Meteorite is defined as a piece of debris from a comet, asteroid or meteoroid that survives its passage from outer space to impact the surface of a planet.
Most meteoroids will burn up and disintegrate whilst entering the earth’s atmosphere and will never become a meteorite that impacts the earth. Those that do make it to the earth’s surface may be grouped into two broad categories, primitive and differentiated.
Primitive – These meteorites are called chondrites, and are the most abundant type. They are stony meteorites that have maintained the original structure and composition of the dust that first formed in our solar system over 4.6 billion years ago. They are even said to contain presolar grains that condensed around other stars in the vicinity before our sun was formed. They can be identified by circular features called chondrules. They come from asteroids that did not experience high enough temperatures to alter its components.
Differentiated – Meteorites that have experienced sufficient heat to cause melting and the separation of the elements.(differentiation) When a planetary body experiences widespread melting, the heavy elements like iron and nickel sink to form a core, while the lighter elements rise to the surface to create the mantle and the crust.
From left to Right:
Chondrite meteorite (Primitive stony meteorite) with noticeable regmaglypts.
Regmaglypts in iron meteorite.
Fusion crust on a chondrite meteorite. The light grey patches are where the black fusion crust has been removed revealing the true colour of the chondrite below.
Achondrites are stony meteorites that have experienced melting and recrystallization, they do not have chondrules and without a fusion crust they would be difficult to differentiate from terrestrial basalts or plutonic rocks.
Stony irons are differentiated meteorites that are made up of almost equal parts rock and metallic iron. These stony iron meteorites can be split into two groups Pallasites and Mesosiderites. Then you get your iron meteorites that are made up of mostly iron and nickel and would be from the core of the planetary body.
Tektites were once thought to come from space although most scientists now agree that they are not meteorites but rather a natural glass that was formed by the impact of large meteorites on the surface of the earth. e.g. – Moldavite.
The vast majority of meteorites have an irregular shape and are not round like a ball, however they do often have rounded features such as rounded indentations (regmaglypts) that look like someone pushed a thumb into putty. As the meteorite passes through the earth’s atmosphere its surface begins to melt and the air pressure forces the molten material back, leaving a featureless molten surface called a fusion crust. Almost all meteorites are magnetic to some degree, even stony meteorites usually have enough iron and nickel in them to be magnetic.
The nickel – iron meteorites may be cut into plates, beads or inlays that can be etched using acid to reveal a pattern (Widmanstatten patterns) of long nickel – iron crystals. These etched meteorites are being used in jewellery by clients who are intrigued by their extra terrestrial origin.