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GEOL0011 Practical 5 2023/24
ART 23/10/23 1
Igneous & Metamorphic Petrology: Practical 5
Mafic rocks (this practical will span both sessions – 2 hours)
Objectives:
Looking at mafic intrusive / extrusive rocks
Identification and description of mafic phases: olivine, pyroxene, amphibole,
plagioclase (new phase(s) this week in bold)
Microtextures in thin section
Recognition of mafic rocks at hand specimen scale
What you will need:
Optical microscope
Thin sections X777, ICE1, X633 (and P5492 – optional extra)
THERE ARE NOT ENOUGH SECTIONS OF ALL SECTIONs FOR EVERYONE TO WORK
ON THEM AT THE SAME TIME, PLEASE ONLY TAKE ONE AT A TIME
pencil(s) & plain paper, compass, ruler
Hand specimens (provide at front)
Exercise:
All thin sections (X777, ICE1 and X633) are mafic rocks. There is one new mineral in these thin sections,
which is amphibole (in this case hornblende). It is distinguished using its pleochroism, moderate-high
birefringence and 2 cleavages that intersect at 120/60° (it looks quite like clinopyroxene).
1. Study thin section X777. Make a thin section sketch highlighting the diagnostic features of the
minerals you see. Measure the average grainsize (in millimetres) and the modal mineralogy. Make
a specific note about the dihedral angle between adjacent grains. Is this rock plutonic or
extrusive Where in an ophiolite section would you expect to see it
[~ 40 mins]
2. Briefly look at the section ICE1 (there are only 13 of them). Identify the grainsize and any textures
you can see. Can you identify the mineralogy What do your observations tell you about whether
it was extrusive of intrusive, and its cooling rate
[~ 10 mins]
3. Hand specimens of basaltic and gabbroic rocks are provided.
i. Looking at the large, rounded basalt samples (which are a bit altered – but shape matters):
1. Describe any surface and interior textures / features you can see
2. Why are they this shape
3. In what tectonic environment did they form
ii. Looking at the large hexagonal-shaped samples:
1. What is their rock type (hint: think about colour and density)
2. Why are they this shape
3. Using a hand lens see if you can identify the mineraly of the sample that is cut in half
(with holes in it).
iii. Looking at the smaller hand specimens:
1. For each sample, using a hand lens, identify the mineralogy.
2. Categorise it as a basalt or a gabbro
iv. What is the key difference between a basalt and a gabbro [~ 15 mins]
GEOL0011 Practical 5 2023/24
ART 23/10/23 2
4. Study section X633. Make an illustrative thin section sketch of the minerals you see in the section
and include some notes about textural features you see in the sample. Estimate the modal
mineralogy of the rock. Give it a name and indicate whether it is extrusive or intrusive.
note some of these sections are a tiny bit thicker than normal – this will make minerals appear to have a
slightly higher birefringence than usual – e.g. feldspars may appear pale yellow. Also some mafic phases
have been slightly altered to green clay-like minerals – don’t worry too much about these features but do
note them down as this type of alteration is unique to certain phases
[~ 30-45 mins]
5. (optional) Study section P5492. Identify its mineralogy, both the phenocrysts and groundmass
phases (which are large enough to be observed at high magnification). Make a sketch of key areas
of this section. When looking at extrusive rocks you often must combine multiple observations
from different regions of the sample to make a representative sketch. Name the rock.
[~ 40 mins]