The Petrological Microscope

1. The Petrological Microscope

2. The use of the Petrological Microscope The use of the microscope allows us to examine rocks in much more detail. For example, it lets us :- • examine fine-grained rocks • examine textures of rocks • distinguish between minerals that are otherwise difficult to identify in hand-specimen (e.g. the feldspars)

3. eyepiece focus A petrological microscope The petrological microscope differs from an ordinary microscope in two ways: • it uses polarised light • and the stage rotates There are two sheets of polaroid: the one below the stage of the microscope is the polariser, the other, above the stage, is the analyser. The analyser can be moved in and out. Most rocks cut and ground to a thickness of 0.03mm become transparent. fine focus analyser lens rotating stage polariser light source

4. Preparing thin sections Rock specimens are collected in the field, then cut into small thin slabs. These are glued on to glass slides and ground down to 0.03mm thickness. At this thickness all rocks become transparent. Only a few minerals, mainly ore minerals, remain opaque, i.e. stay black under PPL. If the sections are too thick, the polarisation colours are affected. Quartz is used to check thickness for this reason – see the next slide

5. amphibole pyroxene muscovite feldspar biotite olivine quartz The colours appear in a series of repeated rainbows across the chart and a mineral may show any colour up to a maximum, reading from the left. Read along diagonal to top for mineral name calcite Read along 0.03mm line to the highest order colour seen in the mineral

6. Identifying MINERALS in thin section • When a slide is examined under the microscope, it is important to identify any mineral properties under plane polarised light (PPL) first (analyser out); then proceed to crossed polars (XPL) where the two polaroid sheets are at right angles to each other (analyser in).

7. Mineral properties under PPL • colour (natural colour) • transparency (clear, cloudy or opaque) • relief (high or low) • crystal or fragment shape • cleavage • fracture • pleochroism (colour change when stage is rotated)

8. Note how the olivine with its high relief stands out from the surrounding low relief plagioclase RELIEF plagioclase PPL olivine

9. 1st set run parallel to line Two sets of cleavage are seen in this amphibole crystal; note the 120o angle between the cleavages CLEAVAGE amphibole PPL 2nd set of cleavage

10. The olivine here shows uneven fractures which appear dark grey in the crystal FRACTURE olivine PPL

11. amphibole The biotite shows its distinct brown shades under PPL against the clear colourless quartz and feldspar COLOUR biotite PPL

12. PLEOCHROISM Two views under PPL showing colour change in biotite on rotating the stage. biotite PPL rotated 90o

13. Mineral properties under XPL • interference colours (under XPL the colours seen are not the natural colours of the mineral but those caused by the interference of two refracted beams of light passing through an anisotropic mineral ; they are called interference colours) • extinction angle (as the stage is rotated, each anisotropic mineral goes extinct every 90o; in cases where there is cleavage in the mineral it is possible to measure the angle of extinction relative to the crosswires) • twinning (may be seen in coloured minerals under PPL, but most obvious under XPL, especially with regard to the feldspars)

14. Interference colours quartz amphibole calcite white/grey/black in quartz, microcline and plagioclase muchbrightercolours of ferro-magnesian minerals including amphibole, pyroxene, olivine pearly grey shades of calcite