Saponite
A swelling clay mineral intergrown with serpentine in CI chondrites and in samples from Ryugu and Bennu.
Key facts
- Formula
- (Ca,Na)0.3(Mg,Fe)3(Si,Al)4O10(OH)2·4H2O
- Family
- Silicates
- Crystal system
- Monoclinic
What it is
Saponite is a smectite clay: a sheet silicate whose layers can hold water and loosely bound sodium or calcium between them, so that it swells when wet. Its name comes from the Latin for soap, because it feels soapy.
Where it is found
With serpentine it is the main mineral of CI chondrites such as Orgueil and of the Ryugu and Bennu samples, where it forms very fine intergrowths. It also occurs in some CR and CV chondrites and in the alteration veins of nakhlites from Mars.
How it is recognised
It can be seen only with an electron microscope. X-ray diffraction reveals the spacing of its layers, which changes as water enters or leaves.
Did you know?
The clays in Ryugu grains formed in water at temperatures probably below about 40 °C.
Saponite-bearing clays on Mars have been mapped by orbiting spacecraft.