Saturday, September 20, 2008

I’d like to thank the alaskademy

It looks like the generous allochthonous Alaskan who blogs as Wayfarer Scientista has given me the Brilliante award. Presumably, this is some sort of baddeleyite allotrope.

Baddeleyite, formula unit ZrO2, is a great mineral, so it is a shame that synthetic cubic zirconia has a higher profile in the more colorful parts of non-geologic human culture. Of course, the trick to getting ZrO2 to grow in a cubic structure is itself interesting- it seems that cubic zirconia is in fact a Zr-rich tazheranite solid solution. But growing big crystals at all is impressive when you consider that zirconia has one of the highest melting points of any known substance. But enough of this lapidarial digression.

In order to crystallize baddeleyite, two conditions must be met. Firstly, the major minerals must be saturated in Zr. Secondly, the SiO2 activity has to be low enough that zircon doesn’t grow. This makes the mineral somewhat uncommon. Never the less, it does have a high U/Pb ratio, so it is used for geochronology for rocks that don’t have zircon.

Interestingly, metamorphic fluids are often silica rich, so in some altered rocks a zircon reaction rim will grow on a baddeleyite, allowing both the crystallization age and the alteration age to be determined. In practical terms, this means that if you want a good date, then natural, monoclinic zirconia is more reliable than its artificial, cubic allotrope.

Evidently I am supposed to pass this award along, so nominate the following blogs as being cubic zirconia-worthy:
The Planetary Society Blog
Molecule of the Day
This non-American Life
Apparent Dip
The Volcanism blog
All my faults are stress related
And if any of y'all have already been awarded, then give yourselves an allotrope instead.

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