Geology Reference
In-Depth Information
The method of combining scattered light (also called
diffused light) with cross‐polarized light brings out the
substructure of the grains including the trapped air and
brine inclusions (as bright objects). An illustration is
given in Figure 6.20 for a thin section prepared soon after
sampling the ice core from Resolute Bay, Canadian cen-
tral Arctic, in May 1992. It shows two micrographs of the
same horizontal thin section of FY Ice. The ice in the bay
was directionally solidified (DS) columnar‐grained, S3
type with the long axis of the grains parallel to the growth
direction in the vertical plane. The
c
axis of the grains
(and hence the subgrains) was oriented in the horizontal
plane and tended to be parallel to the direction of the
water current in the bay caused by tidal actions [
Sinha and
Zhan
, 1996]. The brine pockets shown in Figure 6.20 are
linearly arranged following the direction of the major axis
of each grain. In some cases, for example, the central dark
grain in Figure 6.20a, rotation of the specimen slightly
may alleviate the problems of identifying the inclusions.
Nonetheless, Figure 6.20b illustrates clearly the advan-
tages of combining scattered and polarized lights.
6.3.4. Circularly Polarized Light and Rapid
Crystallographic Analysis
If a quarter‐wave (
λ
/4) retardation plate is introduced
in the path of a linearly polarized light (e.g., after the
polarizer in Figure 6.3), a circularly polarized beam is
obtained. If a thin section is placed in a circularly polar-
ized beam and viewed through the analyzer (i.e., another
linear polarizer), an image of twofold symmetry is
obtained. This is, therefore, simpler than a standard con-
oscopic beam with linearly polarized light, producing an
image exhibiting fourfold symmetry. This arrangement
reduces the number of measurements necessary for deter-
mining the
c
axis.
Jaccard
[1969] adopted the principle of
conoscopic imagery in conjunction with circularly polar-
ized light beam for designing an automated tomograph.
An orthoscopic technique incorporating a rotating thin
section stage tilted with respect to the direction of light
propagation was presented by
Suzuki
[1973].
Lile
[1977]
presented a general mathematical treatment of optical
Brine channel
Figure 6.19
Vertical thin section of the top 130 mm of FY
ice from Wellington Channel, Canadian Arctic, acquired in
May 1991, photographed using only the scattered light (left)
and between crossed polarizers (right). Note how the scat-
tered‐light photography brings out the brine pockets and
the brine drainage channels that are completely obscured
in the cross‐polarized light photograph (N. K. SInha and M.
Shokr, unpublished).
(a)
(b)
Figure 6.20
Photographs of 100 mm diameter, DMT horizontal thin section of S3 ice at a depth of 0.60 m, sam-
pled from Resolute Bay, 15 May, 1992. (a) Photograph using cross‐polarized light exhibiting structure with pre-
dominant grain and subgrain orientation; (b) Photograph using combination of polarized light and scattered light
showing the orientations of brine pockets (white spots) trapped predominantly along subgrain boundaries.
(M. Shokr and N. K. Sinha, unpublished). (For color detail, please see color plate section).
