HOW TO CLASSIFY ROCKS ON SITE? (GEOLOGICAL OR LITHOLOGICAL CLASSIFICATION)

Rocks can be classified on site, on the basis of its physical appearance. By making a through visual inspection we can ascertain the engineering properties of rocks. Field classification of rocks gives us logical insight about the suitability of rocks for engineering purpose. This type of classification is known as geological or lithological classification of rock.

While inspecting the rocks visually, we must describe the following properties of rocks, such as

  • Texture
  • Structure
  • Composition
  • Colour
  • Grain size

Let’s know how to describe the above properties of rock

field classification of rock

Geological or Lithological Classification of Rock

1. Texture

Rock material may be of any of the following textural groups.

Crystalline

  • This type of rocks are composed of visible interlocking crystals or crystal grains
  • When scratched by the knife, particles do not come out of the rock mass.

Indurated

  • Interlocking crystals or crystal grains are not visible by naked eye.
  • Rocks consist of fine grained material
  • Particles do not come out of the rock mass when scratched by the knife.

Crystalline – Indurated

  • Individual crystal grains or crystal aggregates are finer than crystalline structure but coarser than indurated structure.
  • Grains do not come out when scratched by knife

Compact

  • Particles or grains are tightly packed
  • Grains are finer
  • Particles or powder come out of the rock mass when scratched by knife

Cemented

  • Grains are visible by naked eye
  • Size of grains ranges from medium to coarse grained structure
  • Particles come out from the rock mass even when scratched with finger nail.

2. Structure

Structure of a rock mass means the orientation of mineral grains within the rock mass. The various types of structures are as follows.

Homogeneous

If the grains and crystals are having random orientation, the structure will be called homogeneous.

Lineated

If mineral particles are having a preferred orientation in a particular linear direction/directions, then the structure will be known as lineated.

Intact-Foliated

When the minerals in the rock mass are having a preferred orientation of a planner nature

Fracture-Foliated

When the planer structure is having closed or incipient fracture such as bedding planes or cleavage planes

3. Composition

Rock’s mechanical and physical characteristics are greatly influenced by the presence of calcite. Therefore based one presence of calcite the rock may be classified as follows.

Non calcareous

Non calcareous rock materials are those in which calcium carbonate is absent.

Part calcareous

The rock contains mainly non calcareous material. The calcareous material is present as a bond between grains.

Calcareous

The rock material which are mainly composed of calcite

4. Colour

Light colour

  • The rocks are acidic in nature
  • Rocks are generally feldspathic
  • Pure calcareous rocks are light in colour

Dark colour

  • Rocks are not acidic in nature
  • Rocks generally contain ferromagnesium mineral
  • Part calcareous rocks are dark in colour.

5. Grain Size

Sometimes classifications of rocks are done on the basis of their grain size. in such cases origin or type of rock is not so important. Based on grain size, the rock material is classified in 3 groups, as given below.

Coarse grained

When size of particles or grains are larger than 2 mm

Medium grained

When size of particle or grain lies between 2 mm to 0.1 mm

Fine grained

When particles are less than 0.1 mm in size and individual grains are invisible to the naked eye

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