What is the matrix of a chondrite?
The matrix is the fine-grained material that fills the spaces between chondrules and other components in a chondrite, like the mortar between bricks.
In a chondrite, the large, recognisable objects are chondrules, CAIs and metal grains. Everything in between is the matrix: fine-grained material, usually made of grains less than a few micrometres across.
What it contains. In primitive chondrites, the matrix contains a mixture of tiny olivine and pyroxene crystals, amorphous silicates, sulfides, metal, organic matter and, crucially, presolar grains, stardust older than the Sun. In carbonaceous chondrites that were altered by water, the matrix is largely made of clay minerals, carbonates, magnetite and sulfides.
How much. The amount varies. In CI chondrites, nearly the whole rock is matrix. In CM chondrites, it is about half to three-quarters. In ordinary chondrites, it is only about 10 to 15 per cent in the most primitive types, and it recrystallises with heating.
Why it matters. The matrix is a sensitive record. Because its grains are tiny, it reacts easily to heating and water, so it shows the history of the parent body. In the most primitive meteorites, it preserves the original dust from which planets formed, including grains from other stars and interstellar organic matter.
Complementarity. Some studies have found that chondrules and matrix in a given chondrite have complementary chemistry: individually they differ from solar values, but together they average out close to them. This suggests that chondrules and matrix formed from the same local reservoir, although this idea is debated.
The matrix is often dark, giving many chondrites their grey or black interiors.