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MINERALOGY OPTICAL STUDIES Microscopic studies have helped in identifying many minerals. Many are hitherto reported all over the world, some minerals are observed for the first time in fly ashes. They are identified by microscopic and X-ray data. Few are confirmed solely based on X-ray analysis. The physical and mineralogical characters of the produced fly ash is greatly dependent on the inorganic constituents of the feed coal, combustion character inside the furnace and rate of cooling. The mineralogy of fly ash can also play an important role in its behaviour and utilization as a replacement material of cement. In this chapter a detailed account on the mineralogy of the fly ashes of Visakhapatnam and Kothagudem are described. Individual crystalline phases observed in the ash samples provided some apparent relationship between chemical composition and crystalline phase formation. The major mineral phases viz., quartz, mullite, orthoclase, gypsum, pyrite, arsenopyrite, sillimanite, ettringite, tridymite, lepidolite and spinels etc., and also minor 76 recrystallized minerals belonging to zeolite group are identified through microscopic and X-ray studies. High zirconium concentration in the ash samples leads to identify stable oxide of zirconium-baddeleyite for the first time. All major mineral phases are confirmed optically as well as through X-ray data. The mineralogical composition (Vol.%) including the total major and minor crystalline components as well as glass content is given in Table No.4.1. The minerals orthoclase, tridymite, mullite, gypsum, heamatite, magnetitite, kaolinite are the original minerals present or phases developed from the physico-chemical transformation of the inorganic matter associated with coal during the combustion. Quartz is observed in bulk samples and size fractions of both Visakhapatnam and Kothagudem fly ashes. Quartz reflections have shown minor intensity variations in their X-ray patterns. This may be due to varying quartz content in the source coal. Presence of quartz is responsible for abrasive natureof ash of Indian coals (Mukherjee et. al., 1983). The other abrasive minerals, iron pyrites, siderite and zircon etc., are less in amount when compared to low – quartz. 77 Mullite is observed in all the size fractions and bulk samples of the studied ashes. Mullite and sillimanite may appear as an intermediate phase and may be derived from kaolinite (Tripathy and Sahu, 1995). According to Diamond (1984), mullite was not found in ashes whose A12O3 + SiO2 contents were less than 60% and whose A12O3 was less than about 17%. In the studied fly ashes, A12O3 + SiO2 percentage ranges between 89% and 92% with 28% to 35% A12O3. This high A12O3content may be responsible for abundance of mullite in the studied fly ashes. Table: 4.1 MEAN MINERAL/GLASS PHASES OF VISAKHAPATNAM AND KOTHAGUDEM FLY ASHES Mineral Name Visakhapatnam Fly Ash (%) Kothagudem Fly Ash (%) 53.86 54.29 Quartz 7.34 6.92 Mullite 10.51 9.64 Magnetite 6.12 5.59 Feldspar 1.61 1.58 Heamatite 1.25 1.22 Pyrite 0.61 0.52 Spinels 3.12 3.25 Glass Content 78 Sillumanite 1.25 2.61 Gypsum 1.01 1.03 Kaolinite 6.61 7.25 Zeolites (Wairakite, Laumontite Chabazite, Edingtonite etc) 5.15 4.75 Baddeleyite 0.81 0.62 