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 o rientation of the twin boundaries depends on the spontaneous strain. M erohedral twin boundaries from transitions with low strain commonly ha ve smoothly curving surfaces as found in dolomite, ferrian ilmenite, v esuvianite and leucite. Dauphine twins in quartz, although merohedral twins, have straight boundaries because of the large amount of spontan eous strain associated with the transition. Pseudomerohedral twins (fe rroelastic twins) have straight boundaries as found in leucite, cordie rite, feldspar, perovskite and cristobalite. The large spontaneous str ains (up to 5%) associated with these latter transitions form single a nd multiple needle twins at boundary intersections as well as long-ran ge strains which can be imaged in the transmission electron microscope . Many of these transitions provide useful information to the earth sc ientist, as will be discussed.