New Zealand Journal of Geology
and Geophysics abstracts
Petrography of quartz grains
in beach and dune sands of Northland, North Island, New Zealand
J. J. Kasper-Zubillaga1
W. W. Dickinson2
A. Carranza-Edwards1
Y. Hornelas-Orozco1
1Instituto
de Ciencias del Mar y Limnología
Universidad Nacional Autónoma de
México
Apartado Postal 70-305
Mexico, D.F.
Mexico 04510
2School
of Earth Sciences
Victoria University of Wellington
P.O. Box 600
Wellington, New Zealand
Abstract Petrographic
and scanning electron microscopy (SEM) analyses of quartz grains from
beach and dune sands were carried out in the western and eastern
Northland coasts, New Zealand, to examine variations in durability and
surface texture, which are controlled by mechanical and chemical
processes, in profiles across beach and dune environments. This was
done through point counts of quartz grain properties based on
extinction angle and crystallinity. Variations in surface texture were
assessed through SEM observations of mechanical features (conchoidal
fractures, smooth surfaces, groove forms) and chemical features
(solution pits, etching, silica deposits). Mechanically produced
grooves are associated with beach sands affected by the high energy of
the surf zone. Both mechanical and chemical processes occur in the
eastern dune sands. They are associated with the greater abundance of
angular grains in the eastern dune sands than the western dune sands.
In addition, conchoidal fractures produced by the collision of grains
in aeolian environments and linear and curved grooves produced by
quartz grains from the beach support the mechanical processes taking
place in the dunes. Solution pits, etching, and the presence of diatoms
in the quartz grains are associated with pedogenesis and high silica
precipitation in the eastern beach and dune sands. The durability of
coarse-grained polycrystalline quartz relative to fine-grained
polycrystalline quartz suggests that chemical abrasion exerts control
over the distribution of quartz types in the dune sands.
Keywords polycrystalline
quartz; monocrystalline quartz; quartz; beach and dune sands; SEM; New
Zealand
G05001; Received 12 January
2005; accepted 22 July 2005; Online publication date 10 October 2005
New Zealand Journal of Geology
& Geophysics, 2005, Vol. 48:
649–660
0028–8306/05/4804–0649 © The Royal Society of New Zealand 2005
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