Description of Minerals and Rocks :

The terms mineral and rock is often confused. They are frequently used together, and the materials they describe are closely related. In
general, a mineral is a naturally occurring, inorganic, chemical element or a group of elements (a compound). A mineral has a
crystalline structure and a unique chemical composition. A rock is a mixture of particles or grains of one or more minerals.
When we refer to mineral resources or industrial minerals, however, we generally include materials that are technically rocks—such as
limestone, dolomite, and shale. We also include coal and oil, which are in fact organic substances. Conversely, we include in the rock
category high-purity sandstone, which is composed almost entirely of quartz, and high-purity limestone, which is composed largely of
the mineral calcite.
Mineral Groups
A few minerals are composed of only one element, such as diamond (carbon), native copper, and sulfur, but most minerals are
chemical compounds that contain several elements (table 1). Only about 100 of the more than 2,000 identified minerals are considered
to be common minerals. Of those 100, fewer than 20 are found everywhere in the Earth's crust. Scientists group minerals according to
their chemical composition and crystal structure. The main mineral groups, their composition, and examples of each are shown in
table 1.

Group Composition Examples

Elements Pure elements Copper, gold, sulfur

Silicates Metals, silicon, oxygen Quartz, feldspar, mica, garnet, beryl, talc

Carbonates Metals, carbon, oxygen Calcite, dolomite, siderite

Oxides Metals, oxygen Hematite, bauxite

Sulfides Sulfur, metals Galena, pyrite, chalcopyrite

Sulfates Metals, sulfur, oxygen Gypsum, barite, anhydrite

Halides Metals, chlorine, fluorine, iodine, bromine Halite, fluorite

Using Characteristics of Minerals to Identify Them

Most minerals can be characterized and classified by their unique physical propertiesoperties: hardness, luster, color, streak, specific gravity,
cleavage, fracture, and tenacity.
Hardness: The ability to resist being scratched—or hardness—is one of the most useful properties for identifying minerals. Hardness is
determined by the ability of one mineral to scratch another. Federick Mohs, a German mineralogist, produced a hardness scale (table 5) using a set of
ten standard minerals. The scale arranges the minerals in order of increasing hardness. Each higher-numbered (harder) mineral will scratch any
mineral with a lower number (softer).A rough measure of mineral hardness can be made by assembling a kit of handy objects (table 6). A fingernail
has a hardness ranging from 2 to 2.5, a penny is a little harder than 3, window glass ranges from 5.5 to approximately 6 in hardness, and a knife blade
is generally in the range of 5 to 6.5.

Hardness Mineral Common field test

1 Talc Easily scratched with a fingernail

2 Gypsum Scratched by a fingernail (2.5)

3 Calcite Scratched by a penny (3)

4 Fluorite Difficult to scratch by a nail (4); scratched easily by a knife (5)

5 Apatite Difficult to scratch with a knife (>5); barely scratches glass (5.5)

6 Feldspar Scratched by a steel file (6.5); easily scratches glass

 7 Quartz Scratches a steel file and glass

8 Topaz Difficult to test in the field

9 Corundum Difficult to test in the field

10 Diamond Difficult to test in the field

Creating a Mineral Hardness Test Kit

A mineral hardness test kit can be easily created from common household or hardware items (table 6). Wear safety glasses and gloves when cutting
glass. All pieces in the hardness test kit should be compared to one another and specific minerals with a known hardness.

Approximate
Hardness Item

1 Blackboard Chalk

2.5 Fingernail

3 Copper penny

4 Common nail

5.5-6.5 Pocket knife

Window glass (2 inches square; use caution making this scratch plate. Tape the edges of the glass
5.5 with duct tape. You may be able to get this cut at a local hardware store.)

6.5 Steel file or tempered steel. Try to find an old, broken or worn flat file

Luster:
Luster is how a mineral reflects light. The terms metallic and nonmetallic describe the basic types of luster. Table 7 lists the most common terms used
to describe luster and an example of a corresponding mineral. Some minerals that don't exhibit luster are referred to as "earthy," "chalky," or "dull."
Colour:
One of the most obvious properties of a mineral is color. Color should be considered when identifying a mineral, but should never be used as the
major identifying characteristic.
Streak:
Streak is the color of the powdered mineral, which is usually more useful for identification than the color of the whole mineral sample. Rubbing the
mineral on a streak plate will produce a streak. A streak plate can be made from the unglazed back side of a white porcelain bathroom or kitchen tile.
Some minerals won't streak because they are harder than the streak plate.
Specific Gravity:
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 2.65 is 2.65
times as heavy as the same volume of water.
SG = mineral mass/water mass
Cleavage:
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.
Fracture:
When a mineral breaks irregularly, the breaks are called fractures. The breaks can be described as grainy, hackly (jagged), conchoidal (curved), or
splintery.
Tenacity: How well a mineral resists breakage is known as tenacity. Tenacity is described using these terms:
-Brittle - Mineral crushes to angular ragments (quartz).
-Malleable - Mineral can be modified in shape without breaking and can be flattened to a thin sheet (copper, gold).
-Sectile - Mineral can be cut with a knife into thin shavings (talc).
-Flexible - Mineral bends but doesn't regain its shape once released (selenite, gypsum).
-Elastic - Mineral bends and regains its original shape when released (muscovite and biotite mica).
Other Diagnostic Characteristics:Other characteristics may be useful in identifying some minerals:
-Transparency - Objects are visible when viewed through a mineral.
-Translucency - Light, but not an image, is transmitted through a mineral.
-Opaqueness - No light is transmitted, even on the thinnest edges.
-Taste - Taste can be used to help identify some minerals, such as halite (salt).
-Acid reaction - Object reacts to hydrochloric acid. The most distinguishing characteristic of calcite is that it effervesces when hydrochloric acid is
applied. Dolomite shows a reaction on a freshly broken or powdered surface. Testing for calcite, limestone, or dolomite calls for 10% hydrochloric
acid, but strong white vinegar can be substituted for the acid.
-Magnetism - Magnetism is a distinguishing characteristic of magnetite.
-Crystal shape - Cubic, rhombohedral (tilted cube), hexagonal (six-sided), etc. Some crystal shapes are illustrated below.