Graphite 0.12 0.11 Others 0.61 0.62 100.00 100.00 With the moderate CaO content in the studied fly ashes, it is unusual to observe anhydrite (CaSO4). The availability of CaSO4 in these ashes promotes the self pozzolanic reactions such as ettringite (Ca6A12(SO4)(OH)12 25 H2O) formation which is responsible for the stored ash to set-up, upon contact with water. Mineral ettringite was identified in both the studied fly ashes through X-ray studies. Moderate X-ray intensity reflections of Sodalite are observed in the coarser fraction (0.090mm) of Kothagudem fly ash sample. 79 Sodalite is generally observed as a major devitrification product along with heamatile, pyroxene and plagioclase phases (Diamond, 1984). The mechanism of sodalite phase formation may have the reaction of Na and Ca sulphates with the aluminosilicate glass. Xray study of magnetic separated fraction of studied fly ashes reveals presence of heamatite and iron spinels (ferrite spinel). Certain trace elements that were partitioned into pyrite in the coal may also remain partitioned in the ferrite spinel phase in fly ash (Lauf, 1981). Opaque spheres of magnetite, translucent brownish black spheres with finer inclusions of iron oxides, internal recrystallites of mullite and transparent spheres of low relief alkaline silicates are commonly observed during optical studies of both Visakhapatnam and Kothagudem fly ashes (Plates). X-ray patterns of bottom ashes of both the studied areas indicate that graphite is present with moderate intensities. Graphite peaks are partially attributed to the presence of unburnt carbon in the fly ash (Henry and Knapp, 1980). 80 X-RAY STUDIES Fly ash samples of both Visakhapatnam and Kothagudem contain a number of new minerals which occur as minute segregations. through x-ray These mineral occurrences are identified only studies. The occurrence of such mineral assemblages from the studied fly ashes are reported for the first time. These zeolites and other minerals may have recrystallised minerals due to temperature conditions at thermal power station. Gismondine: Rare zeolite, gismondine (CaA12Si2O84H2O) is identified from fly ash samples of both the studied areas. X-ray data (Table No.4.2) is similar to that of JCPDS file number is 20-452. X-ray patten reveals that the mineral crystallizes in monoclinic system with possible space group P21/C. The unit cell parameters were determined as ao=10.021Ao(VF) and 10.028 Ao (KF); bo = 10.562 Ao (VF) and 10.565 Ao (KF); Co= 9.828 Ao (VF) and 9.841Ao (KF);V=1039.15Ao3(VF) and 1041.65Ao3(KF); β =92046’(VF) and 92.465’(KF) with Z=4. The density calculated from the x-ray data is 2.222 (X) (VF) and 2.226 (X) (KF). The strongest intensities are from (300), (121), (202), (004) and (015) hkl values. 81 TABLE 4.2 X-RAY DATA OF GISMONDINI: (CaA12Si2O84H2O) hkl VISAKHAPATNAM FLY ASH KOTHAGUDEM FLY ASH dAo 1/1o dAo 1/1o 020 5.2813 5 5.2825 4 121 4.2695 2 4.2697 26 202 3.4308 24 *** *** 300 3.3406 100 3.3428 100 311 3.0504 14 3.0718 8 032 2.8731 8 *** *** 132 2.7423 22 2.7403 20 213 2.6971 12 2.6893 10 123 2.6558 6 *** *** 004 2.4571 25 2.4603 15 221 2.1994 9 2.3963 11 142 2.2834 5 2.2734 6 214 2.1399 6 *** *** 224 2.0134 12 2.0155 10 333 1.9798 8 1.9815 11 82 015 1.9329 41 1.9375 9 *** 1.7819 9 *** *** *** 1.6409 6 *** *** CRYSTAL