I was looking for this and failed
where can we buy their minerals ?
Go in mining tabs on the civilian mining colonies, should be at the bottom
To be clearer, Open the Colony window, switch the chosen "population" to the mining colony with CMCs, then either the mining, wealth, or commercial shipping tab.
If you so desire, set the mass driver target to deliver the minerals to another colony. Personally, I prefer government freighters on cycling orders to pick them up.