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Skeletal morphologies and crystallographic orientations of olivine, diopside and plagioclase
Journal of Crystal Growth 318 (2011) 135–140
Contents lists available at ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Skeletal morphologies and crystallographic orientations of olivine, diopside and plagioclase Shan-Rong Zhao a,n, Rong Liu a, Qin-Yan Wang a, Hai-jun Xu b, Min Fang a a b
Faculty of Earth Sciences, China University of Geosciences, Wuhan 430074, People’s Republic of China State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Wuhan 430074, People’s Republic of China
a r t i c l e in f o
abstract
Available online 2 December 2010
The morphologies and crystallographic orientations of quench-textured olivine, diopside and plagioclase, artificially crystallized at cooling rates of 3–5 1C min 1, have been investigated by scanning electron microscopy (SEM) and Electron backscattered diffraction (EBSD). Olivine displays skeletal, rod-like and dendrite forms with few branches, whereas diopside dendrites have many branches. Plagioclase forms spherulites or euhedral tablets with a porous texture. The skeletal microstructure of these crystals is nearly the same: it consists of H-shaped and dove-shaped units, as described by Donaldson [7] and Faure et al. [5,6], respectively. When the composition of the starting material lies near the cotectic in the phase diagram, an intergrowth of dendritic diopside and euhedral tablets of plagioclase is formed, and the diopside has a fractal structure. EBSD shows that the skeletal rods of olivine are elongated along /0 1 1S, whereas the branches of the dendritic diopside are elongated along /0 0 1S. These elongation directions are related to the shortest cell parameters in their crystal structures. & 2010 Elsevier B.V. All rights reserved.
Keywords: A1. Non-equilibrium A1. Crystal morphology A1. Dendrite
1. Introduction
2. Experimental procedures
Crystals formed under non-equilibrium conditions usually display skeletal morphologies, such as dendrites and spherulites. These types of crystal morphologies are typical of rapidly cooled rocks with a high degree of undercooling, such as pillow basalts [1], boninite dyke [2] and chondrites [3]. To investigate the influence of intensive parameters on the crystal morphologies, numerous experimental studies, relying on dynamic crystallization, have been carried out on dendrites [4]. Recently, transmission electron microscopy (TEM) studies on the synthetic forsterite dendrite have provided the exact image of the dendrite in three-dimensional images of the dendrites and their crystallographic orientations [5]. The relationship between the morphology and growth conditions for synthetic forsterite has also been investigated [6,7]. However, our knowledge of complex non-equilibrium crystal morphology is still incomplete, especially in regard to the nonequilibrium morphologies of different minerals. In this paper, we report scanning electron microscopy (SEM) observations of skeletal morphologies of synthetic olivine, diopside and plagioclase and the crystallographic orientations of the dendrites.
The starting materials for diopside and plagioclase crystal growth were a mixture of reagent grade oxides and carbonates: SiO2, Al2O3, MgO, CaCO3 and Na2CO3. The weight percents of SiO2, Al2O3, MgO, CaCO3 and Na2CO3 for the points 1, 2 and 3 in the phase diagram (Fig. 1 [8]) are shown in Table 1. The oxides and carbonates were mixed in an agate mortar, then melted for 2 h at 1400 1C (125 150 1C above the liquids temperature) in air in a platinum crucible, then quenched in water to form the glass. The glass was sectioned, polished and analyzed by electron probe microscopy (EPM). The microprobe analyses show that the glass compositions are nearly the same as those of the starting materials: only a small amount of Na2O was lost during the melting process. This small loss does not affect the crystal growth. The glass was then ground in an agate mortar and re-melted at 1400 1C in a corundum crucible. The melts were cooled at a rate of 3–5 1C min 1 to room temperature to facilitate growth of diopside and plagioclase. The starting material for olivine crystal growth was natural olivine from the Hannuoba peridotitic xenoliths, located on northern margin of the intra-North China orogenic belt. The natural olivine has a composition of (Mg1.835, Fe0.142 and Ni0.008)1.985 [Si1.006O4] [9]. Because the melting temperature of olivine is very high, we added NaCl as a flux in a weight ratio of 1:1 with the natural olivine. During heating the NaCl melts at 801 1C and becomes Na + and Cl , which is a very strong fluxing agent. Solid Mg2 [SiO4] (olivine) is readily dissolved into Mg + and [SiO4]4 ,
n
Corresponding author. E-mail address: [email protected] (S.-R. Zhao).
