Saponite
Saponite is a clay mineral that, with serpentine, makes up much of CI chondrites and the Ryugu samples.
Saponite is a magnesium-rich smectite clay, a sheet silicate that can absorb water between its layers. On Earth it forms where water alters basalt and other magnesium-rich rocks.
In meteorites, saponite is a main component of CI chondrites such as Orgueil and Ivuna, intergrown with serpentine at a scale too fine to see without an electron microscope. It also occurs in some CR and CV chondrites and in the samples returned from the asteroid Ryugu by Hayabusa2, which closely resemble CI chondrites.
Saponite forms at fairly low temperatures in the presence of liquid water, typically well below about 150 °C, so it is evidence of long-lived watery conditions inside early asteroids. Its swelling behaviour also matters for collectors and curators: CI meteorites take up moisture from the air, which can make them crumble and grow salts, so they must be stored in dry conditions.
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