A2.2GZ2Geology for Life Sciences Lecture 2 Rocks and Minerals
Overview • Rocks and Minerals - definitions • Mineral families • Igneous rocks • Metamorphic rocks • Sedimentary rocks
Rocks & Minerals Definitions • A mineral is a naturally occuring, chemical compound, often crystalline • Most minerals have a defined range of compositions in terms of their cations • A given mineral may have a defined crystal structure as well as composition.
Rocks & Minerals Definitions • A rock is a defined mixture of minerals • Most rocks contain fewer than three or four principal minerals • A given rock has a defined compositional range in terms of these minerals.
Rocks & Minerals Definitions • An ESSENTIAL mineral is one that MUST be present for a rock to be given a particular name. • An ACCESSORY mineral is one that MAY be present (and often is) but its absence does not affect the name of the rock.
Common Minerals • Over 2000 minerals have been described • Of these, only a few dozen occur frequently and together define all the common rocks • Minerals can be classified in many different ways • The simplest and most common method is classifying (and identifying) the minerals according to their physical properties.
Classification and Identification of Common Minerals • This method is simply identifying the minerals based on common, easily recognised traits such as: • colour (the result of selective absorption of the light spectrum) • size of grains • shape • cleavage surfaces (lines of weakness) or fracture (a measure of how the mineral breaks: cleanly or irregular) • transparency (transparent, translucent or opaque) • lustre - the nature of the reflective surface (metallic, adamantine [diamonds], resinous, vitreous [broken glass], greasy, pearly, silky, earthy).
Classification and Identification of Common Minerals • Some physical properties are a little more difficult to recognize, but still are relatively simple characteristics to observe. • These include: • density, hardness, streak, conductivity, flourescence, magnetism and radioactivity. • density (specific gravity = wt compared to wt of equal vol. of water) • the heavier the atoms and the more closely packed, the greater the density.
Classification and Identification of Common Minerals • hardness (Moh’s scale of 1-10, where talc = 1 and diamond = 10). • Measured by the scratch test:relative test - does a known mineral (known hardness) scratch the unknown mineral? or • can a steel knife blade (hardness = 5.5) or fingernail (hardness = 2.5) scratch the surface of the mineral • minerals 6 will scratch glass • hardness is related to structure and chemical bonding.
Classification and Identification of Common Minerals • streak: the property a mineral has when the powder of a mineral is a different color than the mineral itself • usually determined by drawing the mineral across a piece of white, unglazed porcelain. • useful for identification of opaque and coloured minerals • many minerals have a white streak so this test is often of no use • electrical conductivity, flourescence (emission of light after UV radiation, e.g. fluorite), magnetism (e.g. magnetite, an iron oxide) and radioactivity can also be diagnostic • There are also a number of fairly complicated diagnostic optical tests using the petrological microscope. • There are a number of mineral identification books which list and describe these properties.
Mineral Groups Oxygen Silicon Bonds Viewed from above Tetrahedron • They have atomic structures based on frameworks of the ‘silica tetrahedron’ SiO4 • The silica tetrahedron is a basic building unit for a major group of minerals called the silicates.
Dark-coloured minerals • There are four structural families based on the arrangement of the silica tetrahedron • Separated (isolated) tetrahedra • Single chains of tetrahedra • Double chains of tetrahedra • Sheets of tetrahedra • These structures are formed through direct linkages (bonds) between the tetrahedra vs linkages via metal cations
A. Separated tetrahedra - all linkages are via cations B. Single chains - direct linkages between tetrahedra C. Double chains D. Sheets (micas) Links between individual chains and sheets are via cations E. Framework silicates, (e.g. plagioclase and quartz), are three-dimensional networks of silica tetrahedra
Mineral Groups • It is convenient to divide these minerals into groups based on their atomic structure • Only six groups are needed: • quartz • feldspar • olivine • pyroxine • amphibole • mica Light minerals Dark minerals
Light-coloured minerals • These minerals lack iron (unless present as trace impurities) - their main components are silicon, aluminium, oxygen and the metals sodium (Na), potassium (K), calcium (Ca) Two groups occur: • QUARTZ - pure SiO2 with some impurities (usually oxides). These give a variety of colours • FELSPAR - some replacement of Si by Al, plus a corresponding incorporation of metal ions into the structure • in ALKALI FELSPAR the metal ions are Na+, K+ (0-100%) • in PLAGIOCLASE FELSPAR the metal ions are Na+, Ca++ (0-100%).
Dark-coloured minerals • Dark minerals contain iron (Fe) • This is nearly always associated with magnesium due to their similar ionic size • The Mg:Fe ratio is controlled by the temperature at which the mineral crystallised • In natural minerals this ratio varies from about 95% Mg (higher T) to 95% Fe (lower T ) • This is because Mg is slightly smaller than Fe and therefore more mobile at higher T.
Relative sizes of some of the most common ions in minerals. Compare magnesium and iron.
Dark-coloured minerals • The crystallisation of dark minerals occurs between about 1800ºC to 1200ºC • The temperature of crystallisation has an impact on which minerals are formed and the proportions of elements.
Dark-coloured minerals • Common mineral groups within the four structural groups are: • OLIVINE (separated tetrahedra) • PYROXENE (single chain) • AMPHIBOLE (double chain) • MICA (sheet) • Within each of these groups there is variation due to the Mg:Fe ratio
Dark-coloured minerals • The minerals within a group may look very similar and often can only be distinguished by using a microscope • Thus it is best to focus just on mineral groups • Even the single and double chain groups (pyroxenes and amphiboles) are difficult to distinguish from one another in isolation.
Formation of rocks • Rocks are formed by three processes • Melting of existing material. This creates IGNEOUS rocks • Heating of existing material without melting. This creates METAMORPHIC rocks • Erosion and deposition of particles from existing material. This creates SEDIMENTS and SEDIMENTARY rocks.
Igneous rocks • Igneous rocks are formed by solidification (by crystallisation) of magma (molton rock) either underground or by volcanic eruption • The former are known as intrusive rocks and occur in a variety of underground shapes each with different names • The latter are known as extrusive rocks and include lavas, volcanic debris flows and ash • The final composition of the rock is the product of: • initial composition of the melt • the extent to which the rock minerals have segregated from the liquid melt during crystallisation.
Igneous rocks • There are fine-, medium- and coarse- grained varieties: • fine-grained crystals are visible only under the microscope • medium under the hand lens • coarse-grained are visible to the naked eye. • The crystal size is determined by the rate of cooling • fine grained rocks have solidified within weeks or months, • coarse-grained may have taken tens of thousands to several millions of years to solidify • the larger the igneous body, the coarser it will be (other than at its edges, which will have cooled rapidly) • two igneous rocks may thus have the same mineral composition but be of different grain sizes • they are considered to be different (though related) types of rock and therefore are given different names (e.g. basalt and gabbro).
Igneous rocks - intrusive Intrusive rocks • Magma that forms intrusive rocks solidifies (underground) relatively slowly; most intrusive rocks have larger crystals than do extrusive rocks • They can only be seen in mines and tunnels or on the surface where they have been exposed by geological uplifting and by erosion. • Intrusive rock formations vary from thin sheets to huge, irregular masses: • sills • batholiths • dykes • volcanic plugs