2Petrological Microscope The use of thePetrological MicroscopeThe use of the microscope allows us to examine rocks in much more detail. For example, it lets us :-examine fine-grained rocksexamine textures of rocksdistinguish between minerals that are otherwisedifficult to identify in hand-specimen (e.g. thefeldspars)
3A petrological microscope The petrological microscopediffers from an ordinarymicroscope in two ways:it uses polarised lightand the stage rotatesThere are two sheets of polaroid:the one below the stage of themicroscope is the polariser, theother, above the stage, is theanalyser. The analyser can bemoved in and out.Most rocks cut and ground toa thickness of 0.03mm becometransparent.eyepiecefocusfine focusanalyserlensrotating stagepolariserlight source
4Preparing thin sections Rock specimens are collected in the field, then cut into smallthin slabs. These are glued on to glass slides and grounddown to 0.03mm thickness. At this thickness all rocksbecome transparent. Only a few minerals, mainly oreminerals, remain opaque, i.e. stay black under PPL.If the sections are too thick, the polarisation colours areaffected. Quartz is used to check thickness for this reason –see the next slide
5mineral may show any colour up to a maximum, reading from the left. amphibolepyroxenemuscovitefeldsparquartzbiotiteolivineRead along diagonal to top for mineral namecalciteRead along 0.03mm line to the highest order colour seen in the mineralThe colours appear in a series of repeated rainbows across the chart and amineral may show any colour up to a maximum, reading from the left.
6Identifying 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).
7Mineral properties under PPL colour (natural colour)transparency (clear, cloudy or opaque)relief (high or low)crystal or fragment shapecleavagefracturepleochroism (colour change when stage is rotated)
8RELIEFplagioclasePPLolivineNote how the olivine with its high relief stands out from the surrounding low relief plagioclase
91st set run parallel to line CLEAVAGEamphibolePPL2nd set of cleavageTwo sets of cleavage are seen in this amphibole crystal; note the 120o angle between the cleavages
10FRACTUREolivinePPLThe olivine here shows uneven fractures which appear dark grey in the crystal
11amphiboleCOLOURbiotitePPLThe biotite shows its distinct brown shades under PPL against the clear colourless quartz and feldspar
12biotitePPLrotated 90oPLEOCHROISMTwo views under PPL showing colour change in biotite on rotating the stage.
13Mineral properties under XPL interference colours(under XPL the colours seen are not the natural colours of the mineral butthose caused by the interference of two refracted beams of light passingthrough an anisotropic mineral ; they are called interference colours)extinction angle(as the stage is rotated, each anisotropic mineral goes extinct every 90o; incases where there is cleavage in the mineral it is possible to measure theangle 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)
14Interference colours white/grey/black in quartzamphibolecalcitewhite/grey/blackinquartz, microcline and plagioclasemuch brighter coloursofferro-magnesian minerals including amphibole, pyroxene, olivinepearly grey shadesofcalcite