P1: Rock identification (I)

Transcription

1 P1: Rock identification (I) 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. 1 Granite from Ballyedmonduff Quarry, Stepaside, Co. Dublin age: early Devonian (~400 Ma) Granite is a common kind of intrusive igneous rock. This sample contains three different types of mineral grain: quartz, feldspar and mica (biotite). The composition of a granite is sometimes described as felsic because its principal constituents are feldspar and silica (quartz). The grey, glassy material is quartz, the remaining light-coloured mineral is feldspar and the dark flakes are mica. Diagnostic properties of quartz are given below; identify all three minerals in your sample and note down the diagnostic properties of the feldspar and mica (think about colour, lustre, crystal habit, presence of cleavage, hardness). Quartz: Pale grey, glassy, scratches knife (hardness=7), no cleavage. Feldspar: Mica (biotite): Granite is an important constituent of the continents but it is almost entirely absent from oceanic crust. Engineering Value: excellent dimension stone, decorative stone and aggregate.

2 2a Gabbro Gabbro is a coarse-grained, intrusive igneous rock containing two main minerals: pyroxene (augite) and feldspar (plagioclase). Identify these minerals. The composition of a gabbro is sometimes described as mafic because its principal constituent (pyroxene) is an Mg-Fe silicate. The most obvious difference between granite and gabbro is that granites are relatively light in colour and gabbros are relatively dark. What is the mineralogical reason for this observation? Gabbro is rare on the continents but very abundant globally, since it forms the lower part of oceanic crust. 2b Basalt from near Giant s Causeway, Co. Antrim age: Tertiary (~60 Ma) Basalt is the most abundant type of extrusive igneous rock. It forms from cooling of lava flows. Basalt has exactly the same mineralogical composition as gabbro, explaining its dark colour. Briefly explain the physical process that makes extrusive igneous rocks finegrained and intrusive igneous rocks coarse-grained. The upper part of oceanic crust is made of basalt. Around 20 km 3 /yr of gabbro and basalt is produced at the world s mid-ocean ridges, where diverging tectonic plates allow the underlying mantle to well up and melt. On the continents, basalt mostly occurs as plateau basalts within large igneous provinces (e.g. Antrim Plateau Basalts within the North Atlantic Igneous Province; Deccan Plateau Basalts which make up most of the Deccan Large Igneous Province of India). Engineering Value: Good aggregate; valuable roadstone.

3 3 Tuff from Martello Tower, Portrane, Co. Dublin age: late Ordovician (~ 440 Ma) Tuff is a term used to describe a rock composed of fragments (clasts) of volcanic debris such as ash, lava etc. It can therefore be classed as both an igneous and a sedimentary rock; sometimes the term pyroclastic is used. Volcanic activity was quite widespread during late Ordovician times in Ireland. 4 Serpentinite from the lower slope of Croagh Patrick, Co. Mayo age: probably early Ordovician (~490 Ma) Serpentinite is a relatively rare kind of metamorphosed igneous rock. The original rock (protolith) was peridotite, a rock composed mostly of pyroxene and olivine. Peridotite is the main rock in the uppermost mantle. If exposed at Earth s surface, both olivine and pyroxene react with water to form serpentine, a soft, green, magnesium-rich mineral. Describe two mechanisms by which mantle rocks like peridotite might be emplaced in the uppermost continental crust. Engineering Value: Massive serpentine, which is translucent and of a light to dark green colour, is often used as an ornamental stone. Chrysotile is a fibrous variety of serpentine that is known as white asbestos (NB white asbestos is considerably less hazardous than blue asbestos, which is a type of amphibole).

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5 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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