Two referees thoroughly evaluated the manuscript, and the authors answered in detail to the comments. Most of these comments have been addressed appropriately. Still, I would like to emphasize some important issues the authors still should focus on when revising the manuscript. Beside all given comments by the reviewers these include particular the following points:

1) possible challenges with field application should be put into the discussion and not the conclusion chapter
2) I agree to keep the rather long theory because it is helpful for readers not very familiar with the topic; still try to shorten parts if possible as it indeed is very lengthy
3) it should be at least mentioned in the manuscript that synthesis procedures for the iron oxides have been verified earlier
4) thoroughly check for recent, important references to strengthen your introduction.

All reviewer comments require some substantial rewriting of the manuscript according to the given answers and my comments above. A revised version of the manuscript can be uploaded and, I am happy to then reconsider the manuscript for publication after these major revisions were done.

The authors improved the manuscript according to the reviewer comments. In particular, they added more relevant literature to the introduction, emphasized the objectives, shortened the methods and added a new paragraph about field applications to the discussion. Therefore, the manuscript can be accpetd for publication in HESS.