Introduction
In nature, we generally see the earth crest which is made up of rocks. Unconsolidated weathered and fragmented parts of rocks are called regolith. Regolith is the precondition of soil formation. On the earth, the thin topmost layer from where plants collect their foods is called Soil. So after the erosion of soil by different natural agencies the hardest part exposed on the earth is the primary rock or mainly igneous Rock. Somewhere it may be metamorphosed. Igneous and metamorphic rocks create sedimentary rocks. It is easier to identify the rocks in hand specimen than minerals. In the article, we will discuss the techniques of identification of rocks in hand specimens.
List of Rocks to be identified
| Rocks Samples | ||
| S.L. | (B) Rock : | Types |
| 1 | Granite | IGNEOUS |
| 2 | Basalt | IGNEOUS |
| 3 | Dolerite | IGNEOUS |
| 4 | Laterite | SEDIMENTARY |
| 5 | Limestone | SEDIMENTARY |
| 6 | Shale | SEDIMENTARY |
| 7 | Sandstone | SEDIMENTARY |
| 8 | Conglomerate | SEDIMENTARY |
| 9 | Slate | METAMORPHIC |
| 10 | Phyllite | METAMORPHIC |
| 11 | Schist | METAMORPHIC |
| 12 | Gneiss | METAMORPHIC |
| 13 | Quartzite | METAMORPHIC |
| 14 | Marble | METAMORPHIC |
Rocks at a glance
Properties of the Rock:
•Rocks made of mineral particles bonded together.
•Can also be made up of rocks particles (Conglomerates).
•Can also be made up of a single mineral (limestone: calcite or aragonite or quartzite: by quartz)
•May is not a definite chemical composition & Structure like mineral
•Natural body with or without organic particles (Coal & quartzite)
Igneous Rock
The Latin word ‘IGNIS’ means fire. Igneous rocks are actually of fiery origin. They are made by the cooling and solidification of hot molten material either magma within the earth’s crust or lava or on the earth’s surface.
Ex. Granite, Basalt Dolerite etc.
Rock Specimen no: 1
- Colour: Light-colored with spots of white, pink, and black (Leucocratic)
- Grain size: Moderate to coarse
- Specific gravity: Low
- Forming minerals: Quartz, feldspar, mica
- Other characteristics: Hard & compact
- Structure: Massive
- Special characteristics: Shining surface when polished
Hence the specimen is an Igneous Rock and it is Granite.
Rock Specimen no: 2
- Colour: Dark black/grayish black (Mesocratic)
- Grain size: Very fine-grained
- Specific gravity: Moderate to high
- Forming minerals: Pyroxene, biotite
- Other characteristics: Hard
- Structure: Massive
- Special characteristics: Individual identification of minerals is quite impossible due to the fineness of the grains.
Hence the specimen is an Igneous Rock and it is Basalt.
Rock Specimen no: 3
- Colour: Dark-colored (Mesocratic)
- Grain size: Moderate to coarse
- Specific gravity: Moderate to high
- Forming minerals: Pyroxene, feldspar
- Other characteristics: Hard & compact
- Structure: Massive
- Special characteristics: Have inter-fingering texture (amygdaloidal).
Hence the specimen is an Igneous Rock and it is Dolerite.
Sedimentary Rocks
The Latin word “SEDERE’ means “SETTING DOWN” Deposition of eroded materials of pre-existing rocks by natural agents like river glacier, wind at distant places below the river, lake, sea, or oceans underwater in layers and solidification of these sediments (deposited particles) in layers from sedimentary rocks. • •Ex. Laterite, Limestone, etc.
Rock Specimen no: 4
- Colour: Brown, ogre, Teddies brown, red. Line prone (Leucocratic)
- Grain size: Moderate to coarse
- Specific gravity: High
- Forming minerals: Quartz, zircon, and oxides of titanium, iron, tin, aluminum, and manganese
- Other characteristics: Bricks colored
- Structure: Vesicular structure with spots of nodule
- Special characteristics: Porous in Structure
Hence the specimen is a Sedimentary Rock and it is Laterite.
Rock Specimen no: 5
- Colour: Grey / Grayish white / Mixture / Reddish-brown/ Liver red (Leucocratic)
- Grain size: Medium to fine
- Specific gravity: Moderate
- Forming minerals: Calcite
- Other characteristics: Can be scratched heavy knife
- Structure: Clastic in nature
- Special characteristics: Reacts with HCl
Hence the specimen is a Sedimentary Rock and it is Limestone.
Rock Specimen no: 6
- Colour: Yellowish green/Grey/Brown/Greenish brown (Mesocratic)
- Grain size: Very fine
- Specific gravity: Moderate
- Forming minerals: Clay minerals (Kaolinite)
- Other characteristics: Moderate to soft
- Structure: Non-clastic rock
- Special characteristics: Gives smell of clay when water is applied
Hence the specimen is a Sedimentary Rock and it is Shale.
