Gs 553. Thermodynamics and phase equilibria




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March 18, 2003

GS 553. Thermodynamics and phase equilibria

Lecture 20: The Good Experiment

Wednesday: Case Study I. The system TiO2

readings: Withers et al. (2003) reprint provided

lecture today: from Holloway & Wood (1988), Chapter 2; Fyfe (1960)

establish why to do the experiment

model the system thermodynamically and ask whether the experiment is worthwhile

ask whether the experiment can be done

plan carefully for a minimum number of runs with very specific target in hand

sensitivity analysis

can the experiments be done with sufficient precision to solve the problem?

select the appropriate equipment for the problem at hand

consider time needed for approach to equilibrium

characterization of starting materials

are appropriate and well characterized starting materials available or can they be synthesized?

careful analysis, determination of XRD pattern are minimum requirements

characterization of run products

careful analysis, determination of XRD pattern are minimum requirements

if a phase mixture, need to evaluate amounts

evaluation of reaction direction

always inspect run products optically

in immersion oils if transparent

in polished section if opaque

polished grain mount with BSE on SEM or EMP

measure changes in intensity of characteristic X-ray peaks

observe textures growth features, solution pits with SEM (Haas, 1972)

Rietveld analysis for more careful examination with XRD, yields accurate proportions

spectral techniques may be important (FTIR, Raman, MAS-NMR, Mossbauer)

solubility measurements

least soluble polymorph is most stable, plots of solubility vs. e.g. T shows crossover

Jamieson (1953) measured relative solubilities of calcite vs aragonite at 25-75°C, ca. 4 kbar

Fyfe (1960) proposed this as a technique that should be applied to silicates

examples of solubility experiments

Weill (1966) determined the stability relations of Al2SiO5 in molten cryolite in Fyfe’s lab

Hemley et al. (1977a, 1977b) measured a(SiO2) of solutions with assemblages in MgO-SiO2-H2O

inferred the locus of dehydration reactions

weight gain/loss measurements: first proposed in Fyfe (1960)

select large crystalline phase involved in a reaction, monitor its weight before/after experiment

can monitor very small weight changes with modern scales, easily to 0.1 mg, with care to 0.01 mg

plot weight vs. T, changes from gain to loss at equilibrium

must correct for solubility of solid in presence of fluid

critical to be sure that no fine grained phase is added in same run as weighed crystal

typical experiment: monitor quartz weight gain/loss

examples of weight gain/loss experiments

Evans (1965) muscovite + quartz = K-feldspar + Al2SiO5 + V in Fyfe’s lab

Kerrick (1968) pyrophyllite = quartz + Al2SiO5 + V in Fyfe’s lab

Manning & Newton (2000) used weight loss of crystals to measure solubilities of refractory phases

experimental reversals, not syntheses

synthesis implies crystallization from metastable materials such as glasses, gels, oxide mixtures

despite warning of Fyfe (1960), this practice continues to this day

synthesis of a phase or assemblage does not constrain its stability except to the starting materials

case studies in next several lectures will amply demonstrate perils of synthesis approach

examples of reversals

melting of albite (Bohlen et al., 1982) HW61, Table 2.1

pyrophyllite = diaspore + andalusite + V (Haas & Holdaway, 1972, 1973) HW61, Fig. 2.2

References Cited


Bohlen, S.R., Boettcher, A.L. & Wall, V.J. (1982) The system albite-H2O-CO2: a model for melting and activities of water at high pressures. Am. Mineral. 67, 451-462.

Evans, B.W. (1965) Application of a reaction-rate method to the breakdown equilibria of muscovite and muscovite plus quartz. Am. J. Sci. 263, 647-667.

Fyfe, W.S. (1960) Hydrothermal synthesis and determination of equilibrium between minerals in the subliquidus region. J. Geol. 68, 553-566.

Haas, H. & Holdaway, M.J. (1974) Equilibria in the system Al2O3-SiO2-H2O involving the stability limits of pyrophyllite, and thermodynamic data of pyrophyllite: additional data. Am. J. Sci. 274, 825-828.

Haas, H. & Holdaway, M.J. (1973) Equilibria in the system Al2O3-SiO2-H2O involving the stability limits of pyrophyllite, and thermodynamic data of pyrophyllite. Am. J. Sci. 273, 449-464.

Haas, H. (1972) Diaspore-corundum equilibrium determined by epitaxis of diaspore on corundum. Am. Mineral. 57, 1375-1385.

Hemley, J.J., Montoya, J.W., Christ, C.L. & Hostetler, P.B. (1977a) Mineral equilibria in the MgO-SiO2-H2O system. I. Talc-chrysotile-forsterite-brucite stability relations. Am. J. Sci. 277, 322-351.

Hemley, J.J., Montoya, J.W., Shaw, D.R. & Luce, R.W. (1977b) Mineral equilibria in the MgO-SiO2-H2O system. II. Talc-antigorite-forsterite-anthophyllite-enstatite stability relations and some geologic implications in the system. Am. J. Sci. 277, 353-383.

Holloway, J.R. & Wood, B.J. (1988) Simulating the Earth: Experimental Geochemistry. Unwin/Hyman, Boston, 196 p. Sci. Lib., QE 515 .H721.

Jamieson, J.C. (1957) Phase equilibrium in the system calcite-aragonite. J. Chem. Phys. 21, 1385.

Kerrick, D.M. (1968) Experiments on the upper stability limit of pyrophyllite at 1.8 kilobars and 3.9 kilobars water pressure. Am. J. Sci. 266, 204-214.

Newton, R.C. & Manning, C.E. (2002) Solubility of enstatite + forsterite in H2O at deep crust /upper mantle conditions: 4 to 15 kbar and 700 to 900*C. Geochim. Cosmochim. Acta 66, 4165-4176.

Weill, D F. (1966) Stability relations in the Al2O3 - SiO2 system calculated from solubilities in the Al2O3-SiO2-Na3AlF6 system. Geochim. Cosmochim. Acta 30, 223-237.

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