Volumetric and Optical Studies of High Pressure Phases of MgSO4·H20 With Applications to Europa.

A. J. Dougherty1 , D. L. Hogenboom1, and J. S. Kargel2, 1Dept. of Physics, Lafayette College, Easton, PA 18042, e-mail: doughera@lafayette.edu, hogenbod@lafayette.edu,

2Department of Hydrology and Water Resources, The University of Arizona, Harshbarger Building, PO Box 210011, Tucson, AZ 85721-0011, e-mail: kargel@hwr.arizona.edu

Why study high pressure phases of magnesium sulfate solutions?

Magnesium sulfate was chosen because it, along with other hydrated salts such as sodium sulfate, which we studied previously [1,2], is a likely constituent of Europa's ocean and icy shell [3,4], based on chondrite-evolution models and evidence of excellent matches to NIMS spectra of the non-ice regions of Europa’s surface. These salts may play several major roles: they could depress melting points, alter buoyancy relations of key phases, form thick layers of bedded seafloor sediments, and allow explo-sive aqueous eruptions. It is therefore useful to know their high pressure and low temperature phase behavior. In addition, magnesium sulfate minerals are common terrestrial evaporite phases, and they also probably occur on Mars[5], so our measurements are applicable to those bodies, too.

In previous work, we have focused on pressures less than 200 MPa, or the Ice I regime[8]. In the future, we will examine slightly higher pressures where the phases of ice are denser than liquid water, but less dense than the eutectic liquid. Some preliminary results are presented below. This work also complements our work with the Na2SO4·H2O system [1,2].

Magnesium Sulfate Phase Diagram

Figure 1: Phase diagram for MgSO₄·H₂O at atmospheric pressure.

The main solid phases relevant for this work are epsomite (MgSO₄·7H₂O, indi-cated by MS7) and MgSO₄·11H₂O (MS11), previously identified as MS12.[6]. All data reported here are for the eutectic composition of 17 wt.%. For this concen-tration, MS11 is the stable state, but long-lived metastable states of MS7 can also be observed. Generally, the presence of metastable states and the overall slug-gish dynamics of the hydrated salt systems make accurate measurements of the phase boundaries challenging.

Experimental Apparatus

The apparatus consists of 3 main parts: a central high-pressure fitting con-taining the sample fluid, a pressure system, including both pressure and volume sensors, and an optical system for imaging the sample.

About one mL of sample is contained in a pressure cell consisting of a standard high-pressure fitting called a cross. This stainless steel block has four ports, as shown below. Two opposing ports contain replaceable plugs that have sapphire windows sealed with epoxy. The third port contains a plug in which a silicon diode thermometer is installed, and the fourth port connects the cell to the pressure system.

Figure 2: Sapphire windows in steel plugs are mounted inside a steel block known as a cross. The diagram on the left shows a cut-away view of one of the plugs. The image on the right shows the relative positions of the two plugs with windows and the plug containing the thermom-eter. The window separation is approximately 1 mm. The sample volume is approximately 1mL.

Image Acquisition

The optical system is shown in Figure 3. Light from a tungsten lamp is passed through an optical fiber and then collimated to enter the cross. After passing through the sample, the image is relayed by a pair of matched achromatic doublet lenses, magnified by a 4X mi-croscope objective, and pro-jected onto a CCD camera. The resulting images are recorded at regular intervals through-out the experiment.

Figure 3: Optical system. Collimated light enters the system through an optical fiber on the left. The sample is contained between the two sapphire windows. The image is relayed by a pair of lenses to a microscope objec-tive and then projected onto a CCD camera. The apparatus is kept in an insulated temperature-controlled bath.

Pressure and Volume

The fourth port of the cross is connected to a pipe filled with mercury that connects to the pressure system. A small magnet floats on top of the mercury, and a transducer is used to mea-sure the position of the magnet. As the sample expands or contracts, the magnet rises or falls. A transducer produces a voltage that is propor-tional to the height of the magnet, which, in turn, varies nearly linearly with the volume of the sample. As long as the sample is not frozen solid, measurements of the magnet height and the pressure in the pump fluid allow us to deter-mine the pressure and volume changes of the sample.

