brown mineral deposit in toilet AFTER H2O softener

We’ve recently installed our water softener, which I am quite happy with. One odd thing that has happened is that we are now getting a brown ring of mineral deposit in the toilet. I would have thought the softener removed the iron and other metal ions. Can someone give me the chemistry behind why we are getting this ring now? Is the pH of the water different with the softener and will that cause some residual oxide or something to fall out of solution? Is it organic?

Softening shouldn’t change the pH much.
Your water pipes were equilibrated with hard water. They likely had a build up of mineral deposits. Those deposits need time to re-equilibrate with the new, soft water. Once they do that, the brown stains should go away. Of course, there’s no telling how long it’ll take for that to happen.

Possibly, but I would have thought I would be getting stains before the water softener too if that was the case.

And my neighbor whose house has always had a water softener has the same stains.

one obligatory bump before this falls into my huge pile of uninteresting topics

Water softeners will only remove limited iron from water. If you have more you need an iron filter. Here is a good site that explains things http://www.apswater.com/how_to_select_a_Water_softener.asp

The simple explanation is that the piping system was depressurized to facilitate installation of the water softener, and some rust was knocked loose when everything was powered up.
No different than when hydrant flushing takes place-your water can be rusty for a day or two.