Volatiles in Silicate Melts
Introduction: While the subject of my Ph.D. was the organic geochemistry of coal and I continue to focus my research or organic chemical problems in Earth and Space sciences, I have many years worked with my colleague Bjorn Mysen on the problem of volatiles (principally H2O, but also CO2, CH4, N2, NH3, and H2) and what they do in silicate melts. It should not surprise anyone that volatiles (water, CO2, H2, etc) have a profound effect on the "operation" of this planet- Water effects when rock melts, controls the viscosity of resultant magma, and like is the both influential and elusive parameter in geodynamics.
What I am learning is that we really do not have a robust physical chemical understanding of how volatiles interact with silicate melts- at the molecular level: Most of what one sees is derived from "partitioning experiments" where no description of valence, structure, or mechanism is ever (or rarely) provided.
So What I bring to this problem is many years experience with Solid State Nuclear Magnetic Resonance (SSNMR) spectroscopy- which turns out to be a good technique for this question.
Of course to do SSNMR one needs a solid, so I work with glasses quenched from silicate melts. By definition, a glass represents the structure of a liquid at the "glass transition". To study melts above the glass transition Bjorn Mysen and Dionysis Foustoukos use Raman spectroscopy and "hydrothermal diamond anvil cells"; where NMR is still useful in benchmarking glass structures quantitatively to calibrate the Raman spectra with glasses prior to forming melts at much high T. This is the only game in town.
One of the things I hear all the time is "don't we already know this?" in particular, in regards to how water interacts with silicate melts. I agree that you would resonably think this was the case, people have been studying this for over 50 years, but recent experiments that I have performed with collaborators indicates that we DO NOT UNDERSTAND why water does what it does in silicate melts (Collaborators in Alphabetic order): Mike Ackerson, Celia Dalou, Dionysis Foustoukos, Charles Le Losq, Bjorn Mysen and Ying Wang.
It seems so simple - at least with regards to the silicate oxide network- water has to do one of two different things 1) either dissolve in the melt as H2O or 2) hydrolyze silyl ether linkages (Si-O-Si) via the reaction:
H2O + Si-O-Si <-> 2 Si-OH (there are other reactions that can yield alkali- and alkili-earth hydroxides- but that is about it)
Moving the reaction to the right depolymerizes the melt reducing melt viscosity. To the left water does not actually do anything obvious or at least it should behave like a dissolved gas, e.g. Ar. So if you want to know how far "right" or "left" you are in this reaction all you need to do is to measure the amount of H2O and the amount of Si-OH. How hard could that be?
It turns out not easy at all.
Across the electromagnetic spectrum there are only a couple regions that might provide primary functional group level detection of H bearing functional groups. These are primary X-H vibrational modes and 1-H and 2-H Chemical shielding via NMR:
- Vibrational spectroscopy in the mid-Infrared (FTIR and Raman) that reveal H-X through characteristic PRIMARY vibrational frequencies typically in ~ 2500 - 3600 cm-1 range. Sadly with IR spectroscopy the primary O-H stretching bands cannot be used to directly distinguish between Si-OH and H2O
- NMR where 1-H is a very abundant isotope, with a large magneto gyric ratio (meaning sensitive), and where variations in X (in H-X) results in distinct frequency shifts in the emission spectrum that are primarily based on X-H distance. Sadly NMR cannot directly distinguish between H in H2O and H in Si-OH directly. 1-H NMR is a great ruler of O-H distance, perhaps the best as it is not convolved with multiple vibrational state complexities.
There are some methods, less direct that show promise.
There is an indirect method that is the mainstream method for estimating Si-OH and H2O concentration that relies on some very none primary interactions that yield very weak absorption features in the near IR region. These are the so-called "sum bands" at XXXX and XXXX cm-1; these bands are ~ 100 x weaker than the primary O-H vibrations and are believed to be the primary means of determining Si-OH and H2O abundances. Independent of the robustness of the assignments, there is an issue with strongly hydrogen bonded OH, where the OH primary vibrational modes are shifted to low and very low frequencies. In systems with a broad distribution of hydrogen bonding strengths, only OH and H2O experiencing weak H-bonding interactions will be detected- all other OH and H2O sum bands will be hidden in the primary stretching.
If you have been following our work you will know that for even simple systems like Na2O•4(SiO2) [NS4} there are predominantly two hydrogen environments (at ~ 50 % - 50 %), one that exhibits very intense H-bonding and another that exhibits week H-bonding. So, with regards to the near IR features traditionally used, for NS4 only the environments that have low H-bonding will be reported, the 50 % that exhibit strong H-bonding will be invisible- THIS IS A PROBLEM.
We argue that the true understanding of how water controls melt structure is actually not known! Yes this is a radical statement and we are working hard to add foundation to our concerns.
STAY TUNED- this page is in progress!