0022-0248/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2010.11.137
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which can then crystallize back to olivine during rapid cooling process. NaCl is very useful for this purpose because neither Na + nor Cl cannot enter the olivine crystal structure. NaCl has been used in the same manner for crystallization studies of Mg-perovskite [10] and of b-barium borate [11]. Our melting temperature was 1400 1C, which is a little below the volatilization temperature (1413 1C) of NaCl, thus avoiding loss of Na + during the melting. Even if a small amount of NaCl is lost, the crystal growth on cooling will not be affected. The natural olivine crystals were ground in an agate mortar, mixed with NaCl and ground again. The powder was melted in a corundum crucible for 2 h at 1400 1C in air. The melt was then cooled at a rate of 3–5 1C min 1 to room temperature. The samples of glass containing crystals were analyzed by powder X-ray diffraction, confirming that the crystals are olivine, diopside and plagioclase. The samples were sectioned, polished and coated with carbon. The crystal morphologies were observed under a scanning electron microscope (SEM, Quanta 200) and the orientation analysis was carried out using a scanning electron microscopy equipped with a Nordlys-II Channel 5.0 EBSD system at an accelerating voltage of 20 kV.
We used EBSD to determine the orientation of the olivine grains. The Kikuchi diffraction patterns confirm that the dendrites are structurally consistent with forsterite. The data in the EBSD system used to digitize and analyze the diffraction patterns are: Forsterite, space group: 62, cell parameters a ¼1.025 nm, b¼ 0.601 nm and c¼0.477 nm (Source: Dokl. Akad. Nauk SSSR [Dankas], 1984, 276: 873–877). In order to determine the crystallographic orientations of each crystal, we obtained the Kikuchi diffraction patterns at several points on different rods (The point numbers are shown in Fig. 3a), and then generated /1 0 0S, /0 1 0S, /0 0 1S, /1 1 0S, /1 0 1S and /0 1 1S pole figures for each point using the EBSD
3. Morphological characteristics and crystallographic orientation of the quenched crystals 3.1. Olivine Most olivine crystals display a rod-like habit with skeletal structure, as shown in Figs. 2(a) and 3(a). Some olivine crystals display a dendritic texture, as shown in Fig. 2(b).
Fig. 1. Liquids surface (temperature contours in 1C; composition in moles) and the cotectic line in the system of Di–Ab–An [8]. Inset upper left, the composition of points 1, 2 and 3.
Fig. 2. Skeletal rod and dendrite of olivine (SEM: the cooling rate is 5 1C min 1. The starting material is the nature olivine and NaCl).
Table 1 Starting material composition (wt%). Point number in Fig. 1
SiO2
Al2O3
MgO
CaCO3
Na2CO3
Ab–An–Di composition
1 2 3
47.76 47.99 48.36
12.05 13.81 16.52
8.08 7.00 5.34
27.95 26.41 24.06
4.17 4.79 5.72
Ab22An22Di56 Ab25.5An25.5Di49 Ab31An31Di38
S.-R. Zhao et al. / Journal of Crystal Growth 318 (2011) 135–140
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Fig. 3. EBSD determination of the orientation of the olivine rods. (a) SEM image (numbers indicate positions, where the Kikuchi diffraction patterns were obtained. Inset upper right, gives the sample stage coordinates.). (b) Kikuchi diffraction pattern of the point 9; and (c) /0 1 1S pole figure of point 9 (X–Y is the coordinate of sample stage, not crystallographic axes).
software. Based on the analysis of the pole figure and the elongation direction of each rod, we have found that the elongation direction of each rod olivine is /0 1 1S. Because the analysis results of the elongation direction of each rod are the same for all the rods, we show data only for point 9 (Fig. 3b and c). From the pole figure, we can see that the elongation of this rod (with number 9 on it) is consistent to the /0 1 1S direction (We used a dashed line to show the /0 1 1S direction in Fig. 3c).
analyzed points in Fig. 5(a), the Kikuchi diffraction pattern of point 1 in Fig. 5(b) and the pole figure of point 1 in Fig. 5(c). Points 1–5 have the same Kikuchi diffraction pattern, so we show only the pattern and pole figure for Pt. 1. The main branches of the diopside dendrites are elongated along /0 0 1S (We also used a dashed line to show the /0 0 1S direction in Fig. 5c).
