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Caused by incomplete or late division of the cells before they form a foetus, similar twins occur only once in every 200,000 live births.
Very similar twins of sal ammoniac were reported from Arniston in Great Britain by Shand (1910), and from Mount Vesuvius in Italy by Scacchi (1874).
There are isolated examples of similar twins of sphalerite and sal ammoniac, but only a few other species are likely to form such twins.