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  • Author: Michael A. Carpenter x
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Abstract

A diversity of twinning and exsolution textures has been observed by transmission electron microscopy in natural kalsilite from volcanic and metamorphic rocks (Mount Nyiragongo, Zaire; Alban Hills, Italy; Kerala, southern India). The phase transitions responsible for these microstructures were examined by high-temperature powder and singlecrystal X-ray diffraction. Pure kalsilite has P63mc symmetry at high temperatures but transforms to its P63 state through a two-phase field, between ~870 and ~920°C. In this two-phase field, the low form coexists with a structure that has a sixfold repeat of its a unit-cell dimension. For ~KS₈₈,the high-temperature state is high tetrakalsilite with possible space group P63mc; a two-phase field between ~890 and ~930°C involves the coexistence of this phase with P63 kalsilite. High tetrakalsilite with composition ~KS74 reverts to low tetrakalsilite on quenching to room temperature from 950°C. Latticeparameter variations, and estimates of the AI-O-Si angles that can be derived from them, imply that the stability limit for the high structural states occurs when the angles for apical and basal 0 atoms converge. At lower temperatures, pure P63 kalsilite appears to have transformed during a metamorphic evolution in nature to an intergrowth of the P63 structure and a structure for which P31c symmetry is proposed. The latter can be thought of as a polytype of kalsilite, with (001) layers stacked in an eclipsed array rather than in the staggered array of normal low kalsilite. In this regard, KAlSiO4 seems to be remarkably similar to KLiSO4. If ~3-12 mol% NaAlSiO4 is present in solid solution, a P63 structure with an a parameter √3 greater than normal low kalsilite develops. An anomaly in the lattice parameters of KS₈₈ suggests that the transition temperature below which this superstructure develops may be ~500°C. Natural nepheline ex solved from kalsilite displays merohedral twinning, which can be accounted for by a P63mc → P63 transition during cooling. Transformation behavior in the Ne-Ks system may be explained, at least qualitatively, in terms of Na-K ordering between cavity sites, ordering of basal O atoms between two sites on either side of the mirror plane parallel to the c axis of P63mc structures, ordering of apical O atoms between three sites, and coupling between all these processes.

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Abstract

A new KAlSiO4 polymorph was found in a granulite facies gneiss from the Punalur district, southern India. The structure was solved and refined on a twinned crystal to an R index value of 1.98% for 265 independent reflections. Metamorphic kalsilite is trigonal, space group P31c with a 5.157 (1) Å , c = 8.706 (3) Å , V = 200.52 (9) Å3, Z = 2. The overall diffraction symmetry 6/mmm exhibited from all the crystals examined arises from a {0001} twinning, related to a mistake in the ordered Al-Si-Al-Si sequence along the c axis. The crystal structure is a stuffed derivate of tridymite, and is characterized by sixmembered tetrahedral rings with ditrigonal shape. Individual layers of this structure are the same as those of P63 kalsilite, but are stacked along the c axis in an eclipsed manner rather than in the staggered manner of the P63 structure.

Abstract

Elastic anomalies that accompany cation ordering and displacive phase transitions can be understood in terms of coupling between strain components and the driving order parameter in Landau free energy expansions. Non-convergent cation ordering in spinel, MgAl2O4, is accompanied by changes in individual elastic constants, shear modulus, and bulk modulus that vary linearly with the order parameter. Convergent cation ordering, such as Al/Si ordering in anorthite, is expected to give changes in elastic properties that scale with the square of the order parameter. The elastic anomalies that develop in association with displacive phase transitions show greater diversity, due to the additional influence of the order parameter susceptibility. These are illustrated for the cases of the proper ferroelastic transition at high pressure in stishovite and the improper ferroelastic transition in SrTiO3 perovskite. Low temperature transitions in lawsonite show a more complex pattern of softening and stiffening that depends on coupling with both cation ordering and displacive processes. Variations of the spontaneous strain and elastic constants are indicative of the underlying thermodynamic mechanism for a phase transition. If any such transitions occur in minerals of the Earth's crust or mantle they should be identifiable from their distinctive influence on seismic velocities.

Abstract

Transmission electron microscopy (TEM) has revealed dense, complex twins in synthetic anorthite crystals formed by annealing CaAl2Si2O8 glass. The crystals are dominated by Carlsbad and Carlsbad-albite twins, with small amounts of albite twins. All the composition planes are parallel to (010). High-resolution TEM images show the twin boundaries to be coherent. Selected-area electron diffraction (SAED) patterns containing the b*- c* reciprocal plane are useful for distinguishing these twins. Extremely dense Carlsbad twins at the unit-cell scale indicate that additional energy caused by the Carlsbad twin boundaries in highly disordered anorthite is lower than that caused by albite twin boundaries. We propose that for anorthite with an initially disordered structure the total energy induced by Carlsbad twinning is lower than that caused by albite twinning and higher than that caused by albite twinning in anorthite with an initially ordered structure.