Under supergene process influencing, kaolinites invered dickites, siderite were been ferritization, part of carbonate mineral have been dissoluted producing calcite.
There is siderite, iron dolomite , calcite, Barite, pyrite, anatase, rutile, piece quartz, the mica ( or hydromica ), diaspore,etc. in the impurity in the mineral .
It seems that iron is the paleontologist's friend so any kind of iron cementation, iron stone cementation, so siderite, iron carbonate helps to preserve structures within sediment.