Rocks and Minerals – Definition and Identification

Identifying properties of rocks and minerals in hand specimen

There are two types of minerals & rocks specimen identification. One is microscopic identification and another is hand specimen identification. In the subject of Geography, the 2nd process is significantly used. In the field of Geology the 1st and 2nd, both processes are significantly used. Here we will discuss the identifying properties of rocks and minerals in hand specimens.

List of rocks and minerals to be identified:

There is a huge number of rocks and minerals in nature. Some important selected rocks and minerals are listed below. We will discuss their characteristics gradually. Here is the list of the rocks and minerals samples, to be identified.

S.L.MineralsS.L.Rocks
M1BauxiteR1Granite
M2 CalciteR2Basalt
M3ChalcopyriteR3Dolerite
M4FeldsparR5Laterite
M5GalenaR6Limestone
M6GypsumR7Shale
M7HematiteR8Sandstone
M8MagnetiteR9Conglomerate
M9MicaR10Slate
M10QuartzR11Phyllite
M11 TalcR12Schist
M12TourmalineR13Gneiss
R14Quartzite
R15Marble
Rocks and minerals samples

Definition of Rocks and minerals:

  • Definition of Rocks: – Rock can be defined as a material made of mineral particles bonded together. Rock is a hard and clastic substance. Therefore, rocks are aggregate of minerals.

Ex. Granite. Basalt, Dolerite, Shale. Sandstone. Limestone. Conglomerate. Laterite, Slate. Phyllite. Quartzite, Gneiss, Marble. Schist.

  • Definition of Minerals: – Mineral is an inorganic homogeneous substance usually crystalline with a definite chemical composition. Therefore minerals are natural body without organic particles.

 Ex. Mica, Tale, Quartz, Feldspar, Gypsum, Hematite, Magnetite, Calcite, Chalcopyrite, Galena, Bauxite.

Basic identifying characteristics of rocks and minerals:

We can identify the rocks and minerals on the basis of their physical and chemical characteristics or properties in the hand specimens. In that case, we find out the properties of a particular specimen by our naked eye, not under the microscope. The main properties are as follows.

  1. Colour.
  2. Lustre.
  3. Specific gravity
  4. Hardness,
  5. Cleavage
  6. Streak
  7. Special characteristics

 Now, we can elaborately discuss the characteristics one by one.

1. Colour

One of the most important physical properties of minerals, reflecting the nature of the interaction of the electromagnetic radiation of the visible region with the electrons of the atoms, molecules, and ions of the crystals and with the electron system of the crystal as a whole.

2. Lustre

The term luster refers to the quantity and quality of the light which is reflected from a mineral’s exterior surfaces. Luster provides an assessment of how much the mineral surface ‘sparkles’.

  • Metallic or splendent: Minerals have the luster of polished metal, and ideal surfaces will work as reflective surfaces. Examples include galena, pyrite, and magnetite.
  • Vitreous: The term is derived from the Latin for glass, vitreum. This type of luster is one of the most commonly seen and occurs in transparent or translucent minerals with relatively low refractive indices. Common examples include calcite, quartz, topaz, beryl, tourmaline, and fluorite, among others.
  • Resinous: A surface of resinous luster possesses a sheen resembling that of resin. Such materials have a refractive index greater than 2.0. Sphalerite (ZnS) demonstrates a resinous luster.
  • Dull or Earthy: Minerals of dull or earthy luster reflect light very poorly and do not shine. This type of luster is often seen in minerals that are composed of an aggregate of tiny grains.
  • Pearly: Pearly luster appears iridescent, opalescent, or pearly. This is typically exhibited by mineral surfaces which are parallel to planes of perfect cleavage. Layer silicates such as talc often demonstrate a pearly luster on cleavage surfaces.
  • Greasy: A surface that possesses a greasy luster appears to be covered with a thin layer of oil. A light-scattering surface that is slightly rough, such as that of nepheline, may exhibit greasy luster.
  • Silky: Silky luster occurs when light is reflected off of an aggregate of fine parallel fibers; malachite and serpentine may both exhibit silky luster.
  • Adamantine or brilliant: A brilliant luster such as the sparkling reflection of diamond is known as adamantine. Minerals of adamantine luster have high refractive indices (1.9-2.6) and are highly dispersive and translucent.

