Instructions for teachers and techniciansThis practical activity is composed of two parts; a teacher/technician section and the student activity which can be found on page 6. This practical activity supports OCR AS/A Level Geology.
When distributing the activity section to the students either as a printed copy or as a Word file you will need to remove the teacher instructions section.
This is a suggested practical activity that can be used as part of teaching the OCR AS and A Level Geology specifications helping to fulfil the requirements of the Practical Endorsement.
These are not required activities, nor are they coursework tasks.
You may modify these activities to suit your students and centre. Alternative activities are available from, for example, ESTA, Earth Learning Idea, CLEAPSS and publishing companies. Support for mapping
activities to the requirements of the Practical Endorsement is available from OCR – see www.ocr.org.uk/positiveaboutpractical or email us at [email protected].
Students can collaborate during the activities but each student must individually demonstrate competence in each of the practical skills being assessed (see Practical Skills below).
It is possible for a student to achieve some but not all of the practical skills involved in an activity (and this can be recorded as individual skills in the OCR PAG Tracker).
Further details are available in the specifications (Practical Skills Topics).
OCR recommendations:
Before carrying out any experiment or demonstration based on this guidance, it is the responsibility of teachers to ensure that they have undertaken a risk assessment in accordance with their employer’s requirements, making use of up-to-date information and taking account of their own particular circumstances. Any local rules or restrictions issued by the employer must always be followed.
CLEAPSS resources are useful for carrying out risk-assessments: (http://science.cleapss.org.uk).
Centres should trial activities in advance of giving them to students. Centres may choose to make adaptations to this practical activity, but should be aware that this may affect the Apparatus and Techniques covered by the student.
This activity has been adapted from a Royal Microscopical Society activity. Please visit their website for further information on this practical activity or the RMS Microscopes for Schools project www.rms.org.uk
DISCLAIMERThis resource was designed using the most up to date information from the specification at the time it was published. Specifications are updated over time, which means there may be contradictions between the resource and the specification, therefore please use the information on the latest specification at all times. If you do notice a discrepancy please contact us on the following email address: [email protected]
IntroductionHaving conducted a visual examination of a rock sample in the field or laboratory geologists will carry out a more detailed examination of samples under a microscope. Using a diamond saw a sample of rock is cut, glued to a microscope slide and polished to a thickness of 30 µm. At this thickness light will pass through the thin section. The optical properties can be used to determine the composition of the key minerals present and the processes (e.g. diagenesis, lithification, crystallisation, metamorphism, deformation) that created the rock sample.
Students will examine photomicrographs of contrasting rock samples. The online Virtual Microscope tool (www.virtualmicroscope.org/) or printed photomicrographs from university collections or the BGS catalogue can be used. Most university geology departments have outreach programmes and it may be possible to arrange to use petrological (polarising) microscopes and thin sections. Students have many misunderstandings about rocks in thin sections and initially may find the images quite confusing. It is important for students to appreciate that the colour of a rock forming minerals in thin section can be quite different to the colour of a hand sample. Linking the students learning in module 2.1.1 on the crystalline nature of minerals to the optical properties of thin sections, will reinforce prior learning and help them to understand what they are seeing.
Aims to use photomicrographs to recognise and record the optical qualities of sedimentary, igneous
and metamorphic rocks and rock textures in thin section to identify common rock forming minerals in thin section using their optical qualities and a
classification key to produce scientifically annotated drawings of rocks in thin section
Intended class time 1 hour
Practical Skills – competence assessed by the teacher1.2.1 (c) follow written instructions1.2.1 (d) make and record observations/measurements1.2.1 (f) present information and data in a scientific way1.2.1 (j) use a wide range of experimental and practical instruments, equipment and techniques appropriate to the knowledge and understanding included in the specification1.2.2 (f) application of classification systems using distinguishing characteristics to identify unknown minerals and fossils1.2.2 (i) use of photomicrographs to identify minerals and rock textures1.2.2 (j) use of appropriate apparatus to record a range of quantitative measurements 1.2.2 (l) use of methods to increase accuracy of measurements, such as timing over multiple observations, or use of a fiducial scale.
CPAC(1) follows written procedures(3) safely uses a range of practical equipment and materials(4) makes and records observations.
This document may have been modified – if in doubt check the master version on OCR Interchange.
