Namesake Minerals #11

Today’s mineral is formed under intense heat and pressure – quite apt considering what’s been going on in the world lately!

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Coesite eclogite from Norway (red = garnet, green = omphacite). This contains minute crystals of coesite which cannot be seen by the naked eye (image by Carl Hoiland from travelinggeologist.com)

Quick facts:

  • Silicate mineral
  • Formed in high pressure-temperature environments
  • Type locality: Meteor Crater, Coconino County, Arizona, USA
  • Formula: SiO2
  • Crystal system: Monoclinic
  • Hardness: 7.5-8
  • Density: 2.92g/cm3

Coesite is one of the high pressure polymorphs of quartz. The intense pressure and temperature needed to form it occurs during meteorite impacts or ultra-high metamorphism. If coesite is the result of metamorphism, it is evidence of subduction or continental collision that has carried continental crust to a depth of at least 70km. The rocks formed are eclogite xenoliths from the mantle (see picture above).

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Quartz phase diagram (image from quartzpage.de)

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Thin section of coesite with a polycrystalline quartz rim. The small colourful grain near the top is clinopyroxene. Field of view ~3mm (image from ruby.colorado.edu)

Coesite forms tabular and prismatic crystals which can only be seen in thin section and is usually preserved as inclusions in minerals. The picture on the right shows a quartz rim which exerts pressure on the coesite, preventing it from reverting to quartz. This sample is a coesite eclogite from a kimberlite pipe in the Roberts Victor Mine, South Africa. Kimberlite pipes bring up xenoliths from great depths; this sample is from a depth of 150km where the temperatures reached 1050ºC and the pressure was 4.9GPa. Coesite is found in other ultra-high pressure rocks from the Western Alps, the Erzgebirge in Germany and the Kokchetav Massif in Kazakhstan. However, the first natural occurrence was found in Meteor Crater, Arizona, USA.

This mineral was named after the scientist who synthesised it. Loring Coes Jr. was born in 1915 in Worcester, Massachusetts, USA. He had a love for chemical reactions and even built his own laboratory in his house when he was still a student. Coes Jr. was employed at the Norton Company which researched superhard materials and diamonds. The company focused on high pressure experiments but Coes decided to use a mixture of high pressure and temperature. When he subjected quartz to 800°C and 3.55GPa, he created coesite. He announced his discovery on 31st July 1953 and also hypothesised that it must be possible for this mineral to form in nature under similar conditions. His hypothesis was proven to be true when in 1960, Edward C. T. Chao found coesite insandstone ejecta from Meteor Crater. The mineral was named after Coes in the same year.

Due to a smoking and drinking habit, Coes Jr. died in 1978 from lung cancer.

 

References
http://www.mindat.org/min-1104.html
http://www.quartzpage.de/gen_mod.html
https://de.wikipedia.org/wiki/Loring_Coes_Jr.

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