Coesite Eclogite


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.

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.

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 at Roberts Victor

Chris Hatton 'sampling' the pile of nodules at Roberts Victor. Sample SRV-1 can actually be seen here as a broken, light-colored nodule in the pile.
 
 

Coesite in SRV-1

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 polycrystalline quartz.
 


Inverted Pigeonite

Coesite grain appoximately 1mm across with small clinopyroxne inclusion.
 

Corundum in Kyanite

Large coesite grain with polycrystalline quartz rim. The small birefirngent grain in clinopyroxene. The mattrix is omphacite and garnet.



kyanite in cpx  Inverted majorite

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.


Pallisade Qaurtz on coesite

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 pseudomorph after K-cymrite (KAlSi3O8 2H2O) and a source for water during decompression.


Migrated Twin 

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).


kyanite in cpx  

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).

CPx inclusion in kyanite

This is a kyanite grain containing an unaltered inclusion of clinopyroxene.  The gray grain at the bottom right is sanidine.

Strain twins in kyanite

The zig-zag pattern is caused by strain (deformation) twinning in kyanite
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Deformation twins in kyanite

Here is another set of deformation twins in kyanite.

Migrated Twin 

The rock contains polyphase sulfide grains. This one contains pyrrhottite, pyrite, and chalcopyrite.


SEM image of diamond

The rock also contains small diamonds. This grain is about 150 micrometers across and protrudes above a polished section of the rock.


References

J. R. Smyth and C. J. Hatton (1977) A coesite-sanidine grospydite from the Roberts-Victor Kimberlite. Earth and Planetary Science Letters 34, 284-290. reprint

J. R. Smyth (1977) Quartz pseudomorphs after coesite. American Mineralogist 62, 1252-1257.

J. R. Smyth (1980) Cation vacancies and the crystal chemistry of breakdown reactions in kimberlitic omphacites. American Mineralogist 65, 1257-1264.

K. H. Wohletz and J. R. Smyth (1984) Origin of a sanidine-coesite grospydite. In J. Kornprobst, (ed.) Kimberlites II: The Mantle and Crust-Mantle Relationships, Elsevier, 33-42. reprint

T. A. Scambos, J. R. Smyth, and T. C. McCormick (1987) Crystal structure refinement of a natural high sanidine of upper mantle origin. American Mineralogist 72, 973-978.

J. R. Smyth, G. Artioli, J. V. Smith, and A. Kvick (1987) Crystal structure of coesite, a high-pressure form of SiO2, at 15 and 298 K from single-crystal neutron and X-ray diffraction data: test of bonding models. Journal of Physical Chemistry 91, 988-992.

R. J. Angel, R. M. Hazen, T. C. McCormick, C. T. Prewitt, and J. R. Smyth (1988) Comparative compressibility of end-member feldspars. Physics and Chemistry of Minerals 15, 313-318.

G.R. Rossman and J.R. Smyth (1990) Hydroxyl contents of accessory minerals in mantle eclogites and related rocks. American Mineralogist 75, 522-527.

J. R. Smyth, G. R. Rossman and D.R. Bell (1991) Incorporation of hydroxyl in upper mantle clinopyroxenes, Nature 351, 732-735. reprint

Z.D. Sharp, E.J. Essene, and J.R. Smyth (1992) Ultra-high temperatures from oxygen isotope thermometry of a coesite-sanidine grospydite. Contributions to Mineralogy and Petrology 112, 358-370.