Links to Specifications2.1.1 (b) rock-forming silicate minerals as crystalline materials built up from silicon–oxygen tetrahedra to form frameworks, sheets or chains and which may have a range of compositions2.1.1 (c)(ii) the classification of samples, photographs and thin section diagrams of minerals using their diagnostic physical properties2.1.2 (a)(ii) the diagnostic properties of rocks to identify igneous rocks in samples, photographs and thin section diagrams2.1.2 (b)(ii) the diagnostic properties of igneous textures and crystal shape in samples, photographs and thin section diagrams2.1.2 (b)(iii) the representation using drawings and annotated diagrams of igneous textures and crystal shape in samples2.1.3 (c) the diagnostic properties of rocks to recognise and measure grain sizes in samples, photographs and thin section diagrams2.1.3 (d)(iii) the diagnostic properties of rocks to identify siliciclastic and carbonate rocks in samples, photographs and thin section diagrams5.4.1 (b)(ii) the diagnostic properties of metamorphic fabrics in samples, photographs and thin section diagrams
Mathematical Skills – learning opportunity within activity Mathematical skills must be applied in the recording of the data and calculations, and in
analysing the data. These steps require the appropriate application of the following mathematical skills:o M1.1 Recognise and make use of appropriate units in calculations.o M1.2 Recognise and use expressions in decimal and standard form.o M1.3 Use an appropriate number of significant figures.o M3.2 Change the subject of an equation. Use and manipulate equations e.g.
magnification.
EquipmentEach student or pair of students will require: photomicrographs of three contrasting rock samples under plane and cross polarised light Mineral Identification Key – for some common minerals in thin section, Resource sheet 1 It may help students to have access to Resource sheet 2 How a petrological (polarising)
microscope works, and Resource sheet 3 Glossary of terms used with thin sections and the polarising microscope
Health and Safety Health and safety should always be considered by a centre before undertaking any practical
work. A full risk assessment of any activity should be undertaken including checking the CLEAPSS website (http://www.cleapss.org.uk).
You should follow your centre’s practice on using computers and the internet. If you are using petrological microscopes and thin sections please refer to PAG 5.2
Microfossils for advice on the safe use and handling of microscopes.
NotesCentres are advised to trial this activity before using it with students. In particular: This activity has been adapted with permission, from a resource created by Royal
Microscopical Society (RMS). Please visit their Microscopes for Schools webpages for further information on this practical activity, including diagrams, photomicrographs, notes for teachers
This document may have been modified – if in doubt check the master version on OCR Interchange.
and notes and questions for students. There is also a wealth of microscope related material and advice on the main website www.rms.org.uk;
This is an activity to introduce the use of petrological (polarising) microscopy where students are aiming to become familiar with the techniques and the optical characteristics of rocks/minerals in thin section. The use of specialist terminology in this activity does not imply that students are required to know these terms nor will they will be assessed in written examinations on any understanding beyond that required in the published specification;
http://www.virtualmicroscope.org/content/uk-virtual-microscope the UK Virtual Microscope is a good teaching set for example: chloritoid schist, Arran; granite, Loch Doon; oolitic limestone, Weldon. The Irish University Rocks has a wider selection of sedimentary rock samples;
If you are using www.virtualmicroscope.org it will need to provide students with a list of pre-selected samples to use and have familiariser yourself with using the virtual microscope;
The BGS Rock Collections (mineralogy and petrology collection database) contains thousands of images and can be searched in multiple ways, including rock type, BGS 50k map and grid reference. http://www.bgs.ac.uk\data\britrocks.html can also be used to create map based exercises. It has more limited functions than virtualmicroscope.org, however the plane and cross polarised images can be printed off to use with students;
Many universities have made their teaching sets available online: Liverpool University GEO–OER www.esta-uk.net/geooer/photomicrographs.htm; Oxford Earth Sciences Image Store www.earth.ox.ac.uk/~oesis/micro/index.html; University of Glasgow, Hunterian collection www.gla.ac.uk/hunterian/collections/collectionsummaries/rocksandminerals/.
Competent students can be expected to identify most of major minerals present on the photomicrographs that you have selected for them to use. It is better to allow students to succeed with a limited palette (e.g. quartz, feldspar, calcite, olivine, pyroxene, mica) than stun them with spectacular images that are beyond an introductory exercise.
Answers and Guidance to Extension Activities9. See pages 16 and 17 of the Drawing Skills Handbook for examples of student drawings.10. For example a phenocryst that appears to be 30mm on a photomicrograph and whose real
length is 0.5 mm:
magnification= ¿image¿object
= 300.5
=60
11. Suitable reference resources to use with students include: MacKenzie, W. Adams, A. and Brodie, K. (2017) Rocks and Minerals in Thin Section, 2nd
ed, CRC Press/Balkema, Leiden. McLeish, A. (1992) Geological Sciences, Thomas Nelson and Sons, Walton-on-Thames.
