organisms are more likely to be more preserved in mud than sand because mud is moist and wet, while sand is very dry.
the balanced equation for reaction of ca metal with o2 to produce cao is as below
2 ca+ o2→ 2 cao2
the reaction is balanced since the number of atoms in reactant side is equal to number of atoms in product side. for example they are 2 atom of ca in reactant side and 2 atoms of ca in the product side.
the process is a meet the requirement of redox reaction because : calcium is oxidized from oxidation state 0 to 2+ while oxygen is reduced from oxidation state 0 to 2-