Physical Properties of Minerals

Minerals are the building blocks of rocks, and therefore are the building blocks of our planet's structure. They are specifically defined as naturally occurring, crystalline (as used in mineralogy, this means that they have an ordered internal structure) solids that are made inorganically, not by biological methods. For example, the bismuth shown below is not a mineral, because it does not naturally occur in this form; this crystal was created in a laboratory. Minerals each have their own specific chemical composition and structure that gives them distinction from other similar minerals. They also have specific physical properties that scientists can use to identify them without resorting to looking at them under a microscope. Let's look at each of these distinguishing properties and how minerals are identified.

A mineral’s color can be very distinctive at times. Take azurite (in the picture below), known for its deep blue color, or olivine, named for its olive green color. However, not all minerals come in one specific color. Some, like quartz, come in many tints and hues. Two or more different minerals may be a similar color. Weathering can also alter the color of minerals. The color you see may just be a coating over the mineral, like rust on a hematite or surface weathering on clays. Opaque and metallic minerals tend to come in certain distinctive colors, while translucent and transparent minerals seem to experience color changes from chemical impurities more readily. But even then, color is not the most reliable method of identifying a mineral. You have to look at specific details: is it pale or a deeper color? Is it a smooth color or are there bands or mottled markings? Is it all one color or does it contain several different shades that are blended together? Looking closely at the evidence at hand and all the possible origins of that evidence will give you more clues.

Luster is a description of how much a mineral reflects light. There are two main kinds of luster: metallic (shiny) and nonmetallic (dull). Luster is also related to atomic structure and bonding within the mineral itself: metallic lusters tend to correspond with ionic bonds and nonmetallic lusters with covalent bonds. This makes it a fairly reliable way to identify minerals as it shows some of the chemical characteristics of the mineral. Metallic minerals are usually opaque, but nonmetallics can be opaque, translucent, or transparent. Minerals may also be described as glassy (or vitreous), silky, waxy, or resinous, among other things. The amethyst shown here is an excellent example of vitreous luster.

Hardness is a mineral’s resistance to scratching, and shows the strength of a mineral’s atomic bonds. For example, take a human fingernail. It has a hardness of 2.5 on the Mohs hardness scale, which is the standard for measuring a mineral’s hardness; 1 is really soft and 10 is extremely hard. If you were to scratch that fingernail against talc, which has a hardness of one, there would be a mark on the talc because the atoms in your fingernail were bonded more tightly than the loose atoms in the talc. However, if you tried to scratch your fingernail on a piece of orthoclase, with a hardness of 6, you would wear away part of your fingernail because those atoms are more strongly bonded. Hardness tends to increase with the structural complexity of the arrangement of atoms in a mineral, or by packing the atoms more tightly together. In general, hardness is tested by scratching things of known hardness against each other until you find the range it falls into. Diamond is the hardest mineral in the world due to its tight atomic packing and strong covalent bonds. The gypsum shown here is much softer, with a hardness of 2.

Cleavage is the tendency for a mineral to break into smooth planes. This is governed again by the internal structure of the mineral, because breakages occur along weak planes between atoms. It is a very good indicator of a mineral’s identity for this reason. Minerals can cleave into thin sheets (mica), or rods (some types of asbestos), or octahedrons (fluorite), or rhombic prisms (calcite), as well as other forms. Some minerals don’t cleave; instead, they fracture unevenly. Some minerals like quartz display conchoidal fracture, which looks sort of like the inside of an oyster, smooth and curving. Others are fibrous, with fine parallel crystals, or splinter into oddly shaped pieces. Smithsonite, as shown in the picture below, is often botryoidal, meaning it forms rounded, layered bubbles that can be broken off. If you have a sample of an unidentified mineral, you may try hitting it with a rock hammer to better see where the planes of weakness are. Just be careful not to hit it too hard!

The definition of a mineral's streak is that it is the color of a powdered mineral. Streak is commonly examined by using a small ceramic tile called a streak plate and scratching the mineral across its surface. The color produced here is a better diagnostic than the color you see when you look at the mineral, because the color you see is affected by impurities in the mineral, but when streaked, the crystals are randomly arranged and it is less likely for impurities to affect light absorption. A streak can only be produced by minerals that are softer than the streak plate, which is typically around 7 on the hardness scale. For harder minerals, you can crush them to produce powder. These tend to have a white streak. Not all minerals leave a streak similar to their natural color. The mineral hematite produces a deep red streak because it is basically solid rust, even though solid pieces of hematite are black.

Specific gravity is the density of a material, in this case a mineral, compared to an equivalent volume of water. If a piece of galena has a specific gravity of 7.58, that means that it is 7.58 times heavier than a volume of water identical to the volume of that particular piece of galena. These are standard to each sample of that particular mineral, which makes specific gravity a good diagnostic criteria for identification. Metallic minerals tend to be denser than their nonmetallic counterparts. Pycnometers (the little beaker on the scale seen below) can used to measure the specific gravity of a mineral by using the mass of water at a certain temperature and the volume of the pycnometer to calculate the density of a mineral.

Minerals with carbonate, or CO3, in them will dissolve and produce bubbles when a solution of diluted hydrochloric acid (typically 5-10% HCL) is poured on them. This is known to geologists as the acid test, and it can be of great diagnostic help in identifying carbonate minerals. Calcite will fizz more violently than dolomite, and has more of an immediate reaction, so you can use the acid test to figure out if your mineral is one or the other. Some minerals may also require heat to start this reaction, like magnesite and siderite. The video below shows how immediate the reaction is with calcite.