Download P1: Rock identification (I)

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.
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 km3/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 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).
Rocks & Minerals: Definitions
Minerals you need to know:
Olivine
(Mg,Fe,Ca) Silicate
Pyroxene
(Mg,Fe,Ca) Silicate
Amphibole
(Mg,Fe,Ca) Silicate + OH
Feldspar
(K,Na,Ca) Al-Silicate Ig
Quartz
Silica
Mica (biotite & muscovite) (K,Mg)
Al-Silicate + OH
Clay
Al-Silicate + OH
Chlorite
(Mg,Fe) Al-Silicate + OH
Serpentine
Mg Silicate + OH
Calcite & Dolomite
CaCO3 and (Ca,Mg)CO3
Gypsum
Calcium Sulphate + H2O
Hematite & Magnetite Iron Oxide
Ig
Pyrite
Iron Sulphide
Ig
Ig
Ig
Ig
Sed
Ig
Ig
Met
Met
Sed
Sed
Sed
Met
Met
Met
Met
Sed
Sed
Sed
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.