3. Specific gravity:

Specific gravity is the ratio of the density of a substance to the density of a reference substance; equivalently, it is the ratio of the mass of a substance to the mass of a reference substance for the same given volume. 

Apparent specific gravity is the ratio of the weight of a volume of the substance to the weight of an equal volume of the reference substance. The reference substance for liquids is nearly always water at its densest (at 4 °C or 39.2 °F).

Specific Gravity

Where:-

                     w1  = wait of a mineral in air

                     W2 = wait of a mineral in water

4. Hardness:

Hardness is measured by the resistance which a smooth surface offers to abrasion. The degree of hardness is determined by observing the comparative ease or difficulty with which one mineral is scratched by another.

In 1812 by German geologist and mineralogist Friedrich Mohs, it is one of several definitions of hardness in materials science, some of which are more quantitative.

5. Cleavage:

Cleavage, in mineralogy, is the tendency of crystalline materials to split along definite crystallographic structural planes. These planes of relative weakness are a result of the regular locations of atoms and ions in the crystal, which create smooth repeating surfaces that are visible both in the microscope and to the naked eye. Cleavage forms parallel to crystallographic planes.

  • Basal or pinacoidal cleavage occurs when there is only one cleavage plane. Ex. Graphite, Mica (muscovite or biotite). This is why mica can be peeled into thin sheets.
  • Cubic cleavage occurs on when there are three cleavage planes intersecting at 90 degrees. Ex, Halite, Galena.
  • Octahedral cleavage occurs when there are four cleavage planes in a crystal. Octahedral cleavage is common for semiconductors. Ex. Diamond, Fluorite.
  • Rhombohedral cleavage occurs when there are three cleavage planes intersecting at angles that are not 90 degrees. Ex. Calcite.
  • Prismatic cleavage occurs when there are two cleavage planes in a crystal. Ex. Spodumene.
  • Dodecahedral cleavage occurs when there are six cleavage planes in a crystal. Ex. Sphalerite.

6. Streak:

The streak of a mineral is the color of the powder produced when it is dragged across an un-weathered surface. Unlike the apparent color of a mineral, which for most minerals can vary considerably, the trail of finely ground powder generally has a more consistent characteristic color and is thus an important diagnostic tool in mineral identification. If no streak seems to be made, the mineral’s streak is said to be white or colorless. Streak is particularly important as a diagnostic for opaque and colored materials.

Some minerals leave a streak similar to their natural colors, such as cinnabar and lazurite.

Other minerals leave surprising colors, such as fluorite, which always has a white streak, although it can appear in purple, blue, yellow, or green crystals. 

Hematite, which is black in appearance, leaves a red streak or cherry red which accounts for its name, which comes from the Greek word “haima”, meaning “blood.” 

Galena, which can be similar in appearance to hematite, is easily distinguished by its gray streak.

7. Special characteristics:

  • Feelings: Soapy feeling. Ex. Talc
  • Acid reaction:  Object reacts to dilute hydrochloric acid (HCl). Ex. Calcite, Dolomite & limestone.
  • Magnetism – Magnetism is a distinguishing characteristic of magnetite. Hematite is slightly magnetic.
  • Crystal shape –
    • Cubic(Galena),
    •  Rhombohedral (tilted cube: Calcite)
    • Hexagonal (six-sided: apatiteberyl)
    • Pisolitic and oolitic: Bauxite
  • Transparency – Objects are visible when viewed through a mineral (Muscovite Mica).
  • Translucency – Light, but not an image, is transmitted through a mineral (Biotite Mica).
  • Opaqueness – No light is transmitted, even on the thinnest edges (Pyrite and magnetite).
  • Taste – Taste can be used to help identify some minerals, such as halite (salt).

Conclusion:

On the basis of the above properties, we will easily identify the minerals and rocks in the hand specimen. It is comparatively difficult than microscopic identification. In spite of this, the method is useful in field identification without getting a strain of the specimen for microscopic view.           

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