3.3. Plagioclase 3.2. Diopside Diopside crystals also have dendritic patterns, as shown in Fig. 4. Unlike the olivine, which forms rods or dendrites with few branches, the diopside dendrites have many branches. However, both the olivine crystals and the diopside dendrites also have a skeletal microstructure inside the branches, indicating that this is a common feature of crystals formed under non-equilibrium conditions. We also used EBSD to determine the orientation of the diopside branches. The Kikuchi diffraction patterns confirm that the dendrites are structurally consistent with diopside. The data in the computer of EBSD system used to digitize and analyze the diffraction patterns are: diopside- CaMgSi2O6, space group C2/c, cell parameters a¼ 0.975 nm, b¼0.892 nm, c¼0.525 nm; b ¼105.861 (Source: American Mineralogist, 1981, 66: 315–323). We show the
In the quenched samples, plagioclase formed spherulites (Fig. 6a), not dendrites. When the starting material has the composition of point 2 in Fig. 1, near the cotectic line, plagioclase and diopside form an intergrowth, as shown in Fig. 6(b). In this example, the plagioclase forms euhedral tablets with porous texture that enclose some dendritic diopside. The shape of tablet is characteristic of equilibrium crystals but the porous texture inside the tablet suggests a skeletal structure, typical of nonequilibrium. Thus, it appears that plagioclase can develop this special type of morphology with both non-equilibrium and equilibrium characteristics. We used EBSD to determine the orientation of the plagioclase tablets, but the elongating direction is not unique. Some of the tablets are elongated along /1 0 0S, whereas others extend along /0 1 0S or /0 0 1S. This may be due to the fact that plagioclase
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Fig. 4. Dendritic morphologies of diopside (SEM: The cooling rate is 3 1C min 1. The starting material is that for point 1 in Fig. 1).
Fig. 5. EBSD determination of the orientation of the diopside branches; (a) SEM image (numbers indicate positions where the Kikuchi diffraction patterns were obtained. Inset upper right, gives the sample stage coordinates); (b) Kikuchi diffraction pattern; and (c) Pole figures (X–Y is the coordinate of sample stage, not crystallographic axes).
structure has a uniform bond strength in different directions [12]. Because of the uncertainties, we do not discuss the EBSD results for plagioclase in this paper.
4. Discussion Many types of non-equilibrium shapes for olivine, such as hopper, branching, chain, lattice, feather, skeletal, and swallowtail, along with a
systematic change from one shape to another linked to the degree of supercooling have been reported [6,7]. However, all of these various shapes are really just dendrites. For example, some shapes mentioned above are just the different sections of the three-dimensional olivine dendrites [6]. The growth conditions of olivine in our experiment were relatively constant, so we did not produce many of the shapes described by Donaldson and Faure. The rod-like olivine in Figs. 2(a) and 3(a) corresponds to the chain olivine reported by Donaldson.
S.-R. Zhao et al. / Journal of Crystal Growth 318 (2011) 135–140
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Fig. 6. (a) Spherulite morphology of plagioclase (PL 50. The cooling rate was 5 1C min 1. The starting material is that for point 3 in Fig. 1); (b) tabular plagioclase with many diopside dendrite inclusions (SEM). (the cooling rate was 3 oC min 1, the starting material is that for point 2 in Fig. 1).
Fig. 7. Sketch of the two generations of H-shaped units inside the diopside branches, showing a fractal character.
Donaldson [7] described the microstructure inside the chain as: H-shaped units stacked together. Faure et al. [6] described the microstructure in rod olivine as dove-shaped units. However, the microstructure of the rod-like olivine reported in this paper (Fig. 2a) is very similar to stacked H-shaped units described by Donaldson. Interestingly, Donaldson and Faure only reported this kind of microstructure in olivine but not in any other crystals. In this paper we have shown that diopside and plagioclase can both develop this type of microstructure. Fig. 4(a) and (b) shows that the skeletal microstructure inside the branches of the dendritic diopside consists of both H-shaped and dove-shaped units (we indicate ‘‘H-shaped’’ and ‘‘dove-shaped’’ in Fig. 4a). In Fig. 6(b), the shape of the euhedral plagioclase tablets with porous texture can also be considered as being formed from H-shaped units. So, H-shaped or dove-shaped units appear to be the common skeletal microstructure of non-equilibrium crystals of several different minerals. There are two generations of H-shaped units in dendritic diopside (Fig. 4b): a large H-shaped unit contains some smaller H-shaped units. We sketched this kind of generation structure in Fig. 7. These two generations of H-shaped units have a kind of the self-similarity, so they can be considered to have a fractal structure.
