Way beyond my knowledge base to even speculate. The NPS website has a bit of info on how fossils form, and the important bit seems to be that the material has pores which mineral carrying water can flow through. The site does mention that softer tissues like cartilage can undergo permineralization as well, but... Silicones are pretty permeable to gas, but fairly impermeable to liquids. They'd probably lose the bits that 'decayed' from the outside in, instead of the permineralization happening throughout as the material decays.