In May, 1976, I was a scientific collaborator with Stefan Hafner at
Phillips-Universitaet Marburg and with Ahmed ElGoresy at the
Max-Planck-Institut fuer Kernphysik (Heidelberg) and made several thin
sections of my South African eclogites. I could not identify the low-
birefringece phase with polycrystalline rims thyat made up about 10% of
the sample so I did microprobe work on the rocks with Prof. El Goresy.
It turned out to be pure SiO2, but optically it was biaxial.
These are images of coesite eclogites from South African kimberlites. I wrote the first desription of coesite (the high pressure form of SiO2) in a metamorphic rock with Chris Hatton (Smyth and Hatton, 1977). In 1975 I was a Senior Lecturer at University of Cape Town. We collected the sample from the Roberts Victor mine about 50 km east of Kimberley in October, 1975 with Arch Reid and John Gurney. The mine operator collected and put aside the large nodules from the kimberlite and saved them for the geologists who came to visit.
The original paper describing the petrology of the sample was
published in 1977 (Smyth and Hatton, 1977). This was the first
petrographic description of coesite in a metamorphic rock. The coesite
was positively identified by single crystal X-ray diffraction. The
description of quartz pseudomorphs after coesite was also given from
this rock (Smyth, 1977). The original equilibration conditions of this
rock are extimated to be 4.9 GPa and 1050ºC which corresponds to a
depth of about 150 km (Wohletz and Smyth 1984). The oxygen isotope
fractionation are also iconsistent with an equilibration temperature of
about 1100ºC (Sharp et al 1991). The sample contains the largest
coesite grains ever described and they were sufficiently large (up to
3mm) for a neutron single-crystal structure refinement of coesite
(Smyth et al., 1987). The trace OH contents of the phases have been
measured (Rossman et al 1990), and this omphacite is the most hydrous
ever reported (Smyth et al 1991). The rock has recently been found to
contain small (<300 micron) diamonds.
Chris Hatton 'sampling' the pile of nodules at Roberts Victor.
can actually be seen here as a broken, light-colored nodule in the pile.
Coesite in eclogite. This is a grain of coesite in an eclogite I
collected in South Africa in 1975. It was the first time that coesite
had been described in a metamorphic rock (Smyth and Hatton, 1977). The
description led to the recognition of coesite in many other localities
around the world. The colored grain at the center is a hydrous
clinopyroxene. The gray is coesite, and the mottled gray rim is
Coesite grain appoximately 1mm across with small clinopyroxne
Large coesite grain with polycrystalline quartz rim. The small
birefirngent grain in clinopyroxene. The mattrix is omphacite and
Twinned coesite grain at extinction in crossed polars with
polycrystalline quartz rim. The matrix is omphacite
Same grain as above in plane-polarized light showing the index of refraction contrast (relief) between the coesite and the lense dense quartz rim.
Here we see pallisade texture in the quartz rim in a coesite grain.
The sanidine grain at lower left is thought to be a sanidine
K-cymrite (KAlSi3O8 2H2O) and a source for water during decompression.
Sanidine (KAlSi3O8) (gray) with polycrystalline quartz (mottled)
kyanite (bright), garnet (extinct), and omphacite (finely fractured).
The sanidine in this rock has the highest degree of Al-Si disorder of
any recorded (Scambos et al, 1987).It is nearly pure KAlSi3O8, but
contains 1 to 2 mole percent celsian (BaAl2Si2O8). The sanidine in this
rock contains molecular water as extensive fluid inclusions. Under the
TEM these inclusions appear as a swiss-cheese texture. There are also
veins of hydrous alteration containing Ba that extend away from the
sanidine in this rock. It is possible that the sanidine existed in the
mantle as K-cymrite (KAlSi3O8 H2O).
SRV-1 altered clinopyroxene (omphacite). This omphacite
contains a large component of Ca-Eskola pyroxene which correlates with
H content. This sample contains approximately 1800 ppm by weight H2O
(Smyth et al 1991).
This is a kyanite grain containing an unaltered inclusion of
clinopyroxene. The gray grain at the bottom right is sanidine.
The zig-zag pattern is caused by strain (deformation) twinning in
Here is another set of deformation twins in kyanite.
The rock contains polyphase sulfide grains. This one contains pyrrhottite, pyrite, and chalcopyrite.
The rock also contains small diamonds. This grain is about 150
micrometers across and protrudes above a polished section of the rock.