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.


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.


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.



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.


Stishov, S.M., 1995. Memoir on the discovery of high-density silica. International Journal of High Pressure Research, 13(5), pp.245-280


On Twitter, there are several ‘hashtag games’ where someone tweets a picture and others have to guess what it is. I’ve played quite a few of these games, such as #NameThatTrack by @LisaVipes and #CougarOrNot by @drmichellelarue, but as far as I could see, there were no geology based games.

I decided to create a geology themed game called #AreYouSiO2. Every Monday, I tweet a picture of a mineral at 14:00 GMT and players have until 21:00 GMT to guess whether it’s a variety of quartz (SiO2). Players get bonus points if they can name the variety.

#AreYouSiO2 is still in its infancy – there have only been two rounds so far – but I hope that people will enjoy seeing the wide range of quartz varieties. Make sure you don’t miss the next round this Monday!

(Title image is quartz with some pyrite, accession number 8A:1(109), from the University College London Geology Collections)

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!


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)


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.


Geo Advent Calendar Day 25

O Christmas tree
O Christmas tree
How lovely are thy minerals!

Sadly we’ve reached the end of the Geo Advent Calendar… but that means it’s Christmas! The best way to celebrate Christmas is with a mineral tree. The baubles have pictures of thin sections on them and were made by Ruth Siddall. You can download them here.

Merry Christmas everyone! (more…)