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Science, Maths & Technology

What rock is that? Just the facts

Updated Wednesday 27th September 2006

Step by step rock analyser will help you identify your sample. For a richer experience, you can explore with our interactive Geology Toolkit.

What rocks are these? Copyrighted image Icon Copyright: OU

Have you got a sample - collected in the field, or perhaps found at a rock and mineral fair - that you'd like to identify? Our rock analysis feature can help you. It assumes that you've got a sample from the UK - if the rock is from elsewhere the analysis might fail. Read through our guide to using the analyser, and then have a go...

First of all - have you got a rock or a mineral?

A 'mineral' is a solid material, formed by natural processes and with a chemical composition that falls within certain narrow limits. Minerals are made up of atoms which are arranged in a regular pattern, so they form 'crystals' with characteristic shapes, like cubes, sheets or pyramids.

A 'rock' is a solid collection of mineral grains. These may be fragments of crystal or whole crystals and they can be mm to cm in size. A 'rock' may have only one type of mineral, but usually it consists of several different minerals.

So how do you know whether you're looking at a rock or a mineral?
A rock is an aggregate of one or more types of minerals which may be present as crystals or grains.

If your specimen is a single (or collection of) crystals, all made up of the same thing, you're probably looking at some kind of mineral.
If it's metal, you could be in luck, it might be a native element like gold or silver, or more likely, it's one of the sulphide minerals like pyrite - often known as fool's gold.

Alternatively, there's a very small outside chance it's part of a meteorite! In which case you should send it with details of when and where you found it to either the Natural History Museum or to the Open University, and get us to identify it for you. (Don't worry, we'll send it back!)

If you're fairly sure you're looking at a rock, before you try to work out its mineral composition, you should first look at its texture.

Texture is the term used to describe the physical relationships between the particles from which the rock is made. Virtually all rocks have textures that are either crystalline (made of crystals) or fragmental (made of grains).

In rocks with a crystalline texture, the minerals have grown together as interlocking crystals. In rocks with a fragmental or granular texture, individual mineral and rock grains have been transported and deposited, and may be quite rounded. In either case the size of the individual grains can vary considerably.

Rocks with crystalline texture are usually harder and more compact than those with granular texture. When crystalline rocks are broken they tend to fracture along smooth, angular surfaces within individual crystals, rather than between crystals. The result is that the broken surfaces of crystalline rocks have lots of flat faces that reflect and glint as they catch the light. Rocks with granular texture usually fracture between individual grains because the 'cement' holding them together is often weaker than the grains themselves.

After texture, grain size and colour, you should consider if there are any 'structures' in your rock. By 'structure' we mean are there any bands, sheets or layers?

Layers can be bands of different coloured crystals, different sizes of grains, or the rock may simply split into thin sheets, like slate.

Layers in rock appear in different ways -

Ribbon-like layers of crystals;
Grain size layers
or Thin plate-like layers or sheets

Ribbon-like layers of crystals:
In some rocks different coloured minerals are lined up in ribbons. Usually in two colours, black and white, or green and white, or black and pink.

Ribbon-like layers are often typical of metamorphic rocks where the minerals grow in layers in response to pressure.

Bands and layers of crystals are usually an indication of a metamorphic rock.

Grain size layers: In sedimentary rocks, grains of different sizes are deposited as the strength of the current waxes and wanes (gets stronger or weaker). Larger grains are deposited by stronger currents. Smaller grains are deposited by weaker currents. Often when grains are sorted by wind or water, they will be the same composition and shape, but different sizes; and therefore different shades of the same colour.

Bands and layers of grains usually indicate a sedimentary rock.

Thin plate-like layers or sheets:
Some minerals grow naturally to form platy sheets - but it takes added temperature and pressure to bring them all into line so that they form sheets or layers. Clay minerals are typically flat and platy, but if they undergo metamorphism they will line up into sheets and form a rock which easily splits - like slate.

For example: mudstone contains lots of flat platy clay minerals - but it doesn't easily split into layers because the minerals are arranged in a random pattern. However, if you add heat and pressure all the clay minerals will align themselves so they are roughly parallel to each other and the rock will split very easily giving you "slate".

What are fossils?
The word fossil comes from the latin word 'fossilis' which means 'dug up'. A fossil is the remains or impression of any plant or animal which has been buried in sediment and preserved in a sedimentary rock. Less commonly, fossils can also be preserved in ice, e.g. woolly mammoths, or even in lava flows and volcanic ash, e.g. charred tree stumps.

There are two main kinds of fossil: a body fossil which is the actual remains of a plant or animal and a trace fossil, which records the activities of the animal, like footprints, burrows and bit marks.

How do fossils form?
In principal forming a fossil is quite easy. A plant or animal just has to get buried in sediment. Sadly the process is not that simple. Not all organisms get buried, and not all of those that are buried are preserved as fossils. Normally the first thing that happens is the death of the organism. The soft body parts then decay or get eaten by carnivores. Only in very rare and exceptional cases are the conditions right to preserve the soft parts of an organism.

If the remaining hard parts, like the skeleton or shell, are then quickly buried by sediment they have a chance of being preserved. If not, they may be scattered by predators or broken up by wind and water.

Even when buried, potential fossils may be destroyed by chemical solutions percolating through the sediment and dissolving the hard parts.

As the sediment continues to compact and turn into sedimentary rock, the remaining hard parts may themselves undergo change. For example, the original skeleton may be dissolved and replaced by a new mineral.

Why are fossils important?
Fossils tell us something about the biology of now extinct plants and animals and sometimes also about the environment in which they lived. By comparing fossils with present animals and plants we can also begin to map out parts of the evolutionary tree of life. Fossils are also useful for the relative dating of sedimentary rocks and in some cases they also serve a practical economic purpose. The oil, gas and coal industries make extensive use of fossils in rock formations to help them date and correlate rocks so that they can then locate and exploit reserves.

Grain Size
Once you have decided if a rock is crystalline or granular it is helpful to work out the grain size - the size of crystals or grains in the rock. There are three broad categories:

Coarse Grained: most of the rock is made of crystals or grains as large as rice, or larger. An example of a coarse grained rock is granite.

Medium Grained: you can see the individual crystal grains without a magnifier, but most of the rock is made of grains smaller than rice, for example, sandstone.

Fine Grained: the individual grains cannot be seen without a magnifier or microscope.

Rocks come in a multitude of colours, for simplicity we have divided them into three catgories:

Dark coloured rocks: may be black, dark brown, dark grey or dark green.

Intermediate coloured rocks: are pale grey, reddish brown or green.

Light coloured rocks: are pale pink, pale green, light grey or white.

Tip: the colour of a rock can also be influenced by its grain size. Very fine grained rocks often appear much darker than their coarse grained or coarsely crystalline equivalents.

Now you know what you're looking for, move on to the analyser - we'll ask you a series of questions about your sample; click on the link that most closely matches your rock.

When you're ready, move on to the first question

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For further information, take a look at our frequently asked questions which may give you the support you need.

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