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Module 1 Part 2 - Minerals
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Figure 3 - Mica crystal displaying one direction of cleavage. Mineral cleavage occurs because crystal lattices break along planes of weak chemical bonds. Minerals with one direction of cleavage break into sheets.

Figure 3 - PG. Mica crystal displaying one direction of cleavage. Mineral cleavage occurs because crystal lattices break along planes of weak chemical bonds. Minerals with one direction of cleavage break into sheets.

Figure 4 - Halite crystals display 3 directions of cleavage. These mineral crystals break into cubes.

Figure 4 - PG. Halite crystals display 3 directions of cleavage. These crystals break into cubes.

Mineral Properties - How We Identify Minerals

We must be able to identify minerals in order to recognize the different kinds of rocks. So we can use different mineral properties like hardness, weight or density, and fracture to help us identify minerals.

Color and/or streak.  The color of the mineral you see are the wavelengths of visible light that are not absorbed by the mineral. Some minerals always have the same color (such as malachite, a copper sulfate) but most have variable colors (such as quartz, which can be many colors). Streak is the color of the powder of the mineral and is obtained by scratching the mineral across an unglazed white ceramic tile.

Luster. Luster is the way a mineral surface scatters light. Often the luster is compared to a common material, such as a “pearly” luster, or a “metallic” luster. The first division of luster is metallic versus non-metallic. There are many non-metallic lusters: silky, vitreous (or glassy), pearly, resinous, earthy.

Hardness. A scale of mineral hardness that is commonly used is the Mohs Scale. The Mohs scale has 10 levels of hardness based on minerals. The softest mineral level (Talc) has a hardness of 1; Diamond is the hardest with a level of 10.  The scale is given below:

Mineral Hardness   Mineral Hardness   Mineral Hardness
Talc 1   Apatite 5   Topaz 8
Gypsum 2   Orthoclase 6   Corundum 9
Calcite 3   Quartz 7   Diamond 10
Fluorite 4            

Don’t worry, you won’t have to memorize these. You can use some common materials to scratch your mineral specimen instead of these minerals. Your fingernail has a hardness of about 2.5. The steel of a pocket knife blade or a glass plate is about 5.5.

To get the hardness of your mineral specimen, see if you can scratch it with your fingernail. If you can, its hardness is less than 2.5. If you can’t, it is harder than 2.5 and you then try to scratch a glass plate. If the mineral scratches the glass plate it is harder than 5.5. The mineral identification charts always separate minerals by luster (metallic vs. non-metallic), then by hardness, so you can narrow down your search.

Figure 5 - Fluorite crystals displaying four directions of cleavage where the mineral crystal will often break into 8-sided fragments called octahedrons.

Figure 5 - PG. Fluorite crystals displaying four directions of cleavage where the mineral crystal will often break into 8-sided fragments called octahedrons.

Figure 6 - Obsidian (volcanic glass) displaying the curved fracture known as conchoidal fracture. This shell-shaped fracturing property of glasses made obsidian and chert useful to stone-age people for manufacturing sharp tools.

Figure 6 - PG. Obsidian (volcanic glass) displaying the curved fracture known as conchoidal fracture. This shell-shaped fracturing property of glasses made obsidian and chert useful to stone-age people for manufacturing sharp tools.

Figure 7 - The prism crystal form is a common one in nature. It has one long and two short dimensions.

Figure 7 - MII. The prism crystal form is a common one in nature. It has one long and two short dimensions.

Figure 9 - Cubic crystals have three equal dimensions.

Figure 9 - AM. Cubic crystals have three equal dimensions.

Mineral cleavage or fracture

Minerals break along planes of weakness in the crystals that are caused by planes of weak bonds in the crystal lattice. Some minerals always break in flat, planar fractures called cleavage planes. This property is called mineral cleavage. This is recognizable when you see multiple flat parallel surfaces like stair steps that all reflect light back at you. Some minerals have one direction of cleavage, meaning that the crystal splits into sheets, like mica minerals - Figure 3. Other minerals may have two, three (Figure 4), or 4 directions of cleavage (Figure 5).

Some minerals, on the other hand, break with curved or irregular fractures. If a crystal breaks with a curved fracture, it is said to have a conchoidal fracture (shell-like). Glasses break with a conchoidal fracture—look at the edge of a piece of glass and you may see small curved chips broken from the edge. Native Americans used this property to chip arrow heads and stone tools. See Figure 6.

Whether a mineral has cleavage or another kind of fracture is an important property which will help you identify the mineral.

Crystal form and habit

Minerals have crystals that have certain characteristic shapes or forms. Some minerals have needle-like crystals; some have plate-like crystals, etc. The characteristic crystal forms of minerals are given in mineral identification tables. A prism is shown in Figure 7, platy/tabular crystals in Barite in Figure 8, and cubic crystals in Figure 9.

Crystal habit refers to the most common crystal occurrence of a mineral. For instance, a mineral may have a certain crystal form, such as a needle or blade, but commonly occurs in massive form, that is, without clearly recognizable crystals.

Density or specific gravity

Minerals can be light or heavy when hefted. Heavy minerals are said to have a high specific gravity, which is a kind of density measurement. Density is the weight per unit volume. Very dense minerals are heavy for their size and easy to identify, like galena (PbS, a lead sulfide).

Special properties

Minerals as you can see from above, have many properties. Some properties are peculiar to certain minerals, such as:

Magnetism: some minerals are very magnetic, that is, a magnet will stick to them. Example: magnetite.

Acid reactivity: some minerals react strongly to acids. Calcite, for instance, will fizz or bubble when a drop of acid is put on it.

Crystal striations: some minerals have crystals that have many tiny parallel lines called striations cut onto their cleavage planes. Plagioclase feldspars are an important type of igneous mineral and are an example of this. Use your hand lens to see if the cleavage planes have striations. You can use this property to tell plagioclases from other feldspars.

Part 2 - Minerals - Module 1 Home | Page 1 | Page 2 | Page 3 | Web Sites and References

Part 1 - Plate Tectonics | Part 2 - Minerals | Part 3 - Igneous Rocks

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