Kyanite Formation

Rosing, M. T. Frei, R. 1999. Late Archaean metasomatism and kyanite formation in the >3700 Ma Isua supracrustals, west Greenland. Journal of Conference Abstracts, 4, 144. 

Table 1.4 The enthalpy of formation of the three polymorphs of AI2Si()5, kyanite, andalu-site and sillimanite at 298.15 K [3].

Table 1.5 The enthalpy of formation of kyanite, andalusite and sillimanite from the binary constituent oxides [3]. The enthalpy of transition between the different polymorphs is also given. All enthalpies are given for T = 298.15 K.

Figure 1.10 The standard Gibbs energy of formation from the binary constitutent oxides of the kyanite, sillimanite and andalusite modifications of A SiOs as a function of pressure at 800 K. Data are taken from [3]. All three oxides are treated as incompressible.

ArV is not necessarily positive, and to compare the relative stability of the different modifications of a ternary compound like AGSiOs the volume of formation of the ternary oxide from the binary constituent oxides is considered for convenience. The pressure dependence of the Gibbs energies of formation from the binary constituent oxides of kyanite, sillimanite and andalusite polymorphs of A SiOs are shown in Figure 1.10. Whereas sillimanite and andalusite have positive volumes of formation and are destabilized by pressure relative to the binary oxides, kyanite has a negative volume of formation and becomes the stable high-pressure phase. The thermodynamic data used in the calculations are given in Table 1.7 [3].1... 

Figure 7.1 Typical thermal expansion trace kyanite (AljCVSiOj) + quartz (Si02) at 5°C/min. The a-0 quartz inversion is apparent at 573°C. Kyanite converts to mullite (3Alj03-2Si02) and residual glass starting at 1100°C, reaching a maximum rate at 1400°C [1]. The sharp contraction starting at 1100°C is interpreted to correspond to sintering. At 1320°C, the rapid formation of the less dense decomposition products of kyanite cause a temporary expansion [2].

Mohr D. W. and Newton R. C. (1983) Kyanite—staurolite metamorphism in sulfidic schists of the Anakeesta Formation, Great Smoky Mountains, North Carohna. Am. J. Sci. 283, 97-134. 

This can be illustrated by a natural example. In the coarse-grained Allanin magnesium-gabbro, infiltration of fluid caused the formation of reaction rims around olivine (Chinner and Dixon, 1973). The succession is olivine anthophyllite (2 wt.% H2O) — talc (4wt.% H2O + kyanite — chloritoid (8wt.% H20- -talc + kyanite. This reaction rim is H2O undersaturated, and the succession of mineral assemblages corresponds to an increase of H2O content towards the rim and can only be modeled by an increase in the availability of water towards the rim. The H20-undersamrated character of the inner rim zones does not necessitate (or justify) a CO2 component in the fluid, but rather reflects limited availability of an H2O fluid. 

The heat of the reaction Al OgCcr, a) + Si02(quartz) Al2S10g(cr, kyanite) has been determined at 968 K by Holm and Kleppa (, 2) to be -2.37 0,15 kcal mol". This value was obtained from heat of solution measurements in a high temperature oxide melt calorimeter. Using the JANAP functions, this reduces to Aj,H"(298.15 K) -1.96 kcal mol", which is used to calculate the adopted value for the heat of formation of kyanite. 

The three sillimanite minerals are structurally similar and have structures that are related to that of mullite. It is not surprising that they all form mullite upon decomposition. Kyanite crystallizes in the triclinic system, while sillimanite, andalusite, as well as mullite have orthorhombic crystal structures. In these structures, all the Si4+ cations are in fourfold coordination with 02 anions, but the Al3+ cations exist in four-, five-, and sixfold coordination with 02 anions, and therein lie the structural differences. The fivefold coordination of some Al3+ cations within A105 polyhedra is rather unusual, perhaps the result of formation at high pressures. The other structural differences among the three minerals are quite small. They are associated with the double chain structures of these three minerals and the linkages of the chains to one another by different alumina and silica polyhedra. Those concepts are readily extended to mullite. 

The decompositions of the three sillimanite minerals occur over different temperature ranges and with different volumetric expansions. As the sillimanites are formed at high pressures, it is natural that they exhibit large volumetric expansions when they decompose at 1 atm pressure. As expected, kyanite, the highest-pressure form, undergoes the largest volumetric expansion (about 15%). The P-T formation densities can be viewed as the driving force for the decompositions. Kyanite also initiates its decomposition at the lowest temperature of the three. 

Holm and Kleppa (1966) made some calorimetric determinations of the heats of formation of kyanite, andalusite, and sillimanite from their constituent oxides (quartz and corundum) in an oxide melt calorimeter at 968 K. For andalusite, the result for the reaction given above was =... 

Enthalpies of formation (from the oxides) of MgAl204 and AlgSiOs (kyanite and sillimanite) have been determined by oxide melt solution calorimetry at 965 and 1173... 

Stump (1986) is supported by the occasional presence of apatite, rutile, corundum, kyanite, and tourmaline. Stump (1995) also noted that the La Gorce Formation differs from the Goldie Formation of the central Transantarctic Mountains (Section 5.2) by the absence of carbonate beds except for calcite veins near faults. 

A variety of minerals such as garnet [Fe3Al2(Si04)3], kyanite (Al2Si05) and staurolite [FeAl4Si20io(OH)2] may be present in these schists, indicating formation at increasing temperatures. 

Above 800 C an important chemical change takes place with the formation of one of the three aluminosilicate polymorphs (Al SiOj), i.e., andalusite, kyanite, or sillimanite, and free silica according to the overall chemical reaction ... 

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