How can hardness be used to identify minerals

In the same way that a headache is a symptom for a whole host of problems from the flu to a head injury, many minerals share the same color. For example, several minerals are green in color — olivine, epidote, and actinolite, just to name a few. On the other extreme, one mineral can take on several different colors if there are impurities in the chemical composition, such as quartz , which can be clear, smoky, pink, purple, or yellow.

Part of the reason that the color of minerals is not uniquely diagnostic is that there are several components of the crystal compositions and structure that can produce color.

The presence of some elements , such as iron, always results in a colored mineral, but iron can produce a wide variety of colors depending on its state of oxidation — black, red, or green, most commonly. Some minerals have color-producing elements in their crystal structure, like olivine Fe 2 SiO 4 , while others incorporate them as impurities, like quartz SiO 2.

All of this variability makes it difficult to solely use color to identify a mineral. However, in combination with other properties such as crystal form, color can help narrow the possibilities.

As an example, hornblende, biotite, and muscovite are all very commonly found in rocks such as granite. Hornblende and biotite are both black, but they can be easily distinguished by their crystal form because biotite occurs in sheets, while hornblende forms stout prisms Figure 2.

Muscovite and biotite both form in sheets, but they are different colors — muscovite is colorless, in fact. The external shape of a mineral crystal or its crystal form is determined largely by its internal atomic structure, which means that this property can be highly diagnostic. Specifically, the form of a crystal is defined by the angular relationships between crystal faces recall Steno's Law of Interfacial Angles as discussed in our Minerals I module.

Some minerals, like halite NaCl, or salt and pyrite FeS have a cubic form see Figure 3, left ; others like tourmaline see Figure 3, middle are prismatic.

Some minerals , like azurite and malachite, which are both copper ores, don't form regular crystals, and are amorphous Figure 3. Unfortunately, we don't always get to see the crystal form.

We see perfect crystals only when they have had a chance to grow into a cavity, such as in a geode. When crystals grow in the context of cooling magma , however, they are competing for space with all of the other crystals that are trying to grow and they tend to fill in whatever space they can. The shape of the crystal can vary quite a bit depending on the amount of space available, but the angle between the crystal faces will always be the same. The hardness of a mineral can be tested in several ways.

Most commonly, minerals are compared to an object of known hardness using a scratch test — if a nail, for example, can scratch a crystal, than the nail is harder than that mineral. In the early s, Friedrich Mohs , an Austrian mineralogist, developed a relative hardness scale based on the scratch test. He assigned integer numbers to each mineral, where 1 is the softest and 10 is the hardest. This scale is shown in Figure 4.

There are some properties that only help to distinguish a small number of minerals, or even just a single mineral. Calcite fizzes or effervesces as the HCl solution dissolves it and creates CO 2 gas.

Calcite is easy to identify even without testing the reaction to HCl, by its hardness, luster and cleavage. Another special property is magnetism. This can be tested by seeing if a small magnet responds to the mineral.

The most common mineral that is strongly magnetic is the mineral magnetite. A special property that shows up in some sample of plagioclase feldspar is its tendency to exhibit striations on cleavage surfaces. Striations are perfectly straight, fine, parallel lines. Magnification may be required to see striations on plagioclase cleavage surfaces. Other special properties may be encountered on a mineral to mineral basis.

Some minerals have other unique properties, some of which are listed in Table 3. Hint: It is most likely found on your dinner table. Minerals are classified according to their chemical properties. Except for the native element class, the chemical basis for classifying minerals is the anion, the negatively charged ion that usually shows up at the end of the chemical formula of the mineral.

For example, the sulfides are based on the sufur ion, S 2—. Pyrite, for example, FeS 2 , is a sulfide mineral. In some cases, the anion is of a mineral class is polyatomic, such as CO 3 2— , the carbonate ion. The major classes of minerals are:.

Based on the polyatomic anion, SiO 4 4— , which has a tetrahedral shape. All silicate minerals are built of silicon-oxygen tetrahedra SiO 4 4— in different bonding arrangements which create different crystal lattices. You can understand the properties of a silicate mineral such as crystal shape and cleavage by knowing which type of crystal lattice it has.

These are based on the sulfide ion, S 2—. Some sulfides are mined as sources of such metals as zinc, lead, copper, and tin. These are based on the carbonate ion, CO 3 2—. Carbonate minerals tend to dissolve relatively easily in water, especially acid water, and natural rain water is slightly acid. These are based on the oxygen anion, O 2—.

These have a halogen element as the anion, whether it be fluoride, F — , chloride, Cl — , bromide, Br — , iodide, I — , or astatide, At —. Halite, NaCl, is a halide mineral. These have the polyatomic sulfate ion, SO 4 2— , as the anion. Anhydrite, CaSO 4 , is a sulfate. These have the polyatomic phosphate ion, PO 4 3— , as the anion. Fluorapatite, Ca 5 PO 4 3 F, which makes your teeth hard, is a phosphate mineral. These are made of nothing but a single element. Gold Au , native copper Cu , and diamond and graphite, which are made of carbon, are all native element minerals.

Recall that a mineral is defined as naturally occurring. Therefore, elements purified and crystallized in a laboratory do not qualify as minerals, unless they have also been found in nature.

In tables 1—3, hardness is measured on Mohs Hardness Scale. As you read through the tables, you can click on the images of minerals to see a larger version of the photo. First, you need good light and a hand lens or magnifying glass. Minerals are identified on the basis of their physical properties, which have been described in the the previous section.

To identify a mineral, you look at it closely. At a glance, calcite and quartz look similar. Both are usually colorless, with a glassy luster. However, their other properties they are completely different.

Quartz is much harder, hard enough to scratch glass. Calcite is soft, and will not scratch glass. Quartz has no mineral cleavage and fractures the same irregular way glass breaks.

However, these habits can also aid in identification. Some commonly found habits include: botryoidal which resembles a cluster of grapes , striated parallel grooves on crystal faces , and acicular needlelike.

Some minerals, such as calcite, gypsum, halite, uranium minerals, and fluorite, will fluoresce in brilliant colors when viewed with an ultraviolet UV light. UV light is not normally visible to the human eye, and you should avoid looking directly at the UV source, as it can damage eyesight. Reference: Utah Geological Survey: How do geologists identify minerals? Saturday, November 13, Sign in. Forgot your password?

Get help. Password recovery. Geology Page. Home Latest News Video. Debris Flow Dynamics. Sampling Hot Molten Lava. Incredible moment Anak Krakatau erupts, Oct Download Google Earth For Free. Remote Sensing Downloader. Thunder Egg. Mineral Researches Top Posts. Specific gravity is the ratio between the mass weight of a mineral and the mass weight of an equal volume of water. A mineral's specific gravity SG can be determined by dividing its weight in air by the weight of an equal volume of water.

For instance, quartz with a density of 2. The way in which a mineral breaks along smooth flat planes is called cleavage. These breaks occur along planes of weakness in the mineral's structure.

However, if a mineral breaks along an irregular surface, it does not have cleavage. When a mineral breaks irregularly, the breaks are called fractures. The breaks can be described as grainy, hackly jagged , conchoidal curved , or splintery.

How well a mineral resists breakage is known as tenacity. Tenacity is described using these terms:. Richard C. Berg, Director E.