Page 204 - A Practical Introduction to Optical Mineralogy

TRANSMITTED-LIGHT CRYSTALLOGRAPHY INTERFERENCE FIGURES
plate is fast, then subtraction (or compensation) will occur and the Note that n ~ is always at right angles to the OAP. Light entering the
retardation of the plate is subtracted from the retardation of the crystal section is resolved into two components along each major semi-
mineral. For example a mineral showing second order green will be axis as follows:
changed to first order grey.
( 4) The fast component identified from step 3 is brought parallel to the Bx.
direction along which an accessory plate can be inserted. The
Bertrand lens may be inserted. A length slow quartz wedge is I
inserted until a black band is obtained. In fact the colour may oc Bx,
I
appear more like dark grey than black and represents the begin- oc-
n~
ning of Newton's Scale. The black band marks the position where
the retardation of the quartz wedge exactly compensates that of
the mineral. The quartz wedge is then slowly withdrawn and the
orders of colours counted until the original interference colour of
the mineral section is restored. Alternatively the thin section may
be removed and the order assessed by viewing the order of colour
of the quartz wedge. I
oc-
n~ I
oc-
Bx"
4. 7.4 Abnormal or anomalous interference colours
n~
These are obtained occasionally because a particular mineral is isotropic
for a particular wavelength (or colour) ; this wavelength is removed and
The crystal is isotropic along OA; along the optic axis light moves with a
the complementary colour appears. Thus, for example, melilite and
velocity proportional to 1/n~ . Note that, if the mineral is +ve, Bxa
chlorite have yellow removed and appear dark blue.
corresponds ton , and Bx 0 ton., and vice versa if the mineral is -ve.
When a substage convergent lens is present in the optical train, as is
usually the case, it has the effect of producing light rays which enter the
4.8 Interference figures crystal fragment at every angle from nearly horizontal to vertical:
4.8.1 Biaxial minerals
A mineral section with a vertical optic axial plane is first considered. If roo~
Bxa is vertical, that is parallel to the microscopic axis, the situation is as len~cte:Se:Ut
follows:
light from
polariser
OA OA
With this lens in position the plan view is as follows:

I
I
---+ 1 oc- oc-
I
oc-
n~
n ~
n~
____ o~-+l .?t---~+----OAP
oc- ex- oc- oc-
l Bx" Bx,, Bx 0 Bx,,
oc- oc _I_ oc _ I_
n~ n ~ n ~ n~
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