Page 202 - A Practical Introduction to Optical Mineralogy

FAST AND SLOW COMPONENTS
TRANSMITTED-LIGHT CRYSTALLOGRAPHY
is, one component (a: lin. ) is faster than the other (a: 11np), or one
Figure4.15 ()),
Quartz wedge ~-violet component is fast (a: lin. ) and the other slow (a: l lnp)·
spectra. 4 10 In any anisotropic crystal, light is polarised into two components, one
(~5o) of which will be fast and the other slow. For example, in a positive
Ol\ c\...,._.,...~
uniaxial indicatrix, the vertical semi-axis ne is greater in value than n 0 ,
and therefore lin. will be less than lln 0 • Light entering a positive
indigo uniaxial crystal at right angles to n. will be split into two components,
445
one pf which has a velocity proportional to line (slow) and the other with
a velocity proportional to 1ln 0 (fast). Sincen 0 is coincident with the c axis
in uniaxial crystals, a section at right angles to thee axis is a prism section,
and a prism section (of a positive uniaxial, mineral) is length slow. (A
blue
:i. 480 uniaxial negative mineral will be length fast on a prism section).
~ ("-~6)
o;,
., 4. 7.2 Quartz wedge and first order red accessory plate
"
Q)
~ green These microscope accessories are length slow. The quartz wedge is cut
:;: 535
"§ (1'J6) with its length parallel to the prism zone. If the first order red plate (also
.... called gypsum plate or sensitive tint plate) is examined under a micro-
:l scope, with both analyser and polariser in position, the colour transmit-
0
0 ted by the plate is red of the first order of Newton's Scale, corresponding
U yellow
580 to a retardation of 560 nm.
( 5? 3c-)
4. 7.3 Determination of order of colour
orange
620 To determine the maximum order of colour displayed by a mineral, a
(9-,o) section showing maximum birefringence giving the highest order of
interference colour is needed.
In biaxial crystals such a section has n . and n , in its plane, and the
red section is at right angles to the semi-axis np. Light will be polarised into
710 two components, one a: lin. (fast) and the other a: lin, (slow).
v;~v) ~=-----~~~~~----~~~~---4
In a uniaxial crystal the section required is a prism section. To deter-
mine, in any mineral grain, which component is fast and which is slow,
the procedure is as follows:
4. 7 Fast and slow components, and order determination
(1) Rotate the mineral being examined into extinction and make a
sketch of the mineral in this position.
4. 7.1 Fast and slow components (2) The two components will then be parallel to the vibration direc-
In biaxial crystals the refractive indices are n ,, np and n. with n, > np tions of the polariser and analyser.
> n •. Refractive index is inversely proportional to the velocity of light (3) Rotate the section through 45° and insert a first order red plate
for a particular wavelength: therefore l in , < llnp < lin •. For example, which will be parallel to one of the components. If this accessory
along the vertical semi-axis of the biaxial indicatrix shown in Figure 4.5 plate is length slow, then addition (or increase in retardation) will
the two components into which the light is polarised have velocities occur if the component is also slow, i.e. the retardation of the plate
proportional to llnp and l in •. Because lin. is greater than lln p, the will be added to the retardation of the mineral. For example a
component with a velocity proportional to l in. will move further than mineral having second order green interference colours will be
the component with a velocity proportional to llnp in a given timet. That changed to third order green. If the component lying parallel to the
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