The eight major characteristics used to identify rocks?

Answer 1

Minerals possess a number of physical properties that aid in their identification. These are listed below with a brief description:

Color--The color of the specimen as it appears to the naked eye under normal lighting conditions. Some minerals such as gold will only appear as one color, but due to impurities and Crystal Light distortion, many minerals can appear in multiple colors. Therefore, observable specimen color is the least effective property in identification.

Streak--The color of a mineral in powdered form. A streak test is performed by dragging a freshly cleaved mineral surface across an unglazed porcelain surface. If the mineral is less hard than the porcelain, it will leave a stripe of color (the mineral in a powdered state). This is the true color of a mineral specimen as it lessens the impurity impact and eliminates the light distortion from the crystal. Although a mineral may have multiple observable specimen colors, it will only have one streak color.

Luster--Refers to the absorption, reflection, or refraction of light by the surface of a mineral. Minerals possess various lusters that are somewhat self-explanatory such as dull, earthy, adamantine, metallic, glassy, metallic, silky, greasy, pearly, and resinous.

Fluorescence--Some minerals fluoresce in ultraviolet light, and some minerals specimens may appear to fluoresce due to fluorescent mineral impurities, or due to locality. This is not an extremely reliable indicator property but can be used in conjunction with other identification properties.

Chemical Composition--Minerals are chemicals, either elements or compounds found naturally in the Earth. Some minerals have a fixed chemical composition, and some minerals have a rigid set of chemical composition parameters that allow for the replacement of certain elements with other elements. Chemical properties can be tested with the use of a blowpipe, flame color, bead test, and laboratory solution tests.

Hardness--Minerals are identified roughly by their hardness based on the Mohs scale of mineral hardness, a list of ten minerals from #1 (softest) to #10 (hardest). All minerals will fall somewhere along the scale, based on their ability to scratch any mineral with a number lower than theirs and their inability to scratch any mineral with a number higher than theirs.

Mohs Scale of Mineral Hardness

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

Specific Gravity--Is the relative weight of a mineral as compared to the weight of an equal volume of water. The specific gravity is also referred to as density and is expressed normally as an average of a small range of densities.

Some Common Minerals and Their Specific Gravity

1. Halite (2.1)
2. Gypsum (2.3)
3. Quartz (2.7)
4. Talc (2.8)
5. Muscovite Mica (2.8)
6. Corundum (4.0)
7. Cinnabar (8.0)
8. Gold (19.3)

Crystal Form--A mineral's crystals form into one of six mineral crystal systems: cubic, tetragonal, hexagonal, orthorhombic, monoclinic, or triclinic. The crystal form reflects the structure of the molecules and formation environment of the mineral. Although sometimes difficult, identification of a mineral's crystal form is an invaluable aid to mineral identification along with the other mineral properties.

Optical Properties--Used mainly be scientists, X-rays are sent through thin slices of mineral, producing identifying patterns of light which measure their index of refraction which is distinct for each mineral.

Cleavage--The way some minerals split along planes which are related to their mineral structure, parallel to any crystal faces. Cleavage is measured in degrees of perfection from poor, to fair, good, perfect, and eminent, and are described by the number and direction of cleavage planes. Some minerals do not show cleavage, but fracture in a way other than along any cleavage plain.

Magnetism--Certain minerals exhibit magnetism or an attraction to a magnet, and some will display this property only when heated.

Electrical Properties--Some minerals when heated or rubbed will develop an electrical charge. Others will be able to control radio frequencies.

Melting Point--A pure mineral will posses a melting point which can be tested with a high temperature flame until it fuses at the edges.

Some minerals can be identified by their unique characteristics. Sulfur can be identified by its smell, while halite (salt), by its taste.

Answer 2

1. Color: The hue of a rock can provide clues to its composition.
2. Texture: Describes the size and arrangement of mineral grains within the rock.
3. Hardness: Determined by the rock's resistance to scratching.
4. Mineral composition: The specific minerals present in the rock.
5. Grain size: Refers to the size of the mineral grains within the rock.
6. Density: The mass of the rock per unit volume.
7. Cleavage and fracture: How the rock breaks under stress.
8. Luster: The way light interacts with the rock's surface.