shells are made of aragonite. During the fossilization
process aragonite reverts to a more stable form of calcium
carbonate called calcite. Thus, recrystallization from
aragonite to calcite represents a type of replacement.
Some shells are made of layers of calcite and aragonite.
into larger calcite crystals. The overall shape of the shell
may remain, but the effect of recrystallization on microscopic
texture is evident (Prothero, 2004, p. 9).
that fills in empty spaces or replaces structure in organic
materials during permineralization undergoes a recrystallization
process. In this case, it is not the original organic
matter that is recrystallized. Permineralized
fossils form when solutions rich in minerals permeate porous
tissue, such as bone or wood. Minerals precipitate out
of solution and fill the pores and empty spaces. During
the initial stages of permineralization amorphous silica
pits connecting cells and cell lumina (the cavity enclosed
by the cell walls). At this early stage no replacement
of cellulose in the walls of cells may occur as permineralization
continues. Cellulose that degrades leaves room for the emplacement
of silica between and within cells walls. The more decay
lignin that remains in the cell walls continues to act
as a guiding framework
that initially permeates the porous tissue is amorphous.
Silica that replaces cell wall material is also amorphous.
This amorphous silica is unstable and slowly crystallizes
more stable forms over millions of years. The transition to more stable
forms of silica involves continued polymerization and water
forms of opal are created through
this process and eventually lead to the thermodynamically more stable silica
quartz (Stein, 1982, p. 1277). The quality of preservation usually, but
always, declines during successive stages of silicification
(Mustoe, 2003, p. 36). Read
our article on Permineralization in
the Fossil Types section of our museum to learn more.
G.E. (2003). Microscopy of Silicified Wood. Microscopy
Today, vol 11, no 6, pp. 34-37.
D.R. (2004). Bringing Fossils to Life: An Introduction
to Paleobiology [2nd edition]. New York: McGraw-Hill.
C.L. (1982). Silica Recrystallization in Petrified Wood. Journal
of Sedimentary Petrology, vol 52, no 4. pp. 1277-1282.