Saturday, May 17, 2014

Tourmaline lemonade



crystallographic sector zoning is a phenomenon that causes all sorts of headaches for geochemists and petrologists.  Basically, as different crystallographic faces grow in a medium (e.g. magma), they have different selectivities for different elements.  If you want to measure how much of a particular element a growing crystal scavenges from its surrounding, and you don’t measure all sectors, or don’t know their true relative volume, this can cause errors.

However, at least two scientists have turned this around, and used the zoning as a feature, not a problem.  Hinsberg and Schumacher (2007) treat the different sectors as co-existing minerals, calculate D values between them, and note that the D values are temperature dependent. Ta da!  They now have a single crystal geothermometer that records T over the growth of the mineral.  If life hands you lemons, compare the sections and build a new tool.


5 comments:

Chris Phoenix said...

That is SO COOL! Thanks!

Isotopic said...

That's pretty weird! Tough to say where the temperature relationship comes from. Seems unlikely to be a real equilibrium process - more likely a temperature controlled kinetic effect?

Chuck Magee said...

As a general rule, partitioning should get smaller at higher T in any generalized thermodynamic system, because less segregated systems have higher entropy.

Isotopic said...

Sure, but equilibrium thermodynamics plays a backseat here, right? Approach to equilibrium can't be driving the partitioning (at least, not in the tourmaline) because there is no exchange between the two "phases". If it's an equilibrium effect, I guess it's between the interstitial medium and the crystal interface, so the effective "partition coefficient" is actually the ratio of two different equilibrium processes (e.g., the two interfaces are different). That (the ratio of the two different processes) seems less likely to be temperature dependent than some other kinetic process, right?

Chuck Magee said...

Yes, the equilibrium is between two independent faces and the"medium". So you divide D c-axis/melt by D a-axis/melt. But all xals get less choosy at high T, due to the TdS term in Gibb's free energy. It is the same reason isotopic fractionation decreases at higher T.