*Review* **Ferroelastic Twinning in Minerals: A Source of Trace Elements, Conductivity, and Unexpected Piezoelectricity**

**Ekhard K. H. Salje**

Department of Earth Sciences, University of Cambridge, Cambridge CB2 3EQ, UK; ekhard@esc.cam.ac.uk

**Abstract:** Ferroelastic twinning in minerals is a very common phenomenon. The twin laws follow simple symmetry rules and they are observed in minerals, like feldspar, palmierite, leucite, perovskite, and so forth. The major discovery over the last two decades was that the thin areas between the twins yield characteristic physical and chemical properties, but not the twins themselves. Research greatly focusses on these twin walls (or 'twin boundaries'); therefore, because they possess different crystal structures and generate a large variety of 'emerging' properties. Research on wall properties has largely overshadowed research on twin domains. Some wall properties are discussed in this short review, such as their ability for chemical storage, and their structural deformations that generate polarity and piezoelectricity inside the walls, while none of these effects exist in the adjacent domains. Walls contain topological defects, like kinks, and they are strong enough to deform surface regions. These effects have triggered major research initiatives that go well beyond the realm of mineralogy and crystallography. Future work is expected to discover other twin configurations, such as co-elastic twins in quartz and growth twins in other minerals.

**Keywords:** twin wall; twin boundary; minerals; emerging properties; piezoelectricity in minerals; surface relaxations; anorthite; Pamierite; perovskite
