Just for the fun of it I decided to see how different is the residue after evaporating the water coming directly from the well, vs passed through the softener, vs passed through the reverse osmosis filter (iSpring under-the-sink system).
I have filled in 3 glasses, covered them with a napkin and left to evaporate slowly in a secluded corner in the basement.
To my surprise it took a while (about 2-3 months, probably due to high humidity in the basement in the summer).
The filter has (quote) “… Alkaline mineralization filter cartridge with Mineral Stone, Calcite, and Corosex …”. The content inside is of brown/yellow color, so I guess that’s where the brown crystals come from.