Table 2.5 pages 42–49. Out of print but good second hand copies are cheaply available. Faithfull, J. (1998) Identification Tables for Common Minerals in Thin Section,
Hunterian Museum, University of Glasgow. This excellent online resource is written by the Curator (Mineralogy/Petrology) at the Hunterian Museum and is widely used on introductory petrology courses. It covers most minerals that students are likely to see and can be downloaded as a pdf from ow.ly/fAJE30nmvsN
RecordsAs evidence for the Practical Endorsement, students: should not need to re-draft their work, but rather keep all of their notes as a continuing record
of their practical work, dating their work clearly, should produce correctly annotated drawings of their photomicrograph – this means the
drawings need to be drawn with a sharp pencil with no sketching, be labelled with a ruled line, include a scale bar and be correctly titled,
should include any information provided on when and where the sample was collected
This document may have been modified – if in doubt check the master version on OCR Interchange.
Extension questions help students develop their understanding of the underlying geological theory and are a preparation for the written examinations. They also help students to develop the practical science skills assessed indirectly in the written examinations and they should be encouraged to record their data appropriately, for example showing full workings in calculations, and stating final answers to the appropriate number of significant figures.
Document updatesv1.0 April 2018 Original version.v1.1 January 2019 Minor edits for clarity.
This document may have been modified – if in doubt check the master version on OCR Interchange.
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OCR Resources: the small printThis formative assessment resource has been produced as part of our free A Level teaching and learning support package. All the A Level teaching and learning resources, including delivery guides, topic exploration packs, lesson elements and more are available on the qualification webpages.
If you are looking for examination practice materials, you can find Sample Assessment Materials (SAMs) on the qualification webpages: http://www.ocr.org.uk/qualifications/as-a-level-gce-geology-h014-h414-from-2017/
OCR’s resources are provided to support the teaching of OCR qualifications, but in no way constitute an endorsed
teaching method that is required by the Board, and the decision to use them lies with the individual teacher. Whilst
every effort is made to ensure the accuracy of the content, OCR cannot be held responsible for any errors or omissions
Student activityIntroductionHaving conducted a visual examination of a rock sample in the field geologists will carry out a more detailed examination of samples under a microscope. A sample of rock is cut, glued to a microscope slide and polished to a thickness of 30 µm. At this thickness light will pass through the thin section. By looking at the optical properties of the sample, geologists can determine the minerals present in the sample and the processes that created the rock (e.g. diagenesis, lithification, crystallisation, metamorphism, deformation). In this practical activity you will examine photomicrographs of at least two contrasting rock samples to identify minerals and rock textures.
Aims To use photomicrographs to recognise and record the optical qualities of sedimentary, igneous and metamorphic rocks and rock textures in thin sectionTo identify common rock forming minerals in thin section using their optical qualities and a classification keyTo produce scientifically annotated drawings of rocks in thin section
Intended class time1 hour
EquipmentIn your pair you should have access to:
photomicrographs of three contrasting rock samples under plane and cross polarised light Mineral Identification Key – for some common minerals in thin section – Resource sheet 1 How a petrological (polarising) microscope works – Resource sheet 2
Health and Safety If you are using www.virtualmicroscope.org or other online resource you should follow your
centre’s policy on the use of computers and the internet.
ProcedureBefore starting your practical work, read the information below. 1. Select your first sample, if you are using photomicrographs you should have two images, one
under plane polarised light and one under cross polarised light. Look at the largest crystals or grains in the image. Are these phenocrysts (igneous rock), aggregates (sedimentary rock) or porphyroblasts (metamorphic rock)? Now work your way through the optical of the crystals/grains in the image.
This document may have been modified – if in doubt check the master version on OCR Interchange.
2. Size and shape: the absolute and relative sizes of the crystals and grains present. In addition to the igneous and sedimentary textures that you are familiar with from hand samples. can you recognise any patterns in the crystals/grains? Metamorphic minerals may show preferred orientations; igneous crystals can show zoning and inclusions; sedimentary grains may show the over growth of cements or the effects of chemical compaction (pressure dissolution).
3. Relief: certain minerals appear to stand out strongly in plane polarised light, with a marked outline and clearly defined internal features (such as fractures and cleavage). Relief is a measure of the relative differences between the mineral grain and its surroundings.