Fractal structure is common for non-equilibrium morphology, such as the fractal structure of olivine in the pillow margin of a lava flow [13] and the ‘‘Sierpinski gasket’’ fractal morphology in LiAlSiO4– SiO2 system [14]. The orientation of the rod-like olivine is /0 1 1S, which is between axes Y and Z. Considering the cell parameters of a¼1.025 nm, b¼0.601 nm and c¼0.477 nm, we conclude that the elongation direction is perpendicular to the longest cell parameter X and lies between the short cell parameter directions Y and Z. Note that the orientation of rod or dendritic olivine reported by Faure et al. [5,6] is /1 0 1S. The cell parameters of olivine in their paper are: a ¼0.475 nm, b¼ 1.019 nm and c¼0.598 nm. That means, that the X, Y and Z axes reported by them are reversed from our, so our /0 1 1S is equal to their /1 0 1S. This study confirms the results obtained by Faure et al. Based on the point group of olivine (mmm), /0 1 1S includes two directions: [0 1 1] and [0 1¯ 1]. These two directions are equal, so the rods must develop along these directions. Thus, the olivine grows along these two directions to form dendrites. This means that the rodlike olivine shown in Figs. 2(a) and 3(a) is a section of the dendrite. The dendritic olivine shown in Fig. 2(b) develops two branches with an angle of 7717. The two directions of /0 1 1S ([0 1 1] and [0 1¯ 1]) also have an angle of 771, so we deduce that the branches in Fig. 2(b) are [0 1 1] and [0 1¯ 1] (Unfortunately we could not find the dendrite in Fig. 2(b) when the sample was being examined under EBSD, so we could not determine its branch directions). Using EBSD we determined that the branches of the dendritic diopside are oriented along /0 0 1S. Again, based on the cell parameters a¼ 0.975 nm, b¼0.892 nm, c¼ 0.525 nm; and b ¼105.861, we conclude that the elongation direction is the shortest cell parameter, direction Z. On the other hand, the Z axis is also the strongest bond chain direction in the diopside crystal structure [12]. Therefore, we can suggest that the elongation direction of diopside branches is related to the shortest cell parameter direction and the strongest bond chain direction.
5. Conclusion Our study shows that olivine, diopside and plagioclase develop different morphologies when cooled at the same rate 3–5 1C
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min 1. Olivine displays skeletal rod-like forms and dendrites with few branches. Diopside displays dendrites with many branches, whereas plagioclase forms spherulites or euhedral tablets. Within the olivine rods, the branches of the dendritic diopside and the euhedral plagioclase tablets, there is a skeletal microstructure consisting of H-shaped and dove-shaped units. H-shaped and doveshaped units are common non-equilibrium morphologies for these three minerals. When the starting material is near the cotectic line of the phase diagram, an intergrowth develops, composed of dendritic diopside and euhedral plagioclase tablets. A fractal structure was found in the branches of the dendrite diopside. EBSD determinations show that the rod olivine is elongated along /0 1 1S, whereas the branches of the dendritic diopside are elongated along /0 0 1S. These elongation directions are related to the short cell parameters. For dendritic diopside the elongation direction is also the strongest bond chain direction.
and Mineral Resources, China University of Geosciences. We also express our gratitude to Prof. Paul Robinson for his helpful English revision of our manuscript.
