File:Arcicardium acardo rhodochrositized fossil bivalve (Pliocene; Kerch Peninsula, eastern Crimea, southern Ukraine) (15360692248).jpg

Arcicardium acardo Deshayes, 1838 rhodochrositized fossil bivalve from the Pliocene of Ukraine.

This fossil cockle shell has had its original aragonite shell replaced by pinkish rhodochrosite (MnCO3 - manganese carbonate). Rhodochrosite is a rare fossil replacement mineral.

Classification: Animalia, Mollusca, Bivalvia, Cardioidea, Cardiidae, Lymnocardiinae

Age: Pliocene (possibly from the Kerchenskoe Deposit)

Locality: Kerch Peninsula (possibly from the Chernomorskiy Mine), eastern Crimea, southern Ukraine

Replacement is a fossil preservation style involving the crystal structure and the mineral of an organism's hard parts being changed.

The most common replacement mineral is quartz (silica) (SiO2) - fossils that have been replaced by quartz are said to be silicified (silicification). Many silicified fossils have rounded to pustulose structures covering their surfaces. These are called beekite rings, but they're composed of ordinary quartz.

Other common replacment materials include the mineral pyrite (FeS2 - iron sulfide) and calcium phosphate. These replacement styles are called pyritization and phosphatization.

Numerous other minerals have been found replacing minerals - many of them are quite rare. 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.