Mechanism of decomposition of dolomite, Ca0.5Mg0.5CO3, in the electron microscope

Ultramlcroscopy 18 (1985) 241-252 North-Holland, Amsterdam

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M E C H A N I S M O F D E C O M P O S I T I O N O F D O L O M I T E , Cao.sMgo.sCO3, IN T H E E L E C T R O N MICROSCOPE E. David C A T E R Department of Chemistry, The University of Iowa, Iowa City, Iowa 52242, USA and

Peter R. BUSECK Departments of Chemistry and Geology, Arizona State University, Tempe, Arizona 85287, USA Received 18 June 1985; presented at SymposiumJanuary 1985

Decomposition of dolomite, Ca0.sMg0.sCO3, induced by the electron beam in the transmission electron microscope, was studied by means of electron diffraction patterns and high-resolution images. Radiolysis of the carbonate ion gives a metastable, fee solid solution, Ca0.sMg0.~O, which is formedby a topotacticprocess so that a body diagonal of the fee unit cell is coincidentwith the original dolomite c-axis. The fee lattice constant of 0.461+ 0.006 nm is 2% larger than the mean value for CaO and MgO. Subsequently, the fee domains lose orientation, the specimen passes through an amorphous state, and randomly oriented crystallites of CaO and MgO are formed. The diameters of the crystallites of both the solid solution and the final CaO and MgO are in the range 1 to 10 nm. Thermal decompositionin vacuum differs from electron-beamdecomposition in that the range of solid-solution compositions is much more limited in the former case. Electron-beam decomposition of minerals may be a useful method of preparation of highly reactive solid solutions that are not otherwise obtainable.

1. Introduction Structural damage, resulting from irradiation by the electron beam, frequently occurs in minerals and other inorganic materials being examined in the transmission electron microscope (TEM) [1-3]. Indeed, this is a perennial problem in obtaining microstructural information, as significant damage can occur during the first few minutes o r even seconds of the initial alignment and orientation procedures. We encountered this problem during a study of natural and synthetic dolomite, Ca0.sMg0.sCO3. By studying the TEM images and electron diffraction patterns (EDPs) of thirty dolomite grains that had suffered beam damage, we were able to elucidate the mechanism of decomposition, and we have performed additional experiments to verify it. Dolomite is a widespread carbonate mineral whose thermal decomposition products are of cur-

rent interest for removing SO 2 from combustion gases [4] and as components of refractory materials for the steel and other industries [5]. Our results on decomposition of dolomite in the electron beam help explain some details found in recent investigations [6-8] of thermal decomposition in vacuum.

2. The structure of dolomite The dolomite structure is hexagonal (rhombohedral, space group R3) and consists of alternating, parallel planes of calcium and magnesium ions, interleaved with planes of carbonate ions. These planes are normal to the threefold c-axis. The Miller indices used in this paper are for the hexagonal unit cell with lattice parameters a 0 = 0.481 nm and co - 1.602 nm. Figs. l a and l b show the dolomite structure viewed along the [001] and

0304-3991/85/$03.30 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

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E.D. Cater, P.R. Buseck / Decomposition of dolomite in electron microscope

~

8.

b.

CO3 z=.~ Ca z-, Mg z =,/6

2

,7"/C.

7

3 3im

Fig. 1. The structure of dolomite, showing the relation of cleavage rhombohedron of dolomite to foe unit cell of the solid solution decomposition product. (a) Dolomite structure viewed in [001] direction. The lowest three layers of the unit cell are shown. The hexagonal unit cell is outlined by dashed lines. The projection of the cleavage rhombohedron with its {104} faces is outlined by the solid lines. (b) Dolomite structure viewed in [110] direction. The projected cleavage rhombohedron is shown. The face on the left side in the line of sight is (ila). (c) Projection of foe Ca0.aMg0.50 solid solution structure in [101] direction. The projected unit cell is outlined. Note that: (a) the body diagonal of the cubic cell has the same orientation as the dolomite c-axis; (b) the spacing of cation planes is the same as in dolomite; (c) the spacing in the cation planes is decreased 32%; (d) in actual specimens Ca and Mg ions are randomly

