from The Open University
Alternatively you can skip the navigation by pressing 'Enter'.
Life In Cold Blood: Armoured GiantsMonday, 31st August 2015 08:00 - YesterdayThe heavyweights of the cold-blooded world. Read more: Life In Cold Blood: Armoured Giants
More or Less: Chinese market crash, e-cigarettes and runnersAvailable until Tuesday, 29th September 2015 20:30More or Less investigates the Chinese market crash, e-cigarettes, engineering and how sprinters run so fast. Read more: More or Less: Chinese market crash, e-cigarettes and runners
The world’s busiest railway 2015 – Mumbai Railway: Episode 1Available until Tuesday, 29th September 2015 19:00
Are our kids tough enough? Chinese school: Episode TwoAvailable until Monday, 28th September 2015 01:55
Canals: The Making of a Nation: EngineeringAvailable until Friday, 2nd October 2015 01:50
OpenLearn Live: August Bank Holiday Special 2015Not so much live, but with a great collection of free courses, things to watch and listen to, and... Read more: OpenLearn Live: August Bank Holiday Special 2015
Are our kids tough enough? Chinese schoolIn a unique experiment, five teachers from China take over the education of 50 teenagers in a... Read more: Are our kids tough enough? Chinese school
Challenges in advanced management accountingThis free course, Challenges in advanced management accounting, focuses on strategic management... Try: Challenges in advanced management accounting now
Forensic psychologyDiscover how psychology can help obtain evidence from eyewitnesses in police investigations and... Try: Forensic psychology now
- You are here:
- Science, Maths & Technology
- An introduction to minerals and rocks under the microscope
- 3.5.1 The mica group
An introduction to minerals and rocks under the microscope
Experience the study of minerals using a polarising microscope. While the study of...
Experience the study of minerals using a polarising microscope. While the study of minerals can involve electron or ion beam chemical analysis, the polarising microscope remains the prime tool for the study of rock thin sections and is the foundation of learning to recognise, characterise and identify rocks.
After studying this unit, you should be able to:
- understand the facts, concepts, principles, theories, classification systems and language associated with minerals and rocks
- use the essential terms, concepts and strategies of mineralogy
- apply knowledge and understanding of the study of rock thin sections using a polarising microscope
- work with and recognise a variety of minerals and microtextures in igneous, metamorphic and sedimentary rocks
- make systematic descriptions and identifications of minerals in rocks, observing them using images of thin sections viewed under a polarising microscope, and deduce how and in what environments the minerals and rocks were formed.
- Learning outcomes
- 1 Minerals and the crystalline state
- 1.1 Introduction
- 1.2 States of matter
- 1.3 Physical properties of minerals in hand specimen
- 1.4 The atomic structure of crystals
- 1.5 Crystal defects and twinning
- 1.6 Crystal symmetry and shape
- 1.7 Summary of Section 1
- 1.8 Learning outcomes for Section 1
- 2 Minerals and the microscope
- 2.1 The nature of light
- 2.2 Minerals and polarised light
- 2.3 Minerals and the polarising microscope
- 2.4 Summary of Section 2
- 2.5 Learning outcomes for Section 2
- 3 Rock-forming minerals
- 3.1 Introduction
- 3.2 Silicate mineral structures
- 3.3 Minerals with isolated SiO4 tetrahedra
- 3.4 Chain silicates
- 3.5 Sheet silicate minerals
- 3.6 Framework silicates
- 3.7 Non-silicate minerals
- 3.8 Summary of Section 3
- 3.9 Learning outcomes for Section 3
Study this free course
Enrol to access the full course, get recognition for the skills you learn and track your progress. Make your learning visible!
3.5.1 The mica group
Mica is a general name given to a range of sheet silicate minerals that are commonly found in igneous, metamorphic and sedimentary rocks. In igneous rocks, they crystallise from hydrous magmas with medium to high silica contents; in metamorphic rocks the parallel alignment of mica crystals defines the foliation found in slates and schists.
Micas have sandwich structures, weakly bonded by interlayer ions. Each sandwich contains a tetrahedral sheet on each side of an octahedral sheet (Figure 48d). Commonly one in four of the tetrahedra contains Al instead of Si (although the number of oxygen atoms remains unchanged), with the result that sheets have an excess negative charge. This is balanced by the presence of interlayer cations, such as K+, between the sandwiches (Figure 49a).
The bonding inside a sandwich is very strong, but between sandwiches it is very weak (due to the interlayer ions), permitting one sandwich to slide past another. Thus, mica has one perfect cleavage, parallel to the layers, so it is easy to split a mica crystal into very thin flakes (see).
You have seen that there are two options for making the octahedral layers: either an Al(OH)3 dioctahedral layer (Figure 48c), or a Mg(OH)2 trioctahedral layer (Figure 48b). These give rise to two important mica minerals: muscovite (‘white’ mica; Figure 51a), which contains dioctahedral layers, and biotite (‘dark’ mica; Figure 50a), which contains trioctahedral layers in which Fe2+ commonly substitutes for Mg2+.
Why should biotite commonly be dark and muscovite be white?
Silicate minerals containing large amounts of Fe tend to have dark colours, and this is also true of biotite, which is often Fe rich. Muscovite contains little if any Fe, and so is usually pale coloured.
The structures of muscovite and biotite are given, in schematic form, in Figure 49a.
Micas are striking minerals under the microscope; biotites are often strongly coloured, and muscovites are colourless in plane-polarised light, but both have a perfect cleavage and vivid second- to third-order interference colours. The difference in colour between biotite, with strong pleochroism, and muscovite is diagnostic in thin section (Figures 50b and c; 51b and c). Note that basal sections show neither cleavage nor pleochroism.
This is an extract from an Open University course which is no longer available to new students. If you found this interesting you could explore more free Science courses or view the range of currently available OU Science courses.
Copyright & revisions
Originally published: Tuesday, 9th December 2014
Last updated on: Tuesday, 9th December 2014
- Creative-Commons: The Open University is proud to release this free course under a Creative Commons licence. However, any thirdparty materials featured within it are used with permission and are not ours to give away. These materials are not subject to the Creative Commons licence. See terms and conditions. Full details can be found in the Acknowledgements section.
- This site has Copy Reuse Tracking enabled - see our FAQs for more information.
If you enjoyed this, why not follow a feed to find out when we have new things like it? Choose an RSS feed from the list below. (Don't know what to do with RSS feeds?)
Remember, you can also make your own, personal feed by combining tags from around OpenLearn.
- Latest OpenLearn pages
- Latest pages from OpenLearn - Science
- Latest pages tagged - microscopes
- Latest pages tagged - minerals
- Latest pages tagged - igneous rock
- Latest pages tagged - metamorphic rock
- Latest pages tagged - sedimentary rock
- Latest pages tagged - rock
- Latest pages tagged - rock analysis
- Latest comments on this page