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Quartz replaced anhydrite nodules (‘Bristol Diamonds’) from the Triassic of the Bristol District

Published online by Cambridge University Press:  01 May 2009

Maurice E. Tucker
Maurice E. Tucker
Affiliation:
Department of Geology, University of Newcastle upon Tyne, Newcastle NE1 7RU, U.K.
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Summary

Quartz nodules and geodes, known as Bristol Diamonds, occur in the Triassic Dolomitic Conglomerate of the Bristol district of England. They are composed of either, megaquartz, milky in the outer part and clear towards the centre, or fibrous quartz, chalcedony and lutecite. Much of the quartz, particularly in the outer parts, is full of anhydrite inclusions which are relics of larger lath-shaped anhydrite crystals. The Bristol Diamonds formed by silica replacement of anhydrite nodules, with replacement proceeding from the outside inwards. In many cases anhydrite solution was faster than silica replacement, so that a central void developed into which grew crystals with fine terminations. The anhydrite formed by precipitation of sulphate from hypersaline sediment-porewaters, probably in a marginal playa situation.

Type
Articles
Information
Geological Magazine , Volume 113 , Issue 6 , November 1976 , pp. 569 - 574
Copyright
Copyright © Cambridge University Press 1976

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