A crystal's form may be open or closed.
Many people use the word "form" to describe a crystal's appearance, though "habit" is the correct term to describe it. In crystallography, "form" actually refers to a group of crystal faces with the same relationship to the elements of symmetry within a certain crystal system.
The dimensions of a crystal are symmetrical and comprised of a pattern. There are two forms of crystals, "open" and "closed." An open crystal is flat and does not take volume. A closed crystal has volume and closure to the symmetrical pattern. Within these two classifications are three forms of open and closed crystals.
Open Forms
Open forms of crystals have many faces of symmetry but do not close that symmetry in a symmetrical volume. These crystals include peridions, pinicods, domes, sphenoids, pyramids and prisms. A peridon must have two parallel faces, like a ruby. The topaz is an example of a dome crystal.
Closed Isometric Forms
Diamonds, garnet and spinel are good examples of closed isometric forms of crystals. They have points and edges perpendicular to one another and make 90-degree angles. Pyrite, tetrahedrite and garnet are examples of closed isometric forms.
Closed Non-Isometric Forms
Closed non-isometric forms differ from closed isometric forms because they lack perpendicular angles. There are six sub-categories of this form. They include hexagonal closed forms, tetragonal closed forms, rhombohedral closed forms, orthorhombic closed forms, momclinic closed forms and triclinic closed forms.
Identifying Closed Non-Isometric Forms
Isometric forms are often part of a cubic system, The three axes are all at right angles to one another. Pyrite is an example. Tetragonal crystals have four sides and form prisms and pyramids. Zircon is an example. Hexagonal crystals are six-sided prisms like beryl. Othorhombic crystals are small and wide with an unequal axes. Topaz is an example. Monoclinic crystals have tilted faces on the on the ends of the crystal. There are three unequal axes and the third axis is at an incline. An example is gypsum. Triclinic crystals are flat and sharp and have no right angles. An example is feldspar.
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