Petrological Microscope The Petrological Microscope

Petrological Microscope 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)

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. eyepiece focus fine focus analyser lens rotating stage polariser light source

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

mineral may show any colour up to a maximum, reading from the left. amphibole pyroxene muscovite feldspar quartz biotite olivine Read along diagonal to top for mineral name calcite Read along 0.03mm line to the highest order colour seen in the mineral 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.

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).

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)

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

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

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

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

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

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)

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