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I just learned that the cryptocrystalline form of silica, chalcedony, usually contains 1-20% of the monoclinic $\ce{SiO_2}$ mineral mogánite as well as the much more common hexagonal quartz. With time the monoclinic part alters to quartz.

Why do the two crystal structures form together? My guess is that the mogánite is not stable but an aqueous solution is supersaturated with respect to quartz so mogánite can precipitate out. But that doesn't explain why it forms simultaneously with quartz. If quartz is readily precipitating, it doesn't make sense to me that it would typically reach mogánite saturation - if that is what's happening. Perhaps mogánite is the stable form but has lousy crystallization kinetics and can only nucleate on a colloidal quartz surface.

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