Transmission electron microscopy of interfaces and defects ...have been interpreted through extensive computer-generated image simulations. The results illustrate the complexity of

American Mineralogist, Volume 74, pages 1070-1083, 1989

Transmission electron microscopy of interfaces and defects inintergrown pyroxenes

KBNNnrs J. T. Lrvr, Dlvro R. Ynnr.rNDepartment of Earth and Planetary Sciences, The Johns Hopkins University, Baltimore, Maryland 21218, U.S.A.

Arsrucr

High-resolution and conventional transmission electron microscopy has been used tostudy exsolved augites from the l,aramie Anorthosite Complex, and the HRrEM imageshave been interpreted through extensive computer-generated image simulations. The resultsillustrate the complexity of interface structures and morphologies in exsolved pyroxenesand confirm that defects such as dislocations and stacking faults play an important role inpyroxene exsolution processes.

Pigeonite in the Laramie augite occurs as both " 100" and "001" lamellae, and the degreeof coherency of the interfaces is inversely related to the lamellar size. Both sets of lamellaegrew by the propagation ofgrowth ledges, and nnrBu images ofthe dislocations on "100"growth ledges are consistent with unit Burgers vectors of [001]. The interfaces of "001"pigeonite lamellae have a stepped morphology where they intersect (100) stacking faultsin the pigeonite; the faults have the stacking sequence + - - + and therefore can be de-scribed as single blocks of orthopyroxene structure. The'%r[001] partial dislocations atthe terminations of the stacking faults are dissociated into two equal partials approximately5 nm apart, and the stacking sequence ofthe faults between the two partials is +-+.Where "100" and "00l" pigeonite lamellae approach each other, the "l00" lamellaetypically have bulges on both sides, and the "001" lamellae are hooked, probably at leastpartially because of interaction between the strain fields of the growing lamellae. In anadditional unrelated observation, narrow orthopyroxene lamellae grew from their tips asentire lamellae, rather than by a ledge mechanism. Each growing orthopyroxene unit cellis led by two partial dislocations, the strain fields ofwhich cancel each other.

Most of the stacking faults observed in orthopyroxene involved skew reversal of an evennumber of (100) octahedral layers, and most of the faults observed in pigeonite alsoconsisted of even numbers of octahedral layers and were separated by slabs of ordinarypigeonite containing an even number of octahedral layers. However, a number of odd-layered faults also were observed in both orthopyroxene and pigeonite, indicating that afull description ofthe stacking phenomena in real pyroxenes requires the use of0.45-nm(100) slabs of structure, rather than the 0.9-nm layers used by some authors. HRrEM imagesindicate that the stacking faults in pigeonite nucleate symmetrically about the B chains ofthe pigeonite structure. The structures and distribution of the stacking faults show thattheir positions are structurally controlled and confirm that they formed below the C2/c -

P2, / c lr ansrtion temperature.

INrnonucrroN

It has been long recognized that rock-forming pyrox-enes commonly exhibit a wide variety of intergrowthstructures that result from exsolution (also called precip-itation) (Poldervaart and Hess, 1951; see also review byBuseck et al., 1980). Although these intergrowths in or-thopyroxene, augite, and pigeonite were initially reportedto form in rational orientations parallel to (001) and (100),it is now recognized that these intergrowth orientationsare variable, that they depend on the detailed crystalstructures and unit-cell parameters at conditions of nu-cleation and during growth, and that exsolution micro-structures observable with the petrographic microscope

can be quite complex (e.g., Robinson et al., 1977; Rob-inson, 1980). Furthermore, transmission electron mi-croscopy (rertr) studies have demonstrated that featuresbelow the resolution of the light microscope, such asgrowth ledges, stacking faults, and interface dislocations,can play an important role in the development and ther-mal equilibration of pyroxene intergrowths during cool-ing or annealing (Champness and Lorimer, 1976; Nordet al., 197 6; Iijima and Buseck, 1975; Kohlstedt and Van-der Sande, 1976; Champness and Copley, 1976; Robin-son et al., 1977; Buseck et a1., 1980; Kitamura et al.,198 la; Nord, 1980, 1982; Nobugai and Morimoto, 1979;'Rietmeijer and Champness, 1982; Crawford et al., 1983;Mori and Takeda, 1988). These features are in general a

0003-004x/89/09 I 0-l 070$02.00 1070

response to the strain involved in the fitting together ofthe host and precipitate structures.

