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The Geometry of Mineral Crystals

Many different mineral crystal geometric shapes exist in the world. In fact, there are too many to list in a simple article without turning this into a book. Determining the geometry of a mineral sample, if there is one, can help you uncover its identity, making it one of the unique crystal properties that geologists use in the evaluation process of a new sample. Crystal geometry is where math and nature collide.

Mineral Crystal Shapes

Many of the shapes that minerals form are common patterns that you learned in in high school geometry class like cubes, rhombohedrons, octahedrons, and hexagons. Halite, pyrite, and galena are classic cube forms. Diamond and fluorite are two well-known minerals that form as octahedrons while calcite is one of the best-known rhombohedra. The rhombohedral calcite shape gives the clear variety, known as Iceland Spar Calcite, 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 a six-sided prism. Finding a perfectly shaped quartz with the prism and both pyramids are 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 exist, but only the ones in Herkimer County are the true “Herkimer Diamond.” Although, you will find that most people refer to any perfect quartz crystal by that name.

Staurolite Mineral - Mini Me Geology

The most interesting part about the geometry of minerals is that crystals can sometimes form more than one shape depending on how they grows or the environment. Calcite, for example, as mentioned previously, can form nice rhombohedrons but it can also form shapes like scalenohedrons, hexagonal prisms, and pinacoids. Fluorite also forms in multiple shapes including octahedrons and cubes.

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

Amethyst Mineral - Mini Me Geology

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” is two staurolite crystals that grow perpendicular to one another. At times, the crystals may be slightly less than perpendicular, as shown in the photograph above.

A mineral “cluster” or “druze” forms when many crystals grow in a group or layer and are attached to one another side by side. Geodes often have druze of quartz, amethyst, and citrine inside. The amethyst shown in the photo is a druze with numerous side-by-side crystals.

Crystal Shape vs. Mineral Cleavage vs. Fracture

People sometimes confuse mineral shapewith mineral cleavage but the two are actually different. The shape is a natural form that the mineral will take as the crystal grows. Cleavage is the tendency of a crystal to break along planes within the crystal. Minerals with cleavage break into particular shapes 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.

Mineral Cleavage Examples

Minerals that do not have 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.

Mineral Conchoidal Fracture

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 Detectives

At Mini Me Geology, we love the geometry of minerals so much that we created a fun activity kit for kids that focuses on these fun properties. We designed the Rock Detectives Crystal Geometry kit 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.
Rock Detectives Crystal Geometry Sample Activity 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.