What are unit cells and why do they matter in mineral crystals?

Unit cells are the smallest repeating atomic groupings that make up a crystal lattice. Because unit cells are identical within a crystal, they create symmetry at both micro and macro scales. This ordered repetition of unit cells extends infinitely in three spatial directions and defines the uniformity and physical properties of the entire crystal.

How do the seven basic crystal systems differ from each other?

The seven basic unit cell types vary in edge lengths and angles between edges. The cubic system has equal edges and right angles, while tetragonal and orthorhombic systems have different edge lengths. Rhombohedral structures lack right angles, monoclinic and triclinic systems have varied angles and lengths, and hexagonal structures feature two parallel hexagonal faces with six rectangular faces.

What determines whether a mineral will cleave or fracture?

Cleavage depends on bond strength differences within and across crystal planes. Good cleavage occurs when bonds within a plane are stronger than those across it, creating predictable breaking surfaces. Poor cleavage or irregular fracture results when bonds are equally strong in all directions, preventing the crystal from breaking along preferred planes.

What is basal cleavage and which minerals display it?

Basal cleavage occurs when a crystal breaks along a single plane due to dramatically different bond strengths between sheets of atomic groupings. Biotite and muscovite mica display basal cleavage, splitting into thin sheets. This property results from strong atomic bonds within the sheets but weak bonds between them, making mica useful as a substrate for high-resolution imaging.

How do cubic and rhombohedral cleavage patterns differ?

Cubic cleavage results from three cleavage planes at 90-degree angles, producing cubic fragments. Rhombohedral cleavage also involves three planes but at 120 and 60-degree angles, creating rhombus-shaped pieces. Halite exhibits cubic cleavage, while calcite displays rhombohedral cleavage, reflecting their different crystal structures and bond orientations.

What is conchoidal fracture and why does quartz exhibit it?

Conchoidal fracture occurs when a crystal has no preferred breaking planes because bond strengths are comparably equal in all directions. Quartz displays conchoidal fracture, breaking with smooth, curved surfaces rather than along flat cleavage planes. This property reflects the uniform distribution of strong atomic bonds throughout the quartz crystal lattice.

How can X-ray diffraction reveal crystal structure information?

X-ray diffraction produces scattering patterns that indicate crystallinity through smooth or spotty Debye rings. The location of each ring corresponds to a specific crystal plane, allowing researchers to determine the crystal structure of minerals. This quantitative analysis technique is more precise than visual observation and can identify crystal structures in synthesized or complex mineral samples.

Physical Properties Of Minerals I: Crystals and Cleavage

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1. Establish a Group of Mineral Samples

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

Place a sample onto the observation surface.
Rotate in order to observe all sides. Look for crystal faces, crystal edges (lines where faces meet), and crystal vertices (points where edges meet).
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.
Compare to the set of characteristic crystalline polyhedra.
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)).

Crystallography; clear crystal; geometric facets; mineral study; educational; research analysis.
Figure 1. Quartz displaying hexagonal dipyramidal form.

Hands holding a raw mineral crystal specimen, showcasing crystal structure and clarity.
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.

Salt crystal close-up held in hand, illustrating crystallization, study of crystal structures.
Figure 3. Halite displaying cubic crystal form.

Rock sample examination; geology study, textural analysis, mineral composition identification.
Figure 4. Garnet displaying dodecahedron form.

Polished mineral specimen, geology study, rock crystal structure analysis, educational research.
Figure 5. Biotite displaying pseudo-hexagonal form.

3. Observe and Analyze Cleavage

Put on eye protection.
Place a piece of quartz on breaking surface.
Using a hammer, break the piece of quartz in half.
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.
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)).
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).

Raw calcite crystal sample demonstrating mineral structure and optical properties.
Figure 6. Quartz displaying conchoidal fracture, without cleavage surfaces.

Calcite crystal showing natural cleavage planes; mineralogy study for crystallography analysis.
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.