SYSTEM MONOCLINIC SPACE GROUP P21/C Z 4 ao = 10.021 AO ao = 10.028 AO bo = 10.562 AO bo = 10.565 AO co = 9.828 AO co = 9.841 AO V = 1039.15 AO V = 1041.65 AO β = 92o46’ β = 92o465’ Dx = 2.222 (X) Dx = 2.226 (X) 83 Mordenite : Mordenite, an alkali zeolite ((Ca, Na2, K2) Al2Si10O247H2O) is identified in the studied fly ashes. Its x-ray data (Table No. 4.3) is comparable with that of JCPDS file No. 6-239. X-ray analysis of mordenite discloses that the mineral crystallizes in orthorphombic system with possible space group C*/C. The strongest diffractions are at (202), (200), (060) and (402) hkl values. The unit cell parameters were calculated as ao = 18.202 Ao (VF) and 18.222 Ao (KF); bo = 20.364 Ao (VF) and 20.393 Ao (KF), Co = 7.525 Ao (VF) and 7.518 Ao (KF); V=2789.26 Ao3 (VF) and 2793.70 Ao3 (KF) with Z=4. Density calculated from x-ray data is 2.122 (X) (VF) and 2.127 (X) (KF). TABLE 4.3 X-RAY DATA OF MORDENITE: ((Ca,Na2, K2), Al2,Si10O247H2O) hkl VISAKHAPATNAM FLY ASH KOTHAGUDEM FLY ASH dAo 1/1o dAo 1/1o 200 9.1013 71 9.1111 70 002 3.7624 16 3.7591 15 202 3.4879 101 3.4910 100 060 3.3947 46 3.3989 18 84 402 2.8982 39 2.8953 9 152 2.7423 8 *** *** *** 2.4687 8 2.4636 17 *** 2.0134 12 2,0154 8 *** 1.9376 13 1.9374 9 *** 1.8824 4 1.8869 6 *** 1.7218 6 *** *** *** 1.6628 7 1.6665 8 *** 1.5927 8 1.5918 5 CRYSTAL SYSTEM ORTHORHOMBIC SPACE GROUP C/C Z 4 ao = 18.202 AO ao = 18.2222 AO bo = 20.364 AO bo = 20.393 AO co = 7.525 AO co = 7.518 AO V = 2789.26 AO3 V = 2793.70 AO3 Dx = 2.122 (X) Dx = 2.127 (X) 85 Leifite: This mineral leifite (Na2AISi5O)2 (F,OH) 1-2 is identified in both the studied fly ashes through x-ray analysis. X-ray data of this mineral is given in Table No.4.4 and the data is similar to that of JCPDS file number is 18-710. X-ray pattern indicates that the mineral crystallizes in hexagonal system. The strongest diffractions are at (301), (211), (101) and (220) hkl values. The unit cell parameters are determined as ao= 14.295 Ao (VF) and 14.296 Ao (KF); Co = 4.815 Ao (VF) and 4.815 Ao (KF); V=852.08 Ao (VF) and 852.20 Ao (KF) with Z=3. The density calculated from x-ray data is 2.47 (X) (VF) and 2.51 (X) (KF). 86 TABLE 4.4 X-RAY DATA OF LEIFITE: (Na2AlSi5O)2 (F,OH)1-2 hkl VISAKHAPATNAM FLY ASH KOTHAGUDEM FLY ASH dAo 1/1o dAo 1/1o 200 6.1899 19 6.1909 11 101 4.4950 26 4.5015 21 220 3.5813 16 3.5716 28 211 3.3406 75 3.3444 81 301 3.1314 100 3.1298 100 400 3.0753 5 3.0787 10 311 2.7864 9 *** *** 401 2.6014 8 2.6013 8 102 2.3963 9 2.3903 9 411 2.3666 7 2.3574 12 510 2.2134 15 *** *** 212 2.1399 6 2.1475 8 *** 1.9086 7 *** *** *** 1.6409 6 1.6415 5 87 CRYSTAL SYSTEM ORTHORHOMBIC SPACE GROUP C/C Z 4 ao = 14.295 AO ao = 14.296 AO co = 4.815AO co = 4.815 AO V = 852.08 AO V = 852.20 AO Dx = 2.47 (X) Dx = 2.51 (X) Palygorskite Magnesium aluminium hydroxy silicate palygorskite ((Mg.Al)5 (SiAl)5 O2 (OH)2 8 H2O)) is identified by x-ray data. The data (Table No.4.5) is similar to that of JCPDS file No.29-855. The unit cell parameters are determined as ao= 12.720 Ao (VF) and 12.714 Ao (KF);bo = 17.866 Ao (VF) and 17.835 Ao (KF); Co = 5.215 Ao (VF) and 5.213 Ao (KF); V=1185.14 Ao (VF) and 1182.07 Ao3 (KF) with Z=2. X-ray data reveals that the mineral crystallizes in orthorhombic system. The strongest diffractions are at (231), (110), (200) and (121) hkl values. 