Mineralogy 101: Crystal Systems

As previously discussed (see Mileralogy 101: Crystals), crystals have an ordered internal arrangement that is reflected by the external shape of the crystal. The elements that are the building blocks of the crystal can be translated, rotated, inverted, or mirrored to create 32 unique crystal classes. These crystal classes are the basic forms from which all minerals take their shape.

As mineralogists studied the different crystal classes, they learned that many of the classes share characteristics with other classes. These similarities allow the crystal classes to be grouped into six crystal systems. Understanding the crystal systems, and how they differ from each other allows mineralogists to use the crystal system as a diagnostic tool in the classification of minerals.

To be able to visualize the crystal systems, mineralogists often refer to a set of three reference axes when describing them. Known as crystallographic axes, these imaginary reference lines are usually taken parallel to the major crystal faces. All of the crystal systems, except those belonging to the hexagonal system, have three axes which are designated as a, b, and c. The a and b axes are horizontal axes, the a being front-to-back, the b being left-to-right. The c axis is vertical. The end of the axes is denoted as positive (+) or negative (-), the positive ends being the front of a, the right-end of b and the upper part of c, the opposite ends are designated as negative. The six different systems are illustrated at the right, and can be described as follows.

Triclinic. Has three unequal axes that all intersect at oblique angles.
Monoclinic. Has three unequal axes, two of which are inclined to each other obliquely, the third being perpendicular to the plane of the other two.
Orthorhombic. Three perpendicular axes all of different lengths.
Tetragonal. Three perpendicular axes, the horizontal axes being of equal length, while the vertical axis is either shorter or longer than the other two.
Hexagonal. Has four crystallographic axes, three horizontal axes of equal lengths that intersect at angles of 120^o; the fourth (vertical) axis is perpendicular to the plane of the other three.
Isometric. Three axes all perpendicular to each other and of equal lengths.

When you hold up a mineral specimen to determine its crystal system, you can imagine the intersection of the crystallographic axes with the face of the crystal. Several methods have been developed to describe this intercept, but the most universally accepted is the system proposed by William H Miller (1801-1880), referred to as Miller indices. The Miller index is expressed as a ratio, but is generally written with whole numbers and corresponds to the a, b, and c axes. The indices refer to the face of a crystal and how that face is aligned with the axes. For example, if a crystal, say an octahedron (or 8-faced crystal), is examined, we see that each face intersects all three axes. We arbitrarily assign one face as the unit face (generally the largest crystal face). The Miller index for this face is then expressed as (111). If a face intersects the negative end of the axis, it is written with a bar over the number. If a face is parallel with an axis and does not intersect it, a 0 is used to denote this. So in a cube, a face that is parallel to the b and c axes is expressed as (100).

Go To Page: 1 2

Articles in this Topic    Discussions in this Topic