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Pressure-Temperature Stability Studies of Talc
and 10-Å phase using x-ray diffraction.
Arianna E. Gleason1, Martin Kunz1, Stephen
Parry2, Alison Pawley2, and Simon M.
Clark1,2. 1Advanced Light Source,
Lawrence-Berkeley National Laboratory, MS 4R
0230, 1 Cyclotron Rd, Berkeley CA 94720 2School
of Earth, Atmospheric and Environmental Sciences,
The University of Manchester, Oxford Road,
Manchester, M13 9PL, UK.
Abstract
Experimental Setup
-ALS Beamlines 12.2.2 and 11.3.1 probed with
energies 20 to 30 keV -Diffraction patterns
acquired using MAR345 image plate and Bruker
CCD -Resistively heated, membrane driven DAC was
used to reach P and T -contained in gasket
natural talc water gold (pressure calibrant)
The pressure-temperature phase boundary for talc
plus water to 10-Å phase conversion has been
determined. Our data suggest that 10-Å phase may
form at room temperature while at elevated
pressures. X-ray diffraction measurements of
natural talc plus water at combined temperatures
of 27oC 450oC and pressures of 0-15 GPa were
made at the Advanced Light Source beamlines
12.2.2 and 11.3.1. Elevated temperatures and
pressures were achieved with a resistively
heated, membrane driven diamond-anvil cell. The
data were modeled using a monoclinic unit cell.
Our data and proposed phase diagram are presented.
Data Analysis
Isotherms we note the pressure range where the
002 peak discontinuity occurs. Up is
compression of the DAC and down is
decompression. The down points are not
necessarily considered due to hysteresis.
Straight lines drawn are only a guide for the eye.
Lattice parameters for each phase (talc and gold)
were determined using Le Bail whole pattern
refinement with the GSAS program.
Introduction
Talc, Mg3Si4O10(OH)2, is a sheet silicate
commonly formed as an alteration product from
ultramafic rock. At elevated pressures and
temperatures, H2O can fit between the TOT
(tetrahedra-octaheadra-tetrahedra) layers of talc
to form 10-Å phase. Volatile recycling, via
oceanic slab subduction, plays a major geodynamic
role triggering mass transfer, melting and
volcanism.
Basal spacing talc peaks 002 and 006 Diffraction
pattern taken at ALS BL12.2.2 with Bruker CCD at
30 keV
Unit Cell of Talc.
The pressure-temperture P-T stability field for
10-Å phase, as a potential water carrier within
the slab, is important to shed light on the
Earths water budget.
When comparing our room temperature isotherm
(27oC) data to compression data collected by S.
Parry for both 10-Å phase and talc, we note a
conversion of talc to 10-Å phase with only the
increase of pressure. Based on the transition
pressures seen in the above isotherm data, we can
plot a more accurate phase boundary between talc
and 10-Å phase and generate P-T diagram.
Schematic Diagram extracted from Gill 1981
Method
To determine the phase boundary between talc and
10-Å phase we can collect isothermal data across
the zone where we expect a change. This change
is noted by monitoring the basal spacing of talc
during x-ray diffraction. We should be able to
see a discontinuity in the 002 spacing with
pressure when water starts fitting into the talc
structure for each isotherm.
Plot extracted from Parry S, Pawley A, Clark S,
Jones R. 2004. In-situ high pressure synchrotron
and X-ray diffraction study of talc and
10-Angstrom phase.
Discussion and Conclusions
10

• References
• - Parry S, Pawley A, Clark S, Jones R. 2004.
  In-situ high pressure synchrotron and X-ray
  diffraction study of talc and 10-Angstrom phase.
  COMPRES Annual Meeting 2004.
• - Pawley, 2004 unpublished personal
  communication pdf received Dec.3, 2004.
• - Pawley A, Redfern S, Wood B. 1995. Thermal
  expansivities and compressibilities of hydrous
  phases in the system MgO-SiO2-H2O talc, phase A
  and 10-Å phase. Contributions to Mineral
  Petrology Vol 122 301- 307.
• - Shim S, Duffy T, Kenichi T. 2002. Equation of
  state of gold and its application to the phase
  boundaries near 660 km depth in Earths mantle.
  Earth and Planetary Science Letters Vol. 203
  729-739.

9.35
Prior to this study, the stability region of 10-Å
phase within a subducting slab was thought to be
here. However, this new version of the phase
boundary would indicate that 10-Å phase is also
stable at lower pressures. The boundarys
decrease in slope from 150 500oC could mean
that 10-Å phase is less sensitive to temperature
increases. 10-Å phase would reside longer inside
the slabs center (where it is cooler) and could
carry more water in the slab as it subducts.
Extracted from Poli and Schmidt 2002