Tornoceras uniangulare aldenense House, 1965 pyritized fossil goniatite from the Devonian of New York State, USA (lateral view, 2.7 cm across).

This is a pyritized internal mold of a coiled goniatite cephalopod in a pyrite concretion, from the famous Alden Pyrite Bed of New York State.

Classification: Animalia, Mollusca, Cephalopoda, Ammonoidea, Goniatitida, Tornoceratidae

Stratigraphy: Alden Pyrite Bed, Ledyard Shale Member, Ludlowville Formation, Middle Devonian

Locality: western New York State, USA

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The most common replacement mineral is quartz (silica) (SiO2) - fossils that have been replaced by quartz are said to be silicified (silicification).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.