During a run, the pressure, voltage, and tem-perature are recorded at regular intervals, along with an image of the sample.

Figure 4: Pressure and volume measure-ments. We monitor the height of the magnet and the pressure in the pump to measure the pressure and volume changes in the sample throughout an experimental run.

Insulated container

CCD Transducer

Mercury

Sample

Preliminary Results

The results for a run at a nominal pressure of 350 MPa are shown in Figure 5. After pressurizing and warming the liquid to point (a), we super-cooled the sample down to about 242K. Approaching point (b), a small in-crease in voltage signaled the formation of a less dense mixture just before the main freezing transition at point (c).

Figure 5: Transducer voltage (varies approximately linearly with volume) versus Temperature for a run at a nominal pressure of 350 MPa.

We have not yet identified the phases in-volved. In this run, only a small portion of the crystal was visible in the corner of the window, as shown in Image (b1).

Image (b1): Crystals formed at point (b) on Figure 5.

However, in another run at a nominal pressure of 250 MPa, we ob-served the crystals shown in Image (b2). In both the runs at 250 and 350 MPa, the growth of these crystals is accompanied by an overall decrease in the density of the sample.

Image (b2): Crystals formed at 250 MPa in conditions otherwise similar to point (b) in Fig. 5. The overall density of the mixture decreased as the crystals grew.

Additional cooling pro-duced a sharp drop in volume (c), presumably due to the formation of a dense hydrate along with

additional ice, and perhaps a eutectic solid mixture of the two. Further cooling resulted in no significant changes. Upon warming to about 251K,

eutectic melting com-menced, and the volume began to quickly increase (point d).

Image (d): Crystals ob-served at point (d) on Figure 5, during eutectic melting. The thin crystals in the upper half of the image are less dense than the surrounding liquid, and presumably are ice.

However, unlike the melting transitions we typically observed at lower pressure, this transition was not reversible. The crystals in the upper por-tion of the image dissolve at this temperature, but the crystal in the lower left corner (the same as in Image (b1)) did not.

After slight cooling, crystals in the image began to resolidify, but the volume did not decrease. (Note the small horizontal line segment at point (d).) Subsequent warming again dissolved most of the material, but the crystals from point (b) remained at point (e).

After cooling again until about 250K, the system froze rapidly. Thin dendrites, presumably of ice, grew out ahead of the interface until the system froze solid at point (f ).

Image (f): Dendritic ice crystals growing at a pressure of 350 MPa. The MgSO4 hydrate crystals are at the lower left.

Further warming (back to (d) again) repeated the previous stages, in-cluding a second unsuccessful reversal attempt between (d) and (e). Even-tually, this low-density state dissolved along a liquidus back towards (g).

We observed such a low-density state in every run at high pressure. At this stage of the project, we are unable to conclude whether this low-density state is a metastable mixture of ice III or ice V together with MS7 or MS11, or whether it is a previously unreported sulfate hydrate of lower density than MS11.

These findings are consistent with our earlier study of this system, where we also found possible evidence of a low-density sulfate[7]. With improvements currently under way, we anticipate being able to perform additional experiments in this interesting regime to further study the possible phases involved.

References: [1] Hogenboom, D.L. et al. LPS XXX, Abstract #1793. [2] Dougherty, A. J. et al. LPS XXXVII, Abstract #1732. [3] McCord, T.B. et al. (1998) Science, 280, 1242. [4] Kargel, J.S. et al. (2000) Icarus 148, 226-265. [5] Squyres, S.W. et al. (2004) Science, 306. [6] Peterson, R.C. and Wang, R. (2006) Geology 34, 957-960. [7] Hogenboom, D.L. et al. (1995) Icarus 115: 258-277. [8] Hogenboom, D.L. et al. LPS XXXVI, Abstract #1825.

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