Fractionation of Oxygen Isotopes at the Faces of Smoky Quartz

Recently we reported on a new isotope effect [1]: During the growth of natural amethyst, Si02 with different lsO content is incorporated at crystallo­ graphically different faces of the crystal, the 180 / 160 ratio increasing in the order z-face, r-face, m-face. Meanwhile a similar effect in natural calcite was re­ ported by J. A. D. Dickson [2]. Care must be taken if one wants to single out such an effect because during the hydrothermal growth of the crystal the isotope composition of the mother liquid may have varied, this leading to a corresponding iso­ topic layering ("mantle zoning") inside the crystal, which is superimposed on the sought for isotopic "sec­ tor zoning". In a polished plane-parallel slice of quartz, which is cut out from the crystal parallel to the c-axis and perpendicular to an m-face, one sees the boundary between the rand m-sectors if during the growth impurities, which cause colouring, have been incorpo­ rated preferentially at one of the two faces. This was the reason why for our first study we had chosen amethyst and in the present one we deal with smoky quartz. The colours of both smoky quartz and amethyst are due to aluminium and iron impurities. If the aluminium impurity Al3 + substituting silicon ex-