unaltered preservation (like insects or plant parts trapped in amber, a hardened form of tree sap)
permineralization=petrification (in which rock-like minerals seep in slowly and replace the original organic tissues with silica, calcite or pyrite, forming a rock-like fossil - can preserve hard and soft parts - most bone and wood fossils are permineralized)
replacement (An organism's hard parts dissolve and are replaced by other minerals, like calcite, silica, pyrite, or iron)
carbonization=coalification (in which only the carbon remains in the specimen - other elements, like hydrogen, oxygen, and nitrogen are removed)
recrystalization (hard parts either revert to more stable minerals or small crystals turn into larger crystals)
authigenic preservation (molds and casts of organisms that have been destroyed or dissolved).

thanks trixie,thats some great info.you undoubtedly do your home work !!