The Geometry of Minerals: How Some Crystals Form Unique and Beautiful Mathematical Shapes

Mineral ShapesThere are many, many different crystal shapes in the world. In fact, there are too many to list in a simple blog post without turning this into a book. Determining the shape of a mineral sample can help you uncover its identity, making it one of the unique crystal properties that geologist use in the evaluation process of a new sample.

Mineral Crystal Shapes

Many of the shapes that minerals can form are common patterns that you learned in geometry class in high school like cubes, rhombohedron, octahedron, and hexagons. Halite, pyrite, and galena are classic cube forms. Diamond and fluorite are two well-known minerals that can form as octahedral while calcite is one of the best-known rhombohedra. The rhombohedral shape of calcite is what gives the clear varieties its optical properties.

Quartz and its other varieties like amethyst and citrine form a unique shape that has a six-sided pyramid on each end of the crystal stacked on a six-sided prism in the center. Finding a perfectly shaped quartz with the prism and both pyramids is somewhat rare. The famed Herkimer Diamonds are examples of these perfectly formed crystals. Their name comes from Herkimer County, New York where many beautiful samples were first discovered. Other locales of perfect quartz crystals have been found since, but only the ones in Herkimer County are the true “Herkimer Diamond” although you will find that most people refer to any perfect quartz with that name.

The most interesting part about crystal shapes is that minerals can sometimes form more than one shape depending on how it grows or its environment. Calcite, for example, as mentioned previously can form nice rhombohedrons but it can also form in shapes like scalenohedron, hexagonal prism, and pinacoid. Fluorite also forms in multiple shapes including octahedrons and cubes.

Mineral crystals also grow in some unique forms when two or more crystals grow together.

Sometimes you will see a mineral that looks like two crystals that have grown together at slightly different angles. This is called a “twinned” crystal. Staurolite is a commonly twinned mineral. A staurolite cross, which you may hear people discuss, is two staurolite crystals that grow perpendicular to one another. At times, the crystals may be slightly less than perpendicular, as shown in this photograph.

Minerals can also form where many crystals form in a group or layer and are attached to one another side by side. This is called a mineral “cluster” or “druze.” Quartz, amethyst, and citrine are commonly found in a druze form inside geodes.

Crystal Shape vs. Mineral Cleavage vs. Fracture

The mineral shape is sometimes confused with mineral cleavage but the two are actually different. As discussed, the shape is a natural form that they mineral will take as the crystal grows. Cleavage is the tendency of a crystal to break along natural planes within the crystal. Minerals with cleavage will break into particular shapes when the crystal is broken, such as cubes, rhombs, octahedrons and thin layers.

Some minerals have “perfect cleavage,” which means the mineral breaks smoothly along the cleavage plane without any rough edges. The diagram below shows some of the common cleavage types and associated minerals.

Minerals that do not have good cleavage (do not break into specific shapes) will fracture when broken. Even minerals that naturally grow in perfect shapes, such as cubes or prisms, may fracture when they are broken and will no longer be a cube or prism. Two minerals that can have beautiful crystal forms but fracture when they break are quartz and pyrite.

A common fracture pattern is a conchoidal fracture. Minerals and rocks with a conchoidal fracture will have a distinctive swirl pattern on the surface after it breaks. In addition to quartz and pyrite, the igneous rock obsidian exhibits conchoidal fracture patterns when broken.

Learn more about Mineral Crystal Geometry

Crystal Geometry Rock DetectivesAt Mini Me Geology, we love the shape of mineral so much that we created a fun activity kit for kids that focuses on these fun properties. The Rock Detectives Crystal Geometry kit was designed for kids ages 6 to 12, but older kids and adults secretly love it too. The Crystal Geometry Rock Detectives kit comes complete with:

  • 6 Large Mineral Samples: muscovite, fluorite, calcite, citrine, pyrite and agate geode
  • Mini Me Geology custom Hand Lens with 3X and 6X magnifications
  • 30 page, full-color eBook on CD that includes mineral information, sample identification activities, puzzles, coloring pages, experiments, an adventure story writing exercise for your geology detective, and activities including making a personal geologist’s field notebook and your very own rock collection box.

Crystal Geometry Sample Pages

If you have any questions about mineral crystal shapes or any of our kits, please contact us and we will be happy to answer your questions.

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