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Rocks under the Microscope

Pictured below are some common rock types as seen under the microscope. The images are classified according to rock type (sedimentary, igneous, metamorphic). Each thumbnail has a short caption and links (by clicking on the thumbnail itself) to a medium-resolution image. Use the links below to jump down the page to the section you want:

[ Sedimentary ]

[ Igneous ]

[ Metamorphic ]

Sedimentary Rocks

This sandstone is made of quite well rounded grains of quartz, cemented together by calcium carbonate. Cambrian, NW Scotland.
Field of view 3.5 mm, polarising filters.
Sandstone (with mica)
A fairly fine-grained sandstone made of rather angular grains of quartz and feldspar (feldspar looks more cloudy). Narrow flakes of mica, seen edge-on, and slightly crumpled, lie on bedding planes. Precambrian (Torridonian), Raasay, Inner Hebrides, Scotland. Field of view 3.5 mm.
The green colour that this Cretaceous sandstone has in hand specimen comes from the rounded grains of the mineral glauconite, seen here among quartz grains in a matrix of clay and calcium carbonate. Field of view 3.5 mm.
Greywacke (impure sandstone)
Greywackes are impure sandstones in which the grains are commonly made of feldspar and rock fragments as well as quartz. Notice that the grains have a wide range of sizes, and that some are rounded, some not. Field of view 5 mm, polarising filters.
Limestone (with fossil fragments)
This is a fairly typical limestone deposited in shallow water. A good proportion of the particles are tiny shells and worm tubes, and much of the rest is very small particles of calcium carbonate. Where there was empty space in the sediment, such as inside the worm tube on the right of the picture, larger crystals of calcite have grown. Field of view 3.5 mm, polarising filters.
Limestone (oolitic)
Oolitic limestone is made up largely of sand-sized, rounded pellets of calcium carbonate, which are formed in warm shallow water where carbonate sediment is moved about by currents. Isle of Skye, Scotland. Field of view 4.5 mm, polarising filters.
Limestone (oolitic)
Oolitic limestone is made up largely of sand-sized, rounded pellets of calcium carbonate. In this closer view we can see that some of the pellets have grown by adding layers of calcium carbonate onto a tiny sedimentary grain of quartz. Isle of Skye, Scotland. Field of view 3 mm.
Limestone (dolomitic)
In this limestone, diamond-shaped crystals of dolomite (calcium magnesium carbonate) have grown after deposition, while the sediment was being changed into rock. They replace the fine calcium carbonate mud (dark material in the photo) that makes up the rest of the rock. Field of view 3.5 mm.
Coal is mostly opaque under the microscope, as you might expect. The red-black material (vitrinite) in the thin section is the part of the coal that appears shiny black in hand sample, and is made from compressed wood tissue. The loop-shaped orange-yellow objects are the flattened large spores of plants. Field of view 3 mm.

