They are therefore an indication of relatively unchanged, or primitive materials. Achondrites have no chondrules. Chondrites are divided into two main groups, the carbonaceous chondrites and the ordinary chondrites. The presence of these volatile elements indicates that these meteorites have never been heated and that their components have not separated or differentiated. Such meteorites are said to be undifferentiated.
Achondrites show even greater signs of heating and differentiation than the ordinary chondrites.
The chondrules are recrystallized and sometimes the whole meteorite looks like an igneous rock in Earth, indicating that the rock has been completely or partially melted in its history. The internal structure of iron meteorites often includes large iron crystals, indicating that the iron was molten and solidified very slowly, much more slowly that would be expected unless the iron were buried under 10 or more kilometers of rock during the cooling and solidification. Iron is a major component of chondritic meteorites, therefore iron meteorites are differentiated, similar to the achondrites. The model that explains this range of meteorites compositions and textures is that all meteorites were originally carbonaceous chondrites. Some of these meteorites never became part of a large body and kept their original characteristics.