sity of California at San Diego. Four samples (Nos. 0, 1, 2, and 3) are of finely divided synthetic crystals (0.1 to 20/~m diameter), produced hydrothermally in sealed tube reactions from orthorhombic CaCO 3 and aqueous MgC12 [10,11]. The fifth specimen (No. 6) is a natural, hydrothermal dolomite from Pen field, NY. Lattice constants of specimens 1-6, determined from Guinier X-ray diffraction films, are those expected for stoichiometric dolomite with various degrees of order [12-14]. Lattice parameters of specimen No. 0 indicate [13,14] it has the composition Ca0.53Mg0.46CO3. Ratios of C a / M g determined by energy-dispersive X-ray spectroscopy (EDS) in the microscope are in the range expected from stoichiometric dolomite [15], except for No. 0, which is generally rich in Ca but showed some variation in C a / M g from grain to grain. The profiles and intensities of the "ordering reflections" 015 and 021 indicated an increasing degree of order from No. 0 to No. 6, with Nos. 3 and 6 being well ordered. The grains used for this study gave well defined selected area electron diffraction patterns (EDPs) without satellite reflections or streaking of the diffraction spots. All behaved similarly with respect to damage in the electron beam, although the more poorly ordered specimens damaged more rapidly than the well ordered ones. Lattice fringes in the TEM images are sharply defined in one or more directions. We conclude that our specimens are representative dolomites, and that their behavior in the electron beam is not the result of the synthetic origin of some of them.

mixed. 4. Electron microscopy

[110] directions. The hexagonal unit cell is outlined by the dashed line in fig. la. The projection of the cleavage rhombohedron (morphological unit cell [9]) is outlined in figs. la and lb by solid lines for later reference.

3. Materials

The dolomite specimens studied were kindly provided by Professor M. Kastner of the Univer-

Crushed-grain specimens dispersed on holey carbon grids were examined at 200 keV with a JEOL 200CX TEM equipped with a top-entry, high-resolution stage, and at 100 and 120 keV with a Philips 400T analytical TEM. Images and EDPs recorded with the electron beam normal to the dolomite c-axis required specimen tilts of about 40 ° and were obtained only in the Philips instrument. Diffraction camera lengths of the TEMs were calibrated as a function of objective lens

E.D. Cater, P.R. Buseck / Decomposition of dolomite in electron microscope

current by use of EDPs from evaporated gold films and dolomite specimens. Observed d-spacings agree to better than one percent with those calculated for ordered, stoichiometric dolomite, for almost all single-crystal EDPs, and no systematic differences between microscopes or specimens were found within this uncertainty. Larger variations were obtained for d-spacings from polycrystalline EDPs of beam-damaged specimens because of the difficulty of measuring the faint, poorly defined diffraction arcs, as discussed below.

5. Experimental results Electron-beam damage was usually observed only after a crystal had been oriented, initial EDPs had been taken, and a through-focus series of high-resolution images was begun. Typically, this was a period of 5 to 15 rain, depending on the beam intensity and specimen. Initial EDPs were typical single crystal patterns, and were easily indexed as dolomite diffraction zones. Initial images showed corresponding lattice fringes. Then, over a period of perhaps half an hour, the dolomite images degraded, with disappearance of detail and simultaneous appearance in various regions of the crystal of many domains 1 to 10 nm across, without discernable structure in many cases. In other instances these small regions contain groups of parallel fringes of 0.27 and 0.23 nm spacing, running in directions not characteristic of the original dolomite. During this time EDPs showed decreasing intensity of the dolomite patterns, without other significant changes in them, but with simultaneous superposition of diffuse diffraction patterns from a face-centered cubic (fee) phase. The fee patterns typically exhibit partial or complete rings of a polycrystalline nature, but with symmetrically placed arcs of enhanced intensity. The latter indicate that the domains of fee structure are preferentially oriented, with their [111] directions approximately parallel to the original dolomite c-axis. On longer or very intense exposure the dolomite spots eventually disappeared from the EDP, and the fee pattern became more intense, without other changes in its general appearance. The d-spacings

