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TECHNICAL PAPER

properties of minerals leading to their identification are This hypidiomorphic, and tiled texture is the characteristic
presented in Table 4 [1, 7-11,15-20] . These petrographic features will texture of granite, and in a broad way it is the common texture
lend a hand in characterizing rocks, and rock forming minerals in many of igneous siliceous rocks. The CPL, and PPL views
pertaining to aggregates using scanning electron microscopy of these mineral assemblages elucidate the mineral habit of
(SEM) or thin section petrography. Typical crystal habits, and quartz which is interstitial and vermicular. The peculiar quartz
cleavage, or fracture may aid in studying the morphological, habit in thin sections can be correlated with the sequence of
and topographical features of fracture surfaces of different their crystallization during the formation of rock. The feldspars
types of aggregates. Optical properties such as indicative crystallize in an early stage; whereas, the quartz occupies the
mean refractive indices (RI) and order of birefringence are interstitial voids in a later stage [7-9] . Figure 3 (k and l) provides a
also furnished. To characterize aggregates, the knowledge of typical thin section representing possible mineral assemblages
their mineral assemblage is essential. In this context, colour in granite [22] . Apart from vermicular quartz, potash, plagioclase,
interferences of minerals in thin section of (standard 30 µm and orthoclase feldspars, this image also exhibits microcline
sections) are included in Table 4. Additionally, Table 4 is feldspar with twinning lamella. Another major mineral present
supplemented by X-ray diffraction (XRD) to characterize the in the assemblage is biotite with light brown colour in PPL.
crystalline phases present in the aggregates. The detailing Hornblende is characterized by its cleavage rich dark brown
includes two major peaks (2θ, with Cu Kα radiation) with colour. Euhedral or subhedral magnetite is also present in the
their intensities and corresponding d-spacing (Å) analyzed system with isotropic mineral behavior (very dark, or black in CPL
using PANalytical’s X’Pert highscore plus software and other and PPL).
references [19,20] .
The colour, and textural patterns of the aggregates are mostly
4.1 Aggregate minerals in thin section governed by their mineral composition, and systematic, or
less systematic mineral assemblage. For example, granite
Some common minerals in aggregates are explained in this is a natural rock classified under the igneous rock, and is a
section with the help of thin section petrography. These
microscopic images are obtained on standard thin sections of conglomerate of minerals with hypidiomorphic texture in thin
30 µm thickness using plane polarized light (PPL) and crossed sections. The colour of granite varies in a spectrum of colours
polarized light (CPL). The field under the microscope measures from black to white to pink. Usually, they form as pink, white,
4 mm × 3 mm (length × width). Under cross polarized light, the and different shades of gray, and black. The colour changes
minerals can be identified with their interference colours based are attributed to the mineralogy of granite as a result of the
on refractive index and birefringence. The colour interference cooling of molten magma. The visual appearance or colour of
starts from the first order to the last based on birefringence rock forming minerals is presented in Table 3. The colour of
classification such as very low, low, high, and very high. The granite varies based on the relative proportions and assembling
interference colours ranges from white, or gray colour to dark patterns of these minerals during the crystallization period. If
through yellow, brown, blue, and orange, or red shades. The the granite is majorly composed of potash feldspars, it attains
plane polarized light enables the identification of mafic, dark a salmon pink colour. A granite with other feldspars appears to
coloured, and isotropic minerals in thin sections [9-11] . be off white or dull/gray white with quarzitic white appearance.
Presence of other accessory minerals artistically raffle the
Figure 3 (a and b) pictorially depicts magnetite (Fe 3 O 4 ) in a patterns in granite. Figures 4 [(a and b, c and d)] demonstrate
thin section. This dark coloured mineral is typified by a high thin section petrography of granite with more amphibole
interference colour and exhibits opacity with subhedral, or mineral hornblende, micaceous mineral biotite, and euhedral
euhedral habit in thin sections under polarized light. In the same magnetite. The distribution may be uniform [Figure 4 (a and b)]
way, another iron oxide mineral hematite (Fe 2 O 3 ) exhibits a deep or discrete single massive mafic mineral [Figure 4 (c and
red colour interference in CPL and nearly isotropic behavior in d)]. These mafic textures turn the colour of granite to dark.
PPL [Figure 3 (c and d)]. The petrographic view of sedimentary Figure 4 [(e and f, g and h, I, j, k and l)] describes in what way
rock laterite exemplifies a blended view of hematite, and the accessory (or impurity) minerals draw the textural patterns
magnetite [Figure 3 (e and f)]. However, laterite also includes in aggregates. Biotite and magnetite provide an inclined black
silica, and alumina (or bauxite) as well in its composition (Table streak pattern to the granite in Figures 4 [(e and f and g and
3). Feldspars, and quartz are some common minerals present h)]. Along with feldspar, and quartz, biotite, and magnetite
in the aggregates. Figure 3 [(g and h) and (i and j)] provides are evenly distributed in the hypidiomorphic tiled texture,
petrography of typical Indian granites which essentially contains and eventually provide a dark dot pattern in white (Figure 4i).
60 % feldspar, 35 % quartz, and the remaining other accessory Figures 4j to 4l explicate the presence of iron bearing minerals
minerals [21] . The yellowish white indicates quartz whereas the (magnetite, biotite and hornblende) in a very dark coloured
plagioclase and potash feldspars exhibit light gray. Orthoclase granite. Additionally, the larger proportions of iron bearing
feldspars provide a dark grey interference in thin sections. minerals impart high density to the aggregate as well.

12 THE INDIAN CONCRETE JOURNAL | AUGUST 2022