Stone meteorites (Chondrites and Achondrites)
Stone meteorites are the largest and most extensive group, and definitely the most interesting as regards structure. This group includes the commonest but also the rarest meteorites. They consist mainly of silicate minerals. Some have a little triolite (iron sulphide), and most have up to 30 % of nickel-iron, while a few are entirely without. Even though they contain minerals that are not unknown on earth, they have an inner structure that indicates their cosmic origin.
Chondrites (OC – ordinary chondrites)
This group is the commonest, and comprises more than 80 % of all stone meteorites and 85 % of all observed falls. These meteorites contain characteristic millimetre-large spherical grains called chondrules. They consist of the minerals olivine and pyroxene, and in quantity they can vary from 5 to 70 % of the meteorite. The word chondrules originates from the Greek word chondros meaning corn. They are thought to have been formed in the solar cloud in an early stage in the development of the solar system about 4.5 billion years ago. The nickel-iron content in these meteorites is between 10 and 30 %, either as a distinct element or as a constituent of the minerals. Some chondrites can contain various carbon compounds and are important for our understanding of how organic compounds can develop. Chondrites can be divided into a series on sub-groups.
Achondrites, being without chondrules, are the opposite of chondrites. Achondrites are rare, and some in this group have their origin in large asteroids such as Vesta, the planet Mars or the moon! They also have an appearance and content that strongly resemble earthly volcanic rock types as, for example, basalt. Achondrites are younger than chondrites, and most of them were formed about 3.5 – 3.7 billion years ago.
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