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calculated from the diffraction arcs correspond to the 0.27 and 0.23 nm fringes mentioned above. Typical EDPs from several specimens at increasing stages of alteration in the electron beam are presented in fig. 2. In figs. 2a and 2b the single crystal dolomite patterns are still visible, with superimposed diffuse fee arcs. In fig. 2a the coincidence of the fee 111 and dolomite 006 reflections is indicated by the arrow. In figs. 2c and 2d the dolomite spots have disappeared, leaving only fee patterns. Fig. 2c shows an intense fee (211) zone with a weaker (100) zone superimposed. Fig. 2d shows rings from randomly oriented crystallites of fee material with two bright spots corresponding to 002 and 002 reflections from a relatively large crystallite. Those fee EDPs that show the strongest preferred orientations have diffraction maxima that correspond to (100), (110), or (211) diffraction zones. We ascribe these fee patterns to equimolar solid solutions of Cao.sMg0.50 that formed as CO2 was lost from the dolomite. Such solid solutions would be expected to have the rocksalt structure (as do both CaO and MgO), with lattice parameter a 0 intermediate between those of CaO (a 0 = 0.4811 nm) and MgO (a 0 = 0.4213 nm) [16]. Measurement of 28 EDPs gave the average and standard deviation for the Ca0.sMg0.50 solid solution a 0 -0.461 + 0.006 nm, which is 2% greater than the mean value for CaO and MgO. We discuss this result in more detail elsewhere [17]. Here we merely note that the Ca0.5Mg0.50 solid solution is thermodynamically unstable because of the mismatch of cation sizes [18], and that equilibrium solid solutions in this system, even near the eutectic temperature of 2370°C, contain at most 8 mol% CaO in MgO and 22 mol% MgO in CaO [19]. The initial decomposition reaction is thus Ca 0.5Mg0.5CO3 (s)

--, Ca05Mg050(fee solid soln) + CO2(g).

(1)

Ultimately, for crystals receiving very long or very intense exposure, the hazy, diffuse arcs and rings were replaced by two superimposed sets of more sharply defined fee rings (fig. 2f), that have spacings characteristic of CaO and MgO. In one

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E.D. Cater, P.R. Buseck / Decomposition of dolomite in electron microscope

Fig. 2. Typical EDPs of various, dolomite specimens at different stages of decomposition in the electron beam. (a) EDP from (110) zone of a partially decomposed dolomite crystal showing coincidence of dolomite 006 and fee 111 reflections (arrow). (b) EDP from (151) zone of a dolomite crystal showing superimposed diffuse fcc pattern with predominant (100) orientation. (c) EDP of a dolomite crystal after complete loss of CO 2 to form fcc solid solution with predominant (211) orientation. (d) EDP of a dolomite crystal decomposed to fcc solid solution with crystallites generally randomly oriented. Bright spots are 002 and 002 reflections from a larger crystallite or mosaic. (e) EDP of specimen shown in fig. 3d, in amorphous stage, undergoing phase separation of CaO and MgO from fcc solid solution. (f) EDP of decomposed dolomite specimen of fig. 3e after complete phase separation. The rings are from CaO and MgO crystallites in random orientations. From center outward the first three rings are CaO 111, CaO 200 and MgO 111 superimposed, and MgO 200.

E.D. Cater, P.R. Buseck / Decomposition of dolomite in electron microscope

experiment, described below, an amorphous intermediate stage between the fcc solid solution and the separated CaO and MgO phases was also observed. The EDP from the specimen at this stage (fig. 2e) was a structureless blur with faint partial rings. The processes giving rise to these diffraction patterns are Cao.sMg0.50(fcc solid soln) --, Ca 0.5Mg0.50(amorphous),

(2)

Ca 0.5Mgo.50(amorphous) 0.5 CaO(s) + 0.5 MgO(s).

