You may recall that virtually all chondrites have similar textures, chemistry and ages of 4.56 billion years. When we examine the achondrites, we find more variety. In the first few million years of Solar System history, the achondrites were either fully melted and differentiated into mineral zones within each parent body or they showed partial melting in an interrupted attempt to differentiate.
... [Show full abstract] The heating probably occurred within the first few million years after the chondritic parent bodies accreted. So we see that the ages of the chondrites and these achondrites are approximately the same. But there are also some groups of achondrites which are very different, and these strange meteorites turn out to come not from the asteroid belt but from the planet Mars and the Moon.
Martian meteorites are grouped into three types known by the acronym SNC (pronounced “snick”). A fourth type, represented by one meteorite from Allan Hills in Antarctica was discovered in 1984 and given the name ALH 84001. More will be said of this extraordinary meteorite later. SNC refers to the first letter in each of the names of the three type specimens: “S” is for shergottite, named after a meteorite that fell in 1865 in Bihar, India, near the town of Shergotty; “N” is for nakhlite after the type specimen that fell in Nakhla, Egypt, in 1911; and “C” is for chassignite, named for a meteorite that fell in Chassigny, France in 1815. Most of the SNC meteorites are thought to be cumulate igneous rocks that formed on the floor of a magma chamber.