4. Colour: can only be observed in plane polarised light. In plane polarised light if the minerals change colour when the microscope stage is rotated they are said to exhibit pleochroism. Biotite, hornblende and augite exhibit pleochroism because they absorb different wavelengths of light in different directions. In cross-polarised light the light passing through minerals is split into two paths. Minerals may exhibit colour variations when the microscope stage is rotated, and this is known as birefringence.
5. Cleavage; cleavage planes are the natural planes along which a mineral will fracture, and are a characteristic of minerals. If the minerals orientation is parallel to the thin section then no cleavage lines will be seen. Common minerals which exhibit cleavage in two directions include amphibole, pyroxenes and feldspar. Most carbonate minerals exhibit a rhombohedral cleavage in three directions, while cubic minerals with three cleavage surfaces produce square or rectangular boundaries.
6. Twinning: occurs when a crystal is composed of two or more parts which are orientated (optically or physically) in different directions. Feldspar minerals commonly exhibit twinning: polysynthetic (“zebra” stripes) in plagioclase, and Carlsbad (two crystals growing within each other) is common in orthoclase/K feldspar.
7. Repeat the process by examining the groundmass (igneous and metamorphic rocks) or matrix (sedimentary rocks).
8. Sedimentary particles may be composed of single minerals such as quartz grains; carbonate grains (such as broken shell material), complete organisms (microfossils), or well-rounded oolitic grains; aggregates may appear to contain many grains ‘welded’ together.
9. Produce a scientific drawing of what you see. Annotate the drawing identifying the optical characteristics used to classify the minerals that make up the sample and any rock textures that you have observed. You will need to include a scale.
Extension opportunities10. Using the scale and the dimensions of minerals that you have drawn to calculate the
magnification of the image:
magnification= ¿ image¿ real object
11. Using the reference material available in class attempt to identify any minor minerals in your photomicrograph. If you use a photo atlas it is important to confirm your identification using the optical characteristics as “picture matching” alone is never definitive.
RecordsAs evidence for the Practical Endorsement, you need records in your field notebook of: your annotated drawings of the contrasting thin sections/photomicrographs, any modifications to supplied procedures, or notes you have made to assist you in using the
visual key.All work should be clearly dated. In addition you should have considered the above questions as the answers to these questions will aid you in preparation for your written examinations.
This document may have been modified – if in doubt check the master version on OCR Interchange.
RESOURCE SHEET 1Mineral Identification Key – for some common minerals in thin section
1 What colour is the mineral in thin section under plane polarised light? Colourless go to 2Red or pink go to 7Yellow go to 8Green go to 9Purple, lilac or blue go to 10Brown go to 11
2 What is the relief of the mineral under plane polarized light?Very high positive relief go to 3Moderate to high positive relief go to 4Low positive relief go to 5Very low positive to very low negative relief (lower than the resin mounting medium) go to 6
3 Metamorphic sample, crystal is rounded or of similar dimensions in all directions, with between six and eight sides and may contain inclusions GARNET
4 Crystal is rounded or of similar dimensions in all directions and lacks cleavage OLIVINEPale green or brown tinges, two cleavage planes close to 90° AUGITE PYROXENEPale green or red tinges, two cleavage planes close to 90°, shows pleochroism PYROXENE
5 Forms long flake shaped crystals with one obvious cleavage MUSCOVITERelief can vary across crystal, two cleavage planes up to 75°, twinning common CALCITETwinning forms distinctive stripes along the length of the crystal, indistinct cleavage PLAGIOCLASE
6 Looks grey under cross polarized light, no cleavage or twinning QUARTZTwinning forms distinctive stripes along the length of the crystal, indistinct cleavage PLAGIOCLASENegative relief, turbid appearance, indistinct cleavage, twinning in two halves K FELDSPAR
7 Moderate to high relief, pale green pink with pleochroism PYROXENEVery high relief, crystal is rounded or of similar dimensions in all directions, with between six and eight sides and may contain inclusions GARNET Very high relief, deep blood red to opaque, cement or secondary mineral grains HAEMATITE
8 High positive relief, pale yellow to brownish yellow, often as pophyroblasts STAUROLITE
9 Moderate relief, deep greenish brown to brown with one obvious cleavage BIOTITEModerate to high relief, green–greenish brown–bluish green, marked pleochroism AMPHIBOLEModerate to high relief, pale green–deep green, weak pleochroism PYROXENEModerate to high relief, pale green–pinkish green, pleochroism PYROXENE
10
Very high relief, three good cleavages at 60° or two at 70°,secondary mineral or cement FLUORITE
11
Moderate relief, deep brown–greenish brown with one obvious cleavage BIOTITEModerate to high relief, deep brown–greenish brown, marked pleochroism AMPHIBOLEHigh relief, pale purplish brown, two cleavage planes close to 90° AUGITE PYROXENEVery high relief, pale yellowish brown–deep brown often zoned, crystal is rounded or of similar dimensions in all directions, with between six and eight sides and may contain inclusions GARNET
AcknowledgementsThis mineral key is based on John Faithfull’s Identification Tables for Common Minerals in Thin Section, Hunterian Museum, University of Glasgow. We acknowledge his permission to modify his original content. If you key out and cannot identify a mineral then please refer to the source document: www.hmag.gla.ac.uk/John/teaching/mintable.htm
RESOURCE SHEET 2Geology PAG 3.2 Virtual microscopeHow a petrological (polarising) microscope worksHard rocks can be cut with a diamond saw blade to make a thin fingernail sized chip which is thin enough to let light will pass through. The chip of rock is attached to a microscope slide using epoxy resin and polished to a thickness of 30 μm. The microscope slide with the thin section of rock is placed onto the microscope stage. Light filters are inserted into the light column, both above and below the microscope slide. These filters are polarising sheets similar to the lenses of Polaroid® sunglasses. The lower filter (the polariser) is permanently in the light path. The upper filter (the analyser) can be inserted to examine particular optical properties of a mineral.
