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.
Highlights
Twinning of minerals has intrigued mineralogists as long as mineralogy was established as a discipline by Haüy in 1797 [1]
At the beginning of the 20th century, many textbooks and research articles in mineralogy focused on twinning from a geometrical perspective [2,3,4,5,6,7,8]
Many of the early pioneers of mineralogy and crystallography were involved in this research, which highlights its importance
Summary
Twinning of minerals has intrigued mineralogists as long as mineralogy was established as a discipline by Haüy in 1797 [1]. The experimental discovery was that, in a specific tungstate mineral with perovskite structure, WO3 , the boundary between various twins, the so-called ‘twin boundary’ or ‘twin wall’, has completely different physical properties from the twin domains (Figure 1). In this particular case, the twins are insulators while the twin boundaries are highly conducting at room temperature and become superconducting below ca. Domain walls, which appear dark in the PFM image on the right (horizontal length of the image 1.5 μm, after [13]) This will describe describeother otherproperties, properties,and andthey they were later identified to localized be localThis review review will were later identified to be ized in the twin walls, but not in the twin domains. Different twins in other minerals and highlight some open questions
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