1. Mineral and Rock Thin Sections
GCSE Geology
Mineral and Rock Thin Sections
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2. Guide to Mineral and Rock Thin Sections

3. Preparation of Rock Thin Sections
Rock sample is cut 2 mm thick using a diamond tipped saw
The 2 mm thick slice is then glued to a microscope slide
The 2 mm thick layer of rock is ground down to 30 microns

4. Photomicrographs of rocks have been used in the GCSE Geology Online Examination since May 2012
The same mineral may appear a different colour in a different part of the image and from one image to another
Colours of minerals seen in the photomicrographs
will also vary from those seen in hand specimen
i.e. the colours given on the Mineral Data Sheet

5. Will be used in questions on interpretation of rock textures
Igneous Rocks
Crystal Size (Fine <1mm, Medium 1-3mm, Coarse >3mm)
Porphyritic (Phenocrysts and Groundmass)
Equicrystalline
Random Crystal Orientation

6. 5 Will be used in questions on interpretation of rock textures
Sedimentary Rocks
Grain Size (Fine <1/16mm, Medium 1/16th-2mm, Coarse >2mm)
Grain Shape (Well Rounded to Very Angular)
Sorting (Well Sorted to Poorly Sorted) 5 mm

7. Will be used in questions on interpretation of rock textures
Metamorphic Rocks
The measurement of Crystal Size
Random Crystal Orientation (Contact Metamorphism)
Crystals Aligned or Foliated (Regional Metamorphism)
(Slaty Cleavage, Schistose Texture)

8. Black and white stripes in feldspar Black , grey and white quartz
5 mm
Brown biotite mica
Granite

9. Basalt 0.5 mm Black and white stripes of feldspar
Blue, green, brown and purple augite
Basalt

10. Porphyritic Andesite Feldspar phenocrysts 3mm – 5mm in diameter
Groundmass fine grained < 0.5mm
Small amounts of blue, brown and purple augite
Porphyritic Andesite

11. Black and white stripes of feldspar
Bright blue, green, orange and purple are olivine crystals between 3mm and 5mm
Peridotite

12. Black , grey and white quartz
2 mm
Black , grey and white quartz
Sandstone

13. Black Shale Composed mainly of clay minerals such as kaolinite
Darker layers imply a high organic (carbon) content
Laminations evident which are <0.1 mm thick
Black Shale

14. Spherical ooliths made of concentric shells of calcite
Formed by the evaporation of seawater, calcite is precipitated from solution
Each oolith has a nucleus of a shell fragment or small quartz grain
Oolitic Limestone

15. 1.0 mm
Cleavage traces intersecting at 60°/120° in calcite crystals
Many crystals meet together at triple point junctions of 120°
Interlocking black, grey, pink and white calcite crystals formed by recrystallisation of limestone by contact metamorphism
Marble

16. Many crystals meet together at triple point junctions of 120°
1.0 mm
Many crystals meet together at triple point junctions of 120°
Interlocking black, grey and white quartz crystals formed by recrystallisation of sandstone by contact metamorphism
Metaquartzite

17. 0.5 mm
Black, grey, white and brown mica crystals showing foliation or slaty cleavage
Slate

18. Schist Black, grey and white Quartz
Red, yellow and purple mica showing foliation or schistose texture
Black garnet
8 mm
Schist

19. Blue, green and grey mica showing foliation or schistose texture
5 mm
Black garnet
Schist

20. The Electron Microprobe (EMP)
Analytical tool used non-destructively to analyse the chemical composition of minerals, rocks and meteorites
Sample bombarded with an electron beam emitting X-rays at wavelengths characteristic to elements being analysed

21. The Electron Microprobe (EMP)
EDS scan of a chrome-bearing augite. Scan time 5 seconds
Enables the abundances of elements present to be calculated in parts per million (ppm) in each rock or mineral sample

22. The Scanning Electron Microscope (SEM)
High resolution scanning electron microscopy contributes to mineral identification and textural analysis
SEM machines can magnify up to X 2 million compared with x 2000 of standard light microscopes

23. The Scanning Electron Microscope (SEM)
SEM image of Nashite a new mineral discovered in abandoned uranium mines of Utah and Colorado, USA and named after Professor Barbara Nash of Utah University in 2013 (note scale bar of 40 microns)