REPLACEMENT
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.
Age:
Cretaceous
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)