Igneous Rocks

Granites are coarse-grained intrusive igneous rocks made of two different kinds of feldspar (potassium- and sodium-rich), together with quartz and a small proportion of dark minerals. In this view of a granite from Cornwall, the feldspars have a dusty appearance, and the quartz is clear. The only "dark" mineral is a small amount of yellow tourmaline (right). Field of view 8 mm.
Looking at granite between crossed polarisers makes it easier to distinguish the individual crystals. The rock is made up of interlocking rectangular feldspars and irregular clear quartz, all in shades of dark grey through to white. The crystals showing yellow colours are mica (muscovite) and tourmaline. Field of view 8 mm, polarising filters.
This granite has an interesting texture, which is made visible by viewing it with crossed polarisers. Firstly, it has some very large feldspar crystals, set among smaller ones: part of a 2 cm feldspar is seen at the bottom of the photo. Secondly, at right of centre it has quartz and feldspar forming an intergrowth - pairs of crystals grown through each other in a complex pattern. Field of view 8 mm, polarising filters.
Granodiorite is very similar to granite. It has less of the potassium variety of feldspar, more of the sodium feldspar (plagioclase). The dark minerals in this rock include green hornblende and brown mica (biotite). Field of view 8 mm.
Feldspars and quartz show up in shades of grey. Granite and granodiorite are acid rocks, i.e. they are mostly made of felsic minerals (quartz and feldspars) rather than mafic minerals (pyroxene, hornblende and other dark-coloured minerals). Field of view 8 mm, polarising filters.
Gabbro (olivine gabbro)
Gabbro is a basic igneous rock, the intrusive equivalent of basalt. So, there is no quartz, and about half the rock is made of minerals (pyroxene and olivine) that are dark-coloured in hand specimen. In this cross-polars view they show bright colours. The striped grey rectangular crystals are plagioclase feldspar. Field of view 8 mm, polarising filters.
Feldspar (in anorthosite)
Some varieties of gabbro have few dark minerals, and are made almost entirely of plagioclase feldspar. This rock shows the interlocking texture of the feldspars particularly well. Orange crystals are pyroxene. Field of view 8 mm, polarising filters.
Dolerite is the name given to basic igneous rocks found in small intrusions that are intermediate in grain size between basalt and gabbro. Small intrusions (dykes and sills) cool more quickly than large intrusions, but more slowly than lavas erupted at the surface. Field of view 6 mm.
Much of the Earth's mantle is made of peridotite like this, though it is less common to find such rocks at the Earth's surface. This peridotite is made of irregular interlocking crystals of olivine (bright colours) and magnesium-rich pyroxene (large grey crystals). Field of view 6 mm, polarising filters.
Basalt (olivine basalt)
Like many volcanic rocks, it has larger crystals in a very fine-grained matrix. The large crystals (mostly olivine) grew while the magma was held in a chamber beneath the volcano, but the rest of the rock crystallised only after eruption when the lava flow cooled. Field of view 3 mm.
Andesites are intermediate rocks, i.e. between basic (basalt) and acid (rhyolite) in composition. They are important volcanic rocks in the Andes, Japan, and other regions around the Pacific Ocean (where they occur above subduction zones). Most of the large crystals here are plagioclase feldspar. Field of view 4.5 mm.
Acid lavas such as rhyolite are erupted at a lower temperature than basalt, which means that they are much more "sticky". They flow very slowly, and quite commonly develop a "flow banding" in the matrix that you can see here wrapping around the large feldspar crystals. Field of view 6 mm.
Obsidian (with colour-banded glass)
Obsidian is the name given to acid lavas that did not crystallise, and are made mostly of dark glass. This example has a colour banding that seems to have appeared after solidification, where the fresh brown glass has begun to form tiny crystals in some bands, losing its colour in the process. Field of view 6 mm.
Obsidian (with feathery crystals)
This is a glassy volcanic rock in which the glass has begun to crystallise. Because the new crystals are forming at low temperatures they find it difficult to grow as blocky solid shapes, and instead develop branched, feathery forms. Field of view 3 mm.
Obsidian (with feathery crystals)
This is a close-up view of tiny new crystals forming in volcanic glass. Notice how they radiate from certain points in the rock where they appear to begin growing, and how they branch into feathery shapes and curved bundles. Field of view 1.2 mm.
Tuff (welded tuff)
Tuffs are volcanic rocks composed of particles of volcanic ash and crystals that have settled out of the air, either onto the land surface or into water. In this example, the particles of ash were still so hot that they were still soft: they packed down and welded themselves together. This rock is difficult to distinguish from a lava. Field of view 3 mm.
Kimberlite (tuff)
Kimberlite is a rock formed by the explosive eruption of very gas-rich magma produced in the Earth's mantle. It erupts at the surface so violently that its texture is nearly always that of a tuff, i.e. jumbled fragments of igneous material and broken crystals. Olivine, mica and calcite are common minerals, and diamonds, though rare, are economically important. Field of view 8 mm.

