HAH - right up my ally. The one line
answer is a Permineralization --- often
the word "petrification" is used in normal
discussion, but this carries the implication that
ALL of the fossil has been mineralized.
This is sometimes the case, but more often
minerals entrap original organic matter of the
organism. How does this happen? Well,
minerals are borne in soil moisture
(groundwater). These tend to precipitate out
of the groundwater in open spaces. If, by
example, you bury a tree, the cells lose their
contents but the cell walls may remain for a long
time before rotting. If these cells are invaded
by a mineral-rich groundwater, then the mineral
will precipitate out in the cavities created by
the missing cell contents - trapping the
organic material in the process.
Thus, the best "petrified" wood for
scientific study is black in color, because
it retains the original cell walls, trapped in a
hard mineral matrix. Sometimes after
a primary permineralization occurs, the cell walls
will rot out, creating spaces - a secondary
infiltration of minerals can then occur, indeed
turning the whole fossil to "stone". Often
these take on a variety of bright colors, but in
this process the cellular detail is generally
lost. These secondary replacements thus create
colorful rock-shop items, but are often not of
great scientific value. The most common mineral
involved is silica (generally from volcanic
ash), but calcite, pyrite and iron carbonate
may also create permineralizations.
Actually, it is still called a fossil! When
another mineral replaces original bone or shell
material, we call this process replacement.
Most fossils in rocks are changed from the
original material in some way. In the process of
replacement, molecules of original material are
replaced one at a time. Two common types of
replacement are calcification (replacement by
calcite) and silicification (replacement by
Permineralization is another process that
can change original plant or animal remains.
This is when voids are filled with new minerals.
The most common example is petrified wood where
silica has filled in individual plant cells.
The word 'permineralization' or 'petrification'
leaps to mind, but a permineralization is where
the mineral invades organic matter rather than
replacing it (the original carbon in petrified
wood is still there...).
Pyritization is where the
tissues are being replaced or invaded by pyrite
(pyritization is both rare and requires anaerobic
environments). However, to get genuine replacement
under normal circumstances, I think you would need
your fossil to be mineral to start with - like the
shell of a clam. That's just recrystallization.