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
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".
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
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 silica).
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.