Physical Properties Of Minerals I: Crystals and Cleavage

1. Establish a Group of Mineral Samples

1. Include as many of the following as possible: quartz, halite, calcite, garnet, biotite, and/or muscovite. Some are chosen for crystal growth features and others for crystal cleavage features.

2. Observe and Analyze Crystal Form

1. Place a sample onto the observation surface.
2. Rotate in order to observe all sides. Look for crystal faces, crystal edges (lines where faces meet), and crystal vertices (points where edges meet).
3. Where possible, measure the interfacial angles using the goniometer. This is done by simply laying one side of the goniometer on a particular crystal face, the other side of the goniometer on an adjoining face, and then reading the angle.
4. Compare to the set of characteristic crystalline polyhedra.
5. Repeat steps 2.1 – 2.4 for quartz (note hexagonal dipyramidal form (Figure 1)), calcite (note scalenohedron form (Figure 2)), halite (note cubic crystal form (Figure 3)), garnet (note dodecahedron form (Figure 4)), and biotite (note pseudo-hexagonal form (Figure 5)).

Figure 1. Quartz displaying hexagonal dipyramidal form.

Figure 2. Calcite displaying scalenohedron form. Note how several crystal faces intersect to form crystal edges and the combination of edges forms points known as “vertices.” Symmetric crystal growth forms are generated by repetition of fundamental atomic structures (unit cells) within the crystal lattice. In this case, calcite crystal growth generates the specific polyhedron known as a scalenohedron.

Figure 3. Halite displaying cubic crystal form.

Figure 4. Garnet displaying dodecahedron form.

Figure 5. Biotite displaying pseudo-hexagonal form.

3. Observe and Analyze Cleavage

1. Put on eye protection.
2. Place a piece of quartz on breaking surface.
3. Using a hammer, break the piece of quartz in half.
4. Using a hand lens, observe broken piece of quartz for cleavage surfaces. Notice that quartz has none. Quartz exhibits conchoidal fracture, but no well-defined cleavage surfaces (Figure 6). This is a consequence of the fact that the unit cells in the quartz crystal lattice (SiO₄ groups, called silica tetrahedral) have comparably equal bond strengths in all directions. This uniformity of bond strengths results in a crystal with no preferred breaking planes.
5. Repeat steps 3.2 – 3.4 for calcite (should display rhombohedral cleavage (Figure 7)), halite (should display cubic cleavage (Figure 8)), biotite, and/or muscovite (should each display planar cleavage (Figure 9)).
6. Use a hand lens to evaluate different cleavage qualities. Cleavage can occur at a variety of levels. When there is a dramatic difference in bond strengths in a particular orientation, such as between sheets of SiO₄ groupings in the case of mica, a nearly perfect cleavage is generated between these sheets. As noted above, quartz exhibits a nearly total lack of cleavage. In between these extremes (of perfect cleavage and lack of cleavage), there are minerals that have good cleavage (e.g. feldspar) and poor cleavage (certain faces on amphibole crystals).

Figure 6. Quartz displaying conchoidal fracture, without cleavage surfaces.

Figure 7. Calcite displaying rhombohedral cleavage. Symmetric breaking and fracture surfaces are generated by zones of relative weakness in atomic bonding within the crystal lattice. Calcite cleavage results in the specific polyhedron known as rhombohedron.

Figure 8. Halite displaying cubic cleavage.

Figure 9. Biotite displaying planar cleavage.