References [1] [2] [3] [4] [5] [6] [7] [8] [9]
Acknowledgements We acknowledge financial support from National Nature Science Fund of China (40872040, 40472023). The SEM and EBSD were performed in State Key Laboratory of Geological Processes
[10] [11] [12] [13] [14]
P. Schiffman, G.. Lofgren, Journal of Geology 90 (1982) 49. D. Ohnenstetter, W.L. Brown, Journal of Petrology 33 (1992) 231 Part 1. G. Lofgren, Geochimica et Cosmochimica Acta 53 (1989) 461. G. Lofgren, in: R.B. Hargraves (Ed.), Physics of Magmatic Processes, Princeton University Press, New Jersey, 1980, pp. 487–551. F. Faure, G.. Trolliard, B. Soulestin, American Mineralogist 88 (2003) 1241. F. Faure, G.. Trolliard, N. Christian, Contributions to Mineralogy and Petrology 145 (2003) 251. C.H. Donaldson, Contributions to Mineralogy and Petrology 57 (1976) 187. D.F. Weill, R. Hon, A. Navrotsky, in: R.B. Hargraves (Ed.), Physics of Magmatic Processes, Princeton University Press, New Jersey, 1980, pp. 49–92. C.M Yu, J.P. Zheng, W.L. Griffin, Earth Sciences—Journal of China University of Geosciences 31 (2006) 93 (in Chinese). D.P. Dobson, S.D. Jacobsen, American Mineralogist 89 (2004) 807. Peter F. Bordui, US Patent Publication (1994), Publication no. US 5343827. S.R. Zhao, Crystallography and Mineralogy, Higher Education Press, Beijing, 2004 (in Chinese). A.D. Fowler, H.E. Stanley, G.. Daccord, Nature 341 (1989) 134. S.R. Zhao, J. Tan, J.Y. Wang, X.H. Xu, H. Liu, Fractals 11 (3) (2003) 271.
Contents lists available at ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Skeletal morphologies and crystallographic orientations of olivine, diopside and plagioclase Shan-Rong Zhao a,n, Rong Liu a, Qin-Yan Wang a, Hai-jun Xu b, Min Fang a a b
Faculty of Earth Sciences, China University of Geosciences, Wuhan 430074, People’s Republic of China State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Wuhan 430074, People’s Republic of China
a r t i c l e in f o
abstract
Available online 2 December 2010
The morphologies and crystallographic orientations of quench-textured olivine, diopside and plagioclase, artificially crystallized at cooling rates of 3–5 1C min 1, have been investigated by scanning electron microscopy (SEM) and Electron backscattered diffraction (EBSD). Olivine displays skeletal, rod-like and dendrite forms with few branches, whereas diopside dendrites have many branches. Plagioclase forms spherulites or euhedral tablets with a porous texture. The skeletal microstructure of these crystals is nearly the same: it consists of H-shaped and dove-shaped units, as described by Donaldson [7] and Faure et al. [5,6], respectively. When the composition of the starting material lies near the cotectic in the phase diagram, an intergrowth of dendritic diopside and euhedral tablets of plagioclase is formed, and the diopside has a fractal structure. EBSD shows that the skeletal rods of olivine are elongated along /0 1 1S, whereas the branches of the dendritic diopside are elongated along /0 0 1S. These elongation directions are related to the shortest cell parameters in their crystal structures. & 2010 Elsevier B.V. All rights reserved.
Keywords: A1. Non-equilibrium A1. Crystal morphology A1. Dendrite
1. Introduction
2. Experimental procedures
Crystals formed under non-equilibrium conditions usually display skeletal morphologies, such as dendrites and spherulites. These types of crystal morphologies are typical of rapidly cooled rocks with a high degree of undercooling, such as pillow basalts [1], boninite dyke [2] and chondrites [3]. To investigate the influence of intensive parameters on the crystal morphologies, numerous experimental studies, relying on dynamic crystallization, have been carried out on dendrites [4]. Recently, transmission electron microscopy (TEM) studies on the synthetic forsterite dendrite have provided the exact image of the dendrite in three-dimensional images of the dendrites and their crystallographic orientations [5]. The relationship between the morphology and growth conditions for synthetic forsterite has also been investigated [6,7]. However, our knowledge of complex non-equilibrium crystal morphology is still incomplete, especially in regard to the nonequilibrium morphologies of different minerals. In this paper, we report scanning electron microscopy (SEM) observations of skeletal morphologies of synthetic olivine, diopside and plagioclase and the crystallographic orientations of the dendrites.
