Mixing properties garnets

Ottonello G, Bokreta M., and Sciuto P F. (in preparation). Parameterization of energy and interactions in garnets Mixing properties. 

Berman R. G. (1990). Mixing properties of Ca-Mg-Fe-Mn garnets. Amer. Mineral, 75 328-344. 

Ganguly J. and Saxena S. K. (1984). Mixing properties of aluminosilicate garnets Constraints from natural and experimental data and applications to geothermo-barometry. Amer. Mineral, 69 79-87. 

Ganguly J., Cheng W., and O Neill H. St. C. (1993). Syntheses, volume, and structural changes of garnets in the pyrope-grossular join Implications for stability and mixing properties. Amer Mineral, 78 583-593. 

Hackler R. T. and Wood B. J. (1989). Experimental determination of Fe and Mg exchange between garnet and olivine and estimation of Fe-Mg mixing properties in garnet. Amer. Mineral, 74 994-999. 

The mixing properties of garnet components and many other rock-forming solid solutions is an on-going problem, and we can leave resolution of the problems of grossular-pyrope to the experts. The point made here is simply that determinations of excess properties can be made by calorimetry, measuring phase compositions, and X-ray work, and that these can be combined into values of Wc and hence of activity coefficients. None of this is easy to do. 

Mixing properties of Ca-Mg-Fe-Mn garnets. Amer. Mineralogist, 75 328 4. Berman, R. G. and Brown, T. H., 1984, A thermodynamic model for multicomponent melts, with application to the system Ca0-Al203-Si02. Geochim. et Cosmochim. Acta, 48 661-78. 

EXPERIMENTAL DETERMINATION OF THE MIXING PROPERTIES OF SOLID SOLUTIONS WITH PARTICULAR REFERENCE TO GARNET AND CLINOPYROXENE SOLUTIONS... 

The principal problem associated with the determination of the mixing properties of aluminosilicate garnets is that most of the solutions involved are stable only at high pressures. The apparatus used in experimental studies at these pressures does not, of course, enable as accurate a control of the intensive variables as is possible at 1 atmosphere. Nevertheless, activity-composition relationships determined at high pressures need not necessarily have extremely large uncertainties provided that the equilibria used to fix activities are carefully chosen. 

The metal has very little commercial use. In elemental form it is a laser source, a portable x-ray source, and as a dopant in garnets. When added to stainless steel, it improves grain refinement, strength, and other properties. Some other applications, particularly in oxides mixed with other rare earths, are as carbon rods for industrial hghting, in titanate insulated capacitors, and as additives to glass. The radioactive isotope ytterbium-169 is used in portable devices to examine defects in thin steel and aluminum. The metal and its compounds are used in fundamental research. 

The other oxides listed in Table 9.1 are included because their mechanical properties have received some attention (especially quartz). Quartz is by no means close-packed because of the stability of the [Si04] tetrahedron. Yttria and the rare-earth sesquioxides and the mixed oxide garnet structures are compact, in spite of their complexity, but cannot be described as close-packed in the traditional sense. In fact, the C-type M2O3 structure of yttria can be thought of as having the fluorite structure of urania with one-quarter of the anion sites vacant. 

The extensive research devoted to the physics and chemistry of solids during the last quarter of a century has led to great advances in the understanding of the properties of solids in general. One area of very active research has been mixed oxide systems. These have been of particular interest because of their stability and the fact that a wide range of properties can be obtained by substitution of one ion for another. Two of the most extensively studied groups of compounds are the perovskites and garnets. 

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