Most of the rsNr studies of pyroxene exsolution phe-nomena have been performed using conventional bright-and dark-field imaging techniques or one-dimensionallattice imaging of fringes parallel to (100). In the presentpaper, we present results of [0 10] two-dimensional high-resolution rerra (nnrervr) experiments on exsolved augitesfrom the Laramie Anorthosite Complex (Livi, 1987). Inorder to interpret such images reliably, we have per-formed extensive computer simulations of the nnrrvr im-ages under a wide variety of imaging conditions. The ex-perimental images reveal previously unresolved detailsof quadrilateral-pyroxene interface structures and allowinterpretation of exsolution mechanisms for the Laramiepyroxenes. In addition, we present conventional rru ob-servations on unusual lamellar morphologies that resultfrom interactions among crystallographically different setsof exsolution lamellae.

S,q.N{pr,s DESCRIPTTON AND EXpERTMEN-r'AL

METHODS

Exsolved augites in sample LAC-6A from the fine-grained ferromonzonite border facies of the Laramie An-orthosite Complex have been described by Livi (1987),Fuhrman et al. (1988), and Frosr and Lindsley (1981).The Fe-rich augite crystals are anhedral to subhedral, rangeup to 3 mm in diameter, and exhibit a variety of exso-lution textures. Livi (1987) described in detail the chem-istry and texture ofsubsolidus structures pertinent to theinvestigation ofthe augite cooling history, identifying mostof the exsolved lamellae as very low Ca pigeonites andnoting the presence of small numbers of orthopyroxenelamellae. Although the lamellae exhibit a wide range ofsizes and orientations, their compositions indicate thatall lamellae have re-equilibrated at 600 to 650 .C.

The present paper describes further observations onthe microstructures present in LAC-6A augites. Thinspecimens were prepared using standard Ar-ion millingtechniques and were lightly coated with C. Low-resolu-tion rru was performed with a JEoL 200cx electron mi-croscope at the State University of New York at StonyBrook using procedures reported by Livi (1987). HRrEMwas performed with a Philips +zosr microscope at TheJohns Hopkins University. Optical conditions for thePhilips microscope are described by Livi and Veblen(1987). Proper specimen orientations for nnreu imagingwere obtained with a 3-mm Philips double-tilt specimenholder, and images presented here were recorded at mag-nification settings ofbetween 730000x and I 230000x.

Two-dimensional imaging of pyroxenes parallel to theirb axes requires resolution ofthe (002) fringes, which havespacings of approximately 0.25 nm. Since this spacing isbeyond the Scherzer limit of our microscope (0.30 nm),intuitive interpretation of the Hnrevr images is not pos-sible, necessitating the use of computer-simulation meth-ods to interpret the imaging results. Simulated high-res-olution images of b-axis micrographs were calculated using

1 0 7 1

the 80F version of the sHRLr programs (Self and O'Keefe,1988; O'Keefe et al., 1978). The halftone output routineswere modified for use with a dot-matrix graphics lineprinter. The calculation of electron-diffraction ampli-tudes and phases was fully dynamical, using the multi-slice formulation of Cowley and Moodie (1957).

The input structures for the image simulations werethose refined by Cameron etal. (1973) for augite, by Smyth(1973) for orthopyroxene, by Morimoto and Koto (1969)for another orthopyroxene and a model in which clino-hypersthene was twinned into the ortho structure, and bySmyth (1974) for clinohypersthene. Cation occupancieswere altered for consistency with the pyroxene composi-tions observed in LAC-6A (Livi, 1987). There were noappreciable differences in calculated images for the tworefinements oforthopyroxene, nor were there between thecalculated images for the orthopyroxenes and the modeltwinned clinohypersthene. The orthopyroxene calcula-tions shown here are for the Smyth (1973) structure. Itwas found that varying the model slice thicknesses didnot result in noticeable differences in the simulated im-ages; as a result, a slice thickness equal to the length ofthe b axis was utilized for most calculations. The simu-lations were required to match the experimental imagesof two different structures observed at interfaces underidentical conditions of defocus, crystal thickness, and ori-entation. It was found that the imaging conditions arehighly constrained by this requirement of matching theexperimental and computer-simulated images of twostructures.

