Difference between revisions of "Form"

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==Basic==
 
==Basic==
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The internal make-up of a mineral is reflected by its, ideal, outer shape (form). A multitude of forms make the [[habit]] of the mineral.
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Crystal forms consist of crystal faces that are related by crystal axes. Some faces can enclose a space completely (like a face of a cube) and are named "closed forms, while others need additional faces to totally enclose a space and are called "open forms".
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If one would take one face of a cube (which has 6 faces), that shape would fit on all other faces and therefore a repetition of that face can enclose the space inside the cube. A tetragonal prism consists of 4 prism faces (each identical) which can not substitute the bottom and top of the prism without altering the shape of the prism face. For that two additional faces are needed to "close" the prism. For instance [[emerald]] is made up of a 6-faced (or 6-sided) prism with two hexagonal shaped terminations (a basal pinacoid). This means two different kinds of faces are needed to enclose the space within the crystal and hence the prism and the pinacoid (two open forms) need to work together to enclose the space.
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[[image:pedion.jpg|left|thumb|200px|Pedion (open)]]
 
[[image:pedion.jpg|left|thumb|200px|Pedion (open)]]
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[[image:cube.jpg|left|thumb|200px|Cube (closed)]]
 
[[image:cube.jpg|left|thumb|200px|Cube (closed)]]
 
[[image:octahedron.jpg|left|thumb|200px|Octahedron (closed)]]
 
[[image:octahedron.jpg|left|thumb|200px|Octahedron (closed)]]
[[image:tetrahedron.jpg|left|thumb|200px|Tetrahedron (closed}]]
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[[image:tetrahedron.jpg|left|thumb|200px|Tetrahedron (closed)]]
 
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==Sources==
 
==Sources==
  

Revision as of 08:04, 21 November 2006

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Basic

The internal make-up of a mineral is reflected by its, ideal, outer shape (form). A multitude of forms make the habit of the mineral.

Crystal forms consist of crystal faces that are related by crystal axes. Some faces can enclose a space completely (like a face of a cube) and are named "closed forms, while others need additional faces to totally enclose a space and are called "open forms".

If one would take one face of a cube (which has 6 faces), that shape would fit on all other faces and therefore a repetition of that face can enclose the space inside the cube. A tetragonal prism consists of 4 prism faces (each identical) which can not substitute the bottom and top of the prism without altering the shape of the prism face. For that two additional faces are needed to "close" the prism. For instance emerald is made up of a 6-faced (or 6-sided) prism with two hexagonal shaped terminations (a basal pinacoid). This means two different kinds of faces are needed to enclose the space within the crystal and hence the prism and the pinacoid (two open forms) need to work together to enclose the space.


Pedion (open)
Pinacoid (open)
Dome (open)
Tetragonal prism (open)
Trigonal prism (open)
Hexagonal prism (open)
Tetragonal pyramid (open)
Trigonal pyramid (open)
Hexagonal pyramid (open)
Cube (closed)
Octahedron (closed)
Tetrahedron (closed)


Sources

  • Gemmology 3rd edition (2005) - Peter Read

External links