Page 17 - A Practical Introduction to Optical Mineralogy

THE MICROSCOPIC STUDY OF MINERALS SYSTEMATIC DESCRIPTION OF MINERALS

objectives used. The entire lens system below the microscope stage, accessory plates are inserted at 45° to the crosswires. In some micro-
including polariser, aperture diaphragm and condenser, can often be scopes the slot may be rotatable.
racked upwards or downwards in order to optimise the quality of illumi-
nation. Some microscopes, however, do not possess a separate con- Focusing
vergent lens and, when a convergent lens is needed, the substage lens The microscope is focused either by moving the microscope stage up or
system is racked upwards until it is just below the surface of the micro- down (newer models) or by moving the upper microscope tube up or
scope stage. down (older models). Both coarse and fine adjusting knobs are present.

Stage
The microscope stage is flat and can be rotated. It is marked in degrees,
and a side vernier enables angles of rotation to be accurately measured. 1.3 Systematic description of minerals in thin section
The stage can usually be locked in place at any position. The rock thin using transmitted light
section is attached to the centre of the stage by metal spring clips.
Descriptions of transparent minerals are given in a particular manner in
Objectives Chapters 2 and 3, and the terms used are explained below. The optical
Objectives are magnifying lenses with the power of magnification
properties of each mineral include some which are determined in plane
inscribed on each lens (e.g. xs, X30). An objective of very high power polarised light, and others which are determined with crossed polars.
(e.g. x 1 00) usually requires an immersion oil between the objective lens
For most properties a low power objective is used (up to x 10).
and the thin section.
Eyepiece 1.3.1 Properties in plane polarised light
The eyepiece (or ocular) contains crosswires which can be indepen- The analyser is taken out of the optical path to give a bright image (see
dently focused by rotating its uppermost lens. Eyepieces of different Frontispiece).
magnification are available. Monocular heads are standard on student
microscopes. Binocular heads may be used and, if correctly adjusted, Colour
reduce eye fatigue. Minerals show a wide range of colour (by which we mean the natural or
'body' colour of a mineral), from colourless minerals (such as quartz and
The analyser feldspars) to coloured minerals (brown biotite, yellow staurolite and
The analyser is similar to the polariser; it is also made of polarising film
green hornblende). Colour is related to the wavelength of visible light,
but oriented in a N-S direction, i.e. at right angles to the polariser. When which ranges from violet (wavelength>.. = 0.00039 mm or 390 nm) to
the analyser is inserted into the optical train, it receives light vibrating in
red (>.. = 760 nm). White light consists of all the wavelengths between
an E-W direction from the polariser and cannot transmit this; thus the
these two extremes. With colourless minerals in thin section (e.g.
field of view is dark and the microscope is said to have crossed polars
quartz) white light passes unaffected through the mineral and none of its
(CP, XPOLS or XP). With the analyser out, the polariser only is in
wavelengths is absorbed, whereas with opaque minerals (such as
position; plane polarised light is being used and the field of view appears
metallic ores) all wavelengths are absorbed and the mineral appears
bright. black. With coloured minerals, selective absorption of wavelengths take
place and the colour seen represents a combination of wavelengths of
The Bertrand lens light transmitted by the mineral.
This lens is used to examine interference figures (see Section 1.3.2).
When it is inserted into the upper microscope tube an interference figure
Pleochroism
can be produced which fills the field of view, provided that the con- Some coloured minerals change colour between two 'extremes' when
vergent lens is also inserted into the optical path train.
the microscope stage is rotated. The two extremes in colour are each
seen twice during a complete (360°) rotation. Such a mineral is said to be
The accessory slot pleochroic, and ferro magnesian minerals such as the amphiboles, biotite
Below the analyser is a slot into which accessory plates, e.g. quartz and staurolite of the common rock-forming silicates possess this
wedge, or first order red, can be inserted. The slot is oriented so that property.
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