When the microscope is set up for observing objects in plane polarised light (Figure 1 below) the light passes through the polariser (lower filter) and then the object before the image is viewed. Only light vibrating in an E–W direction can pass through the polariser to generate an image.
Figure 1: Microscope set up to observe under plane polarised light
Figure 2: Microscope set up to observe under cross polarised light
Figure 3: Microscope set up to observe a thin section under cross polarised light
When the microscope is set up for observing in cross polarised light (Figure 2 above) the light passes through the polariser (lower filter), and then through the analyser (upper filter). As the direction of polarisation in the analyser is at 90° to the direction in the polariser light cannot pass through and the viewer sees a black image.
If a thin section of rock is put onto the microscope stage in cross polarised light (Figure 3 above) the direction of polarisation of the light from the polariser is rotated as it passes through the mineral crystals in the thin section. Now some of the light will be aligned with the direction of polarisation in the analyser. As the different wavelengths of light pass through the analyser they generate different colours in the image seen by the viewer.
RESOURCE SHEET 3Geology PAG 3.2 Virtual microscopeGlossary of terms used with thin sections and the polarising microscopeaggregate a sedimentary grain composed of many smaller grains that appear to have been
welded together
analyser the upper polarising filter in a petrological (polarising) microscope
birefringence colour variations seen in the image of a mineral thin section when the microscope stage is rotated under cross polarised light
Carlsbad twin two crystals growing within each other, common in orthoclase feldspar
cleavage planes of weakness in the crystalline structure of the mineral
colour the colours seen in thin sections are much lighter and less intense than those seen in hand samples
cross-polarised light
when the two polarising filters in a petrological microscope are orientated so that the directions of polarisation are at 90° to each other
equant when the diameter of a mineral is more or less equal in all directions on the thin section
field of view diameter of the circle of light that you see when looking into a microscope.
groundmass the smaller crystals that make up the space between the larger crystals in a igneous or metamorphic rock
matrix the smaller grains that make up the space between the larger grains/fossils/aggregates in a sedimentary rock
metamorphic fabric
preferred orientation of minerals, seen in both hand specimen and thin section, which result from changes in pressure and temperature during metamorphism
plane polarised light
when the analyser is not inserted in a petrological (polarising) microscope and the light arriving at the thin section is vibrating in the polarising direction of the polariser (lower polarising filter)
pleochroism colour variations seen in the image of a mineral thin section when the microscope stage is rotated under plane polarised light
polariser the lower polarising filter in a petrological (polarising) microscope
polysynthetic twining
multiple laminar twins which produce a distinctive zebra stripe pattern seen in thin sections of plagioclase feldspar
relief the difference in the refractive index between a mineral and the mounting medium (epoxy resin). Relief is positive when the refractive index of the mineral is higher than the mounting medium; relief is negative if the refractive index is lower
refractive index a measure of how much light is dispersed (refracts) as it passes through a mineral
stage the flat circular plate where the microscope slide is placed. On a petrological microscope they rotate 360°, allowing minerals to pass in and out of extinction
twinning the intergrowth of two (or more) single crystals in a mineral grain
![Analyser [1]](https://static.documents.pub/doc/80x56/587356ca1a28ab280c8b7d14/analyser-1.jpg)