The starting materials for diopside and plagioclase crystal growth were a mixture of reagent grade oxides and carbonates: SiO2, Al2O3, MgO, CaCO3 and Na2CO3. The weight percents of SiO2, Al2O3, MgO, CaCO3 and Na2CO3 for the points 1, 2 and 3 in the phase diagram (Fig. 1 [8]) are shown in Table 1. The oxides and carbonates were mixed in an agate mortar, then melted for 2 h at 1400 1C (125 150 1C above the liquids temperature) in air in a platinum crucible, then quenched in water to form the glass. The glass was sectioned, polished and analyzed by electron probe microscopy (EPM). The microprobe analyses show that the glass compositions are nearly the same as those of the starting materials: only a small amount of Na2O was lost during the melting process. This small loss does not affect the crystal growth. The glass was then ground in an agate mortar and re-melted at 1400 1C in a corundum crucible. The melts were cooled at a rate of 3–5 1C min 1 to room temperature to facilitate growth of diopside and plagioclase. The starting material for olivine crystal growth was natural olivine from the Hannuoba peridotitic xenoliths, located on northern margin of the intra-North China orogenic belt. The natural olivine has a composition of (Mg1.835, Fe0.142 and Ni0.008)1.985 [Si1.006O4] [9]. Because the melting temperature of olivine is very high, we added NaCl as a flux in a weight ratio of 1:1 with the natural olivine. During heating the NaCl melts at 801 1C and becomes Na + and Cl , which is a very strong fluxing agent. Solid Mg2 [SiO4] (olivine) is readily dissolved into Mg + and [SiO4]4 ,
n
Corresponding author. E-mail address: [email protected] (S.-R. Zhao).
0022-0248/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2010.11.137
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which can then crystallize back to olivine during rapid cooling process. NaCl is very useful for this purpose because neither Na + nor Cl cannot enter the olivine crystal structure. NaCl has been used in the same manner for crystallization studies of Mg-perovskite [10] and of b-barium borate [11]. Our melting temperature was 1400 1C, which is a little below the volatilization temperature (1413 1C) of NaCl, thus avoiding loss of Na + during the melting. Even if a small amount of NaCl is lost, the crystal growth on cooling will not be affected. The natural olivine crystals were ground in an agate mortar, mixed with NaCl and ground again. The powder was melted in a corundum crucible for 2 h at 1400 1C in air. The melt was then cooled at a rate of 3–5 1C min 1 to room temperature. The samples of glass containing crystals were analyzed by powder X-ray diffraction, confirming that the crystals are olivine, diopside and plagioclase. The samples were sectioned, polished and coated with carbon. The crystal morphologies were observed under a scanning electron microscope (SEM, Quanta 200) and the orientation analysis was carried out using a scanning electron microscopy equipped with a Nordlys-II Channel 5.0 EBSD system at an accelerating voltage of 20 kV.
We used EBSD to determine the orientation of the olivine grains. The Kikuchi diffraction patterns confirm that the dendrites are structurally consistent with forsterite. The data in the EBSD system used to digitize and analyze the diffraction patterns are: Forsterite, space group: 62, cell parameters a ¼1.025 nm, b¼ 0.601 nm and c¼0.477 nm (Source: Dokl. Akad. Nauk SSSR [Dankas], 1984, 276: 873–877). In order to determine the crystallographic orientations of each crystal, we obtained the Kikuchi diffraction patterns at several points on different rods (The point numbers are shown in Fig. 3a), and then generated /1 0 0S, /0 1 0S, /0 0 1S, /1 1 0S, /1 0 1S and /0 1 1S pole figures for each point using the EBSD
3. Morphological characteristics and crystallographic orientation of the quenched crystals 3.1. Olivine Most olivine crystals display a rod-like habit with skeletal structure, as shown in Figs. 2(a) and 3(a). Some olivine crystals display a dendritic texture, as shown in Fig. 2(b).
Fig. 1. Liquids surface (temperature contours in 1C; composition in moles) and the cotectic line in the system of Di–Ab–An [8]. Inset upper left, the composition of points 1, 2 and 3.
Fig. 2. Skeletal rod and dendrite of olivine (SEM: the cooling rate is 5 1C min 1. The starting material is the nature olivine and NaCl).
Table 1 Starting material composition (wt%). Point number in Fig. 1
SiO2
Al2O3
MgO
CaCO3
Na2CO3
Ab–An–Di composition
1 2 3
47.76 47.99 48.36
12.05 13.81 16.52
8.08 7.00 5.34
27.95 26.41 24.06
4.17 4.79 5.72
Ab22An22Di56 Ab25.5An25.5Di49 Ab31An31Di38
S.-R. Zhao et al. / Journal of Crystal Growth 318 (2011) 135–140
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Fig. 3. EBSD determination of the orientation of the olivine rods. (a) SEM image (numbers indicate positions, where the Kikuchi diffraction patterns were obtained. Inset upper right, gives the sample stage coordinates.). (b) Kikuchi diffraction pattern of the point 9; and (c) /0 1 1S pole figure of point 9 (X–Y is the coordinate of sample stage, not crystallographic axes).
