Page 209 - A Practical Introduction to Optical Mineralogy

TRANSMITTED-LIGHT CRYSTALLOGRAPHY PLEOCHROIC SCHEME
(nearly 90°). The field of view of the microscope is such that for a biaxial 4.8.4 Uniaxial minerals
Bxa interference figure with a 2V of almost 40°, both optic axes can just A uniaxial interference figure can be thought of as a special case of the
be seen at the edge of the field of view. If the 2V is larger (greater than
biaxial figure where 2 V = oo. When this occurs both optic axes become
40°), and especially when 2V nears 90°, a section cut normal to Bxa is not
coincidental and the isochromatic curves appear as circles around the
suitable. This is because it is very difficult with a 2V greater than 70° to
single optic axis.
know whether the mineral section under investigation is cut perpendicu-
The isogyres also coalesce and appear as a black cross, the arms of
lar to Bxa or Bx 0 •
which are parallel to the polariser and analyser.
In this case a single optic axis interference figure must be obtained.
To determine the optic sign a centred cross should be used, but a
This is done by finding an isotropic section which will have an optic axis
slightly off-centred uniaxial interference figure may be rotated until the
vertical. The interference figure will show one optic axis with an isogyre
black cross is placed in the lower left hand quadrant of the field of view.
passing through it.
The length slow first order red plate is then inserted towards the cross,
and the colour in the NE quadrant of the cross noted (Fig. 4.21). Blue
signifies that the mineral is positive. 'Addition' of retardations has
4.8.2 Sign determination for biaxial minerals occurred since the length slow plate has been inserted parallel to the
extraordinary ray direction. In uniaxial positive crystals ne > n 0 and
The interference figure is rotated into the 45° position (Fig. 4.19). Either
therefore line is less than 1/n 0 • Yellow signifies that the mineral is
a single optic axis figure (for a large 2V) or an acute bisectrix figure (for a
small 2V) is used. negative and that 'subtraction' has occurred .
. The first order red plate (which we shall assume is length slow) is If a uniaxial mineral grain in the thin section is too small to allow an
interference figure to be obtained, it should be noted that uniaxial
mserted along the OAP (see lower parts of Figs 4.20a, b, c & d). The
positive minerals are always length slow parallel to the prism zone. A
Sign of the component proportional to 1/Bxa is obtained by observing
the colour which appears on the concave side of the isogyre. Blue means prismatic crystal should be observed under crossed polars with a low or
moderate power objective. The mineral is rotated until the (length slow)
that the mineral is positive ( + ve). Addition has occurred since the
first order red plate can be inserted along the prism zone of the crystal. If
length slow plate has been inserted along the slow component, which is
proportional to 1/n y (i.e. n y = Bx 3 ). A yellow colour means that the 'addition' occurs and the interference colour of the mineral is increased
to a higher order of colour then the crystal is positive. If 'subtraction'
mineral is negative (- ve) with Bxa = n •. (Note that sign determina-
tions are always made in the 45° position, with the first order red plate occurs the crystal is negative.
mserted along the OAP to find the relative velocity ofthe acute bisectrix In some cases the accessory plate may be length fast, and not length
component and hence obtain the sign.) slow as described, and also the direction of insertion of the accessory
plate may be NW to SE (and not NE to SW as described). The upper
parts of Figure 4.20 explain sign determinations using a length slow or
length fast first order red accessory plate which may be inserted either
4.8.3 Flash figures
NE to SW or NW to SE into the field of view.
Sections cut parallel to the OAP (i.e. the OAP is in the plane of the
mineral section) are perpendicular to n p, and an interference figure 4.8.5 Isotropic minerals
called a flash figure is seen. It is possible to obtain the sign of a mineral
Isotropic crystals do not show interference figures since they are iso-
from. such a figure, but this is not recommended. A flash figure is
tropic to light in all directions. This fact enables us to distinguish
obtamed from a crystal fragment containingn. and n y in the plane of the
section. isotropic sections of non-cubic minerals from those of cubic minerals.
towards Bxa
4.9 Pleochroic scheme
Figure 4.19 4.9.1 Uniaxial minerals
Biaxial
interference The pleochroic scheme for a uniaxial coloured mineral (e.g. tourmaline)
figure. can be obtained by first finding a basal section to get the colour for n 0 ,
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