Introduction to Crystallography and Mineral Crystal Systems
by Mike and Darcy Howard

Part 2: Crystal Forms and Symmetry Classes

Let's discuss CRYSTAL FORMS and the 32 SYMMETRY CLASSES! Again we must begin with some definitions. Unfortunately, the term "form" is loosely used by many people to indicate outward appearance. However, we must "tighten up" our definition when discussing crystallography. HABIT is the correct term to indicate outward appearance. Habit, when applied to natural crystals and minerals, includes such descriptive terms as tabular, equidimensional, acicular, massive, reniform, drusy, and encrusting.

Drusy Quartz in Geode Tabular Orthoclase Feldspar Encrusting Smithsonite

As a crystallographer, I use "form" with a more restricted meaning. A FORM is a group of crystal faces, all having the same relationship to the elements of symmetry of a given crystal system. These crystal faces display the same physical and chemical properties because the ATOMIC ARRANGEMENT (internal geometrical relationships) of the atoms composing them is the SAME. The relationship between form and the elements of symmetry is an important one to grasp, because no matter how distorted a natural crystal may be, certain key elements will be recognizable to help the student discern what form or forms are present. The term general form has specific meaning in crystallography. In each crystal class, there is a form in which the faces intersect each crytallographic axes at different lengths. This is the general form {hkl} and is the name for each of the 32 classes (hexoctahedral class of the isometric system, for example). All other forms are called special forms.

Let's look at an octahedron as an example (fig. 2.1). All the crystal faces present are the expression of the repetition of a single form having the Miller indices of {111} about the three crystallographic axes (remember those from the first article?). Each face on a natural crystal (octahedral galena or fluorite are examples), when rotated to the position of the (111) face in the drawing, would have the same shape and orientation of striations, growth pits or stair steps, and etch pits, if present.

The presence of these features is true whether or not the crystal is well formed or distorted in its growth. Note that I did NOT state that the faces are necessarily the same size on the natural crystal!

In fact, due to variations in growth conditions, the faces are usually not. In the literature, you may see a notation, given as {hkl}. This is the notation, presented as Miller indices for general form. The octahedral form is given as {111}, the same as the face that intersects all positive ends of the crystallographic axes. A single form may show closure, as with an octahedron, or may not, as in a pinacoid (an open two-faced form). So every form has an {hkl} notation. In the case of general notation concerning the hexagonal system, it is {hk-il} and is read as "h, k, minus i, l".

Before leaving this discussion of form, here are a couple of examples of how knowledge of the interrelationships of forms and crystal systems may be used. Someone gives you a quartz crystal and says, "Look at this crystal. It's not normal." Normal to this person, we assume means an elongate (prismatic), 6-sided crystal form with a 6-faced termination on the free-growth end. When you examine the "abnormal" crystal, it is highly distorted, broken, and has regrowth faces. Prism faces on quartz crystals almost always have striations at right angles to the c crystallographic axis and parallel to the plane of the a1, a2, and a3 axes. These striations are due to the variable growth rates of the terminal faces as the mineral crystallized. Knowing about the striations and their orientation, you examine the surface of the specimen and, with reflected light, find the prism faces by their striations.

Terminal (or pyramidal) faces on quartz crystals often exhibit triangular pits or platforms. By finding them, you can then determine if any other faces that would be really unusual are present. Not finding any unusual faces, you can return the specimen to the person with the comment, "Well, your crystal is certainly interesting, but it does not have any unusual forms. What it does display is a complex growth history reflected by its less than ideal crystal shape."

Most people and many collectors recognize unusual habits, but not unusual forms. They note that the shape of a crystal is odd looking, but don't have the background in crystallography to know if the crystal is truly unusual. A broken and regrown quartz crystal is not particularly special, but a quartz crystal with a c pinacoidal termination is worth noting, as it is a very uncommon form for quartz. I have only seen a few from one locality. Being the skeptic that I am, I purchased one crystal which had another mineral coating the termination. I mechanically removed the coating mineral with the edge of a pocket knife. There for my examination was the c pinacoid termination {0001}, satisfying me that it was a natural growth form!

