This type of preservation takes place when the original hard parts of organisms are removed after being dissolved by underground water. This is accompanied by almost simultaneous deposition of other substances in the resulting voids. Some replaced fossils will have the original structure destroyed by the replacing minerals. Others, as in the case of certain silicified tree trunks, may be preserved in minute detail.
Although more than 50 minerals have been known to replace original organic structures, the most frequent replacing substances are [calcite], [dolomite] (a calcium magnesium carbonate), [silica], and certain iron compounds.
[Replacement] by [calcareous] material
[Calcareous] [replacement] occurs when the hard parts of an organism are replaced by [calcite], [dolomite], or [aragonite] (a mineral which is composed of calcium carbonate but which is less stable than calcite). The exoskeletons of many corals, echinoderms, brachiopods, and mollusks have been replaced in this manner.
[Replacement] by [siliceous] material
When the original organic hard parts have been replaced by [silica] the [fossil] is said to have undergone [silicification], and this type of [replacement] often produces a very high degree of preservation. This is particularly true of the silicified [Permian] (geologic time scale, [Pl. 1]) fossils from the Glass Mountains in Brewster County. These fossils are embedded in limestone which must be dissolved in vats of acid, and after the enclosing [rock] has been dissolved the residue yields an amazing [variety] of perfectly preserved invertebrate fossils ([Pl. 3]).
Silicified [Cretaceous] fossils have been recovered from the Edwards limestone of central Texas. The silicified [fauna] is restricted to a few scattered localities, each of which may yield many unusually well-preserved fossils.
[Replacement] by iron compounds
Several different iron compounds have been known to replace organic matter. Many Texas limestones contain [fossil] snails and clams which have had their original shell material replaced by iron compounds such as limonite, hematite, marcasite, or [pyrite]. Certain of the [fossiliferous] [Tertiary] sandstones of the Texas Gulf Coast area contain large amounts of [glauconite] which commonly replaced organic material.
In some areas entire faunas have been replaced by iron compounds. Such is the case in the famous “[Pyrite] [Fossil] Zone” of the Pawpaw [formation] (Lower [Cretaceous]) in Tarrant County. The fossils in this part of the formation are very small or “dwarfed” and have been replaced by limonite, hematite, or pyrite. Ammonites, clams, snails, and corals are particularly abundant at this locality.