88 Apart from the commonly known assemblages in fly ashes, four new zeolite minerals viz., wairakite, laumontite, chabazite and edingtonite besides zirconium mineral, baddelecyite were identified for the first time. TABLE 4.5 X-RAY DATA OF PALYGORSKITE: ((Mg, Al)5 (SiAl)8 O2 (OH)2 , 8H2O)) hkl VISAKHAPATNAM FLY KOTHAGUDEM FLY ASH ASH dAo 1/1o dAo 1/1o 110 10.3432 98 10.3398 100 200 6.3602 22 63.569 19 130 5.4025 15 *** *** 040 4.4667 14 4.4588 12 121 4.2623 19 4.2771 32 231 3.3454 85 3.3570 98 331 2.8982 10 *** *** 89 251 2.6708 12 2.6681 15 112 2.5347 7 2.5308 12 CRYSTAL SYSTEM ORTHORHOMBIC Z 2 ao = 12.720 AO ao = 12.714 AO bo = 17.866 Ao bo = 17.835 Ao co = 5.215AO co = 5.213AO V = 1185.14 AO V = 1182.07 AO Dx = 2.34 (X) Dx = 2.36 (X) Wairakite: Wairakite (Ca A12Si4O122H2O) is the calcium analogue of analcime (Steiner, 1955). sample displays an This section of Visakhapatnam bulk icositetrahedra like grain of wairakite. Characteristic feature of the mineral is the presence of fine, discontinuous lamellar twinning with two sets at right angles to 90 each other. The mineral is colourless ion plane polarised light and first order grey under crossed nicols. It is optically biaxial with 2 Vo = 82o. Refractive indices are α = 1.496 χ = 1.501, with birefringence 0.005. X-ray data of wairakite was given in Table No.4.6 and the data similar to that of JCPDS file No. is 7-326. X-ray pattern indicate that the mineral crystallizes in monoclinic system. The unit cell parameters were determined as ao = 13.671 Ao, bo = 13.661 Ao, co = 13.541 Ao; V=2528.85 Ao3 and β = 90o.4 with Z = 8. The density calculated from the x-ray data is 2.274 (X). strongest diffractions are at (400), (211) and (431) hkl values. TABLE 4.6 X-RAY DATA OF WAIRAKITE: ((Ca, Al2Si4 O12 H2O)) hkl VISAKHAPATNAM FLY ASH dAo 1/1o 211 5.5712 75 400 3.4178 61 400 3.3888 100 420 3.0753 4 91 The 332 2.8982 8 422 2.7864 8 431 2.6971 40 600 2.2834 4 631 1.9857 7 543 1.9376 6 CRYSTAL SYSTEM MONOCLINIC SPACE GROUP 1’/A Z 8 ao = 13.671 AO bo = 37.661 Ao co = 13.541AO V = 2528.85 AO3 β = 90O.4’ Dx = 2.274 (X) 92 Laumontite: Calcium aluminium, zeolite, laumontite (CaA12Si4O124H2O) occurs in fly ashes expressing acicular habit and uneven fracture with inclusions. It is colourless in plane polarized light and shows first order pink, blue, yellow etc., colours under crossed nicols. Optically it is biaxial negative, length-slow with 2Vα=32o. Refractive indices are α = 1.507; β = 1.514 and χ = 1.516 with birefringence 0.009. X-ray data of laumontite was given in Table No.4.7 and the data comparable with that of JCPDS file No.26-1047. X-ray data indicates that the mineral may be crystallized under monoclinic system with probable space group Cm. The crystallographic constituents determined from the x-ray data for both the studied fly ashes is as follows. Ao = 14.710 Ao(VF) and 14.795 Ao (KF); bo= 14.072 Ao (VF) and 14.059 Ao (KF); Co = 7.525 Ao (VF) and 7.529 Ao (KF), and cell volumes V=1448.78 Ao3 3 (VF) and 1456.58 Ao (KF) and β = 111o55’ (VF) and 111o-56’ (KF). Calculated density is 2.310 (X) (VF) and 2.311 (X) (KF). Strongest diffractions are at (130), (200), (002), (040), (511) and (241) hkl values. 