Defects along kyanite-staurolite interfaces

The note describes (100) stacking faults in kyanite from a staurolite-kyanite intergrovrth with a displacement vector R : 7zb. Di-ffraction spots k * 2n are streaked and only with those reflections stacking faults are in contrast, which is interpreted as due to a displacement of thb on the,(100) faults. This produces groups of two corner-sharing tetrahedra. If stacking faults occur periodically superstructures develop with a doubled or threefold a axis. These faults are frequently combined with lamellar microtwins adjacent to the kyanite-staurolite interface. Kyanite and staurolite are two silicate minerals which are structurally closely related. Staurolite can be viewed as kyanite (AlrSiOr) with inserted layers which contain Fe in tetrahedral sites and have a composition AIO-OH-2FeO (Fig. l). The interface is nearly coherent, with (010) of staurolite corresponding to (100) of kyanite (Naray-Szabo, 1929; Naray-Szabo et al., 1929; Burnham, 1963). The two minerals often occur as crystallographic intergrowths, including the famous specimens from the Alpe Sponda in the Swiss Alps (e.g. Henriques, 1956; Juurinen, 1956). I decided to study these interfaces with the transmission electron microscope (TEM), aiming to find superstructure variations in staurolite or submicroscopic intergrowths of kyanite and staurolite. Staurolite was not very interesting. The homogeneous crystals are orthorhombic without superstructure reflections (Fig. 2b). The interface between staurolite and kyanite was found to be coherent with a few misfit dislocations (Fig. 2a). In the diffraction pattern D* of staurolite corresponds to a* of kyanite (Fig. 2c). Kyanite, however, displayed a whole spectrum of complications in the vicinity of the staurolite contact. Figure 3 shows a variety of hlc0 selected-area electron diffraction patterns (SAD): (a) normal kyanite, (b) kyanite with streaks for /s odd; (c) streaks as in (b) but with additional reflections for /< odd, causing doubling of a; (d) similar to (c) but with sharp additional reflections; and (e) a threefold a lattic,e constant with extra reflections and satellites for k odd. SAD's (b-e) were obtained on areas about 5000A in diameter which show planar defects (b and Fig. 4a) and lamellar structures (c-e and Fig. 4b,c) parallel to (100). The size of the electron beam was therefore too large to isolate the lamellar phase and all patterns must be regarded as composites. KYANITE STAUROLITE