software. Based on the analysis of the pole figure and the elongation direction of each rod, we have found that the elongation direction of each rod olivine is /0 1 1S. Because the analysis results of the elongation direction of each rod are the same for all the rods, we show data only for point 9 (Fig. 3b and c). From the pole figure, we can see that the elongation of this rod (with number 9 on it) is consistent to the /0 1 1S direction (We used a dashed line to show the /0 1 1S direction in Fig. 3c).
analyzed points in Fig. 5(a), the Kikuchi diffraction pattern of point 1 in Fig. 5(b) and the pole figure of point 1 in Fig. 5(c). Points 1–5 have the same Kikuchi diffraction pattern, so we show only the pattern and pole figure for Pt. 1. The main branches of the diopside dendrites are elongated along /0 0 1S (We also used a dashed line to show the /0 0 1S direction in Fig. 5c).
3.3. Plagioclase 3.2. Diopside Diopside crystals also have dendritic patterns, as shown in Fig. 4. Unlike the olivine, which forms rods or dendrites with few branches, the diopside dendrites have many branches. However, both the olivine crystals and the diopside dendrites also have a skeletal microstructure inside the branches, indicating that this is a common feature of crystals formed under non-equilibrium conditions. We also used EBSD to determine the orientation of the diopside branches. The Kikuchi diffraction patterns confirm that the dendrites are structurally consistent with diopside. The data in the computer of EBSD system used to digitize and analyze the diffraction patterns are: diopside- CaMgSi2O6, space group C2/c, cell parameters a¼ 0.975 nm, b¼0.892 nm, c¼0.525 nm; b ¼105.861 (Source: American Mineralogist, 1981, 66: 315–323). We show the
In the quenched samples, plagioclase formed spherulites (Fig. 6a), not dendrites. When the starting material has the composition of point 2 in Fig. 1, near the cotectic line, plagioclase and diopside form an intergrowth, as shown in Fig. 6(b). In this example, the plagioclase forms euhedral tablets with porous texture that enclose some dendritic diopside. The shape of tablet is characteristic of equilibrium crystals but the porous texture inside the tablet suggests a skeletal structure, typical of nonequilibrium. Thus, it appears that plagioclase can develop this special type of morphology with both non-equilibrium and equilibrium characteristics. We used EBSD to determine the orientation of the plagioclase tablets, but the elongating direction is not unique. Some of the tablets are elongated along /1 0 0S, whereas others extend along /0 1 0S or /0 0 1S. This may be due to the fact that plagioclase
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Fig. 4. Dendritic morphologies of diopside (SEM: The cooling rate is 3 1C min 1. The starting material is that for point 1 in Fig. 1).
Fig. 5. EBSD determination of the orientation of the diopside branches; (a) SEM image (numbers indicate positions where the Kikuchi diffraction patterns were obtained. Inset upper right, gives the sample stage coordinates); (b) Kikuchi diffraction pattern; and (c) Pole figures (X–Y is the coordinate of sample stage, not crystallographic axes).
structure has a uniform bond strength in different directions [12]. Because of the uncertainties, we do not discuss the EBSD results for plagioclase in this paper.
4. Discussion Many types of non-equilibrium shapes for olivine, such as hopper, branching, chain, lattice, feather, skeletal, and swallowtail, along with a
systematic change from one shape to another linked to the degree of supercooling have been reported [6,7]. However, all of these various shapes are really just dendrites. For example, some shapes mentioned above are just the different sections of the three-dimensional olivine dendrites [6]. The growth conditions of olivine in our experiment were relatively constant, so we did not produce many of the shapes described by Donaldson and Faure. The rod-like olivine in Figs. 2(a) and 3(a) corresponds to the chain olivine reported by Donaldson.
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Fig. 6. (a) Spherulite morphology of plagioclase (PL 50. The cooling rate was 5 1C min 1. The starting material is that for point 3 in Fig. 1); (b) tabular plagioclase with many diopside dendrite inclusions (SEM). (the cooling rate was 3 oC min 1, the starting material is that for point 2 in Fig. 1).
Fig. 7. Sketch of the two generations of H-shaped units inside the diopside branches, showing a fractal character.