93 TABLE 4.7 X-RAY DATA OF LAUMONTITE: ((CaAl2Si4O124H2O)) hkl VISAKHAPATNAM FLY KOTHAGUDEM FLY ASH ASH dAo 1/1o dAo 1/1o 200 10.3432 100 10.3398 100 130 6.3602 20 63.569 18 002 5.4025 12 *** *** 131 4.4667 13 4.4588 10 040 4.2623 18 4.2771 31 511 3.3454 80 3.3570 100 241 2.8982 8 *** *** 132 2.6708 11 2.6681 13 403 2.5347 5 2.5308 9 622 2.2184 14 2.2212 12 620 2.1546 8 2.1546 10 332 2.0781 4 *** *** 533 *** *** 1.9879 7 94 CRYSTAL SYSTEM MONOCLINIC SPACE GROUP Cm Z 4 ao = 14.710 AO ao = 14.795 AO bo = 14.072 Ao bo = 14.059 Ao co = 7.525AO co = 7.529AO V = 1448.78 AO3 V = 1456.58 AO3 β = 111o55’ β = 111o56’ Dx = 2.310 (X) Dx = 2.311 (X) Chabazite: A section of Kothagudem fly ash shows a fibrous shaped chabazite ((Ca2Al4Si8O24) 12 H2O) mineral. Colour alterations at the edges of fibres suggests that it may be an alteration product derived during mineral transformations. It is colourless and having moderate dispersion. Optically, it is negative. Refractive indices ranges between 1.486 and 1.492, with birefringence 0.006. X-ray data of mineral chabazite is given in Table No.4.8 for both the studied fly ashes. X-ray pattern reveals that the mineral 95 is crystallized under hexagonal system with possible space group R 3m. The unit cell parameters are ao= 13.780 Ao (VF) and 13.789 Ao (KF) and Co – 15.059 Ao (VF) and 15.069 Ao (KF) with cell volume V=2476.35 Ao3 (VF) and 2481.23 Ao3 (KF). The density calculated from x-ray data is Dx-2.032 (X) (VF) and 2.036 (X) (KF). Maximum intensity peaks are at (401), (101), (003), (122), (223) and (104) hkl values and the x-ray data comparable with that of JCPDS file No.19-208. TABLE 4.8 X-RAY DATA OF CHABAZITE: ((Ca2Al4Si8O24) 12H2O) hkl VISAKHAPATNAM FLY ASH KOTHAGUDEM FLY ASH dAo 1/1o dAo 1/1o 101 10.3431 100 10.3398 100 003 6.3601 20 63.569 18 122 5.4024 12 *** *** 104 4.4666 13 4.4588 10 312 4.2622 18 4.2771 31 401 3.3453 80 3.3570 100 214 2.8981 8 *** *** 96 223 2.6707 11 2.6681 13 042 2.5346 5 2.5308 9 205 2.2183 14 2.2212 12 410 2.1545 8 2.1546 10 232 2.0780 4 *** *** 125 *** *** 1.9879 7 116 2.3655 6 *** *** 330 2.2960 9 *** *** 502 2.2838 9 2.2733 5 054 2.0149 4 2.0154 9 217 1.9471 8 1.9483 7 523 1.7818 8 1.7867 6 CRYSTAL SYSTEM HEXAGONAL SPACE GROUP R 3m Z 3 ao = 13.780 AO ao = 13.789 AO co = 15.059AO co = 15.069 AO V = 2476.35 AO3 V = 2481.23 AO3 Dx = 2.032 (X) Dx = 2.036 (X) 97 Edingtonite : Rare barium zeolite, edingtonite (Ba Al2Si3O104 H2O) is identified in both the studied fly ashes. Micro photograph of Visakhapatnam fly ash sample shows number of yellowish cryptocrystalline grains. Optically it is biaxial negative with 2 Va=52o. It is colourless in plane polarized light, first order grey and yellow under crossed nicols. It’s refractive indices are α = 1.541, β=1.552, χ=1.556 with birefringence 0.015. Interference figures show broad isogyres with moderate dispersion. X-ray data of edingtonite is given in Table No.4.9 which is comparable with the data of JCPDS file No.25-60. The strongest peaks are at (001), (020), (200), (210) and (220) hkl values. X-ray pattern indicates that the mineral crystallizes in orthorhombic system with possible space group P21212. The unit cell parameters were determined as ao = 9.548 Ao (VF) and 9.544 Ao (KF); bo = 9.639 Ao (VF) and 9.642 Ao (KF); co = 6.505 Ao (VF) and 6.495 Ao (KF); V = 598.67 Ao3 (VF) and 597.69 Ao3(KF) with Z - 2. 