Zones, Forms, Habits
Crystallographic Directions:
• Linear feature of crystals: [hkl]
• Referenced to unit cell dimensions of lattice nodes
Zones:
• Collection of common faces
• Parallel to common line
• Line called zone axis
• Identified by index: [hkl]
• Zone axis parallels intersection of edges of faces
Form
– formal crystallographic nomenclature for shapes of crystals
• Collection of crystal faces
• Related to each other by symmetry
• Identified by index: {hkl}
• hkl are values of one of the faces
Example
• There are six face in the cube: (100), (010), (001), (100), (010), (001).
• Form is {001}
• Possible to determine form given:
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- Point symmetry
- location of one face
• Two types of forms:
- open form: does not enclose a volume
- closed form: encloses a volume
• Minerals can have only one closed form
• Minerals must have more than one form if they contain an open form
Isometric Forms:
• 15 possible forms
• 4 common ones:
- cube {001}
- octahedron {111}
- tetrahedron {111}
- dodecahedron {110}
Nonisometric Forms
• 10 types of nonisometric forms:
Pedion (open):
- single face
- no symmetrically identical face
Pinacoid (open):
- two parallel faces
- related by inversion of mirror
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Dihedron (open):
- two non-parallel faces
- related by mirror (dome) or 2-fold rotation (sphenoid)
Prism (open):
- 3, 4, 6, 8 or 12 faces, intersect with mutually parallel edges forming a tube
Pyramid (open):
- 3, 4, 6, 8, or 12 faces, intersect at a point
Dipyramid (closed):
- two pyramids at each end of crystal, 6, 8, 12, 16, or 24 faces
Prisms, pyramids, and dipyramids are named on the basis of the shape of the cross
section.
Trapezohedrons (closed)
- 6, 8, 12 faces, each is trapezoid
- named according to number of faces
Scalenohedrons (closed)
- 8 or 12 faces, each is scalene triangle (no two angles are equal)
Rhombohedrons (closed)
- 6 faces, each rhomb shaped
- looks like stretched or shortened cube
Tetrahedrons (closed)
- 4 triangular faces
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• Restrictions on forms in crystals:
- all forms must be in same crystal system
- all forms must be related by symmetry of crystal class
- e.g. tetragonal prism have single 4-fold rotation, only found in tetragonal crystal
class with single 4-fold rotation axis
- e.g. pedions never occur in mineral with center of symmetry
• Multi-faced forms are not composed of several simpler forms
- e.g. cube is not 6 pedions or 3 pinacoids
• Enantiomorphous Forms
- lack centers of symmetry and mirrors
- come as right and left forms, which are mirror images
- Minerals may be either form, but individual specimen display only one type
- Originate as a type of symmetry called screw axis (may spiral to right or left)
- Quartz is common example
• Positive and Negative Forms
- Two forms related by rotation
- rotation is 60º on 3 fold rotation axis or 90º on 4 or 2 fold rotation axis
Forms in Six Crystal Systems
- Forms control orientation of crystallographic axes of the 6 crystal systems
• Triclinic
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- c axis parallels prominent zone axis
- b and a axes parallel crystal edges
-  and  typically > 90º
- common minerals: plagioclase and microcline
• Monoclinic
- b axis commonly parallel 2-fold rotation or perpendicular to mirror plane
- c axis prallel to prominent zone
- a axis down and to front so  > 90º
- common minerals: amphiboles, pyroxenes, micas
• Orthorhombic
- Crystal axes are parallel to 2-fold rotations or perpendicular to mirrors
- no convention to describe which symmetry equals which axis
- Hermann-Mauguin symbols written, with 1st = a axis, 2nd = b axis, 3rd = c axis
- e.g. mm2, a  mirror, b  mirror, and c parallel 2-fold rotation
• Tetragonal
- c axis always the single 4-fold rotation axis
- a and b coincide with 2-fold rotation or perpendicular to mirror
- Hermann-Mauguin symbols written, with 1st = c axis, 2nd = b and c axes, 3rd =
symmetry on [110] and [110] axes at 45º to a and b axes
- e.g. 42m:
- c = 4 fold rotinversion
- a and b axes [100] and [010] are 2-fold rotation
- there are mirrors  to [110] and [110]
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• Hexagonal
- c axis parallel to 6-fold or 3-fold rotation
- a and b axes parallel to 2 fold rotation or perpendicular to mirror
- Hermann-Mauguin symbols written, with 1st = c axis, 2nd = b and c axes, 3rd =
symmetry perpendicular to a and b
• Isometric
- 3 equivalent symmetry axes coincide with crystallographic axes
- symmetry either 2-fold or 2-fold
- Hermann-Mauguin symbols written, with 1st = crystallographic axes, 2nd =
diagonal axes [111], 3rd = center of one edge to center of another edge
[110].
Crystal Habit
• descriptive terminology to describe individual minerals and aggregates
• shape of individual minerals
• intergrowths of several mineral grains
• shape of masses of grains
- colloform (globular, reniform botryoidal, mammillary)
- drusy
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