Penetration Twin — The Crystal Habit of Mutually Interpenetrating Individuals
Penetration Twin — The Crystal Habit of Mutually Interpenetrating Individuals
A twin in which two crystal individuals share a common volume rather than meeting along a contact plane
A penetration twin is a twinned crystal in which two individual crystals appear to grow through one another, sharing a common volume across a twin plane or twin axis. The two individuals are mineralogically identical and have grown in fixed crystallographic relation to each other, but the geometry of their interpenetration produces shapes that look as though one crystal has been pushed through the body of the other. The form is one of the textbook categories of twinning, distinct from contact twins (where the individuals share a flat surface), polysynthetic twins (multiple parallel lamellae), and cyclic twins (rosettes such as cerussite stars).
How they form
Twinning of any kind arises from a defect introduced during crystal growth. At the moment of nucleation or during a growth event, the lattice of an individual crystal extends in two orientations related by a symmetry operation that is not part of the parent structure's normal symmetry — a reflection, rotation, or inversion across a plane that the parent structure does not already contain. The two regions then continue to grow as separate but lattice-locked individuals. In a penetration twin, the two individuals nucleated in such a relationship that their mutual extension carried each into the other's volume rather than producing a planar contact.
Type examples
Fluorite is the species the field worker reaches for first when explaining penetration twins. The classic interpenetrating cubes habit, in which two cubes appear to pass through each other rotated ninety degrees about the body diagonal, is one of the most recognisable twin forms in mineralogy and is the standard demonstration specimen in undergraduate teaching collections. Staurolite produces the cruciform penetration twin in which two prismatic individuals cross at right angles or at sixty degrees, the latter producing the so-called Saint Andrew's cross twins prized as religious folk-talismans in Brittany. Quartz produces a range of penetration twins under the Brazil law and Japan law, and orthoclase feldspar produces the well-known Carlsbad twin, in which two prismatic individuals interpenetrate along the c-axis.
The standard reference for crystal twinning is Hurlbut and Klein's Manual of Mineralogy, which catalogues the principal twin laws by species. The Mineralogical Society of America publishes more advanced treatments in its Reviews in Mineralogy series.
In gemmology and the trade
Penetration twins are diagnostic features for several species and are used by gemmologists to confirm identification. A staurolite cross or a fluorite interpenetration in rough material is conclusive of the species. Twin planes affect cleavage behaviour and can introduce internal strain that produces optical anomalies under polarised light, a feature exploited in the laboratory's identification suite. Cut stones with internal twin planes can show unexpected birefringence patterns that, once recognised, become diagnostic.
For the lapidary, internal twin planes can be a structural problem, since the lattice mismatch at the twin boundary can be a zone of weakness. For the mineral collector, by contrast, penetration twins are highly desirable specimens, and well-formed examples of fluorite cubes, staurolite crosses, and Carlsbad orthoclase command premiums over single crystals of comparable size.