98 TABLE 4.9 X-RAY DATA OF EDINGTONITE: ((BaAl2Si3O104H2O) hkl VISAKHAPATNAM FLY ASH KOTHAGUDEM FLY ASH dAo 1/1o dAo 1/1o 001 10.3431 100 10.3398 100 020 6.3601 20 63.569 18 200 5.4024 12 *** *** 210 4.4666 13 4.4588 10 121 4.2622 18 4.2771 31 220 3.3453 80 3.3570 100 131 2.8981 8 *** *** 311 2.6707 11 2.6681 13 022 2.5346 5 2.5308 9 202 2.2183 14 2.2212 12 320 2.1545 8 2.1546 10 212 2.0780 4 *** *** 231 *** *** 1.9879 7 032 2.3655 6 *** *** 99 240 2.2960 9 *** *** 322 2.2838 9 2.2733 5 023 2.0149 4 2.0154 9 203 1.9471 8 1.9483 7 CRYSTAL SYSTEM ORTHORHOMBIC SPACE GROUP P 2 1 21 2 Z 3 ao = 9.548 AO ao = 9.544 AO bo = 9.639 Ao bo = 9.642 Ao co = 6.505 AO co = 6.495AO V = 598.67 AO3 V = 597.69 AO3 Dx = 2.810 (X) Dx = 2.815 (X) Baddeleyite: Zirconium oxide baddeleyite (Zro2) is identified in Visakhapatnam fly ash sample for the first time. It is irregular in shape with high relief, under plane polarized light it shows deep pinkish brown colour and under crossed nicols first order, pink 100 yellow and brown colours. It exhibits weak pleaochroism from brownish pink to colourless. X-ray data of baddeleyite is given in Table No.4.10, and compared withJCTDS file No. 13-307. the mineralcrystallize X-ray data indicates that inmonoclinic system with possible space group P21/C. Unit cell parameters are a05.144 Ao, bo5.267 Ao, Cell volume V –141.85 A°3 with β 99o.28. ray data is 5.819 (X). hkl values. Density calculated from x- Strongest peaks areat (111), (111) and (211) The occurrence of such new mineral wassupported by high Zr concentrations in the fly ash samples. TABLE :4.10 X-RAY DATA OF BADDELEYITE (ZrO2) hkl VISAKHAPATNAM FLY ASH dAo 1/1o 111 3.6412 100 111 2.3601 20 002 2.4024 12 020 2.4666 13 200 2.2622 18 210 2.3453 80 101 211 2.8981 8 112 2.6707 11 220 2.5346 5 221 2.2183 14 013 2.1545 8 130 2.0780 4 131 *** *** 112 2.3655 6 213 1.2960 9 322 1.2838 9 023 1.0149 4 203 1.9471 8 CRYSTAL SYSTEM MONOCLINIC SPACE GROUP P 21 /C Z 4 Β = 99O.28’ ao = 5.144o AO bo = 5.267 Ao co = 5.215AO 102 V = 141.85 AO3 Dx = 5.819 (X) DTA STUDIES Kaolinite is present in both the fly ashes of Visakhapatnam and Kothagudem samples. They were identified by the differential thermal analysis at about 105°C. This is understood to be carried forward with the clay material inthe coals during deposition. SCANNING ELECTRON MICROSCOPIC (SEM) STUDIES The study of morphology as well as textural patterns of microfined ash particles helps in understanding the binding as well as other engineering properties. The response of fly ash particle to the concrete type environment is individual, depending on its specific composition and internal structure. The surface of fly ash particle serves as the interface for toxic heavy metal adsorption and biological interaction. Therefore it is necessary to understand the external particle morphology. 