(3)

Complete decomposition of one crystal of specimen No. 3, oriented with (111) normal to the beam, was carried out by exposing it to the 200 kV electron beam for successive periods of 2.5 and 2 h. High-resolution images and EDPs were recorded frequently as reactions (1)-(3) proceeded. Fig. 3 shows high-magnification images of a comer of the crystal taken at successive time intervals during this experiment. The right-hand images are enlargements of the areas inset on the left. In fig. 3a, which shows the crystal after approximately 30 min in the electron beam, lattice fringes in directions A, B, C are seen, corresponding to dolomite planes (101), (0il), (112), with respective spacings of 0.403, 0.403, and 0.370 nm. Fig. 3a shows variations in contrast resulting from thickness variations in the crystal. The upper-center portion of the right-hand image may show early stages of decomposition. The edge of the crystal shows an amorphous layer that might indicate contamination or the onset of decomposition. In fig. 3b, after another 5 min in the beam, the comer had deformed and the amorphous material is more conspicuous. The 0.24 nm fringes running into the amorphous area in the right-hand image are dolomite (110). In fig. 3c, after 66 rain exposure, considerable deformation and shrinkage have occurred. No dolomite fringes remain. Patches of 0.27 and 0.23 nm lattice fringes corresponding to fcc (111) and (202) spacings can be seen, surrounded by regions that appear amorphous or at least not appropriately oriented to allow imaging of the fringes.

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The corresponding EDP shows only diffuse arcs of the fcc (211) zone similar to those in fig. 2c. In fig. 3d, after 160 min exposure, the corner has rounded considerably, only a few areas of fcc lattice fringes are visible, and almost the entire area appears amorphous. Optical diffraction patterns from this image confirm the amorphous nature of the specimen. The corresponding EDP, fig. 2e, is a diffuse blur with broad, incomplete rings. In fig. 3e, taken after 250 min exposure to the electron beam, angular comers have emerged along the rounded exterior of the specimen. Patches of lattice fringes have reappeared, now with spacings corresponding to CaO and MgO. The spacings are 2.8 A, CaO (111), and 2.4 A, CaO (200) and MgO (111). Fig. 2f, which is the EDP corresponding to fig. 3e, shows only rings of CaO + MgO. In figs. 3d and 3e the orientation of the crystal relative to the beam is about 6 ° different from that in figs. 3a-3c. Lattice parameters of CaO and MgO, determined from diffraction rings in four EDPs of completely phase-separated decomposition product, gave average values of 0.483 + 0.003 and 0.427 + 0.004 nm respectively. These are in agreement, within their errors, with the accepted lattice parameters of pure CaO and MgO given above. This indicates that crystallites of the separated oxides are essentially pure, equilibrium phases, though formed in very small domains. Several grains of calcite (CaCO3), decomposed to CaO in the electron beam, gave EDPs with sharper rings than those from any of the dolomite decompositions. The lattice parameters of CaO from four of these gave an average and standard deviation of 0.4817 + 0.0004 nm. The precision of measurement of the EDPs is thus almost an order of magnitude better for CaO from decomposed calcite than from decomposed dolomite. Presumably this is because a much simpler decomposition mechanism to only one product is involved. The excellent agreement here with the accepted lattice parameter of CaO (0.4811 nm) establishes the accuracy of the calibration of the diffraction camera lengths of the TEMs. Towe [20] and Wenk and McTigue [21] have reported on decomposition of calcite in the TEM, studied at lower resolution than the present experi-

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E.D. Cater, P.R. Buseck /Decomposition of dolomite in electron microscope

ments. Wenk and McTigue [21] found that calcite decomposes to CaO by a topotactic reaction similar to that reported here for dolomite to the Ca0.sMgo.50 fcc solid solution. The overall mor-

phology of calcite crystals is generally retained in the decomposition process, which gives an aggregate of partially aligned CaO needles roughly 10 x 100 nm in size [20,22], a product quite different

E.D. Cater, P.R. Buseck / Decomposition of dolomite in electron microscope

247

i

!

