Meteorite Madness 3

Stony Meteorites

Chondrites
The chondrites, stony meteorites containing chondrules, numerically dominate the falls, and may be the most abundant class of meteorite in the known Solar System at the present time. Chondrules are usually spherical to subspherical bodies, composed of primarily silicates, that range in size from less than 0.1 to more than 20 mm. Mineralogically, chondrules commonly are composed to olivine, pyroxene, plagioclase, glass, troilite, nickel-iron, and any combination of these minerals. Texturally, chondrules may be aggregates of crystals, single crystals, pure glass, or crystal and glass in a wide range of proportions and textures in seemingly endless variety. Many chrondrules appear to have been free fluid drops that assumed a spherical or nearly spherical shape due to surface tension and later solidified and crystallized. However, other chondrules clearly have not had this sort of history, but may be rounded clasts, or have had other more complex origins.

The classification of chondrites is based on their chemistry and mineralogy. The most widely accepted classification was first proposed in 1920 by G.T. Prior in Mineralogy Magazine and was later modified by Brian Mason (right) in his book Meteorites and includes the following classes: enstatite chondrites; bronzite chondrites; hypersthene chondrites; and carbonaceous chondrites. Chemical and mineralogical regularities in the chondrites were recognized early by Prior, and these observations led to the formulation in of Prior's Rules, which can be stated as follows:
1. The smaller the volume of nickel-iron in a chondrite, the greater must be the nickel to iron ratio in the metal.
2. The smaller the volume of nickel-iron in a chondrite, the greater must be the iron to magnesium ratio in ferromagnesian silicates.

These rules were landmark in the history of meteoritics and had a profound influence on the discipline from that time forward. But with their emphasis on just a few elements, Prior’s Rules also reflect the technological limitations of the early part of the 20th Century. Advances in meteoritics since that time have led some to treat Prior's Rules as having historical importance rather than contemporary relevance. It is important to remember however, that Prior's Rules recognize, mineralogically, the fundamental importance of oxidation states and by implication oxygen fugacity on the gross character of meteoritic samples.

Earlier work with chondrites and their classification was done notably by G. Rose, who first recognized the chondrites as a distinct group, Gustav Tschermak (left), who was a superb petrographer, and Aristides Brezina, who eventually divided the chondrites into more than 30 classes. The Rose-Tschermak-Brezina classification was widely used by students of meteorites until about 1960. The fallacies and superficial basis of much of the Rose-Tschermak-Brezina classification had been pointed out by Prior, but the system was not fully abandoned until the comprehensive review of meteorites by Mason. Students continue to use Mason's system of classification today, with few revisions.