Large-Volume High-Pressure Phase Synthesis:
My Long-Term Involvement With Silica
Silica (SiO2) is one of the most important and well-studied
systems in the Earth Sciences. Over the years I have been involved (sometimes
only tangentially) in quite a few projects as a result of devising a technique
for synthesizing large crystals and polycrystalline aggregates of coesite
and stishovite, two of the high-pressure phases of silica. The recipe for
doing this was really pioneered by Baosheng Li at Stony Brook in the early
90's, but I stumbled upon the idea independently long before I read his
paper. Basically, the trick is to use a solid cylinder of silica glass as
the starting material. This results in a very low-porosity final aggregate,
which is desirable for a number of types of studies. Also, if you are lucky
you can grow large grains using this technique because nucleation is
inhibited relative to the case of using a powdered starting material (basically
grains grow in from the outside of the cylinder). Even though I am not the
first author on most of the papers listed below I am quite proud of my accomplishment
at having started a "cottage industry" of silica synthesis!
Projects:
1. Kinetics of
the Coesite to Quartz Transformation
2. Pressure dependence of hydroxyl
solubility in coesite
3. Equation of State
of Coesite
4. Equation of
State of Stishovite
5. Structural
Refinement of Stishovite
6. High-Pressure
Shock-Wave Equation of State of Stishovite
7. Deformation
of Stishovite at High Pressure and Temperature
Kinetics of
the Coesite to Quartz Transformation
This was the first first-author paper I published, from my thesis
work. I first started playing around with silica synthesis from glass for
this work and for some unpublished research on the kinetics of the reverse
transition (quartz to coesite). I guess this will likely always be my most
cited publication. I would let you download the pdf but unfortunately I
don't have access to it, so instead the abstract is reproduced below.
Mosenfelder, J.L. , Bohlen, S.R., 1997, Kinetics of the coesite
to quartz transformation. Earth and Planetary Science Letters,
153: 133-147.
The survival of coesite in ultrahigh-pressure (UHP) rocks has important
implications for the exhumation of subducted crustal rocks. We have conducted
experiments to study the mechanism and rate of the coesite quartz transformation
using polycrystalline coesite aggregates, fabricated by devitrifying silica
glass cylinders containing 2850 H/106 Si at 1000°C
and 3.6 GPa for 24 h. Conditions were adjusted following synthesis
to transform the samples at 700–1000°C at pressures 190–410 MPa below
the quartz–coesite equilibrium boundary. Reaction proceeds via grain-boundary
nucleation and interface-controlled growth, with characteristic reaction
textures remarkably similar to those seen in natural UHP rocks. We infer
that the experimental reaction mechanism is identical to that in nature,
a prerequisite for reliable extrapolation of the rate data. Growth rates
obtained by direct measurement differ by up to two orders of magnitude from
those estimated by fitting a rate equation to the transformation–time data.
Fitting the rates to Turnbull's equation for growth therefore yields two
distinct sets of parameters with similar activation energies (242 or 269
kJ/mol) but significantly different pre-exponential constants. Extrapolation
based on either set of growth rates suggests that coesite should not be preserved
on geologic time scales if it reaches the quartz stability field at temperatures
above 375–400°C. The survival of coesite has previously been linked to
its inclusion in strong phases, such as garnet, that can sustain a high internal
pressure during decompression. Other factors that may play a crucial role
in preservation are low fluid availability –– possibly even less than that
of our nominally "dry" experiments –– and the development of transformation
stress, which inhibits nucleation and growth. These issues are discussed
in the context of our experiments as well as recent observations from natural
rocks.
Author Keywords: coesite; quartz; kinetics; phase transitions; high pressure
Pressure Dependence
of Hydroxyl Solubility in Coesite
Mosenfelder, J.L., 2000. Pressure dependence of hydroxyl solubility
in coesite. Physics and Chemistry of Minerals, 27: 610-617.
Download the pdf
When I started this project I knew from my thesis work that coesite
could incorporate small amounts of OH in its structure, even though measurements
on natural coesite crystals failed to reveal the presence of OH. When I
got to Germany I had access to much higher pressures in the multi-anvil
apparatus so it seemed like a good opportunity to look systematically at
the influence of pressure on the solubility of OH in this well-studied mineral.
Later on Monika Koch-Mueller (then at Carnegie) improved upon and supplemented
my work by doing more careful polarized IR measurements and coming up with
an IR calibration for OH in coesite.
