P2: Rock identification (II)

Transcription

1 P2: Rock identification (II) Examine the rocks specimens provided with the aid of these notes. All the rocks come from Ireland, as detailed on the attached map. Answer the short question on each specimen in the space provided; use the notes and definitions at the end this handout. Ask the demonstrators for help if necessary. This sheet will be collected at the end of the practical session and returned to you in time for exam revision. 5 Red sandstone from the shore, Harrylock, Hook Head, Co. Wexford age: late Devonian (~ 350 Ma) This sandstone consists largely of quartz grains. It also contains glistening flakes of another mineral that were laid down flat on the sediment surface; what is this mineral? The sandstone is red because the grains are cemented together by the mineral hematite (red iron oxide). Such red-coloured sandstones are common among the Devonian rocks of Ireland and Britain. Associated Devonian shales and conglomerates are also red, and together with the sandstones these rocks are known as the Old Red Sandstone. Engineering Value: Most sandstones abrade too easily for use as aggregate. Some may yield good dimension stone. Sandstones that split easily along the bedding can be used as flagstones. 6 Greywacke sandstone from Denhamstown, Co. Meath age: Silurian (~420 Ma) Greywacke sandstone is formed from an unsorted mixture of sand, silt and mud, with sand predominating in this case. This kind of sandstone formed on the floor of an ancient ocean when a submarine landslide stirred up a huge

2 volume of dense, turbid water which then flowed down the gently sloping seabed under gravity (known as a turbidity current). Engineering Value: Greywacke can be a good roadstone. 7 Quartzite from Great Sugar Loaf Mountain, Co. Wicklow age: Cambrian (~530 Ma) Quartzite is a hard, light coloured metamorphic rock composed almost entirely of the mineral quartz. It is formed when the grains of a quartz-rich sandstone become tightly welded together under heat and pressure. This sample has not been metamorphosed strongly and the original sedimentary grains can still be discerned. It also contains some quartz veins (see sample 8). Quartzite is found a the peaks of several prominent mountains in Ireland, including the Sugar Loaf (Wicklow), Croagh Patrick (Mayo), Twelve Bens (Connemara) and Errigal (Donegal); why is this? Engineering Value: High strength and resistance to wear make quartzite a good road aggregate. 8 Vein quartz beach pebble Vein quartz forms when hard, brittle, quartz-rich rocks (such as quartzite, sample 7) are deformed and crack open slightly during tectonic movements. Pore water flows into the open cracks carrying with it dissolved quartz, which is then precipitated as a thin lining of quartz crystals on the wall of the crack. In this way, the crack fills completely over time to form a quartz vein. Calcite veins form in the same way within limestones. List two ways you could distinguish between vein quartz and vein calcite. 9 Conglomerate beach cobble from Camp, Co. Kerry age: Silurian (~ 420 Ma) Conglomerate is a clastic sedimentary rock containing rounded pebbles (clasts). It is a sort of natural concrete. A strong current of water is needed to transport pebbles and many examples of conglomerate are thought deposited on ancient river beds. The clast types within each sample give

3 geologists an idea of what rocks were being eroded upstream from where the conglomerate was deposited. Name at least one type of clast contained in this conglomerate. What is the name for a clastic sedimentary rock containing large angular clasts? 10 Shale from Donore Quarry, Co. Meath age: late Carboniferous (~300 Ma) Siltstone and claystone are clastic sedimentary rocks composed of silt- and clay-grade particles respectively. Mudrock is a general term for a clastic sedimentary rock composed of both silt- and clay-grade particles. Mudrocks that split easily along the original sedimentary layering are known as shales. This sample is black because it contains a little carbon left over from the decay of plants and animals. What is the economic value of particularly organic-rich black shales? Engineering Value: Soft clays are used for watertight fill for reservoirs etc. and for making bricks and cement. 11 Slate from Killoran Quarry, Co. Tipperary age: Silurian (~420 Ma) Slate is a metamorphic rock which splits easily into thin, flat sheets. It is formed when shale or fine-grained tuff is squeezed during a period of tectonic compression, often related to mountain building (or orogeny). The compressional force causes tiny crystals of mica (too small to see with the naked eye) to grow parallel to each other perpendicular to the tectonic force. Since mica characteristically forms thin, flakey crystals (see sample 1 for example) the whole rock tends to split into thin, flat pieces. The ability of slate to split in this way is known as slatey cleavage. Note that whereas in shale the fissility is parallel to the original sedimentary layering (or bedding), in slate the cleavage is generally oblique to the bedding.

