Here is a list and description of the most commonly encountered mineral crystal shapes.
1. Cubic (Isometric): Cubic crystals have a symmetrical three-dimensional structure with equal sides, angles, and faces. The crystal faces are all squares, and the mineral exhibits three perpendicular axes of equal length, intersecting at right angles. Regular six-sided dice are this shape. Common minerals with cubic crystal forms include halite (rock salt) and pyrite (fool’s gold).
2. Tetragonal: Tetragonal crystals have a similar structure to cubic crystals but differ in the length of their axes. Two of the axes are perpendicular and of equal length, while the third axis is longer or shorter. The faces of tetragonal minerals are square or rectangular. Examples of minerals with tetragonal crystal forms include zircon and rutile.
3. Orthorhombic: Orthorhombic crystals have three mutually perpendicular axes of different lengths. The crystal faces are rectangles, and the mineral does not exhibit any fourfold rotational symmetry. Sulfur and topaz are examples of minerals with orthorhombic crystal forms.
4. Hexagonal: Hexagonal crystals have a six-sided symmetry with one vertical axis and three horizontal axes forming angles of 120 degrees. The crystal faces are often hexagons. Minerals like quartz and calcite commonly exhibit hexagonal crystal forms.
5. Trigonal (Rhombohedral): Trigonal crystals have a threefold rotational symmetry around a vertical axis, resulting in a rhombohedral shape with three equal angles. The crystal faces are rhombuses. Corundum (including sapphire and ruby) and calcite are examples of minerals with trigonal crystal forms.
6. Monoclinic: Monoclinic crystals have three axes of unequal length, with one axis forming an oblique angle with the other two. The crystal faces are often trapezoidal or parallelograms. Gypsum and feldspar are examples of minerals with monoclinic crystal forms.
7. Triclinic: Triclinic crystals have three axes of unequal length, with all three axes intersecting at oblique angles. The crystal faces are generally not symmetrical. Minerals like plagioclase feldspar and microcline exhibit triclinic crystal forms.
8. Dodecahedral and Octahedral (Isotropic): Dodecahedral and octahedral crystals belong to the isometric system and are characterized by their symmetry. In the dodecahedral form, the crystal has 12 faces, each an equilateral triangle. In the octahedral form, the crystal has eight faces, each an equilateral triangle. Common minerals with these crystal forms include garnet (dodecahedral) and magnetite (octahedral).
Each crystal form is a result of the internal arrangement of atoms in the mineral’s structure. These distinct crystal forms, along with other physical and chemical properties, allow mineralogists to identify and classify minerals, contributing to our understanding of the diverse geological processes that shape our planet.