Abstract
In the system (K,Na)AlSiO4, there are several stuffed derivatives of tridymite, the most common of which are nepheline, kalsilite and kaliophilite. The phase relationships, as described by Tuttle and Smith (1958), are unusual in that the density of symmetry elements is higher in the low temperature polymorphs (nepheline and kalsilite) than in the high temperature polymorphs (orthorhombic KAlSiO4, tetrakalsilite and kaliophilite).
The purpose of this study is to reassess the phase relationships for the potassio side of the system in the light of (1) theoretical considerations regarding Al–Si order–disorder and displacive transformations and (2) a new analysis of the diffraction patterns of some of the polymorphs. It is shown that the orthorhombic KAlSiO4 and tetrakalsilite are most likely metastable, produced during quenching. The stable polymorphs have approximately the same unit cell volume as kalsilite and include a hypothetical, Al–Si disordered P63/m2/m2/c polymorph above 850°C, a hypothetical, Al–Si ordered P63mc polymorph at intermediate temperatures and kalsilite, P63, at low temperatures. The other polymorphs are unstable relative to one or the other of these depending on the temperature and annealing time. The so-called synthetic kaliophilite reported by Tuttle and Smith (1958) may in fact be either the high temperature P63/m2/m2/c polymorph or the intermediate temperature P63mc polymorph.
Rotation twins and mirror twins should occur in kalsilite which has transformed from the high temperature, Al–Si disordered P63/m2/m2/c polymorph. During cooling Al–Si ordering results in a reduction in symmetry to P63mc and this should result in twins related by 2-fold rotation. Natural kaliophilite probably represents a transitional or Ostwald state between P63/m2/m2/c and P63mc. Upon further cooling, displacive transformation of the framework results in another reduction in symmetry to P63 (kalsilite). This last transformation should cause mirror twins.
