Chondrites

Group H, L or LL - type 3, 4, 5 or 6

Planets and large asteroids show a differentiation process with an iron core, a mantle and a crust. All original material has been separated by heating and gravitation. Iron meteorites and achondrites come from these differentiated bodies.

Inside chondrites, the condensed interstellar dust of our early solar nebula is present: metal droplets, fine-grained matrix material and small spherical silicates called chondrules.

Chondrules don't occur in terrestrial rocks

85% of the falls are ordinary chondrites, you might think that these meteorites are really common. That's where you're wrong! The most common meteorites on earth are the iron meteorites with about 300 tons coming from 60 different bodies. The chondrites form a group of 50 tons, they are 6 times more rare than iron meteorites and carry valuable information about our earliest system history.

The classification of stony meteorites depends of their composition and their thermal metamorphism.

The most important minerals in chondrites are olivine and pyroxene. These silicates which contain iron and magnesium can vary widely in composition. The olivine with low iron content is called forsterite (magnesium silicate) and the iron rich olivine is the fayalite (iron silicate). The Fs% and Fa% are used to determine the group: H, L or LL.

To complete the classification, petrologic types from 3 to 6 are used (Van Schmus and Wood, 1967).
Minerals in large asteroidal bodies experience heating processes, such metamorphism has caused recrystallizations and mineralogical changes. Type 6 shows evidence of temperatures from 750° to 1000°, on the other hand, heating events didn't exceed 600° in type 3 which suffered less and remain the most fascinating chondrites meteorites.

Table showing the different classifications of chondrites
	H high-iron content	L low-iron content	LL low-iron content low-metal content
3 400° to 600°	Dar al Gani 862		Dar al Gani 632
4 600° to 700°	Dar al Gani 604	Dar al Gani 841	Dar al Gani 860
5 700° to 750°	Dar al Gani 606	Dar al Gani 970	Dar al Gani 633
6 750° to 1000°	Hamada al Hamra 290	Dar al Gani 608	Acfer 354
	H	L	LL

It is rather difficult to give a preliminary visual classification.

Chondrules are numerous in both type 3 and 4, sparse in type 5 or 6.
Various shapes of chondrules occur in type 3, while round chondrules are often the sign of metamorphism and type 4.
Visible metal doesn't always reflect iron content and terrestrial alteration changes metal into hematite and limonite minerals.
Type 6 without chondrules can be confused with magnetic achondrites.
LL chondrites, also called amphoterites, show low magnetism. Fragments from other material are sometimes embedded in their matrix, result of impacts on the parent bodies.
Chondrite breccias often show various petrologic types, the final classification will quote these two types (H3-5 for example)

To complete this classification, shock stage (Stoffler, Keil and Scott, 1991) and weathering grade (Wlotzta, 1993) are studied.

Shock stage S3
small opaque shock veins are visible
Shock intensity varies from S1 to S6. Collisions between asteroids produce pressures 750,000 times the Earth's surface atmospheric pressure and chondrites keep the traces of these impact events.

Weathering W2
oxidation of metal and rust stains
Weathering effects use a scale from zero to six. Fresh falls are usually in W0 or W1 that means no or minor oxidation of metal and sulfite. When there is a complete (>95%) alteration of metal and troilite, the weathering grade is W4. Old finds with massive replacement of silicates by clay minerals and oxides are in W6.

Orders
Polished slices are available, have a look at our catalog.

You can now have a look at these reference cards with a better understanding of the classification results.

Find all the reference cards in printable definition on the
SaharaMet 2004 Cd-Rom

Chondrites

The original material

Group H, L or LL - type 3, 4, 5 or 6