Spectro 186-312B, Francis 2013
LAB 6 - Identification of Biaxial Minerals Using Interference Figures
Tasks:
1. Place a flake of the phlogopite on a glass slide and examine it under high power with the
Bertrand lens. In lab 4, you were told that the interference figure of phlogopite mimicked
that of a uniaxial opitic axis figure. In reality, phlogopite is a biaxial mineral with very
small 2V (0-10o). Watch the isogyres carefully as you rotate the state. You are looking at a
BXA interference figure in which the isogyres just slightly separate as the stage rotates.
Compare the phlogopite’s BXA interference figure to that of a flake of muscovite, which
has a 2V of 30-47o.
2. Examine the BXA, BXO, OA, and ON interference figures in the old oriented thin sections.
Be careful, these slides are on their last legs. In each case, try to determine the optic sign,
and in the case of OA and BXA figures estimate the minerals’s 2V and check it against the
value listed for the mineral in a reference text.
3. The thin sections CA-n, ML-n, FL-n and SO-n are all of similar porphyritic lavas containing
two different phenocrysts minerals. One is slightly turbid, commonly twinned, and exhibits
concentric compositional zonation, while the other is clear and unzoned. For each of the
two phenocryst minerals determine:
 Optic Sign
 2V
 Birefrigence
With these optic parameters, and the assurance that these are common igneous minerals,
identify the two phenocryst minerals. Note any other properties that may have helped in
your identification.
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Spectro 186-312B, Francis 2013
Working with Biaxial Interference Figures
A. INTERFERENCE FIGURES
l) Optic Axis Section (OA)
When dealing with any unknown anisotropic mineral, one searches for grains exhibiting
lowest interference colours, ideally those which remain black or grey upon rotation under
crossed polars.
If the mineral is uniaxial, such grains should exhibit the uniaxial optic axis interference
figure, a black cross which remains intact upon rotation of the stage.
If the mineral is biaxial, however, only one black isogyre will be observed. In the four
extinction positions, this black shadow will be straight and coincide with one of the
microscope's cross-hairs. In most cases, however, this isogyre will flex upon rotation of the
stage in a sense that is opposed to the sense of rotation. The isogyre will show greatest
curvature at 45o rotation of the stage from an extinction position. The degree of curvature
of the isogyre in this position is inversely proportional to the mineral's 2V, and is convex
towards the BXA direction ("OA view" in attached Figure). If coloured interference rings
are present, they will be concentric about the point of emergent of the optic axis. If the
interference figure is perfectly centred, this point will be in the centre of the field of view. If
not, the point will rotate about the centre.
Obtain oriented OA thin sections and practice imaging Optic Axis interference figures.
Be careful not to confuse an off-centred uniaxial figure
with an OA isogyre of a biaxial mineral with low 2V.
2) Acute Bisectrix (BXA)
Grains with low interference colours under crossed polars may give interference figures
which contain both optic axes. With the Bertrand lens inserted, two parabolic isogyres
will be observed which move upon rotation of the stage. 360 of rotation will yield four
positions in which the two parabolas unite to form a cross similar to a uniaxial interference
figure, with arms parallel to the microscope's cross hairs. One of the arms of the cross,
however, is thinner than the other. This arm contains the minerals's two optic axes which
are positioned on opposite sides of the centre of the cross ("BXA" view in attached Figure).
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Spectro 186-312B, Francis 2013
The parabolic shadows will show maximum separation at 45o of rotation of the stage from
the four extinction positions. The magnitude of the separation is proportional to the 2V of
the mineral. If colour rings are present, they will exhibit a double bull's eye pattern with
each eye concentric about one of the optic axes.
3) Obtuse Bisectrix (BXO)
Grains showing higher interference colours may also give two isogyres. This interference
figure closely resembles that of a BXA. The difference is that the parabolic isogyres will
separate and leave the field of view upon rotation of the stage from the extinction position.
However, for minerals with high 2V, the isogyres will leave the field of view upon rotation
of the stage for both BXA and BXO sections. In this case, one usually observes that less
rotation from an extinction position is required for the isogyres to leave the field of view in
a BXO section that a BXA section ("BXO" view).
As 2V approaches 90o it becomes increasingly difficult to differentiate between BXO and
BXA interference figures. At 2V = 90o , of course, there is no distinction.
4) Optic Normal Sections (ON)
Grains exhibiting the highest interference colours have their optic axes in the plane of the
microscope stage. Such grains have interference figures that are usually indistinguishable
from uniaxial flash figures. In the biaxial case, however, the isogyres leave the field of
view in the direction of the BXA upon rotation of the stage from the extinction position.
Optic normal (OA) figures are distinguished from those of BXA and BXO sections by the
more diffuse character of the isogyre cross at the four extinction positions. In addition, the
cross disappears upon less than a 5o rotation of the stage.
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Spectro 186-312B, Francis 2013
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Spectro 186-312B, Francis 2013
B. MEASUREMENTS USING INTERFERENCE FIGURES
In the following techniques, remember that "perfectly centred" interference figures are such
that for:
OA
BXA
BXO
ON
an optic axis is vertical, ie. perpendicular to the microscope stage
Either N or N is vertical
Either N or N is vertical
Nß is vertical
This condition is recognized by observing that at the four extinction positions, the isogyre
shadows coincide with the cross hairs of the microscope. Any departure from this condition
indicates an off-centred interference figure. We will deal with this aspect in more detail in
the next lab, but for now remember that the world is typically not perfect. Most figures will
be slightly off centred. You find the best one you can
1) Estimation of 2V
a. BXA Interference Figure
The separation of the parabolic isogyres at 45o of rotation of the stage from the extinction
position is proportional to a mineral's 2V. The width of the view corresponds to a 2V of
approximately 50-60o. For 2V's higher than 60o, optic axis figures must be used to estimate
2V.
b. Optic Axis Figure
The curvature of the single isogyre at 45o rotation of the stage from an extinction position is
inversely proportional to 2V (see first Figure). Practice with the oriented thin sections of
known minerals to train your eye to accurately estimate 2V using OA interference figures.
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Spectro 186-312B, Francis 2013
2) Determination of Optic Sign
a. BXA Interference Figures
Use this method only for minerals with 2V's less than 60o ie, those whose isogyres do not
leave the field of view upon rotation of the microscope stage.
Rotate the figure 45o from an extinction position and use either your quartz plate (figures
with few colour rings) or a quartz wedge (figures with numerous colour rings) as indicated
in the figures below. Practice with the oriented thin sections.
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Spectro 186-312B, Francis 2013
b. OA Optic Axis Figures
Use this test only for interference figures in which the single isogyre never leaves the field
of view upon rotation of the microscope stage and flexes or bends in the direction opposite
to stage rotation.
Rotate the interference figure 45o from an extinction position and proceed as illustrated in
the figures below. Practice with the oriented thin sections.
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