Expansivity and dehydration of lawsonite

D. Trots, A. Kurnosov, C. Beyer, and D.J. Frost

Bayerisches Geoinstitut, Universitaetsstrasse 30, 95448 Bayreuth, Germany

Lawsonite, CaAl2Si2O7(OH)2*H2O, is a hydrous calcium aluminium sorosilicate mineral capable to store and convey water to depth in subducting oceanic lithosphere [1]. Precise information on the physical properties and stability fields of lawsonite at high temperatures/pressures is therefore of primary importance to geophysics. In order to calculate stability fields of hydrous minerals thermodynamic parameters are often refined from high-pressure experimental phase relations and equation of state data. Such calculations benefit immensely if parameters such as thermal expansivity and bulk modulus, can be accurately determined independently. Thermal expansion measurements provide particularly useful constraints on elastic properties due to very low experimental uncertainties. Hence, we have performed powder diffraction experiment on lawsonite with the aims of: (i) collection of precise synchrotron-based V(T) data and verification of applicability of Grüneisen approximations for CaAl2Si2O7(OH)2*H2O; (ii) revealing either existence or absence of water release in CaAl2Si2O7(OH)2*H2O at high-temperatures and its possible influence on thermal expansion.

10 20 30 40 50 60

0

4000

8000

12000

16000

Inte

nsity

(arb

.u.)

2θ (deg.)

CaAl2Si2O7(OH)2*H2OT=836 K

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0

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Inte

nsity

(arb

.u.)

2θ (deg.)

CaAl2Si2O8

T=1105 K

Figure 1: Examples of Rietveld fits to powder diffraction data.

Thermal expansion measurements were performed at beam-line B2. The sample was ground in an agate mortar and sieved through a mesh. Quartz capillary 0.5 mm in diameter was filled with powdered lawsonite and sealed. Subsequently the capillaries were mounted inside a STOE furnace in Debye–Scherrer geometry, equipped with a Eurotherm temperature controller and a capillary spinner. The furnace temperature was measured by TYPE–N thermocouple and calibrated using the thermal expansion of NaCl. A wavelength of 0.74925 Å was selected using a Si(111) double flat-crystal monochromator from the direct white synchrotron beam. The x-ray wavelength was determined from eight reflection positions of LaB6 reference material (NIST SRM 660a) measured by scintilation single counter detector with analyser crystal in its front. The diffraction patterns were collected at fixed temperatures during the heating cycles using an image-plate detector. The image plate detector was calibrated using the diffraction pattern of LaB6 and the wave length determinated by scintillation counter/analyser crystal tandem. An additional pattern was taken at ambient temperature after the heating cycle. All diffraction patterns were analyzed by full-profile Rietveld refinements using the software WinPLOTR.

Figure 2 demonstrates dependency of volume per formula unit against temperature. Complete dehydration of lawsonite occurs in the range of 850-1050 K, so that only CaAl2Si2O8 phase was detected above 1050 K.

This last phase was also preserved at room temperature after the heating cycle, thereby revealing non-reversibility of dehydration. Significant broadening of reflection profiles was observed during dehydration that can be explained by increase of lattice strains due to motion, with consequent release, of water through the structure.

300 400 500 600 700 800 900 1000 1100168.5

169.0

169.5

170.0

170.5

171.0

171.5

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172.5

CaAl2Si2O8

V/Z

Temperature (K)

CaAl2Si2O7(OH)2*H2O

Deh

ydra

tion

Figure 2: Volume vs. T dependency of lawsonite.

A physically meaningful parameterization of experimental V(T) dependency can be obtained using 2nd order Grüneisen approximations for the zero pressure equation of state in which the effects of thermal expansion are considered to be equivalent to elastic strain, i.e.:

0'00

0

)()1(5.0/)()( V

TUKKVTUVTV +

−−=

γ

where V0 is hypothetical volume at T = 0, γ is the Grüneisen parameter, assumed to be pressure and temperature independent, K0 and K` are the isothermal bulk modulus at T= 0 K and its first pressure derivate, respectively. The internal energy U(T, θD) can be calculated using the Debye model to describe the energy of the lattice vibrations (θD – Debye temperature). Determinations for K0 = 125 GPa and K` =4, were taken from the literature [2, 3] while V0, γ and θD were allowed to vary during the fitting procedure. The line through the experimental points in figure 2 demonstrates fit result which yield the values V0= 673.9(9) Å3, γ=0.94(1) and θD= 782(50) K. Values for θD and γ from the 2nd order fit are in good agreement with values from the literature determined independently from elasticity measurements [2, 3]. Finally, our attempts to fit Grüneisen approximation to literature V(T) of lawsonite [1] resulted in corrupted values for fitted parameters.

References

[1] A.R. Pawley, S.A.T. Redfern, and T.J.B. Holland, American Mineralogist 81, 335 (1996). [2] S.V. Sinogeikin, F.R. Schilling, and J.D. Bass, American Mineralogist 85, 1834 (2000). [3] J. Chantel, M. Mookherjee, and D. Frost, Earth and Planetary Science Letters 349–350, 116 (2012).