Couldn't find it in the forum, but found this:
https://forums.ross-tech.com/index.php?threads/8529/
Allegedly this is the purpose:
(Still should have a part number though surely!)
"Coolant Expansion Tank with Silicate Repository
The coolant expansion tank contains a silicate repository. Silicate is used to protect the aluminum components in the coolant
system from corrosion. There are silicates in the G13 coolant, but they are used up over time if the engine is subject to high
thermal loads.
To compensate for the silicate consumption, silicate is taken from the repository and added to the coolant. The silicate
repository provides additional protection against corrosion for the aluminum components in the coolant system over the entire
lifespan of the engine."