Page 13 - A Practical Introduction to Optical Mineralogy

(a) llntroduction to the
' .
microscopic study of

minerals

Q
1.1 Introduction

Microscopes vary in their design, not only in their appearance but also in
the positioning and operation of the various essential components.
These components are present in all microscopes and are described
briefly below. Although dual purpose microscopes incorporating both
transmitted- and reflected-light options are now available (Fig. 1.1 ), it is
more convenient to describe the two techniques separately. More details
on the design and nature of the components can be obtained in text-
books on microscope optics.

1.2 The transmitted-light microscope

The light source
In transmitted-light studies a lamp is commonly built into the micro-
scope base (Fig. 1.2). The typical bulb used has a tungsten filament
(A source) which gives the field of view a yellowish tint. A blue filter can
be inserted immediately above the light source to change the light colour
to that of daylight (C source).
In older microscopes the light source is quite separate from the
Frontispiece Photomicrographs, taken using (a) transmitted light and
microscope and is usually contained in a hooded metal box to which can
(b) reflected light, of the same area of a polished thin section of quartzite
containing pyrite (P), sphalerite (S), muscovite (M), apatite (A) and abundant be added a blue glass screen for daylight coloured light. A small movable
quartz (Q). circular mirror, one side of which is flat and the other concave, is
The features illustrated in transmitted light are: (i) opacity- pyrite is the only attached to the base of the microscope barrel. The mirror is used to
opaque phase, sphalerite is semi-opaque, and the others are transparent; direct the light through the rock thin section on the microscope stage,
(ii) relief - very high (sphalerite, n= 2.4), moderate (apatite, n = 1.65), moderate and the flat side of the mirror should be used when a condenser is
(muscovite, n= l.60), and low (quartz, n= l.SS); (iii) cleavage- perfect in mus- present.
covite (n is the refractive index of the mineral).
The feature illustrated in reflected light is reflectance: 54% (pyrite, white- true The polariser
colour slightly yellowish white), 17 % (sphalerite, grey), 6% (apatite, dark grey),
The assumption is that light consists of electromagnetic vibrations.
5% (muscovite, dark grey), and 5% (quartz, dark grey) (reflectance is the
These vibrations move outwards in every direction from a point source
percentage of incident light reflected by the mineral).
of 'white' light, such as a microscope light. A polarising film (the polar-
Note that opaque grains, grain boundaries and cleavage traces appear black in
transmitted light, whereas pits (holes), grain boundaries and cleavage traces iser) is held within a lens system located below the stage of the micro-
appear black in reflected light. scope, and this is usually inserted into the optical path. On passing
through the polariser the light is ' polarised' and now vibrates in a single
xiv 1