Permineralization

Permineralization is a process of fossilization in which mineral deposits form internal casts of organisms. 
Minerals in water fill the spaces inside organic tissue. The process gives a record of soft tissue as well as hard tissues. Fossils with permineralization are useful in studying internal structures, especially of plants.
Water from the ground, lakes, or oceans seeps into the pores of organic tissue and forms a crystal cast with deposited minerals. Crystals begin to form in the porous cell walls. This process continues on the inner surface of the walls until the central cavity of the cell, the lumen, is completely filled. The cell walls themselves remain intact surrounding the crystals. Permineralization is different from petrification in that the organic material is only filled with minerals and not completely replaced. Permineralization can occur in several ways:
Types.
Silicification.
Silicification is the most common type of permineralization.
Carbonate mineralization.
Carbonate mineralization occurs as coal balls. Coal balls are fossilizations of plants and their tissues, usually made when there is seawater or acidic peat. This type of fossilization gives information about plant life in the Upper Carboniferous (325 to 280 million years ago).
Pyritization.
Organisms are pyritized when they are in marine sediments saturated with iron sulfides.