Freshly-fractured crystalline silica, such as that emitted during mining and processing, is significantly more toxic than "weathered" silica (from, e.g., agricultural sources), apparently through reactive oxygen species generated mechanically. This same source suggests a half-life of freshly-fractured silica of about 30 hours.

There is also evidence that freshly-fractured silica causes more lung reactivity and inflammation than weather ("aged") silica: 

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Silicosis Find out more from other research