Primitive and Differentiated Meteorites: Asteroidal Achondrites

In Chap. 4 we looked at the most primitive meteorites, the chondrites. We saw that about 85% of all known meteorites fall into this category. The remaining meteorites are achondrites. In the broadest sense the word “achondrites” means “without chondrules.” In looking over the chondrites, we could not help but notice that there is a certain sameness about them. The same precursor minerals (olivine and pyroxene) are found throughout all the groups. Ironnickel metal is scattered liberally throughout their interiors, so much so that a classification system based upon the amount of iron (oxidized and elemental) was developed years ago. Most important, and with few exceptions, all the chondritic meteorites are composed of chondrules or fragments of chondrules—save one (CI). Some of the chondrules are sharply defined while others have all but faded away into the matrix. We further noted that many of these chondrites have been subject to thermal metamorphism in which the chondrule fields were heated to a temperature of 950 °C or more, not enough to melt the chondrules but sufficient to slowly alter them from their primordial state. One of the most plausible means by which thermal metamorphism could change the texture of chondrites is through heat generated by the short-lived radioisotope 26 Al. Recrystallization of the primary chondritic minerals was the result of these severe heating conditions.

On the left side of the temperature gradient in Table 4.2, beginning at about 400 °C and extending below about 150 °C (some as low as 20 °C), we noticed alteration due to the presence of water at some time in the past. This is especially noticeable in type 1 and type 2 chondrites. Here, fluids have entered the rocks through tiny fractures, subsequently reacting with the original primary minerals and producing hydrated minerals such as magnetite and clay-like phyllosilicates. The least affected chondrites are those of type 3 wherein there is scarcely any sign of aqueous alteration or thermal metamorphism that might have changed the primary characteristics of the meteorite.