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Fig. 11. X-ray fluorescence mapping using high-energy synchrotron radiation very close to the top of the Obi 84
sample. (a) Energy spectrum resulting from excitation with 23 keV radiation. Only Zn, Pb, Ca and Sr can be reliably
quantified using a deconvolution procedure. (b) scale bars in maps in counts (logarithmic for Zn and Pb and linear for Ca
and Sr). (c) Elemental maps of an area 1.1 mm by 0.04 mm at 2 mm resolution (top of section at left side). Probably eight
annual layers are shown by Zn and Pb with a complex pattern of enriched micro-laminae because of excitation of
crystallites at different depths below the surface: hence this is a pseudo-3D image. Levels of Zn and Pb are more
uniformly high in two annual cycles on the far right.
Discussion
the beginning of growth), or following a hiatus
(when heterogeneous re-nucleation is often
observed). Hence, the similar optical orientation of
adjacent composite crystals at the top of Figure 2b
implies that competition has occurred.
Speleothems often are formed of composite
(rarely of single) crystals with uniform extinction
(Frisia et al. 2000). Commonly, the columnar crys-
tals will be characterized by an overall orientation of
the z axis (the ,0001. direction) roughly perpen-
dicular to the speleothem surface. If the system is
not disturbed by the presence of growth inhibitors,
such as organic compounds or trace metals, or by
changes in flow at the top of the speleothem, the
crystallites covering the top of the stalagmites will
be characterized by flat rhombohedral faces. In
the case of columnar crystals (sensu Frisia et al.
2000), the crystallites would be expected to be
strictly parallel in lattice orientation, whereas in
microcrystalline calcite (sensu Frisia et al. 2000),
some mismatches in orientation of crystallites are
Chemical control of crystal fabrics
The Obir stalagmites, and specifically Obi84, which
has been studied in most detail, display a series of
striking fabric characteristics, some of which have
not been previously recorded. However, their pre-
ferred optical orientation is a common characteristic
of speleothems. In freely growing columnar crystals
this preferred orientation arises through geometrical
selection, whereby crystals orientated perpendicular
to the growth surface grow fastest and become
predominant, occluding the individuals that are
not favourably orientated (Dickson 1993; Onac
1997). The resulting aggregates can be optically
length-fast or length-slow dependent on crystal
morphology (Dickson 1993). However speleothem
growth is confined within a mm-scale solution
film (Dreybrodt 1988) that limits competition
either to periods whilst crystals are still small (at
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