For those that land on public properties, those belong to the government.

How to identify a meteorite?

Meteorites fall daily. These occur from very small millimeters to large rock-sized ones. Despite this, meteorites are the rarest rocks to find on earth. Because of their close similarity in appearance, many have been confused as to how to identify them.

As a result, many samples are being sent to laboratories all over the world for confirmation. Small-sized meteorites might not be easily noticed but larger ones with sizes in centimeters and inches are the quickest to pick up. So how do you distinguish a meteorite from just a piece of rock?

Some easy ways include;

• Fusion crust

Meteorites travel down to the ground at very high velocities through the earth’s atmosphere. That is why many of them are seen as shooting stars at night. This causes burning of the crust making them appear greyish to almost charcoal black. These could make them appear as though they have smooth, shiny outlines.

Some others have visible flow lines along which the molten rock flowed as it was heated. Depending on how long they have been on the ground, these sharp features could be removed by weathering agents. It is however to note that it would still retain its burnt exterior appearance.

• Shapes

Many recovered meteorites have irregular shapes with well-rounded corners. Undoubtedly, these are a result of abrasion during its journey through space and down to earth. Marks, where portions of it were chipped off or broken, are common signatures.

There could also be some pits on the surface of the iron meteorites called regmaglypts. These are shallow depressions that are about the size of thumbprints on larger meteorites. These are also features caused by heat activities on the meteorites.

• Textures

Meteorites are solid pieces. When you find a dark-looking rock that has spaces or pores, it definitely is not a meteorite. Pumice and slag from industrial processes could appear like meteorites. Industrial slags are made from the manufacturing of iron and steel. This in particular contains metallic portions but has pores.

The depressions observed on the surface of the meteorites do not go inside and remain just as signatures on the surface.

• Color

Because of the heat differences, meteorites are dark-colored. Originally they are black-colored. The presence of flow lines, fusion crusts, and the weathering of the iron content could stain the meteorite a red hue. This could be spots of red or orange on the black matrix. Consequently, prolonged weathering could change the whole stone to a brown color.

It is not possible to find light-colored minerals within them. Quartz, the main light-colored mineral in terrestrial rocks, is not found in other parts of the solar system. Noticing quartz in any suspected piece strikes it off as a meteorite.

• Streak

When a rock is ground on a streak test plate, it gives off mineral powders. The color of the powder produced is known as the streak. Some minerals or rocks could appear light and have a dark-colored streak when checked. The reverse could also occur in some dark-colored minerals.

Howbeit, when meteorites are rubbed on the streak plate, they always give a brown color. Iron-rich ores could appear similar to meteorites through many other properties, but they give a black or red streak.

• High iron content and heavyweight

Iron is an important component of meteorites as they occur in varying amounts in all types. The iron present is seen as shiny metallic grains and not just in combination with other minerals to form minerals.

This is a very rare phenomenon in earth’s rocks. Acknowledging that all meteorites contain iron, it is to be expected that they would weigh more than rocks of similar size. Many times, these grains might not be easily observed if the meteorite has been rusted. It would be better seen if it was ground on a diamond file.

There are also dense minerals here such as pyroxenes. These also contribute to the overall weight of the space rocks.

• Magnetic Properties

Since all meteorites contain iron in varying concentrations, they are bound to be magnetic. Even stony meteorites would attract a magnet, unlike silicate earth rocks. Other earth rocks such as magnetite and hematite would also attract a magnet nonetheless. Taking the other features into account would help identify what is or is not a meteorite.

An actual meteorite could be worth a lot. If you have found a piece of weird-looking rock you suspect is a meteorite, you should first run these simple assessments so you do not jump on a meteor-wrong.