File:Silicified fossil coral (Middle Paleozoic; gravel bar clast, Chatham, Ohio, USA) (26370187737).jpg

Silicified fossil coral from the Middle Paleozoic. (centimeter scale)

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The most common replacement mineral is quartz (silica) (SiO2) - fossils that have been replaced by quartz are said to be silicified (silicification).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. (List mostly from info. in Hartzell, 1906 and Klein & Hurlbut, 1985)

This fossil is a silicified colonial coral having a honeycomb-like skeletons. Corallites are closely-packed and frequently have hexagonal outlines.

Classification: Animalia, Cnidaria, Anthozoa

Provenance: modern fluvial clast derived from Pleistocene glacial drift; ultimately derived from a Middle Paleozoic unit - probably Devonian