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S260 Geology Block Two - Earth Materials
Block two was also difficult particularly the first sections on crystals, but the practical work is fascinating. I enjoyed the microscope work, although I need far more practice with the thin sections and home kit before I'm confident at identifying rocks and minerals on my own. Unlike my previous technology courses I'm finding that I have to revisit each section of the course before I begin to understand the course material. However, I'm really glad that I'm having a change this year with geology as I'm finding it a really interesting subject.
Introduction to Block two
Several people have said that Block two is the most difficult part of the course, and it's easy to get bogged down with the detail of crystal structures, and optics, etc. However, the most important part of this block seems to be, learning to use a systematic approach to identify physical specimens of rocks and minerals. Some of the theory seems quite daunting, but I hope to focus on the practical aspects of this block.
A mineral is defined as:
...a solid body, formed by natural processes, that has a regular arrangement of atoms which sets limits to its range of chemical composition and gives it a characteristic crystal shape.
The crystalline state
Physical properties of minerals
Colour - not a reliable indicator on its own.
Shape - three broad categories: prismatic (elongated), tabular (squashed), equidimensional (regular in shape e.g. cube, rounded, etc.)
Lustre - surface appearance - glassy, waxy, greasy, metallic, etc.
Cleavage - ability to break in a well defined shape.
Hardness - resistance to scratching or indentation
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Crystal optics
Light
• Colour
• Refractive Index
• Anisotropy of refractive index (isotrophy - no structural variation)
• Double refraction
Minerals in thin section
Interference colours
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Exploring rock forming minerals
Silicates
Olivine
Garnet
Pyroxines
Amphiboles
Mica
Clay minerals
Quartz
Feldspar
Non-silicates
Carbonates - calcite, aragonite, haematite, pyrite.
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What is a rock?
"A rock is a granular or interlocking agregate of one or more types of crystal or grain."
Igneous rocks
Igneous rocks are rocks formed by the cooling and crystalisation of magma. Extrusive igneous rocks are formed by volcanic eruption. Intrusive igneous rocks are formed at depth beneath the Earth's surface.
Texture is usually related to cooling rate and can be used to classify igneous rocks - fast cooling = fine grain size (extrusive); slow cooling coarse grain size(Intrusive).
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Sedimentary rocks
The result of physical and chemical weathering.
Siliciclastic rocks
Carbonates
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Metamorphic rocks
Contact metamorphism - intrusion of hot magma into colder rocks
Regional metamorphism - increase in pressure / temperature
Dynamic metamorphism - caused by frictional heating of local deformation
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The rock cycle
Block two exam revision
• Read through block 2 section summaries and objectives.
• Browse Frances' S260 revision site.
• Browse Digital kit and digital microscope cdroms.
• Get familiar with block two bookmark (allowed in exam).
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URL of this page:[ http://www.larnach.info/S260/two.htm ]
Block two links
S260 thin sections
S260 thin section images
Minerals under the microscope
polarisation experiments with micas
Crystal models (print and cut out)
Books
