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Porcellanite

Sedimentary Rock Types

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Porcellanite ("por-SELL-anite") is a rock made of silica that lies between diatomite and chert. (more below)
Porcellanite
Unlike chert, which is very solid and hard and made of microcrystalline quartz, porcellanite is composed of silica that is less crystallized and less compact. Instead of having the smooth, conchoidal fracture of chert, it has a blocky fracture. It also has a duller luster than chert and is not quite as hard.

The microscopic details are what is important about porcellanite. X-ray examination shows that it is made of what's called opal-CT, or poorly crystallized cristobalite/tridymite. These are alternative crystal structures of silica that are stable at high temperatures, but they also lie on the chemical pathway of diagenesis as an intermediate stage between the amorphous silica of microorganisms and the stable crystalline form of quartz.

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