Subject: What are chemical details of fossil formation.

Fossils often carry details on organic structures that have been replaced. 
What happens chemically that preserves detailed structural info. For 
example: fossil tree rings have variations in mineral structure...some 
rings are opaque some are translucent. Bones start as mineral structures 
but they are transformed too...what happens in transformation at micro 
level. Do partially transformed fossils exist? Why not?
Dinosaur dies in mud or volcanic ash and gets covered with fine ash. Why 
couldn't the covering "slow cook" in a way that seals in organics...like in 
amber. Why is fossil dinosaur bone harder than surrounding 
sediments....aren't the minerals in the sediments the source of the bone 
mineralization?
Is any of bone mineral left behind? i.e. can we see any isotope ratios
that were in original structure....Calcium/Oxygen isotopes for example.
Where does the Calcium go? 

Re: What are chemical details of fossil formation.