Replacement is a fossil preservation style involving a change in the crystal structure and a change in the mineralogy of an organism’s hard parts. The fossil is preserved with a mineralogy that is not the primary biologic mineralogy.
Example: start with clam shell composed of aragonite (CaCO3 - calcium carbonate). After death and burial, the original clam shell dissolves away during diagenesis and a different mineral fills in the space left by the shell. That’s replacement.
Many minerals have been reported as replacement minerals in fossils. Common replacement materials include quartz, pyrite, and calcium phosphate. Often, replacement is accompanied by a slight to significant loss of morphological detail. In the case of phosphate replacement, however, anatomical details down to the sub-micron level can be preserved!
If a fossil is preserved with quartz (silica), the term silicification is used. The fossil shown below is a partial solitary rugose coral (Animalia, Cnidaria, Anthozoa, Rugosa) that’s been silicified. Note that the silicification is in the form of irregularly-concentric discs or truncated spherulites of silica. These are beekite rings. “Beekite” is not a mineral, but the term has long been used to refer to such structures (either quartzose or calcitic).
Beekite rings on a the surface of a silicified solitary rugose coral (Devonian, Ohio, USA) (1.1 cm across). The surface details of the coral have been almost completely destroyed. Donated by Nicole Byrd.
If a fossil is preserved with pyrite (FeS2 - iron sulfide), the term pyritization is used.
Paraspirifer bownockeri spiriferid brachiopod (4.6 cm across) that’s been pyritized (view of dorsal valve). This is from the Silica Formation (Givetian Stage, upper Middle Devonian), from a quarry in the Sylvania area of Lucas County, northwestern Ohio, USA.
Paraspirifer bownockeri spiriferid brachiopod (5.1 cm across) with a heavy pyrite coating on its calcitic shell (view of dorsal valve). From the Middle Devonian Silica Shale of Sylvania, Ohio, USA.
Pyritized brachiopod shell pavement on shale (field of view 5.3 cm across). From the Middle Devonian Silica Shale of Sylvania, Ohio, USA.
Michelinoceras aldenense (4.1 cm across) - a pyritized internal mold of a straight-shelled nautiloid (Animalia, Mollusca, Cephalopoda, Nautiloidea, Orthocerida, Orthoceratidae) in a pyrite concretion. This is from the famous Alden Pyrite Bed (Ledyard Shale Member, Ludlowville Formation, Middle Devonian) of western New York State, USA.
Tornoceras uniangulare (lateral view, 2.7 cm across along the base of specimen) - a pyritized internal mold of a coiled goniatite (Animalia, Mollusca, Cephalopoda, Ammonoidea, Goniatitida, Tornoceratidae) in a pyrite concretion. From the Middle Devonian Alden Pyrite Bed of New York State, USA.
Tornoceras uniangulare (axial view of same specimen as above; 2.7 cm across).
Opal (SiO2·nH2O) is a rare replacement mineral. A famous locality having opalized fossils is the Coober Pedy Opal Field in South Australia. There, the remains of several groups of organisms are known with their hard parts replaced by precious opal, including bivalves, gastropods, belemnites, crinoids, ichthyosaurs, and plesiosaurs.
Opalized bivalve from the Coober Pedy Opal Field (upper Lower Cretaceous Bulldog Shale) of South Australia. (Wayne State University specimen, Detroit, Michigan, USA).
Opalized bivalve from the Coober Pedy Opal Field (upper Lower Cretaceous Bulldog Shale) of South Australia.
Here’s another rare replacement mineral - rhodochrosite.
Fossil bivalve, Arcicardium acardo (Mollusca, Bivalvia, Cardioidea, Cardiidae, Lymnocardiinae), replaced by rhodochrosite (manganese carbonate - MnCO3) from the Pliocene of the Kerch Peninsula (possibly from the Chernomorskiy Mine), eastern Crimea (northern Black Sea), southern Ukraine.
Chaledonized gastropods (specimen at far left is 3.0 cm tall) - these snail shells have been replaced by translucent, fibrous, microcrystalline quartz (SiO2). This variety of quartz is called chalcedony. Chalcedonization is a variety of silicification. Chalcedonized fossils are uncommon.
Locality: unrecorded, variously attributed to either Dakhla, “Western Sahara” (= southern Morocco) or the Gobi Desert of eastern Asia.
Reported fossil replacement minerals include: anglesite, apatite, barite, calamine, calcite, cassiterite, celestite, cerargyrite, cerussite, chalcocite, cinnabar, copper, dolomite, fluorite, galena, garnet, glauconite, gumbelite, gypsum, hematite, kaolinite, limonite, magnesite, malachite, marcasite, margarite, opal, pyrite, romanechite/psilomelane, siderite, silica/quartz, silver, smithsonite, specular hematite, sphalerite, sulfur, uranium minerals, and vivianite.
(List mostly from info. in Hartzell, 1906 and Klein & Hurlbut, 1985)