Namesake Minerals

Namesake Minerals #12

Another high pressure quartz polymorph features in today’s post, but this one is even more extreme than coesite. Stishovite has a similar history to coesite – it was first created in the laboratory and a natural occurrence was discovered soon after.

mindat-stishovite

Stishovite (black) in the Muonionalusta Meteorite (image from mindat.com)

Quick facts:

  • Oxide mineral
  • Formed in high pressure-temperature environments
  • Type locality: Meteor Crater, Coconino County, Arizona, USA
  • Formula: SiO2
  • Crystal system: Tetragonal
  • Hardness: 7.5-8
  • Density: 4.35/cm3

Stishovite occurs as small crystals up to 2mm in size which are usually found in rocks from impact craters or as inclusions in diamonds. Stishovite formed by impact metamorphism experiences pressures of over 10GPa and temperatures exceeding 1200°C.

silica_phase_diagram

Quartz phase diagram (image from quartzpage.de)

Even though the chemical formula is SiO2, its crystal structure is different to quartz and other silicates. Silicate minerals have tetrahedral coordination where each silicon atom is surrounded by four oxygen atoms, but in stishovite, the coordination is octahedral so silicon atoms are surrounded by six oxygen atoms. Because of this, stishovite is classed as an oxide mineral; it is isomorphous with rutile (has the same crystal structure). This crystal arrangement also makes stishovite denser than the other quartz polymorphs.

 

structures

Ball and stick models of a silicon dioxide tetrahedron (left) and octahedron (right). Blue = silicon and red = oxygen (images by Ben Mills, left image from commons.wikipedia.org, right image from commons.wikipedia.org)

Stishovite is named after Sergei Mikhailovich Stishov who first synthesised the mineral. He was a geochemistry graduate student at Moscow State University in the early 1960s. A few years earlier, Francis Birch had predicted a high pressure phase of silica, so Stishov decided to create it with high pressure-temperature experiments. In 1961, he synthesised stishovite. Just one year later, Edward C. T. Chao discovered the mineral in Meteor Crater, Arizona, USA and the mineral was named in Stishov’s honour.

 

References
https://www.mindat.org/min-3790.html
http://www.minerals.net/mineral/stishovite.aspx
https://en.wikipedia.org/wiki/Stishovite
Stishov, S.M., 1995. Memoir on the discovery of high-density silica. International Journal of High Pressure Research, 13(5), pp.245-280
http://www.hppi.troitsk.ru/meetings/news/2005/bridgman.htm

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)

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Namesake Minerals #10

Happy New Year everyone! 2017 is finally here as well as the return of fortnightly namesake minerals posts! It’s been quite a while since the last post so today you’ll be treated to three minerals named after famous petrographers.

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Namesake Minerals #9

Sadly, it’s the final mineral in the mythological namesake series. But don’t worry, I’ll return to posts about minerals named after mortals in the coming weeks. If you happen to come across any more minerals named after myths and legends, please let me know! Today’s post features chabazite which gets its name from an ancient poem.

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Chabazite-Ca, from Imilchil, Errachidia Province, Morocco (image by Parent Gery from commons.wikimedia.org)

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Namesake Minerals #8

Today’s post features the penultimate mineral in the mythological namesake series. Tapiolite comes from Finland and is named after a figure from Finnish folklore.

tapiolite

Tapiolite from Baixao de Laje Mine, Parelhas, Rio Grande do Norte, Brazil (image by Rob Lavinsky from wikipedia.commons)

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