On the Rocks: Crystal Habits

0
Rhombic crystals of calcite are the regular habit formed in normal atmospheric temperatures. The Arkenstone Gallery of Fine Minerals, www.irocks.com

By Bob Jones

Whenever we talk about minerals, especially our favorite minerals, we use all sorts of descriptive terms that don’t seem to have anything to do with one of the six crystal systems. Mineral specimens are commonly described as dendritic, acicular, columnar, striated, botryoidal, banded, and prismatic, acicular.

These terms are the language of crystal habits and are an integral part of a conversation when we describe mineral specimens. While using such terms, we seldom think about why a particular term works for us. I doubt we realize these terms are the direct result of two things, the mineral’s internal atomic structure and the role it plays in a mineral’s development and the effects of the environment on a mineral during formation. We already know when a mineral forms, it develops as one of the recognized crystal systems: isometric, hexagonal, monoclinic, triclinic, tetragonal, or orthorhombic. You may also come across a seventh system, trigonal. This is actually a sub-system based on two basic crystal forms in the hexagonal system.

Mineral Types and Crystal Formation

Scalenohedrons, like this specimen from Tsumeb, is just one crystal habit of rhodochrosite.

Regular calcite can show an obvious hexagonal form while some calcites develop in rhombic crystals, so they are identified as trigonal crystals. You have undoubtedly seen this in some rhodochrosite specimens, which is another carbonate. This mineral type shows the same two hexagonal systems. When we describe a mineral, we may start by naming its crystal system. But we need to use terms that describe the specimen in far more detail, and that’s when we describe the mineral’s crystal habits. Native copper is an example of this. It is a cubic or isometric mineral.

Cubic copper crystals that have six faces are very uncommon. More common are copper crystals that form as a twelve-sided dodecahedron. Even more frequently, we find copper in an arborescent or dendritic crystalline form. In its dodecahedron form, it can almost look like a rounded ball when the faces are really tiny. The more common arborescent and dendritic forms develop as the result of the influence of the environment. In a rich solution, as the copper crystallizes rapid molecular electric attraction and growth can create slight irregularities in the unit cells, which causes repeated branching and elongation of the crystallizing copper and arborescent growth. If the growth space is restricted to two dimensions as in a narrow crack, a dendritic form emerges.

The reason crystals develop is due to the electron attraction between metals and non-metals that loan, borrow, or share electrons. This ionic sharing during crystal growth creates an imbalance in electron charges that attracts more molecules during crystal growth. The attraction does not extend in all directions, which can determine the direction of growth. If the crystal growth is dominant in one direction, growth develops into a prismatic form we see in tourmaline, quartz, and other prismatic minerals. If that direction of growth is exclusive to one direction, the crystals are needle-like, which we see in some zeolites, rutile, and even some stibnite.

Time, Pressure, Temperature's Role in Crystal Formation

The energy in an environment, usually in the form of high heat, where crystals form, also determine the formation. Some crystals develop from vapors, while many others form in solid rock while it is in a fluid or plastic state, which allows molecules to slowly migrate toward each other. This migration allows the molecules to move and connect to form a crystal. However, the vast majority of minerals often appreciated by collectors form in watery solutions that vary widely in mineral content or richness, temperature, and pressure. In addition, even the direction the solution is moving can influence how a crystal grows and what form it takes.

If you handle enough specimens of a particular mineral, you learn minerals prefer a particular crystal habit growth. For example, stibnite is always found in long slender needle-like crystals indicating rapid or more persistent growth in one direction due to molecular attraction. Gem tourmalines are almost always striated, which may be due in part to what is called oscillatory growth — during which two different crystal forms vie for dominance. Hematite, on the other hand, is often found in botryoidal form as it has a penchant for very rapid growth forming radiating needles from a common starting point of nucleation. When it comes to malachite, this mineral prefers to form in velvety needle coatings rather than in discrete lengthy prisms. The discrete prisms are the common habit of many species like epidote, kyanite, beryl, and quartz, among others. Again, this is a function of molecular attraction that is strong in one direction.

When you read about mineral deposits, they are always described as low, medium, or high-temperature deposits. This is important as that energy has a profound influence on what species crystallize out of a solution first. It also affects what crystal habits a mineral may choose. Each species has its own temperature of crystallization, so some species form first in a pocket or open seam following by crystals that form at a lower temperature. Zeolites, for example, are very late forming species, so are often found having formed last in a cooling pegmatite pocket full of species that formed first at higher temperatures.

