London Fieldwork Glossary

Augite, Biotite, Calcite, Muscovite, Olivine, Orthoclase, Plagioclase, Igneous Rocks, Sedimentary Rocks, Metamorphic Rocks, Fossils

This glossary gives information about the common rock-forming minerals, major rock types and some of the fossils that you may encounter in common building stones. 

Minerals

Minerals are naturally occurring chemical elements or compounds with a specific composition enabling them to be classified according to their chemical group (e.g. silicate) or family name (e.g. quartz). Each mineral therefore, can be determined by its particular physical and chemical properties, some having scientific and industrial importance. By studying mineral properties such as cleavage, hardness and specific gravity, geologists can discover how the mineral was formed and use them, along with colour and habit, to identify minerals. Information about some of the major rock-forming minerals can be found on this site. For each mineral described, extra information is given in the fact box at the bottom of each page. These give details about the family name, chemical group and a number of characteristics or properties that can aid identification.

For further information about mineral properties you may find the following links useful: 

• mindat.org
• Mineralogy Database

Augite is the commonest pyroxene and occurring widely in igneous rocks. It is dark green to black, although weathered specimens may look faded. It forms prismatic crystals in volcanic rocks and longer flat crystals in basic dykes and sills such as dolerites.

Biotite is an important constituent of many igneous rocks like granite, diorite and andesite. It is also a rock forming mineral in metamorphic rocks like gniess, schist and hornfels. In felsic and intermediate igneous rocks biotite may be confused with amphibole but biotite has flat, slick, shiny faces not broken by the numerous cleavages typical of amphibole. 

Calcite is the principal component of limestones, marble and other calcareous sedimentary rocks. The calcite in limestones may originate as an accumulation of shells or as a precipitate. It is also a common gangue mineral in hydrothermal veins. Calcite can be mistaken for quartz but a simple test of hardness can distinguish the two. Quartz has a hardness of 7 but calcite is only 3 and can be scratched by a coin. Calcite can also be determined by testing with dilute hydrochloric acid, a small drop will dissolve and effervesces readily.

Muscovite occurs in igneous rocks such as granite and pegmatite. It occurs widely in metamorphic rocks like gneiss and mica-schist. It is also a common mineral resistant to weathering and occurs in sedimentary rocks.

The first window panes were made of muscovite ('muscovy glass') and it is still used today in oil lamps and stoves because of its resistance heat.

Olivine is a rock-forming mineral, and occurs in igneous rocks with low silica, such as a gabbro, basalt, or dolerite. This is a relatively rare mineral in most igneous rocks, or one that is hard to see without a microscope. Large accumulations form the igneous rock dunite or, when mixed with pyroxene, peridotite. Olivine is rare as a detrital mineral - on Earth* - owing to its low stability in sedimentary environments. Forsterite can occur in metamorphic marble. Fayalite is dark brown or black; it is uncommon but can occur in granite pegmatites.

*Olivine appears to be stable as a detrital mineral under the cold, dry conditions at the surface of Mars.

Orthoclase is a typical feldspar that is the dominant mineral in coarse-grained silica-rich igneous rocks like granite and syenite. It can also form particularly large crystals in pegmatites.  Common in many metamorphic rocks, it also occurs to a lesser extent in sedimentary rocks.

In igneous rocks orthoclase is common and may sometimes, when white, be confused with Na-plagioclase.

The feldspar crystal structure is complex but consists of rings of four tetrahedra strung into chains and held together ionically by metallic cations. The specific cations determine the particular variety of feldspar obtained.

Plagioclase is an important rock-forming mineral and occurs widely, primarily in igneous rocks such as basalts. It can sometimes be confused with alkali feldspars (such as orthoclase) but can be distinguished by striations on the cleavage surfaces caused by the intersection of lamellar twins with the cleavage surface.

In rocks, quartz often appears light grey to almost black. It occurs widely in silica rich igneous rocks like granite. It is very resistant to weathering due to its hardness and accumulates in sands as well as in sedimentary rocks like sandstone. It is widely distributed in metamorphic rocks like gneiss and quartzite. Quartz also occurs as a prominent matrix mineral in hydrothermal veins.

There are many varieties of quartz, due to its different colours and forms, for example, rose quartz is pink in colour and amethyst is purple. These attractive varieties are often used in jewellery and for ornaments. Quartz is also used in glass-making, ceramics, building materials and abrasives.

Igneous Rocks

Igneous rocks form by direct crystallization of minerals from a molten magma. When this magma cools slowly beneath the surface of the Earth, coarse-grained intrusive (plutonic) rocks crystallize: extrusive (volcanic and pyroclastic) rocks which crystallize quickly after the magma erupts at the earth's surface are typically very fine-grained. If the melt cools too quickly for crystals to grow, the result is a 'glass'.

Igneous rocks are classified according to their silica (SiO₂) content. Expressed as a percentage of silica, four categories of igneous rocks exist; acid* (>66%), intermediate (52-66%), basic (44-52%) and ultrabasic (<44%).

*Not to be confused with acidity in the sense of pH.

Left: This granite is a biotite granite with a black and white appearance. It contains milky-white orthoclase and white plagioclase. There is grey coloured quartz and approximately 10% biotite. This granite, from the South-West of England, is often used as a decorative building stone because it is hard and crystalline.

