Stony Meteorites The largest of the three main types of meteorite group is that of the stones: over 90% of all meteorites that fall on the Earth are stony. The major division of the stones is into melted and unmelted material. A third set of stony meteorites is that of melted rocks from non-asteroidal sources, meteorites from the Moon and Mars. Achondrites Melted stony meteorites record the earliest processes of melting and igneous rock formation on asteroids. Differentiated meteorites record the extent, timing and duration of melting and subsequent magmatic activity on their parents, indicating the short timescale on which parent bodies aggregated, then differentiated. Broadly speaking, an achondrite is a stony meteorite that formed from a melt on its parent body; they resemble basalts produced by igneous processes on the Earth. Thus achondrites have differentiated compositions, having lost a large fraction of their primordial metal content, and generally do not contain chondrules. There are many different groups of achondrites. One of the largest groups is thought to come from the asteroid (4) Vesta, or asteroids similar to Vesta. Chondrites The most interesting stony meteorites are from asteroids that have not melted since their formation. Unmelted stony meteorites are known as chondrites, a description deriving from their texture. The Greek word, chondros, means "little grain", "seed" or "droplet"; looking at the surface of the stone meteorite, we can see that the stone has the appearance of a droplet texture. The chondrules The droplets (0.1 to 10 mm in size) are known as chondrules, and the precise way in which they formed is not known. It is possible that as the pre-solar nebula collapsed into the protoplanetary disk, dust grains collided with each other. At the speed at which they were travelling during the collisions, the dust grains would have instantly melted then quenched into silicate droplets. The grains were then swept up into asteroidal bodies. These asteroidal parents experienced mild heating, and also alteration by fluid from melting ice, but have never been completely melted since their formation. The composition of chondrites Meteorites from unmelted asteroids are undifferentiated. In other words, they have a primitive composition, which, apart from the most volatile of elements like hydrogen and helium, matches the composition of the Sun. Chondrites are composed of high temperature components (CAIs, chondrules) set in a matrix of fragmented chondrules mixed with minerals formed at lower temperatures. The CAIs (for Calcium, Aluminium-rich Inclusions) are refractory inclusions (up to ~ 1 cm in size) of spinel, hibonite, melilite, etc. These objects are believed to be the oldest solids formed: the first materials to condense from the pre-solar nebula. Chondrules are spherical to sub-spherical silicate assemblages, up to 1 mm in diameter, that have been partially or totally melted prior to parent-body accretion. The main component of chondrites, the chondrules, are made from Fe- and Mg-rich silicates, the minerals olivine and pyroxene. They are thought to have formed about three million years after the CAIs. Chondrules and CAIs are set in a matrix of broken chondrules plus metal and sulphide grains. When chondrites are studied in thin section, the internal structure of the chondrules can be seen. Looking at this structure and the relationships between the different mineral types within the chondrules gives information on the composition of the dust from which the Solar System formed, and the physical conditions (pressure, temperature, oxygen abundance) of the protoplanetary disk. Interstellar dust grains in chondrites Also preserved within the chondritic matrix are unaltered interstellar dust grains. These grains are tiny diamond crystallites, no more than about three nm (1 nm = 1 millionth of a millimeter) across. The interstellar diamonds are accompanied by silicon carbide, or carborundum, graphite and aluminium oxide. These different grains have been formed in stars other than our own Sun. The evidence to support this statement comes from measuring the isotopic composition of the silicon, carbon, nitrogen, oxygen and noble gases in the grains. Some grains were formed in the outflowing wind from stars such as red giants, whilst others were formed during a supernova. It is possible that this might have been the supernova that initiated the collapse of the molecular cloud, leading to the formation of the Solar System. These materials were introduced into the pre-solar nebula from neighbouring stars, prior to parent-body aggregation, and thus pre-date the major chondritic components. Populations of interstellar grains in primitive asteroidal meteorites reveal the constituency of the neighbourhood in which the Sun initially formed, helping to constrain astrophysical models of stellar evolution. Carbonaceous chondrites One sub-group of chondrites, the carbonaceous chondrites are so-called because they have a high concentration of carbon (up to 5% by weight); they are also rich in water and sulphur. The carbon occurs as organic molecules including amino and carboxylic acids, plus a complex, highly cross-linked species. During the final stages of Solar System formation, it is believed that the Earth was bombarded by asteroids and comets rich in organic molecules. It is possible that the organic components in carbonaceous chondrites were the precursor molecules, the building blocks, from which life on Earth eventually arose.

A stony meteorite which landed in India in 1865. Its total weight is 5 kg.