The asbestos and picrolitic veins of the asbestos deposits in the Thetford-Black Lake District are very similar in structure and tend to grade into one another. Field and laboratory evidence suggests that the original vein serpentine was in an amorphous or nearly amorphous state, and that the veins are in many cases of a composite nature, resulting partly from fissure-filling and partly from wall rock replacement. It is proposed that the picrolite and asbestos were derived through crystallization of this vein material, and that two stages of crystallization were involved such that a first stage gave rise to picrolite and a second resulted in the conversion of picrolite to asbestos.

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