Right: This specimen from the Shap area in North-West England is an example of a pink granite. It has an equigranular texture with much pink orthoclase, grey quartz and biotite.    

Sedimentary Rocks

About 75% of the Earth’s continental crust is covered by sedimentary rocks. Sedimentary rocks form from pre-existing rock particles - igneous, metamorphic or sedimentary. The parent rock undergoes weathering by chemical and/or physical mechanisms into smaller particles. These particles are transported by ice, air or water. Deposition takes place as a result of a lowering of energy, organic biochemical activity or chemical changes.

Once deposited, the sediments, over time, are lithified (turned into rock) through compaction (decrease in rock volume due to the weight of overlying sediment) and cementation (chemical precipitation in pore spaces between grains which "glues" the rock together). The primary mineralogical and textural characteristics of the rock can be modified as the sediments are buried deeper in the earth's crust and undergo an increase in both temperature and pressure. These low-pressure, low-temperature changes are termed diagenesis. 

Limestone is a sedimentary rock comprising at least 80% calcium carbonate (CaCO₃). Limestones can be deposited in both marine and fresh water environments, the former being more common, and can take many forms such as those described below.

CHALK

SHELLY LIMESTONE

Carboniferous limestone is an example of a shelly limestone and is found predominantly in the Peak District and Yorkshire Dales where it forms a distinctive landscape known as karst. The limestone was deposited in the Carboniferous period, over 300 million years ago, and tells us that Britain once lay in warmer latitudes. This tropical environment was similar to the Caribbean of today with lots of algae, corals, and brachiopods.

OOLITIC LIMESTONE

Portland limestone is an example of an oolitic limestone. This limestone formed in a shallow sea, rather like the modern Bahamas, near the end of the Jurassic period (~135 million years ago). The rock has an even structure rather like cod roe and it can therefore be cut or sculpted in any direction. This feature, coupled with hardness, colour and durability, gives the limestone its quality as a building stone. 

To find out more to do with Portland limestone and how it was once quarried and how it has been used please see the Jurassic Coast website

Metamorphic Rocks

These rocks get their name from the Greek words ‘meta’ and ‘morph’, meaning ‘change of form’. Metamorphic rocks are produced by the alteration of pre-formed rocks by pressure, temperature and migrating fluids, often in environments deep in the Earth’s crust. Because of the severe conditions which rocks undergo during metamorphism, the original minerals may become unstable and change to maintain equilibrium with the new environment. This can involve changes in mineralogy (recrystallization of existing minerals or formation of new ones) and usually changes in texture from the original rock.

Three main types of metamorphism are recognized. Regional metamorphism results from mountain-building events with large scale folding and burial of pre-formed rocks. Contact metamorphism is produced by direct heating of rocks around an igneous intrusion, baking the surrounding country rocks. Dynamic metamorphism occurs where large scale faulting breaks and deforms rocks next to the fault.

A famous white marble comes from Carrara in the Italian Appenines where it has been quarried for two thousand years. It is probably the finest in the world for sculpture, and has been used by famous sculptors such as Donatello, Michelangelo and Canova for their masterpieces. The Taj Mahal in India is built of white marble from Makrana.

This rock type is very widespread in metamorphic regions, such as Scotland, Norway or the Alps.

Slate was commonly used a roofing material on old houses because it splits so readily. Slates from the quarries of North Wales were once widely used for roofing in Britain. Slate was also used for billiard tables and blackboards, and decorative fireplaces made from enamelled slate can sometimes be found in nineteenth Century houses. Nowadays, it is rare to see a new slate roof, probably because alternative materials are cheaper.

Fossils

WHAT IS A FOSSIL? The word fossil is derived from the Latin ‘fossilis’ meaning, an object that has been dug up from the ground. Fossils are the evidence of once-living animals and plants that may be either the preserved remains of an organism, such as body fossils, or evidence of activity, such as trace fossils. They range from tiny plants and animals that can only be seen under a microscope to huge skeletons of dinosaurs.

Body fossils are the remains of living organisms and are direct evidence of past life. Usually only hard tissues are preserved, for example shells, bones, teeth or wood. Trace fossils are the preserved impressions of biological activity. They provide indirect evidence for the existence of past life. They are the only indictors of fossil behavior.

Corals are marine animals with a sac-like body (polyp), mouth, tentacles and skeleton. It is the skeleton that survives in the fossil record. They may be solitary (living by themselves) or colonial (many polyps joined together).

There are three main coral groups, two groups are extint, Rugosa and Tabulata; the Scleractinia can still be found in the oceans today. Coral reefs are inhabited by countless animals and are the most diverse marine environments.

These organisms are easily preserved in limestones and shales, especially those that formed on the extensive continental shelves.

Spirifer, such as those shown above, can reach up to 12cm in width when adult. The hinge line is very distinctive, being wide and straight. Spirifer has a world-wide distribution.

Echinoderms are often fossilized because their skeletons are made of resistant calcite. They have a long fossil record and are known as far back as the Cambrian period (~500 million years ago).

Crinoids (like the ones shown on the right) are common fossils and their broken remains build great masses of limestone, for example the Carboniferous Limestone. Because of their delicate structure, crinoids are rarely found entirely preserved. They live in shallow water, attached to the substrate with their arms outstretched, perpendicular to the water flow. They intercept food particles and pass them into food grooves that lead to the mouth. A small number of crinoid species have survived to the present day.