Crystal Forms: Open, Closed & Isometric Crystal Forms
Types of Crystal Forms
Closed & Open Crystal Forms
Any grouping of crystal faces or facets that are arranged in the same symmetry is referred to as a crystal's "form." There are approximately 48 unique crystal forms which are divided into either "open," "closed Isometric," or "closed non-Isometric" form categories.
There are thirty closed Isometric, or closed non-Isometric forms, and eighteen open crystal forms. Closed crystal forms are those groupings of facets that are related by symmetry, and completely enclosing a volume of space.
Open Forms
The eighteen open-forms are those facet groupings that are related by symmetry, but do not completely enclose a volume of space. A crystal with open-form faces also requires some additional closed-form facets to complete a structure. Open-forms include:
- Pedion
- Pinacoid
- Dome
- Sphenoid
- Pyramid
- Prism
A Pedion is a flat face that is not parallel, or geometrically linked to any other faces. A Pinacoid is composed of only two parallel faces, forming tabular crystals such as ruby. A Dome is found in monoclinic and orthorhombic minerals.
Two intersecting faces that are caused by mirroring (topaz) commonly forms domes. Sphenoids are found in monoclinic and orthorhombic minerals, and have two-fold rotational axes. A Pyramid's multiple facets converge on a single crystallographic axis, and pyramid forms are not possible on minerals from the isometric, monoclinic or triclinic systems.
Open Hexagonal & Triangular Prisms
Prisms have a set of facets that run parallel to an axis of a crystal, yet never converge with it. Quartz forms two sets of three sided prisms. Prisms are not possible in isometric or triclinicminerals.
A Hexagonal (trigonal) prism is comprised of two hexagonal bases connected by a set of six rectangular faces that run parallel to, and never converge with an axes in the crystal.
A triangular (trigonal) prism is comprised of two triangular bases connected by a set of three rectangular faces that run parallel to, and never converge with an axes in the crystal. This form is similar to a light-refracting 60¼ prism.
Closed Isometric Forms
There are several crystal forms in the cubic crystal system that are common in diamond,garnet, spinel and other "symmetrical" gemstones. These forms include:
- Hexahedron (Pyrite)
- Octahedron (Diamond, Spinel)
- Tetrahedron (Tetrahedrite)
- Dodecahedron (Rhombic - Garnet)
- Hexoctahedron (Diamond)
A hexahedron (cube) has eight points, six faces, and twelve edges that are perpendicular to each other, forming 90 degree angles. An octahedron has two four sided pyramids lying base to base, and is totally symmetrical with no top, or bottom and has eight faces. A tetrahedronhas four equilateral triangular faces.
A dodecahedron has 12 faces and there are four types of dodecahedrons listed in order of descending symmetry: 1. Symmetrical pentagonal (five edged polygons) dodecahedrons, 2. Asymmetrical (tetartoid) pentagonal dodecahedrons, 3. Delta (four edged polygons) dodecahedrons, and 4. Rhombic dodecahedrons. A Hexoctahedron is a multi-faceted dodecahedron with 48 triangular faces.
Closed Non-Isometric Forms
1. Hexagonal (Trigonal) Closed Forms
- Hexagonal Pyramid
- Hexagonal Bipyramid (Apatite)
- Dihexagonal bipyramid (Beryl)
- Hexagonal Trapezohedron
- Hexagonal Scalenohedron
- Tetrahexahedron
2. Tetragonal Closed Forms
- Tetragonal Disphenoid
- Tetragonal Scalenohedron
- Tetragonal Trapezohedron
- Tetragonal Trapezohedral Trisoctahedron
- Tetragonal Ditetragonal Bipyramidal (Rutile)
3. Rhombohedral Closed Forms
- Rhombohedral Trapezohedral (Quartz)
- Rhombohedral Hemimorphic (Tourmaline)
- Rhombohedral Holohedra (Calcite)
- Rhombohedral Dodecahedron (Garnet, Fluorite)
- Rhombohedral Trisoctahedron
4. Orthorhombic Closed Forms
- Rhombic Prism
- Rhombic Pyramid
- Rhombic Dipyramid
- Rhombic Hemimorphic
- Rhombic Sphenoid
- Rhombic Pyramid
5. Monoclinic Closed Forms
- Prism
- Monoclinic Clinopinacoid
6. Triclinic Closed Forms
- Prism
- Triclinic Dipyramid
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