The crystal structure of hohmannite, Fe\\r sub/2sub\\r (H\\r sub/2sub\\r O)\\r sub/4sub\\r [(SO\\r sub/4sub\\r )\\r sub/2sub\\r O].4H\\r sub/2sub\\r O and its relationship to amarantite, Fe\\r sub/2sub\\r (H\\r sub/2sub\\r O)\\r sub/4sub\\r [(SO\\r sub/4sub\\r )\\r sub/2sub\\r O].3H\\r sub/2sub\\r O