Metamorphic Rocks

Slates are formed from fine-grained sediments such as mudstone and shale. When these are compressed and heated a little, tiny new flakes of mica grow, and tend to line themselves up at right angles to the direction of compression. Although the individual mica crystals cannot be seen, the rock breaks along a particular direction, or cleavage plane. Here you can see the cleavage, and you can also see that it is not parallel to the original bedding marked by dark and light bands. Field of view 2.5 mm.
Slate (with folded layer)
This rock originally consisted of alternating layers of silty material and mud. When it was compressed, the silty layers folded and the rock as a whole became a slate. The cleavage is best developed in the finer layers, but you can see that it cuts right through the folded silty layer too. Field of view 2.5 mm.
A phyllite is similar to a slate, except that it forms at higher temperatures. Now the new mica flakes are large enough to see under the microscope, and form mats of crystals (pink when seen between crossed polarisers) lying parallel to each other. In hand specimen this rock has a glossy sheen, but individual mica crystals cannot be distinguished with the naked eye. Field of view 2.5 mm, polarising filters.
Schist (mica schist)
At higher temperatures of metamorphism, new mica flakes grow larger. If they line up parallel to each other, they form a schistosity - the rock will split along these directions. In this schist you can see both brown and colourless mica flakes. Field of view 1.5 mm.
Schist (garnet mica schist)
In this schist, viewed between crossed polarisers, the parallel mica flakes show up in bright colours, and large rounded garnet crystals appear black. Field of view 6 mm, polarising filters.
Metamorphic minerals
When a sedimentary rock is heated, chemical reactions between the original minerals (clays, quartz) cause new metamorphic minerals to appear. Often these grow into large crystals, which sit in a finer-grained matrix and sometimes trap many small grains inside them. The large crystal in the centre is staurolite, a mineral rich in aluminium and iron. Field of view 3.5 mm.
This rock was originally a basic igneous rock (basalt or dolerite). When metamorphosed, the heating and compression changed the original minerals to hornblende (green) and feldspar (colourless), and gave the rock a banding of minerals. Field of view 2 mm.
Schist, folded
This schist has been very strongly crumpled, after it was first formed as a schist. It shows that metamorphic rocks can be deformed many times during their lifetime. The black material outlining the folds is carbon, in the form of graphite. Field of view 3 mm.
Gneiss (biotite gneiss)
Gneisses are highly metamorphosed rocks that have a banding or an alignment of minerals, but have little mica and so do not tend to split along the banding. This gneiss was formed from a granite during the continental collision that built the Alps. Field of view 6 mm.
Gneiss (pyroxene gneiss)
This type of gneiss is found in some of the oldest parts of the Earth's crust. It was formed from an intrusive igneous rock called tonalite, a variety of granite and an important rock type in the continental crust. The main minerals are pyroxene (greenish and pinkish-grey colours) quartz and feldspar (colourless). Field of view 6 mm.
Rocks close to a large igneous intrusion are heated to high temperatures but not deformed. Their minerals change, but they tend not to develop a new banding or cleavage. This makes a hard, fine-grained rock called a hornfels. This example, a pyroxene hornfels, was formed from a basalt lava. The minerals are plagioclase, pyroxene, and an opaque oxide. Field of view 2.5 mm.
Rocks close to a large igneous intrusion are heated to high temperatures but not deformed. Their minerals change, but they tend not to develop a new banding or cleavage. This makes a hard, fine-grained rock called a hornfels. This example was a fine-grained sedimentary rock, and the horizontal banding you can see is the original sedimentary layering. There are many small mica flakes, but they do not lie parallel to one another, as they would in a schist. Field of view 2.5 mm.
Metamorphosed limestones are called marble. The calcium carbonate re-forms itself into larger, interlocking crystals of calcite (e.g. the pearly-coloured crystals in the centre). The impurities are converted into new metamorphic minerals. In this case, the larger bold-coloured crystals are forsterite (magnesium silicate, a variety of olivine). Field of view 6 mm, polarising filters.

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D.J. Waters, Department of Earth Sciences, June 2004