Why do rock fossils form in sedimentary rock rather than in igneous rock?

Sedimentary rocks where just sediments like sand until an animal died and stuck there. The rock cycle says the rock goes lower in the earth. So when the animal died in just the right conditions it travelled down with the sediments as well. Compaction of this sediment made it rock. Surrounding the animal making a fossil. After a while the fossil turned into oil or coal. Metamorphic rocks are just heat applied rocks. This is the reason why it only happens in sedentary rocks instead of igneous or metamorphic.

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I am afraid that answer is a confused mess. Let us start again.

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Sedimentary rocks form from sand and other materials eroded from rock, and precipitated minerals. These sediments may bury dead animals or plants and in time the sediment becomes rock, by compaction, de-watering and lithifying, with the buried remains becoming fossils within it.

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Oil and coal are fossil fuels formed from organic debris, but other fossils do not turn into them.

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Metamorphic rocks are formed from any other rock by heat and/ or pressure, not necessarily by heat alone; but metamorphism generally destroy any fossils in sedimentary rocks undergoing the change.

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Fossils cannot form in igneous rocks by definition: igneous rock is formed from its minerals in a molten state, mainly underground but also as volcanic lava which would obviously destroy any plant or animal it engulfs in erupting.