103 Bulk samples and their fractions, and samples treated with 2 N. NaOH (at 70°C-l10°C for 6 hours) were studied under scanning election microscope(SEM). Most fly ash particles are spherical. The spheres range in size from µm to over 100 µm. Large portion of spherical particles and plates inVisakhapatnam fly ash bulk sample can be observed. The same view for Kothagudem fly ash bulk samples can be seen. The Plale-3b is giving the contrast in size that may occur between large and small fly ash spheres in Kothagudem fly ash. The accumulation of smaller spheres in significant proportions may not be simply random accumulation of small spheres stuck together. These may be the fragments went through the larger particles and bound together either by partial fusion or by layers of unburned carbon retained between them. The attachment of spheres with each other can be due to partial fusion (Kaufherr, et.al., 1984).Finer fractions (0.011 mm, size) of Kothagudem fly ash show equal sizes ofspheres in a maximum portion. The surfaces of the particles have large variations in size, shape and morphological features. Plate is an SEM photograph of Visakhapatnam fly ash, indicating the deposition of sub-micron 104 size particles over the surface of a relatively very large irregular particle, with vesicles. The same photograph is also indicating the accumulation of large number of sub-micron size particles around large spheres and amorphous material. Spherical cavities, looks like kettle holes on a glacial deposit model, on the surfaces of larger particles may be the gas pockets or the remmants of smaller encapsulated spheres. In Plate, the diameter of the small spherical particle, which isattached on the top portion of a single larger particle. It is about 1 µm and that of the larger particle, about 10 µm at X 7500 magnification. The photograph of the coarser fraction (0.09 mm, size) of the same fly ash (Visakhapatnam) indicate the 1 µm diameter particles with larger single particle (nearly 10 µm diameter) at X 20,000 magnification. These two photographs explained the wide size variations in fly ash particles. Sometimes the underlying structure of crystalline material embedded in glass may remain hidden. To study the surfacial features of crystalline material, magnetically separated fly ash particles were scanned. 105 In Plate (of Visakhapatnam) two different structures can be observed. In the left top quadrant of the photograph, there is a broken, relatively large, magnetic particle with the surface showing a dendritic pattern. This dendritic pattern may indicate the mineral iron-spinel (Diamond, 1986). X-ray diffraction analysis of the fly ash sample (separated with a hand magnet) revealed the occurrence of iron rich spinels such as magnetite (Fe3O4), magnesio-ferrite (Mg Fe204) and maghemite(χ-Fe2O3) etc. Right top corner of the same photograph contains many smaller nonmagnetic spheres. The well rounded spheres are called "Cenospheres"" (hollow spheres).Careful observation of these cenospheres indicates that some of the larger spheres are having many smaller spheres and amorphous material within it. Plate reveals that these spheres are not empty. Hence the name "Plerosphere" (Plero-from Greek pleresfull) is suggested (Fisher el.al., 1976). Plate is like a perforated shell of a fossil, cidaris. The surface of the plerosphere is having the accumulation of larger number of micro crystals. Infact the content of organic carbon residue is very low in fly ashes, but small fragments of these residues are found in places. 106 The surface of the carbon residue particle indicates the remnants of inorganic spheres embedded in it. This type of morphology was described as "Swiss Cheese" morphology (Diamond, 1986). 107