4 Engineering Value: The slatey cleavage makes for excellent roofing and facing stone. 12 Schist from Upper Lake, Glendalough, Co. Wicklow age: Ordovician (~450 Ma) Schist is a metamorphic rock which, like slate, tends to split into flat pieces. The mineral grains are much larger than in slate and the cleavage surfaces shiney and often wavey. Schists are formed when shales are strongly metamorphosed. This sample was once an Ordovician mudrock similar to sample 10 but was metamorphosed by the heat and pressure from intrusion of the nearby Wicklow Granite (e.g. sample 1). Which mineral is responsible for the sheen on the cleavage surfaces of this schist? Engineering Value: Minimal. 13 Chalk from near Larne, Co. Antrim age: late Cretaceous (~ 80 Ma) Chalk is a white, very fine-grained limestone. The term Chalk is usually reserved for this kind of rock that was deposited over enormous areas of Earth s surface during late Cretaceous time. It is composed almost entirely of the nannometre-scale calcium carbonate skeletons of ancient marine plankton called coccoliths. The reason that such very pure calcium carbonate could accumulate is likely because erosion was minimal worldwide during late Cretaceous time, so little clastic sediment (sand, mud) was available to contaminate the chalk. Chalk is usually a relatively soft, powdery rock, for example where it is exposed in the white cliffs of southern England. This sample of Irish Chalk is unusually hard; why might this be? (Remember that the sample 2b immediately overlies the chalk in Antrim). Engineering Value: Chalk is usually much too soft for building stone and aggregates.

5 14 Black, muddy limestone from Belgrad Quarry, Tallaght age: early Carboniferous (~ 340 Ma) This sample is a dark grey to black, fine-grained limestone. It is dark in colour because, unlike chalk, it contains a substantial amount of clastic muddy material. Hence its composition lies somewhere between that of a pure limestone and a mudstone. List several tests that would allow you to distinguish this limestone from other dark, fine-grained rocks such as samples 2b & 10. Engineering Value: Burn with clay to make cement. Good aggregate. 15 Limestone with coral from Strandhill beach, Co. Sligo age: early Carboniferous (~ 340 Ma) Most Irish limestone, like this beach coral, is pale grey in colour. As outlined above, limestone is formed when calcium carbonate shells and skeletons of dead marine organisms accumulate on the sea floor. These shells and skeletons are sometimes preserved as fossils. The white, tube-like stalks in this sample are the fossilised remains of part of a small colony of corals that inhabited the floor of a warm sea during Carboniferous time. Engineering Value: Burn with clay to make cement. Good concrete aggregate since it does not expand with addition of water. 16 Marble from Streamstown Quarry, Connemara, Co. Galway age: late Precambrian (~ 700 Ma) Marble is metamorphosed limestone. Like limestone, it will fizz when a drop of dilute HCl acid is applied, indicating the presence of calcium carbonate. Although metamorphism has not greatly altered the chemical composition of this rock, it has caused the calcium carbonate grains to increase in size. This sample shows a variety of colours including grey, green and white. The white is pure calcium carbonate. The grey is impure calcium carbonate. The green colour indicates the mineral serpentine (see sample 4). Serpentine is rich in

6 magnesium; what component of the original limestone did the Mg likely come from? Engineering Value: Decorative building stone (the Museum Building is an excellent example).

7 Rocks & Minerals: Definitions Minerals you need to know: Olivine (Mg,Fe,Ca) Silicate Ig Pyroxene (Mg,Fe,Ca) Silicate Ig Amphibole (Mg,Fe,Ca) Silicate + OH Ig Met Feldspar (K,Na,Ca) Al-Silicate Ig Sed Met Quartz Silica Ig Sed Met Mica (biotite & muscovite) (K,Mg) Al-Silicate + OH Ig Sed Met Clay Al-Silicate + OH Sed Chlorite (Mg,Fe) Al-Silicate + OH Met Serpentine Mg Silicate + OH Met Calcite & Dolomite CaCO3 and (Ca,Mg)CO3 Sed Gypsum Calcium Sulphate + H2O Sed Hematite & Magnetite Iron Oxide Ig Sed Pyrite Iron Sulphide Ig Sed The labels Ig(neous), Sed(imentary) and Met(amorphic) show the rock groups which commonly contain each mineral. Properties for identifying minerals Colour: Streak: Scratch mineral on back of ceramic tile. Lustre: e.g. dull, earthy, milky white, glassy, irridescent, pearly, adamantine. Transparency: Habit: e.g. cubic, sheets, fibrous. Cleavage: Tendency to split along planes of weakness within crystal lattice. Density: Hardness: Measured using Moh s Scale of Hardness, a scale from 1 (soft) to 10 (hard). Each number corresponds to the hardness of a particular mineral: 1, Talc; 2, Gypsum; 3, Calcite; 4, Fluorite; 5, Apatite; 6, Orthoclase; 7, Quartz; 8, Topaz; 9, Corundum; 10, Diamond. In the field, hardness can be estimated bearing in mind that a fingernail = 2.5, a copper coin = 3 and the blade of a pocket knife = 5.5. The 3 main rock groups: Igneous Frozen magma Sedimentary Consolidated sediment grains derived from igneous and metamorphic rocks, plant and animal material, minerals precipitated from water. Metamorphic Igneous or sedimentary rocks altered by heat and pressure. Describing and Classifying Rocks Most classification schemes for all igneous, sedimentary and metamorphic rocks are based on three features: Grain Size, Mineral Composition and Texture.

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