The

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 microscopediffers 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.

lens

eyepiece

focus

light source

analyser

polariser

rotating stage

fine focus

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

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.

quar

tz

feld

spar

calcite

oliv

ine

amph

ibol

e

pyro

xene

biot

ite

mus

covi

te

Read along 0.03mm line to the highest order colour seen in the mineral

Read along diagonal to top for mineral name

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)

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

RELIEF

PPL

plagioclase

olivine

Two sets of cleavage are seen in this amphibole crystal; note the 120o angle between the cleavages

CLEAVAGE

PPL

amphibole

1st set run parallel to

line

2nd set of cleavage

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

FRACTURE

PPL

olivine

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

COLOUR

PPL

biotite

amphibole

PLEOCHROISM

Two views under PPL showing colour change in biotite on

rotating the stage.

PPL

biotite

rotated 90o

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, microcline and plagioclase

much brighter colours

of

ferro-magnesian minerals including amphibole,

pyroxene, olivine

pearly grey shades

of

calcite

quartz amphibole calcite