Fig. 3. High-magnification sequence showing successive changes in the corner of a dolomite crystal undergoing decomposition in 200 keV electron beam. Beam normal to (1il). The insets in the left-hand images are enlarged at the right. (a) Image after 30 rain exposure. Inset (right) shows lattice spacings and directions corresponding to: (A) 0.403 nm (101); (B) 0.403 nm (0il); (C) 0.370 nm (112). (b) Same crystal 5 min later, showing early stages of decomposition. (c) Same region after a total of 66 min exposure. Insets show small domains of foe lattice fringes of spacings 0.27 nm (111) and 0.23 nm (200). (d) Same region after a total of 160 min exposure. Most areas are amorphous, though some foe fringes are visible in the inset (right). Fig. 2e is the corresponding EDP. (e) Same region after a total of 250 min in the electron beam. Lattice fringes of CaO (0.28 and 0.24 nm) and MgO (0.24 nm) are present in the inset (right). Fig. 2f is the corresponding EDP.

f r o m the r a n d o m l y oriented, 1 to 10 n m C a O and M g O crystalhtes obtained from dolomite.

6. Mechanism of decomposition The preferred orientations of the fcc decomposition p r o d u c t relative to the parent dolomite lattice are revealed by the relations of fcc arcs and dolomite single crystal spots in the EDPs. As decomposition proceeds, the parallel cation planes

retain the same separation as in dolomite. In E D P s of crystals imaged normal to the dolomite c-axis, pairs of {111} arcs of the fcc phase coincide with the {006} dolomite spots (fig. 2a). This coincidence shows that the alternating Ca and Mg planes of dolomite b e c o m e the close-packed metal planes of the fcc solid solution, and one b o d y diagonal of the fcc unit cell is parallel to the c-axis of the parent dolomite lattice. This relationship is shown schematically in figs. l b and lc. The CO32- planes in dolomite b e c o m e planes of 0 2- ions when C O 2

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E.D. Cater, P.R. Buseck / Decomposition of dolomite in electron microscope

is released and escapes from the lattice. Simultaneously, within the cation planes the 0.481 nm C a - C a or M g - M g distance of dolomite is reduced to 0.327 n m in the fcc decomposition product. The cations are redistributed and randomly occupy the formerly distinct Ca and Mg planes. N o evidence is found in the E D P s to indicate the existence of the larger unit cell that would be required by ordered cation planes. As the lateral shrinkage of the structure occurs, adjacent fcc domains must separate. Their orientations, relative to each other and to the original dolomite lattice, cannot then be precisely maintained. This disorientation gives rise to the char-

acteristic short diffraction arcs of 10 to 20 ° extent, rather than the more precisely defined spots that would appear if there were no disorientation. The diffuseness of the arcs probably is caused by strain arising from the mismatch of cation sizes and possibly also by variation of C a / M g ratio within and between crystalUtes. Fig. 4 illustrates this point by means of a highresolution image of a crystal undergoing reaction (1). Both dolomite (114) fringes of spacing 0.289 n m and fcc (002) fringes of 0.231 n m are visible. N o t e that most of the fcc fringes lie in groups directed at about + 10 ° relative to the directions of the dolomite fringes. The inset EDPs taken

Fig. 4. High-resolution image of dolomite crystal undergoing conversion to fcc solid solution. Black arrows show the orientations of some regions of dolomite (114) fringes of 0.289 nm spacing; white arrows show some regions of fcc (002) fringes of 0.23 nm spacing. (a) EDP before beam damage, indexed as dolomite zone (151). (b) EDP (fcc (100)zone) after complete conversion to Ca0.sMgo.50. Note that the fcc 002 diffraction arc lies in the same direction as the original dolomite 114 reflection.