Equation of
State of Coesite
Angel, R.J., Mosenfelder, J.L., Shaw, C.S.J., 2001, Anomalous compression
and equation of state of coesite. Physics of the Earth and Planetary
Interiors, 124: 71-79. Download
the pdf
Ross Angel found out about my OH in coesite project and asked if I
had any large crystals he could use for single-crystal diffraction. Extracting
large (>100 micron diameter) crystals of coesite from my OH solubility
experiments proved quite easy. This papers reports a new and highly precise
room-temperature equation of state for coesite up to a maximum pressure
of 9.6 GPa. These data should supercede previous measurements by Levien
and Prewitt (1981) that were less precise in the high-pressure range.
Equation of State of Stishovite
Andrault, D., Angel, R.J., Mosenfelder, J.L., Le Bihan, T.,
2003, Equation of state of stishovite to lower mantle pressures. American
Mineralogist, 88: 301-307. Download
the pdf
Ross Angel did the low-pressure (<10 GPa) equation of state measurements
reported in this paper on a rather large crystal of stishovite that I synthesized
(ca. 400 microns long? Ross might remember). I was not really involved in
the more meaty part of the paper, which entailed synchrotron X-ray diffraction
experiments in a diamond-anvil cell to determine the room-temperature equation
of state of stishovite to ~60 GPa and the nature of the high-pressure phase
transition to a CaCl2-structured polymorph at about 60 GPa.
Structural Refinement
of Stishovite
Kirfel, A., Krane, H.-G., Blaha, P., Schwarz, K., Lippman, T., 2001. Electron-density
distribution in stishovite, SiO2: a new high-energy synchrotron-radiation
study. Acta Crystallographica A, 57: 663-677. Download
the pdf
Prof. Armin Kirfel contacted Prof. Fritz Seifert, in Bayreuth, who in turn
asked me if I could synthesize some crystals for this study. I used the
same trick as I did for the Andrault study and I guess it worked out. This
paper reports on a very high-tech and detailed structural refinement of
stishovite using synchrotron radiation. I could not confess to understand
much of the physics in this one so I am very grateful that Prof. Kirfel
kindly acknowledged my contribution in the paper!
High-Pressure
Shock-Wave Equation of State of Stishovite
Luo, S.-N., Mosenfelder, J.L., Asimow, P.D., Ahrens, T.J., 2002. Direct
shock wave loading of stishovite to 235 GPa: implications for perovskite
stability relative to an oxide assemblage at lower mantle conditions.
Geophysical Research Letters, 29(14): article no. 1691 Download the pdf
Luo, S.-N., Mosenfelder, J.L., Asimow, P.D., Ahrens, T.J., 2002. Stishovite
and its implications in geophysics: new results from shock-wave experiments
and theoretical modeling. Physics-Uspekhi, 45: 435-439
The first project I bit into at Caltech was synthesis of polycrystalline
aggregates of stishovite (and coesite) for shock-wave experiments, which
were conducted by Shengnian
Luo. The synthesis was challenging because the minimum sample requirements
for shock-wave EoS measurements (with available technology in the Caltech
Shockwave lab) are dimensions of ~3mm in diameter by 1 mm in thickness.
This diameter is too large for the conventional 14/8 multi-anvil assembly
that I use to obtain pressures up to 16 GPa. So I went with a minimalist
approach and gutted the octahedron, dispensing with the ZrO2
insulating sleeve, LaCrO3 heater and MgO insulating sleeve.
Instead I simply surrounded a slug of silica glass with a thin Re heater.
Cranking this assembly above ~1000 °C proved problematic because without
the ZrO2 insulating sleeve furnace stability was hit or miss;
some runs went south in a hurry! Fortunately, fully dense and single-phase
aggregates were easy to make at a temperature of 1000°C, pressure of
~14 GPa and short times (typically 45 minutes to 1 hour). With a Re heater,
the temperature gradient over the sample length was probably quite high,
but we didn't care so much because the goal was simply to synthesize a one-phase
aggregate of the appropriate dimensions.
Deformation of Stishovite
at High Pressure and Temperature
This is a project that Patrick Cordier (Lille, France) started when
he was visiting BGI in the late 1990's. Patrick is a master of transmission
electron microscopy and has done some very interesting characterization
of dislocation microstructures using a technique called LACBED (large angle
convergent beam electron diffraction) that was not previously very popular
in the Earth Sciences. I showed Patrick the trick for synthesizing polycrystalline
aggregates of stishovite, which he later deformed in separate experiments
using the technique of employing tiny bottom and top pistons in a multi-anvil
assembly. There have been a couple abstracts on the results of the work but
I believe the paper is still in preparation.
I've had enough silica to last a lifetime, take me back to Jedley's homepage!