The term "achondrite" is used to describe a stony meteorite without chondrules, and this lack of chondrules is the primary characteristic used to distinguish the two major stony groups, achondrites and chondrites. However, we have already stated that some chondrites don't contain any chondrules, and at least one type of achondrite contains distinct chondrules. These exceptions mark transitions from one class to another, and they remind us of the fact that chondrites represent primordial matter that remained relatively unchanged since the formation of our solar system. Within the process of accretion and differentiation of larger asteroids and planets, this primordial chondritic matter was melted and recrystallized to form achondrites. These evolved rocks are similar to terrestrial igneous basaltic or plutonic rocks.
The achondrites in our collections are samples of other more or less differentiated worlds, and therefore represent a very heterogeneous class of meteorites. Many of them are primitive; that is, nearly chondritic in composition with an age similar to the primordial chondrites. These so-called primitive achondrites are the residues from partial melting that took place on small parent bodies having chondritic compositions. Following an initial heating phase, they were quickly cooled to become geologically inactive. In the respective section, we will elaborate on the different types of primitive achondrites - also known as the PAC group. Other, more evolved achondrites, have experienced a more extensive igneous processing including magmatic processes similar to geological activities encountered on Earth. Some of these achondrites are basalts, calcium-rich volcanic rocks that represent the upper crust of their parent bodies. Others are magnesium-rich plutonic rocks that formed in deeper regions of the crust and experienced prolonged thermal processing.
Several groups of evolved and more differentiated achondrites can be assigned to specific parent bodies. The meteorites of the HED group are believed to be samples of 4 Vesta, one of the largest asteroids in our solar system. Other basaltic achondrites, such as aubrites and angrites, are also considered to have an asteroidal origin, while a few can be assigned to larger parent bodies - the true planets and their moons. The rare meteorites of the LUN group are genuine pieces of our own Moon - a fact that has been proven by comparisons to samples of Moon rocks that were returned to Earth by the Apollo missions during the late 60's and early 70's. The equally rare and most fascinating achondrites of the SNC group are believed to have their origin on our red neighbor, the planet Mars. These meteorites represent highly evolved rocks and resemble terrestrial rocks more than any of the other achondrites.