Meteorite Families

The Canadian stony-iron meteorite Giroux is composed of equal parts silicates and metal.
After finding or recognizing a meteorite, a scientist's first task is to classify it. There are three main families of meteorites: stony meteorites, iron meteorites, and stony-iron meteorites. The proportion of metal contained in the meteorite determines the family to which it belongs. Iron meteorites are composed of 98% iron, stony-irons contain 50%, and stony meteorites contain between 20 and 27%. Each of these families is in turn divided into different sub-classes according to the meteorites' composition, structure, and mineralogy. The most common classes are shown in the following table.

A cut of the stony meteoroid Homestead. The fusion crust is visible.
The different families of meteorites do not fall to Earth in the same proportions. A very large majority of them (93%) are stony meteorites. Irons are much less frequent-they account for only 6% of falls. Finally, stony-iron meteorites are the rarest-just 1% of meteorites.

Certain scientists use a different system of classification based on the origin of the meteorites. Some meteorites come from parent bodies (asteroids and planets) that were modified after their formation-these are differentiated meteorites. In contrast, undifferentiated, or primitive, meteorites are the debris from celestial bodies that were not altered over the course of time. The same classes of meteorites are found in this system, but they are distributed differently.

Small iron meteorite found at Penouille, in Quebec. A piece of it was cut off for analysis.

At the beginning, the composition of the parent bodies was uniform. Planetoids that became big enough heated up and differentiated. During differentiation, heavy metals condensed at the centre, elements that were a little less heavy accumulated around the centre, and the lightest elements formed the crust. Each of these zones gives rise to a specific class of differentiated meteorites.
© Sophie DesRosiers, Planétarium de Montréal

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Credits.  Last Modified: 2005-09-30