Calcite's Unique Crystal Growth

High temperature mineral deposits are where very complex fluorite crystals often form. The Arkenstone Gallery of Fine Minerals, www.irocks.com

One of the most interesting common mineral species to demonstrate this growth habit is calcite. It is found in lovely crystals in every mineral environment with both low and high temperature climates, and from near-surface sedimentary deposits as well as much deeper locations. Because of this, calcite manages to develop in at least five basic crystal forms. This is one reason why collecting calcite is so widespread and varied.

These different crystal forms, all in the hexagonal-trigonal system, have their growth controlled in large part by the temperature of the environment where they form. The five basic forms calcite takes in crystals are scalenohedrons or dog tooth, tabular, simple hexagonal, rhombic, or disc-like or poker chip form. They are all hexagonal but are not always easily recognized. This is because some crystal habits are dramatically different from the textbook hexagonal shape due to different environmental temperatures.

The available energy affects the position of molecules in their unit cell arrangement resulting in different crystal habits within the hexagonal system. Recognizing these different crystal habits helps scientists identify some in the sub-system trigonal of the hexagonal system. The higher temperature solutions are prone to developing calcite that has a scalenohedral or poker chip form.

Slightly lower temperature solutions produce calcite crystals with a tabular calcite crystal form. In somewhat lower temperature deposits, simple hexagonal calcite occurs, and rhombic crystals can form from solutions where the solution temperature is at ambient levels. Solutions around 25 degrees Celsius and lower produce dog tooth crystals. These are common in near-surface sedimentary deposits. This explains why we find small rhombic and dog tooth crystals to be very common in the Midwest limestone deposits. This does not mean a particular deposit produces one crystal habit exclusively. Temperatures within a given deposit can vary over time, producing different crystal habits.

Diverse Crystal Formation

Another popular mineral that shows a wide variation in crystal form is fluorite. Keep in mind that a major influence on the form of a fluorite crystal within its particular system is determined in part by the interplaner distance, which is affected by the energy available during crystallization.

As in calcite, and other species, the variation from low to high energy also affects the complexity of the fluorite crystal formation.

As it happens, octahedrons of fluorite require less energy to form, so we find them as a common form of this calcium fluoride. Cubes only require a bit more energy, which is why it is not uncommon to find both octahedrons and cubes of fluorite in the same deposit. Deposits producing fluorite from higher temperature hydrothermal solutions will produce slightly more complex dodecahedrons. In very high-temperature metal deposits, more complex fluorite crystals are found, some with as many as 24 or even 48 faces. Some of the high-temperature German silver deposits yielded such complex fluorites. More recently, one or two metal deposits in China have produced fairly complex fluorite crystals.

Exploring Molecular Attraction and Crystal Growth

Hoppered crystals are good subjects to use to explain the molecular attraction

Tabular calcite develops this crystal form in mineral deposits that are high temperature.

that is the function of crystal growth. A hopper crystal starts out growing in a solution in a normal way. It attracts like molecules to itself once it starts to grow. Growth continues consistently but stops when there are no molecules of the mineral left in the solution. This situation happens with a hopper crystal before it can complete itself and occurs because as molecules are attracted to a growing crystal, they attach sequentially from the prism faces inward. They are not growing from the inside out. They grow from inward.

Molecules first attach to the top outer edge of the growing crystal then continue the growth by filling in the spaces. A sudden stop in the availability of molecules leaves the crystal with a partially filled crown. Also, in some cases, this unavailability of molecules results in a termination with crystals around the top edges of the prism faces, and the center of the termination unfinished.

There are several unknown factors taking place during the growth of crystals, which contribute to the crystal habits established, particularly if the growth is a bit unusual for the species. Among the factors are imperfections that can occur during growth. All it takes is one atom or molecule to fail to fit precisely where it belongs, and the result can be a change in growth. Sometimes the imperfection marks the beginning of another crystal.

Among the minerals that exemplify this process, and one of my favorites is rhodochrosite. It forms in layers or bands, as well as stalactites of the same species. A change in the composition of such a mineral often takes place, as impurities may enter the solution
during the deposition process.

Botryoidal minerals like malachite, hematite, and smithsonite tend to grow a myriad of crystals, rather than a single or small cluster. They tend to develop spherulites around points of nucleation, and these tiny clusters of molecules attract like molecules equally from all directions. This process allows many small crystals to form at the same time. Fan-like growths developing next to each other such are common.

Crystal growth habits are an integral part of our ability to identify a mineral. Hopefully, you’ll be more encouraged to delve more deeply into the subject of crystal habits. After all, they are what make minerals in our collections more interesting.