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Mineral Properties

Mineral Properties

You have an unknown mineral sample and must identify it. There are some simple tools on hand, but no laboratory equipment. With over 4,000 types of minerals, how will you ever figure out the mineral name? The good news is that with a few simple tests and some careful observations you will most likely be able to identify your mineral! 

Minerals are identified by their properties. Each type of mineral has a unique set of properties that reflects its chemical makeup and structure. If you needed to describe what your school looked like to someone who has never seen it, you might state the building's exterior color, shape, and number of floors. With a mineral, you also need to develop a list that describes its physical properties to help identify it.

Hardness: 1
Color: White, gray, green
Streak: White
Luster: Pearly
Cleavage: Perfect, one direction
Specific Gravity: 2.7 - 2.8
Other: Greasy feel

Mineral Properties

Mineral color is easy to identify, but can be misleading. The same mineral may occur in different colors depending on impurities. A mineral's exterior color may be altered through weathering. Plus, there are many different minerals, all of which have the same color! Overall, color is usually not a reliable property to base mineral identification.

Streak is a more reliable identification property than color. Streak is the color of a mineral in its powdered form. Although a mineral may have many colors, its streak is usually only one color. To find the streak of a mineral, scratch it across a porcelain plate (known as a "streak plate"). The color of the powder left on the plate is the mineral's streak. A mineral's streak is often a different color than the mineral. Some minerals are harder than the plate and will not leave a streak. In this case the streak is called colorless.

Mineral hardness is the tendency of a mineral to resist scratching. Hardness is ranked on a scale from 1 (softest) to 10 (hardest) called Mohs Hardness Scale. Mineral hardness is found by doing a series of scratch tests. A harder mineral will leave a scratch mark when rubbed across a softer mineral. When two minerals have a similar hardness, neither will scratch the other.

Hardness: 4
Color: Purple, white, blue, green, yellow, colorless
Streak: White
Luster: Vitreous
Cleavage: Perfect, four directions
Specific Gravity: 3.0 - 3.2
Other: Fluorescent

Crystal Habit (Form)
Minerals are crystalline solids. Crystal habit is the form, or shape, of the mineral. The shape of a mineral crystal reflects the repeating pattern of its atoms arrangement. There are many types of crystal habit. For example, some minerals form cubes. Others can be fibrous, prisms, or granular. Some minerals crystals may have no distinctive form.

The tendency of a mineral to break along flat planes is cleavage. Whether or not a mineral has cleavage depends on the arrangement of its atoms and their bond strength. Mica minerals split into flat sheets. They have cleavage in one direction. Other minerals split in more than one direction and have multiple cleavage planes.

When minerals do not break along flat planes they show fracture. There are different ways that minerals fracture. Minerals with conchoidal fracture break into smooth, curved surfaces. Minerals with hackly fracture break into jagged, pointy surfaces. Minerals with uneven fracture break into irregular surfaces.

Hardness: 8
Color: White, yellow, blue, green, pink, colorless
Streak: Colorless
Luster: Vitreous
Cleavage: Perfect, one direction
Specific Gravity: 3.5
Crystal Habit: Orthorhombic prisms

Specific Gravity
Specific gravity is the ratio of the weight of a given volume of mineral to the weight of an equal volume of water. Essentially, specific gravity is a mineral's density. A mineral with a specific gravity of 2.5 is 2.5 times heavier than water. Since calculating specific gravity is not practical when identifying minerals in the field, heft is used instead. Heft is how heavy a mineral feels when you pick it up in your hand. A mineral with an average heft, should have a specific gravity around 2 - 3. As a guide, if a mineral feels heavy, its specific gravity is over 3. If it feels light, its specific gravity is under 2.

Luster is how a mineral reflects light. If your mineral contains metals, it most likely has a metallic luster. If it doesn't contain metals, it is nonmetallic. Nonmetallic minerals can exhibit other types of lusters. For example, vitreous luster looks like glass. Minerals with pearly luster reflect light like pearls. Greasy minerals look oily. Silky minerals reflect light like silk does. Minerals with earthy luster look dull or like soil.

Other Properties
There are many other, less common, properties that can be useful when identifying minerals. Some minerals, like magnetite, are magnetic and can be tested by a magnet. Carbonate minerals tend to fizz when a drop of hydrochloric acid is placed on them. Fluorite shows fluorescence, or glows under ultraviolet light. Some minerals even have distinguishing smells or tastes!


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