E.D. Cater, P.R. Buseck / Decomposition of dolomite in electron microscope

before (fig. 4a) and after (fig. 4b) irradiation show that the diffuse 002 arc of the fcc (100) zone covers about 15 ° and lies near the direction of the i 1 4 reflection of the original dolomite pattern (fig. 4a). Fig. 2b shows the same relationship between fcc and dolomite diffraction patterns as figs. 4a and 4b, but from a different specimen. The sizes and ranges of orientation of the fcc domains give rise to the characteristic short diffraction arcs shown in figs. 2b and 4b. The directional relations shown by the arrows in fig. 4 are mirrored in the diffraction patterns. In figs. l b and l c the faces perpendicular to the paper, on the left hand sides of the projected cleavage rhombohedron of dolomite and unit cell of Ca05MgosO, are equivalent to the dolomite (114) and fcc (002) planes under discussion. Slight reorientation of adjacent fcc domains from the position shown in fig. lc would give rise to just the effects shown in the image and EDPs of figs. 2b and 4. Other evidence for the proposed mechanism of

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decomposition comes from analysis of 12 other EDPs with fcc (111} reflections taken with the electron beam oblique to the dolomite c-axis. In each case, within the typical arc lengths in the fcc patterns, the orientations are also consistent with the model of the fcc solid solution forming with the 3-fold cube axis parallel to the dolomite 3-fold axis. Another alignment frequently observed was fcc 202 and dolomite 110. These planes correspond to the plane of the paper in figs. l b and lc. This alignment is thus also consistent with the mechanism being presented. There is no indication of preferred orientation in EDPs of the final, phase-separated CaO and MgO (figs. 2f and 5). Apparently, during nucleation of these phases, all information about orientation of the original dolomite and the solid solution is lost. Fig. 5 shows the texture of a grain that has decomposed completely to CaO + MgO. The regions of darker and lighter contrast have diameters in the range 1 to 10 nm. By examining the original

Fig. 5. Image of completelydecomposeddolomite crystal with its EDP from randomly oriented CaO and MgO domains. Light and dark areas are approximately the sizes of the domains. From center the EDP rings are CaO 111, CaO 200 and MgO 111 superimposed, and MgO (200).

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E.D. Cater, P.R. Buseck / Decomposition of dolomite in electron microscope

prints of this and other images, e.g. fig. 3e, we conclude these are the sizes of the domains of CaO and MgO.

materials of a variety of compositions by electronbeam-induced decompositions of other minerals. Further research in this direction should be fruitful.

7. Implications for thermal decomposition Acknowledgements

Elsewhere [17] we compare our observations on decomposition of dolomite in the electron b e a m with those of investigators who have studied thermal decomposition in vacuum, but it is useful to present some conclusions here. Powell and Searcy [6] reported an intermediate thermal decomposition product that did not give coherent X-ray powder diffraction patterns. If solid solutions physically similar to the material shown in fig. 3c were formed, they would very likely give sufficient X-ray line broadening to appear amorphous. Spinolo and Tamburini [7,8] studied the thermal decomposition in an evacuated, high-temperature X-ray diffractometer. They report that solid solutions of 5 to 15% MgO in CaO, of particle size 2 to 5 nm, and essentially pure MgO are formed initially " i n a direct and diffusionless process". On further annealing they obtained pure oxide phases. Despite the authors' characterization of the thermal process as "diffusionless", it apparently permitted greater cation diffusion in the initial stages than did our electron-beam decomposition, as evidenced by the much smaller concentrations in their solid solutions. This difference shows that electron-beam decomposition initially involves radiolysis of the carbonate ion at a sufficiently low temperature to prevent cation diffusion and consequent phase separation. It would thus be somewhat misleading to use the c o m m o n term " b e a m heating" to describe our experiments. In recent T E M studies of dolomite at 500 keV [23,24] beam damage was not a problem. This is the expected behavior [2,3] if the energy is absorbed principally in the covalently bonded carbonate ion as we suggest. The formation of the metastable Ca0.sMgo.50 solid solution in the electron beam, in contrast to the formation of more nearly equilibrium products in thermal decomposition, has interesting implications for preparative purposes. It may be possible to prepare unstable, finely divided, very reactive

Financial support was provided by Earth Sciences Division, NSF, Grant No. EAR84-08168 to Arizona State University. Electron microscopy was performed at the ASU Facility for High Resolution Electron Microscopy, funded through N S F Regional Instrumentation G r a n t CHE-7916098. E.D.C. thanks the Graduate College of the University of Iowa for support through a Faculty Development Assignment.

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