By Bob Jones
We collect minerals for many reasons, both scientific and aesthetic, whether they are crystallized or massive. We admire crystal groups, single crystals, and their beauty as creations of nature. By and large, we collect and display specimens dominated by groups of a particular mineral species, sometimes accompanied by other species. Such clusters of minerals are called an aggregate of crystals. Under the right circumstances and within those aggregate mineral groups are three general forms of crystals that exhibit twinning.
Crystals can twin in parallel or routine ways involving two crystals, or as in complex twinning, several crystals. To be considered as twins, the crystals must be in a fixed identifiable relationship to each other. How this happens is most impressive. As crystals grow in solution, individual ions of a mineral gather to form a crystal. This is an orderly geometric process that produces a geometric pattern that results in an external crystal form that we recognize.
Influences on Crystal Growth
Several factors influence how crystals grow, not the least of which is the direction of flow of the solution. This direction brings ions in contact with the growing crystal from a path that can influence growth. Such direction causes ions to accumulate first on the side that receives the flow. This development can affect the orderly process of crystallization.
Sometimes, as the ions move to take a proper position in the crystal lattice, one ion may not quite fit right, appearing slightly offset. This occurrence is not enough to disturb the lattice structure but can affect later arriving ions. Especially under rapid crystal growth, the next arriving ions attach to the slightly offset ion. When this happens, this offset results in a second crystal that grows in a different direction but not in just any direction, a direction determined by the orderly rules of crystal growth.
The original crystal continues to grow during this time, and so does the second attached crystal. The result is the two crystals share a common boundary or face and a singular lattice. Most importantly, the crystals attach at a predictable angle to each other. We can measure that angle to prove the pair of crystals are twinned. Specimens with such crystal structure are called contact twins, which is just one of several types of crystal twinning. Contact twins involve two crystals, while more complex twins involve several.
As I mentioned, contact twins share a lattice, the boundary of which is often a visible plane of attachment between the two. These twins also share a face, giving then a flattened look. The key to a twin is the distinct exact angle between the two crystals, called the reentrant angle. In contact twins, the reentrant angle appears as a ‘V’ notch that can range from barely visible to a wide-open span, making the two crystals look like rabbit ears. The reentrant angle is always the same degree of separation, which is critical to identifying a twin because there can be many “paired” crystals that look like contact twins.
This fact is particularly true with quartz in and aggregate. Unless the angle between the two quartz crystals is 84 degrees, they are not twinned. This type of quartz twin is called a Japanese law twin, and regardless of how many Japanese twins you examine, there is a constancy of that angle. Other species twin with different reentrant angles, but each remains exactly constant to that particular species. Contact twins are common in quartz and gypsum, even calcite, and some feldspars like albite.
Examining Penetration Twins
A second twin form of minerals is penetration twins. Simply described,
penetration twins are two crystals of the same mineral that look as if they have been shoved together. Penetration twins occur when one crystal starts to develop, and a second crystal starts to grow inside the first as ions offset along an axis of the first crystal. The second crystal grows at a different angle, determined by the direction of the first crystal axis! The result is one crystal resides within another crystal with the corners of the second crystal protruding from the center of each host crystal’s face. This formation is prevalent in cubes of fluorite as well as pyrite. Note both these species are in the isometric or cubic crystal system, which commonly develops penetration twins.
Isometric crystals also form octahedral crystals. Why is this important to note? Because the two cubic crystals in a penetration twin share a common octahedral axis as part of a valid twin formation.
Staurolite, a monoclinic crystal form, can exhibit a perfect cross twin that is treasured by some people. Again a common axis has to be shared for this to be a real cross and twin. In this formation, the same mineral forms another twin when one crystal is at a diagonal 60-degree axis of the host crystal. Penetration twins can be visualized as one crystal mysteriously passing into and through another. Mother Nature is mysterious but not magical. Penetration twins are a matter of ions in geometry.
An even more interesting twin in the cubic system in a spinel twin because it’s original identification was spinel crystals. A spinel twin crystal, whether it is spinel, copper, sphalerite, or other cubic minerals, has to appear as an octahedron crystal cut in half. One half of the octahedron is rotated 180 degrees and re-attached to the other half. This process forms two very distinctive angular terminations at opposite ends of the crystal, creating a visible shallow contact boundary running the length of the twin.
One of the most beautiful forms of twinning is cyclical twins, which are part of the polysynthetic group of twins. The word cyclical suggests something round and cyclical twins require as many as six or eight crystals to join a flat-sided round donut-like shape. In some cases like aragonite, the cyclical twin has no doughnut hole, but others like chrysoberyl have a center hole around which crystals twin. Cyclical twins only form in the hexagonal, tetragonal or orthorhombic systems. This information sometimes helps identify an unknown mineral since it can guide the researcher toward a group of crystal species in a recognized group of species. Cyclical twins develop when several crystals share axes and faces during growth. They attach side by side as each pair forms at an angle to its neighbor resulting in a complete link to form a rounded or cyclical crystal twin.
Cyclical twins are part of the group known as polysynthetic twins, which is also known as repeating twins. In the feldspar family, polysynthetic twinning is very common, but each crystal is hardly distinctive. Feldspars such as labradorite and albite, are typically polysynthetic twins. In these minerals, flat, wafer-thin crystals form side by side, alternating one against the other, resulting in a solid mass. The result is something substantial that a lapidary can use. The beauty of this twinning is the piece acts as a diffraction grating scattering or bend light, so its component colors are visible. This subtle rainbow effect is the Schiller effect, and these feldspars are suitable for lapidary use.
The most fantastic use of feldspars with a Schiller effect I’ve seen reside within the Kremlin. The beautiful grounds behind the Kremlin walls feature a riot of flowers in huge planters with large blocks of stone forming low retaining walls. These blocks caught my eye because each was of lapidary gem material. One wall, in particular, was sensational, as it featured huge blocks of polished labradorite that shimmered in the sun. Unfortunately, my camera was “borrowed” by security during our visit.
You’ll be able to recognize twin forms as you read more about twins and handle common minerals like quartz and feldspar crystals. We’ve described twinning that occurs during crystal growth in solution, and twins can also form during metamorphic action as the excess heat and pressure turn exiting rock into a plastic almost fluid state in which atoms of minerals can move around, and crystals grow. During this metamorphic action, any existing crystals that form remain subject to heat and pressure once they reach a solid crystal form. Once some crystals form during metamorphic action, they are still subject to change, including forming of twins.
Crystal Habit Awareness
One of the real joys of collecting minerals is to be familiar with crystal habits not only to recognize but also to collect crystals that are a bit unusual. Knowing what and why such twin crystals form is always a job but a real pleasure. The more effort you put into learning about and collecting twinned crystals, the more enjoyable your collection will be.
Just a word of caution: Be well aware of crystals that look like but are not twinned. Crystals that grow parallel to each other may not be twinned, while others that jut in an angle resembling rabbit ears also may not be twinned. These are things you’ll learn to identify as you develop an interest in twinned crystals.