A New Intracrystalline Isotope Effect: ¹⁸O under the Faces of Amethyst

Abstract

The concentration of ls O within well formed crystals of amethyst was found to vary beneath the different growth faces, increasing in the order z -»• r m. The 18 0/ 16 0 fractionation factor between the faces m and z was measured as 1.0007.

Oxygen isotope sector zoning in natural hydrothermal quartz

“…This is in accordance with the studies by Klemm et al (1990Klemm et al ( , 1991 who found differences of 0.4% between m and r and 0.2% between r and z for an amethyst crystal and of 0.2% between m and r in a smoky quartz. Their conclusion put forward the hypothesis of possible growth rate effects, but also a role of attachment energy.…”

“…Previous work on quartz samples by Klemm et al (1990Klemm et al ( , 1991, Onasch and Vennemann (1995), but also the work on natural calcites by Dickson (1991Dickson ( , 1996Dickson ( , and 1997 has indicated that there may be a face control on O-isotope fractionations. Such an effect may be enhanced at lower temperatures of crystallization, and/or amplified by a large trace element content, and/or related to a difference in the rate of crystallization, which in itself may also affect the trace element incorporation.…”

“…6 that, based on volume of growth, face r 2 from sample VA03 has grown faster than r 1 (Poty, 1969;Klemm et al, 1990Klemm et al, , 1991. If growth kinetics affect O-isotope fractionations in the sense described by Watson (2004), smaller d 18 O values would then be expected in the faster growing crystallographic orientation (e.g.…”

“…Structural arrangements of the crystal lattice may affect the incorporation of trace elements into crystals Reeder, 1990, 1995;Jourdan et al, 2009), and hence isotopic fractionation (Klemm et al, 1990(Klemm et al, , 1991Dickson, 1991;Onasch and Vennemann, 1995;Méheut et al, 2009). The three different types of crystallographic orientations À r, z and m in quartz have different surface structures and hence somewhat different lattice arrangements, with m having a tighter arrangement of atoms at the surface (Dowty, 1976).…”

“…Measurements of O-and C-isotope compositions in carbonates have, however, produced contradictory results and it is still not clear if isotopic sector zoning actually exists. Previous O-isotope measurements in quartz have already indicated the possibility of sector zoning, with d 18 O values that can differ by up to 2% between crystallographically different growth sectors of the same growth zone (Klemm et al, 1990(Klemm et al, , 1991Onasch and Vennemann, 1995).…”