TRANSFORMATION-INDUCED TWIN BOUNDARIES IN MINERALS

Two types of twins are formed during symmetry-breaking transitions in minerals, merohedral twins and pseudomerohedral twins. The shape and orientation of the twin boundaries depends on the spontaneous strain. Merohedral twin boundaries from transitions with low strain commonly have smoothly curving surfaces as found in dolomite, ferrian ilmenite, vesuvianite and leucite. Dauphine twins in quartz, although merohedral twins, have straight boundaries because of the large amount of spontaneous strain associated with the transition. Pseudomerohedral twins (ferroelastic twins) have straight boundaries as found in leucite, cordierite, feldspar, perovskite and cristobalite. The large spontaneous strains (up to 5%) associated with these latter transitions form single and multiple needle twins at boundary intersections as well as long-range strains which can be imaged in the transmission electron microscope. Many of these transitions provide useful information to the earth scientist, as will be discussed.