Types of Meteorites
There are three distinct types of meteorites.
Meteorites are so rare, there is no point in going searching for them unless one is known to have fallen in your area.
Should you come across one, it is really important for you to recognise what you have found, because gram for gram, a meteorite is worth more than gold.
Out of the three main types of meteorites, a further subdivision is made depending on whether or not these fragments contain chondrules.
Those meteorites with chondrules are called chondrites.
Those without are called achondrites.
Photos of chondrules
What are chondrules?
Chondrules are small round grains of a different color and texture found in the sample meteorite rock. As 90% of all meteorites are of a stoney composition, they are generally identified by the presence of chondrules.
Chondrules were formed in space possibly thousands of millions of years ago.
While the full process is not fully understood, it is generally believed that chondrules are the result of molten droplets of silicates formed during space dust collisions in the period during which the planets in our solar system were formed.
Carbon dating has placed most chondrites in the period 4,500 million years ago when the solar system was born.
They mainly consist mainly of olivine and pyroxene, with the occasional inclusions of iron and magnesium rich minerals.
Chondrules all come from space. There are none on our planet.
Photo of stone meteorite
Have you ever found a meteorite? if so, was it
90% of all fallen meteorites are made of stone.
Of that number, 90% of them are chondrites, containing chondrules. The remaining 10% are generally grainier and resemble terrestial rocks in color, texture and chemical composition.
Achondrites consist of mainly plagioclase, pyroxene and olivine in varying amounts.
Chondrites are more uniform in that their chemical composition is made up of
- 40% olivine
- 30% pyroxene
- 5 - 20% nickel-iron
- 6% triolite
Their chondrules are made from either orthopyroxene or olivine and the numbers can be abundant or sparse.
Stone meteorites are usually equidimensional with equal sides.
Some can be angular due to fracturing caused by collisions with other extra-terrestial bodies, or where they broke off from their parent meteor.
They can be conical in shape if they kept the same course and direction while travelling through space. The tremendous heat generated as it passed through our atmosphere would shear off and shape the meteorite into something that looked like an Apollo Command Module.
All meteorites develop a fusion crust, which is the area that becomes burnt and blackened by the tremendous heat generated as they enter Earth's atmosphere.
In stone meteorites, the fusion crust is thicker than of those in iron or stony-irons.
It is often black, though not always, and can be either shiny or dull.
The fusion crust can be fluted or furrowed, due to the flow of molten rock material from the front to the back of the meteorite.
Break open a stone meteorite, and the interior is usually gray or dark gray, and granular. If chondrules are present, they were be seen throughout the interior. Patches of nickel-iron may be apparent.
Widmanstätten Pattern of Iron Meteorites
6% of all meteorites that fall to Earth are iron.
The first thing you may notice about iron meteorites is that they are strongly magnetic. If searching for them with a metal detector, iron meteorites will be picked up as a strong signal.
They look very black when freshly fallen and have irregular shapes, with both protrusions and indentations, probably caused by collisions with extra-terrestrial bodies in space.
Fragmentation in flight, weathering and its final collision with the Earth's surface all add to it's unusual shape.
Iron meteorites which have lain undetected for many years may have faded to brown as the iron oxidized and rusted.
Chemically it is made from nickel-iron alloys, but is identifiable as not being from Earth by the presence of complex intergrowths of kamacite and taenite which are known universally as Widmanstätten structure.
The Widmanstätten structure or pattern is found only in iron and some stony-iron meteorites and is not present on Earth.
The fusion crust of iron meteorites is very thin, usually 1mm or less. It only covers one area, the part that was foremost when it entered the Earth's atmosphere.
Pallasite stony-iron meteorite
Just a tiny 4% of meteorites that fall to Earth are of the stony-iron type, and they are among the most beautiful of all. There are two main types of stony-iron meteorites:
- Pallasites, as shown in the photo here, are made up of olivine (space gems) in a nickel-iron matrix. Less than 100 specimens have been recovered, making them a very rare form of meteorite indeed.
- Mesosiderites are composed of 50% nickel-iron and 50% silicate. These are a very rare form of meteorite, and less than 200 have ever been recovered. Early specimens were mistaken for silver ore.
Stoney-iron meteorites are structurally very similar to the irons, with deep pitting and protrusions, and a blackened fusion crust. It is only when they are sliced open that their true beauty is revealed.
Weathered stony-irons take on a rusty-brown color with a rough, pitted coating. Some show the atypical Widmanstätten pattern when sliced open and treated with acid.