Physics
Scientific paper
Mar 1977
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1977e%26psl..34..284s&link_type=abstract
Earth and Planetary Science Letters, Volume 34, Issue 2, p. 284-290.
Physics
21
Scientific paper
Primary crystals of coesite and high sanidine (Or98Ab2) up to 3 mm in greatest dimension have been identified in a grospydite inclusion from the Roberts Victor kimberlite pipe, South Africa. Microprobe analyses show that the coesite is nearly pure silica with less than 0.05 wt.% each of Al2O3, FeO and Na2O. Cell edges determined by single crystal X-ray diffraction techniques are: a = 7.143(2) Å, b = 12.383(3) Å, c = 7.143(2) Å, β = 120.00(3)° and the space group is C2/c. The feldspar has cell dimensions: a = 8.618(3) Å, b = 13.044(3) Å, c = 7.184(3) Å, β = 116.10(3)° and the space group is C2/m indicating a very high degree of Al-Si disorder. Coesite, previously considered diagnostic of impact processes, is reported for the first time from a natural static pressure environment. This sample provides evidence that sanidine can be a stable phase in the upper mantle and that free SiO2 may also exist. The structural state of the feldspar indicates equilibration above 900°C and the presence of coesite indicates equilibration pressures greater than 29 kbars. The preservation of the coesite indicates that the samples never spent more than a few hours above 700°C since crossing the quartz-coesite transition.
Hatton C. J.
Smyth Joseph R.
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