A crystal's form may be completely described by use of the Miller's indices and the Hermann-Mauguin notation of its POINT GROUP SYMMETRY. The latter notation tells us how to orient the crystal, in each specific crystal class, to recognize which axis (a, b, or c) is designated as having the highest symmetry. It also tells us what other symmetry elements may be present and where they are in orientation to the other elements.

I hope I haven't LOST you here. When considering all the literature written about this subject, sometimes I even feel like I'm rotating on one symmetry axis and not really getting anywhere! Point group symmetry is too complicated to get into in this discussion, so I refer you to Klein and Hurlbut's Manual of Mineralogy for detailed information. However, I will discuss some pertinent portions of point group symmetry under each crystallographic class and introduce its notation for selected crystal forms in each crystal-system article.

Types of Crystal Forms

Note that there are TWO GENERAL TYPES OF FORMS: those that by repetition close on themselves creating a complete form (termed closed) and those that do not (termed open).

Now for the BAD NEWS. Every form has a name and there are many of them.

The GOOD NEWS is that you already know a few of them, particularly some of the closed forms because we have been using them in our previous discussions. The cube and octahedron are examples. There are 32 (some say 33) forms in the nonisometric (noncubic) crystal systems and another 15 forms in the isometric (cubic) system. Let's start to familiarize ourselves with them by making a tabulation and including the number of faces (below). I begin the listing with the isometric forms first, all of them being closed forms.

Isometric Crystal Forms
Name Number
of Faces
Name Number
of Faces
(1) Cube 6 9)Tristetrahedron 12
(2) Octahedron 8 (10) Hextetrahedron 24
(3) Dodecahedron 12 (11) Deltoid dodecahedron 24
(4) Tetrahexahedron 24 (12) Gyroid 24
(5) Trapezohedron 24 (13) Pyritohedron 12
(6) Trisoctahedron 24 (14) Diploid 24
(7) Hexoctahedron 48 (15) Tetartoid 12
(8) Tetrahedron 4    

Non-Isometric Crystal Forms
Name Number
of Faces
Name Number
of Faces
(16) Pedion* 1 (32) Dihexagonal pyramid 12
(17) Pinacoid** 2 (33) Rhombic dipyramid 8
(18) Dome or Sphenoid 2 (34) Trigonal dipyramid 6
(19) Rhombic prism 4 (35) Ditrigonal dipyramid 12
(20) Trigonal prism 3 (36) Tetragonal dipyramid 8
(21) Ditrigonal prism 6 (37) Ditetragonal dipyramid 16
(22) Tetragonal prism 4 (38) Hexagonal dipyramid 12
(23) Ditetragonal prism 8 (39) Dihexagonal dipyramid 24
(24) Hexagonal prism 6 (40) Trigonal trapezohedron 6
(25) Dihexagonal prism 12 (41) Tetragonal trapezohedron 8
(26) Rhombic pyramid 4 (42) Hexagonal trapezohedron 12
(27) Trigonal pyramid 3 (43)Tetragonal scalenohedron 8
(28)Ditrigonal pyramid 6 (44) Hexagonal scalenohedron 12
(29) Tetragonal pyramid 4 (45) Rhombohedron 6
(30) Ditetragonal pyramid 8 (46) Rhombic disphenoid 4
(31) Hexagonal pyramid 6 (47) Tetragonal disphenoid 4
*Pedion may appear in several crystal systems
**Pinacoid drawing displays 3 pairs of pinacoid faces from the Orthorhombic system.
Pinacoids appear in several crystal systems.

Now you know why mineralogy students hate idiot blocks! It is important to note that these are simply the possible INDIVIDUAL FORMS, not the combinations of forms seen on a single natural crystal.

Fig. 2.4 Peruvian Pyrite, Various Crystal Forms

Pyrite is a common mineral which often exhibits several forms on a single crystal. One form is usually dominant, presenting the largest faces on the crystal. Peruvian pyrite commonly has cubic, octahedral, and dodecahedral forms on a single crystal; sometimes even pyritohedral and diploid faces may be present. Any of these individual forms may be the dominant one. Crystals with the same forms present, but with different dominant forms will each appear very different (fig. 2.4). As we explore each crystal system, there will be illustrations displaying most of the ideal forms and some drawings showing combinations of forms often exhibited by individual mineral crystals.

Part 3: The Cubic (Isometric) System

Index to Crystallography and Mineral Crystal Systems

Table of Contents

Bob Keller