Elastic properties of garnet solid-solution series

Compositions ofgarnet from 18 tungsten-bearing skarns in the western Cordillera of North America, with additional examples from the literature, indicate that solid solution between grandite and plralspite is far more extensive than previously reported; garnet with up to 75 mole 9o andradite component can contain significant almandine - spessartine ("subcalcic") component. Detailed microprobe analyses show that skarn garnets are typically zoned toward more subcalcic compositions from core to rim and from periphery of skarn toward the inferred fluid-source. l\mong tungsten skarns, those that possess subcalcic garnet are more abundant, they generally possess higher tungsten grades and tonnages, and they appear to be more typical of occurrences in the Cordillera. A model for the occurrence of subcalcic gamet in a skarn requires relatively low oxidation-state, high manganese-ion activity, and low calcium-ion activity in skarn-forming fluids. These conditions are best met in skarns formed at moderate depths, in highly carbonaceous host-rocks and in zones chemically isolated from calcite marble. Semiquantitative calculations suggest that at oxidation states approximately at or below pyrite- magnetitepyrrhotite, changes in activity of calcium hydroxide in solution by a factor of 2 can produce the observed composi tional variations. Subcalcic garnet generally is characteristic of tungsten skarns because such skarns are tlpically found associated with relatively reduced plutons or wallrocks or found at relatively great depths.

The formation of subcalcic garnet in scheelite-bearing skarns

Silicate garnets, general formula E(3)G(2)Si(3)O(12), form an important class of rock-forming minerals and, in nature, most are solid solutions. Their crystal-chemical and solid-solution properties are sometimes interpreted in terms of the widely used Pyralspite-Ugrandite classification scheme, and this can lead to erroneous conclusions. In this study, published data are reviewed and analyzed to achieve a synthesis of relevant experimental and computational results and to construct a working "crystal-chemical model" for describing aluminosilicate garnet, E3Al2Si3O12, over different length scales. The pyrope-grossular (Py-Gr) solid solution is given special attention, because it has received a great deal of study. It also shows interesting crystal-chemical and thermodynamic mixing behavior. Computational and experimental investigations made on Py-Gr garnets indicate that the shorter Ca/Mg-O2 bond lengths appear to remain roughly constant in length across the binary and can be described as showing "Pauling limit-type" behavior. The longer Ca/Mg-O4 bonds behave differently, because they lengthen with increasing Gr component in the solid solution. Bond behavior in almandine-spessartine (Al-Sp) garnets appears to be partly different, because both Fe/Mn-O2 and Fe/ Mn-O4 bonds show "Pauling limit-type" behavior. E-O bond-length variations are continuous. The bonding type in all aluminosilicate garnet end-members is similar. An analysis shows that various computational simulations on Py-Gr solid solutions are consistent with each other with respect to E-O bond behavior and also with experimental IR, Raman, NMR spectroscopic, and X-ray diffraction results, but not completely with XAS studies made at the Ca edge. Ca/Mg-O4 bond behavior can be used to explain, partly, the nature of various micro/nanoscopic crystal-chemical and strain properties and macroscopic excess thermodynamic mixing behavior of Py-Gr garnets. Micro/nanostrain for the Py-Gr binary is asymmetric in nature, as are the various thermodynamic mixing functions Delta H-ex, Delta S-ex, and Delta V-ex. The widely cited Pyralspite-Ugrandite classification scheme has limited use in terms of explaining many physical and chemical properties of garnet and it should not be used to predict or describe, for example, solid-solution behavior.

http://www.minsocam.org/MSA/AmMin/TOC/Abstracts/2008_Abstracts/FM08_Abstracts/Geiger_p360_08.pdf

Silicate garnet: A micro to macroscopic (re)view

The phase relations in the system grossular-spessartine-H2O were investigated at 2.0 Kb aqueous fluid pressure and at subsolidus temperatures down to 420 ° C. Despite metastable persistence of a compositional gap found in some intermediate members, a complete solid solution between grossular and spessartine exists.

Hydration and phase relations of grossular-spessartine garnets at P H 2O=2 Kb

![][1] 

This article reviews the development of mineralogy as a science by focusing largely on the common silicate garnets of general formula {X3}[Y2](Si3)O12. It tells of important discoveries, analyses, and proposals by various scientists relating to crystallography, crystal structures, isomorphism, and solid solution starting in Europe in the late 1700s. The critical recognition of the importance of ionic size of atoms in determining crystal-chemical properties and solid-solution behavior is emphasized. The two garnet species “pyralspite” and “(u)grandite,” which were considered to represent two independent solid-solution series, were introduced by N.H. Winchell and A.N. Winchell (1927) in their well-known book Elements of Optical Mineralogy . Critical comments on the assumptions behind the classification scheme have been pointed out for at least 50 yr, but it remains in use. There is more, though, behind this garnet classification scheme than just simple terminology. There are a long series of scientific discoveries and advances that are largely forgotten by the broader mineralogical community. They begin, here, with the work of the “father of crystallography,” Rene-Just Hauy, concerning the microscopic nature of crystals around 1780 and include later discoveries and proposals by Mitscherlich, Beudant, Wollaston, and Kopp relating to isomorphism and solid-solution behavior all before 1850. A second key era started with the discovery of X-ray diffraction in 1912 that allowed the atomic structures of crystals and, furthermore, atomic and ion radii to be determined. In terms of isomorphism and solid solution, the proposals and studies of Vegard, Zambonini, Wherry, A.N. Winchell, and the “father of crystal chemistry” Goldschmidt are briefly discussed. The recognition of the sizes of atoms and ions, along with an understanding of chemical bonding behavior in crystals, was critical in the establishment of what can be termed “modern mineralogy,” a quantitative science as it is largely understood today that emerged by the mid-1930s. The silicate garnet system pyrope-almandine-spessartine-grossular-andradite-uvarovite shows extensive homovalent substitutional solid solution over two structural sites and complete compositional variation between “pyralspite species” and “ugrandite species” has been documented. Thus, the prerequisites behind the terms “pyralspite” and “(u)grandite,” as originally formulated and often accepted even today, are incorrect and use of this classification is not recommended. Diffraction determinations of the volumes of garnet end-members and volumes of mixing of garnet solid solutions give physical insight into solid-solution behavior. Today, investigations of local structural and crystal-chemical properties, together with determinations of lattice strain and thermodynamic mixing properties, of silicate solid solutions are leading to an ever more quantitative understanding of mineral behavior from the microscopic to macroscopic level.

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/pdf/a-tale-of-two-garnets-the-role-of-solid-solution-in-the-3phh6qikdg.pdf

A tale of two garnets: The role of solid solution in the development toward a modern mineralogy

The miscibility of the pyralspite and grandite molecules in garnets.

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The miscibility of the pyralspite and grandite molecules in garnets.