I am studying crystals and I learned that there are 32 crystal classes with well-described data regarding symmetries. I have visited a few sites and am having difficulty determining the significance of these classifications. For example, how can any crystal be categorized under these 32 classes? I understand that the information exists somewhere within the symmetry data, but it's difficult to tell what it says; I'm trying to learn a lot of foreign concepts at once (space groups, lattices, symmetry concepts, etc.).

I suppose I'm asking two questions: how can I determine the class a mineral falls under and how were the 32 classes determined?

  • $\begingroup$ There isn't much information on the internet? What about starting with en.wikipedia.org/wiki/Crystal_system and en.wikipedia.org/wiki/Point_groups_in_three_dimensions ? $\endgroup$ – farrenthorpe Jul 21 '15 at 6:11
  • $\begingroup$ @farrenthorpe there is no need to be rude. Yes, I did look at those sources before posting. In fact, I based most of my initial knowledge from there. If you read them, you would quickly realize that they talk all about what the crystal families / systems / lattices / classes are without discussing the significance of the classes. $\endgroup$ – person27 Jul 21 '15 at 13:21

The references you and @farrenthrope cited (or a good introductory mineralogy text) should have answers.

In brief:

The crystallographic class is significant to mineralogy because it is direct result of the mineral's crystalline lattice (its most fundamental property.) Each mineral can belong to only one crystallographic class.

How can I determine the [crystallographic] class an [unidentified] mineral [specimen] falls under?

You'll need to inspect your specimen and try to recognize the symmetry operations (like rotations and reflections) present in some physical properties of your specimen, commonly the symmetry of euhedral crystal habit, the symmetry of optical properties, or the symmetry of an x-ray diffraction pattern etc. Each crystallographic class is defined by a unique set of symmetry operations.

How were the 32 classes determined?

A combination of observation and mathematics. The law of rational indices was deduced by Haüy (1784, 1801) from the observation of the stacking laws required to build the natural faces of crystals by piling up elementary blocks, for instance cubes to construct the faces of the rhomb-dodecahedron observed in garnets or the pentagon-dodecahedron observed in pyrite, or rhombohedrons to construct a scalenohedron of calcite. Hessel (1830) proved that, as a consequence of the law of rational intercepts, morphological forms can combine to give exactly 32 kinds of crystal symmetry in Euclidean space, since only two-, three-, four-, and six-fold rotation axes can occur.

  • $\begingroup$ Thanks, I get it now. I'm not sure now why it wasn't clicking before. $\endgroup$ – person27 Jul 25 '15 at 14:12
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    $\begingroup$ @FizzledOut: let me misuse this comment to bring your attention to the proposal of crystallography.SE site. $\endgroup$ – marcin Oct 15 '15 at 17:44

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