Donaldson [7] described the microstructure inside the chain as: H-shaped units stacked together. Faure et al. [6] described the microstructure in rod olivine as dove-shaped units. However, the microstructure of the rod-like olivine reported in this paper (Fig. 2a) is very similar to stacked H-shaped units described by Donaldson. Interestingly, Donaldson and Faure only reported this kind of microstructure in olivine but not in any other crystals. In this paper we have shown that diopside and plagioclase can both develop this type of microstructure. Fig. 4(a) and (b) shows that the skeletal microstructure inside the branches of the dendritic diopside consists of both H-shaped and dove-shaped units (we indicate ‘‘H-shaped’’ and ‘‘dove-shaped’’ in Fig. 4a). In Fig. 6(b), the shape of the euhedral plagioclase tablets with porous texture can also be considered as being formed from H-shaped units. So, H-shaped or dove-shaped units appear to be the common skeletal microstructure of non-equilibrium crystals of several different minerals. There are two generations of H-shaped units in dendritic diopside (Fig. 4b): a large H-shaped unit contains some smaller H-shaped units. We sketched this kind of generation structure in Fig. 7. These two generations of H-shaped units have a kind of the self-similarity, so they can be considered to have a fractal structure.
Fractal structure is common for non-equilibrium morphology, such as the fractal structure of olivine in the pillow margin of a lava flow [13] and the ‘‘Sierpinski gasket’’ fractal morphology in LiAlSiO4– SiO2 system [14]. The orientation of the rod-like olivine is /0 1 1S, which is between axes Y and Z. Considering the cell parameters of a¼1.025 nm, b¼0.601 nm and c¼0.477 nm, we conclude that the elongation direction is perpendicular to the longest cell parameter X and lies between the short cell parameter directions Y and Z. Note that the orientation of rod or dendritic olivine reported by Faure et al. [5,6] is /1 0 1S. The cell parameters of olivine in their paper are: a ¼0.475 nm, b¼ 1.019 nm and c¼0.598 nm. That means, that the X, Y and Z axes reported by them are reversed from our, so our /0 1 1S is equal to their /1 0 1S. This study confirms the results obtained by Faure et al. Based on the point group of olivine (mmm), /0 1 1S includes two directions: [0 1 1] and [0 1¯ 1]. These two directions are equal, so the rods must develop along these directions. Thus, the olivine grows along these two directions to form dendrites. This means that the rodlike olivine shown in Figs. 2(a) and 3(a) is a section of the dendrite. The dendritic olivine shown in Fig. 2(b) develops two branches with an angle of 7717. The two directions of /0 1 1S ([0 1 1] and [0 1¯ 1]) also have an angle of 771, so we deduce that the branches in Fig. 2(b) are [0 1 1] and [0 1¯ 1] (Unfortunately we could not find the dendrite in Fig. 2(b) when the sample was being examined under EBSD, so we could not determine its branch directions). Using EBSD we determined that the branches of the dendritic diopside are oriented along /0 0 1S. Again, based on the cell parameters a¼ 0.975 nm, b¼0.892 nm, c¼ 0.525 nm; and b ¼105.861, we conclude that the elongation direction is the shortest cell parameter, direction Z. On the other hand, the Z axis is also the strongest bond chain direction in the diopside crystal structure [12]. Therefore, we can suggest that the elongation direction of diopside branches is related to the shortest cell parameter direction and the strongest bond chain direction.
5. Conclusion Our study shows that olivine, diopside and plagioclase develop different morphologies when cooled at the same rate 3–5 1C
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min 1. Olivine displays skeletal rod-like forms and dendrites with few branches. Diopside displays dendrites with many branches, whereas plagioclase forms spherulites or euhedral tablets. Within the olivine rods, the branches of the dendritic diopside and the euhedral plagioclase tablets, there is a skeletal microstructure consisting of H-shaped and dove-shaped units. H-shaped and doveshaped units are common non-equilibrium morphologies for these three minerals. When the starting material is near the cotectic line of the phase diagram, an intergrowth develops, composed of dendritic diopside and euhedral plagioclase tablets. A fractal structure was found in the branches of the dendrite diopside. EBSD determinations show that the rod olivine is elongated along /0 1 1S, whereas the branches of the dendritic diopside are elongated along /0 0 1S. These elongation directions are related to the short cell parameters. For dendritic diopside the elongation direction is also the strongest bond chain direction.
and Mineral Resources, China University of Geosciences. We also express our gratitude to Prof. Paul Robinson for his helpful English revision of our manuscript.
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Acknowledgements We acknowledge financial support from National Nature Science Fund of China (40872040, 40472023). The SEM and EBSD were performed in State Key Laboratory of Geological Processes
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