Rock Specimen no: 7
- Colour: White, light yellowish white, mixture, reddish brown (Leucocratic)
- Grain size: Medium to fine
- Specific gravity: Moderate
- Forming minerals: Sand (Silicon dioxide) quartz
- Other characteristics: Clastic surface
- Structure: Massive
- Special characteristic: Rough surface
Hence the specimen is a Sedimentary Rock and it is Sandstone.
Rock Specimen no: 8
- Colour: Brown / White / Pinkish white (Leucocratic)
- Grain size: Moderate to coarse
- Specific gravity: Moderate
- Forming minerals: Pebbles, sand/quartz
- Other characteristics: Chore (numbers of rocks into a small bowl)
- Structure: Clastic
- Special characteristics Easy to identify due to the presence of rock fragments.
Hence the specimen is a Sedimentary Rock and it is Conglomerate.
Metamorphic Rocks
The Greek word ‘meta’ means ‘altered’ and ‘morpho’ means ‘form’. Due to temperature or pressure a pre-existing either igneous or sedimentary rock changes into a new form of rock which is called metamorphic rock.
Ex. Slate, Phyllite, etc.
Rock Specimen no: 9
- Colour: Greenish black/Black/Grey (Mesocratic)
- Grain size: Fine-grained
- Specific gravity: Moderate
- Forming minerals: Biotite, feldspar
- Other characteristics:
- Structure: Prominent foliation
- Special characteristics: Gives metallic sound when hammered.
Hence the specimen is a Metamorphic Rock and it is Slate.
Rock Specimen no: 10
- Colour: Greenish Grey/ Grey/ Green(Mesocratic)
- Grain size: Very fine to medium
- Specific gravity: Moderate
- Forming minerals: Chlorite and mica
- Other characteristics: Hard & compact
- Structure: Foliated rock
- Special characteristics: Shining surface due to presence of mica
Hence the specimen is a Metamorphic Rock and it is Phyllite.
Rock Specimen no: 11
- Colour: Silver/White/Black/Green (Leucocratic)
- Grain size: Medium grained
- Specific gravity: Low to moderate
- Forming minerals: Muscovite biotite chlorite
- Other characteristics: Hard & compact
- Structure: Foliated rock
- Special characteristics: Shining surface for the presence of mica
Hence the specimen is a Metamorphic Rock and it is Schist.
Rock Specimen no: 12
- Colour: Alternate band of light and dark-colored (Leucocratic)
- Grain size: Medium grained
- Specific gravity: Moderate
- Forming minerals: Mica, quartz and feldspar
- Other characteristics: Very hard and compact
- Structure: Non-foliated
- Special characteristics: Alternate band of colors
Hence the specimen is a Metamorphic Rock and it is Gneiss.
Rock Specimen no: 13
- Colour: White / Ash / Grey / Brownish grey (Leucocratic)
- Grain size: Fine to medium-grained
- Specific gravity: low to moderate
- Forming minerals: sand, quartz
- Other characteristics: sharp edge
- Structure: Massive
- Special characteristics: Non-foliated
Hence the specimen is a Metamorphic Rock and it is Quartzite.
Rock Specimen no: 14
- Colour: White/Pink, oranges white (Leucocratic)
- Grain size: Moderate to coarse
- Specific gravity: Low to moderate
- Forming minerals: Calcite
- Other characteristics: Hard & compact
- Structure: Non-foliated
- Special characteristics: React with HCl
Hence the specimen is a Metamorphic Rock and it is Marbel.
Easy Identifying clue
- Granite: Colourful, Minerals distinct Quartz, feldspar, mica
- Basalt: Black and very fine-grained
- Dolerite: Black but Moderate to coarse-grained
- Laterite: Bricks colored and porous
- Limestone: Liver red-colored and react with HCl
- Shale: Black and earthy smell when water added
- Sandstone: Distinct sand particles
- Conglomerate: Rocks within the rock
- Slate: Foliated and metallic sound when hammered, metamorphosed from shale
- Phyllite: Greenish, foliated, and presence of mica, 2nd order metamorphism from slate
- Schist: Shining surface for the presence of mica, metamorphosed from granite
- Gneiss: Alternate black & white band, 2nd order metamorphism from schist
- Quartzite: Hard, sharp edge metamorphosed from sandstone
- Marble: White/Pink, oranges white, metamorphosed from limestone so react with HCl
Conclusion
On the basis of different identifying characteristics rocks in hand specimens can be easily identified and predicted their name. Microscopic identification is necessary to make ultimate discussion. In the field we, the geographers, generally use the hand specimen identification process. Geologists generally use microscopic observation for extracting ore in the branch of mineralogy.
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