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September-October 2012

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Let's get it Right: Not Japan-Law Twinning

Quartz crystals are known to twin in many different ways; some are quite common, but most are rare. To quote Frondel (1962), “A twin is a geometrical position of intergrowth of two or more crystals of the same species with which is associated a frequency of occurrence greater than that of chance.” The three most common twin “laws” for quartz are the Dauphiné, Brazil, and Japan laws (not Japanese law, please!). With quartz, twinning tends to be the rule rather than the exception, to the extent that the famous French mineralogist Alfred Lewis Oliver Legrand Des Cloizeaux (for whom descloizite was named) is quoted as stating that an untwinned quartz crystal is one of the greatest rarities of the mineral kingdom. The Japan law is the most familiar to collectors because it is the easiest to recognize, being the only one of the three mentioned above in which the axes of the twin members are inclined with respect to each other (fig. 1). Dauphiné- and Brazil-law twins have parallel axes.

John S. White, a consulting editor of Rocks & Minerals, operates Kustos, a museum/collector consulting business.

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