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Starting in 1996, Alexa Internet has been donating their crawl data to the Internet Archive. Flowing in every day, these data are added to the Wayback Machine after an embargo period.
TIMESTAMPS
The Wayback Machine - https://web.archive.org/web/20170201060656/http://www.webmineral.com:80/crystall.shtml
Crystal Groups and Classes - Crystallography groups are composed of 32
classes of symmetry derived from observations of the external crystal form. From
these 32 classes, 230 space groups are
distinguishable using x-ray analysis. For additional information on crystal systems,
please review an excellent on-line treatment of this subject, the
Introduction to Crystallography and
Mineral Crystal Systems by Mike and Darcy Howard.
Example Crystal Morphology - To further illustrate these symmetry
elements, the example crystalline forms for each symmetry class were constructed using Faces (version 3.7) by Georges
Favreau for older browsers and a JAVA language program called JCrystal by Steffen Weber
for the java-capable browsers. In addition, Steffen Weber has a new version of
JCrystal called KrystalShaper that can
produce paper models of the example crystals.
The paper models for each
crystal class have been reproduced in .pdf format (see below) and can be printed
on heavy card stock to construct a physical model. Please note: the paper model
forms may be simplified from the JCrystal forms to avoid overly-complicated
cutting.
The example crystals in each group and class were constructed using the
following parameters:
Crystallographic axes: a=1, b=1, c=1.
Forms: [214], [104], [024],
[100], [010]
(Color coded in all example crystals).
This morphology was used in all the crystal groups and classes for drawing the examples
and the stereo images. The higher order forms were selected to best illustrate some of the
subtle differences in the symmetry classes. Remember, most real crystals will
probably not have similar forms when compared to the examples.
Table Crystallography (Mason 68, p18) -
The thirty-two crystal classes are summarized by symmetry elements in the following table
which is linked to:
Example crystal images within the crystallography group in the yellow columns.
Stereo crystal images/java example and mineral listings of each group and
class in the green columns.
JAVA crystal examples on the individual crystal classes for detailed study. Please note
that the rotational inversion axes of 3,(3BAR) 4
(4BAR), and 6 (6BAR) may not be visible to older
browsers which don't use cascading style sheets (.css).
The paper models for each
crystal class have been reproduced in .pdf format (see below) and can be printed
on heavy card stock to construct a physical model. Please note: the paper model
forms may be simplified from the JCrystal forms to avoid overly-complicated
cutting.
