Rock Identification

On this page is some basic help with rock identification.

 

 



 

First Step


First, decide whether the rock you have picked up is a sedimentary, metamorphic, or igneous rock.

An igneous rock has got an arrangement of mineral crystals. It has got no foliation, and no fossils.

igneous rock
Granite - a typical igneous rock with interlocked mineral
grains.
 

A sedimentary rock can be identified by visible sedimentary layers. But not all sedimentary rocks do have sedimentary layers. They do, however, have a special look - fine-grained ones have a very homogenous structure, others contain larger pebbles or fossils.
They are not foliated. Sedimentary rocks can be quite weak, you can try its hardness by hitting it with a hammer.

sedimentary rock
A typical, "sandy" sedimentary rock with fossil.

A metamorphic rock is often foliated. If it's not, it is still different from igneous rocks because it does not have mineral crystals; and it differs from sedimentaries because it lacks bedding layers, fossils, and the sandy, homogenous structure.

metamorphic rock
A typical "shisty" (layered/foliated) metamorphic rock.


Rock Identification - Second Step 


Once you know what type of rock it is, establish whether it is coarse, medium or fine grained. A rock is coarse-grained if you can see its grains with your bare eyes. With medium-grained rocks, you need a hand lens to see the grains separately. And with a fine-grained rock you need a microscope.
 

Rock Identification - Third Step



If you rock was an igneous rock, look at its colour.
 

Coarse Grained Medium Grained Fine Grained
Light Granite Quartz porphyry Rhyolite
Medium Diorite, Syenite Andesite Porphyry, Monzonite Andesite, Trachyte
Dark Gabbro Dolerite Basalt

igneous rock basalt
Basalt - typical mafic igneous rock. By Dave Schumaker
via Flickr.com


If you rock was a sedimentary rock, look at what minerals you find in the rock.
 

Coarsest Grained Medium Grained Finest Grained
Rock Fragments Conglomerate Breccia Greywacke
Quartz Sandstone Mudstone Siltstone, Shale 
Carbonates Limestone Travertine Chalk, Marl 
Other Minerals Ironstone, Dolomite Rock salt, Rock Gypsum  Flint, Chert, Coal, Clay, Amber

sedimentary conglomerate
Conglomerate - coarse-grained sedimentary rock.
By Molas via Flickr.com


If you rock was a metamorphic rock, look at whether it is foliated or not.
 

Coarse Grained Medium Grained Fine Grained
Foliated Eclogite, Amphibolite, Migmatite, Gneiss Schist, Phyllite Slate
Not Foliated Skarn, Marble, Granulate Hornfels, Marble, Slate Mylonite, Halleflinta

metamorphic gneiss
Gneiss - a typical metamorphic rock. By xrichx via Flickr.


 


 






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