IuacB sTMULATIoNS

Figure I presents selected simulation results for augite,pigeonite, and orthopyroxene images viewed down the bcrystallographic axis at various focus settings and at crys-tal thicknesses of 4.5 and 9.0 nm. It was necessary tocalculate images at defocus intervals of approximately 5nm, since the image character can change significantlywith small changes in focus. However, owing to spacelimitations, Figure I presents only selected simulationsthat generally illustrate the manner in which pyroxeneimages change with focus and thickness for a microscopewith the optical characteristics of the Philips +zosr (i.e.,point-to-point resolution of approximately 0.3 nm).

Contrast details of simulated images

The correspondence of simulated-image characteristicsto the pyroxene structures can be summarized as follows.With crystals 4.5-nm thick, focus settings between *20and 0 nm produce white spots that lie close to the planesof the octahedral sheets, either over or between the oc-tahedral cation positions (see *20-nm images). At focus-es from approximately -20 to -65 nm, lines of whitespots lie over the tetrahedral chains. At -65 nm, themore prominent lines of dots correspond to the positionsof the A chains for both the pigeonite and the orthopy-roxene structures. The B-chain spots for pigeonite andorthopyroxene and the spots for augite are all tilted, and

LIVI AND VEBLEN: INTERFACES AND DEFECTS IN INTERGROWN PYROXENES

t0'72

T


o=' AUGITE4.5 nm 9.0 nm

PIGEONITE4.5 nm 9.0 nm

ORTHOPYROXENE4.5 nm 9.0 nm

+20 nm

-35 nm

-55 nm

-65 nm

-80 nm

-100 nm

-135 nm

ffiffiNffi

ffiffiffilKHill ffilllfl ffinl|Il$lllllluilgffiw$*E$ffiffi

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the tilt direction of the spots indicates the skew of theadjacent octahedral strips (see Thompson, 198 I, p. 146,for a definition of the * and - stacking vectors thatdenote the direction of octahedral skew). It is thus pos-sible, at least in principle, to determine the exact stackingsequence of a pyroxene from images taken under focusvalues near -65 nm. At defocus values of approximately- 90 to - I 30 nm, the brightest spots in the image againlie in or near the planes of the octahedral sheets, eitherover or between the octahedral cations. At greater defo-cus values, the bright spots switch back to the tetrahedralchains.

The orthopyroxene simulations in Figure I show evenintensity ofalternate (100) rows ofspots (i.e., an apparent0.9-nm periodicity parallel to the d axis in the intensitiesof the rows of spots). However, experimental high-reso-

lution images commonly exhibit an asymmetry in thesizes and intensities of the spots corresponding to alter-nate (100) layers of A chains, even though these layersare symmetrically identical. Our diffraction calculationsshow that in the case of an orthopyroxene crystal per-fectly oriented down the b axis and a parallel (i.e., notconvergent) incident beam, dynamical scattering (multi-ple diffraction) into the 100 beam is negligible. Thus, un-der such ideat illumination conditions, those diffractedbeams that are kinematically forbidden by the orthopy-roxene allide plane (e.g., the 100 beam) would havevery low intensities. Since it is primarily these odd-orderft00 diffractions that contribute to the image informationwith 1.8-nm periodicity, the resulting images exhibit onlya symmetric 0.9-nm periodicity in spot intensities.

In order to determine the cause for the differences be-

llllllt ffirittA B A B

Sffiffiffi

1llllllllilllllllillIll|IIffiffi

1 . 8 n m

Fig. l. Representative computer-simulated D-axis nnreu images for augite, pigeonite, and orthopyroxene at crystal thicknesses

of4.5 and 9 nm and at various focus settings. The vertical arrows denote positions ofthe tetrahedral chains. A and B refer to the

chain type. A 1.8-nm scale bar is in the lower left corner of the diagram.

to'13

tween calculated and experimental orthopyroxene im-ages, we performed a series of simulations in which aparallel electron beam was tilted slightly with respect tothe orthopyroxene b axis. The calculations at -65-nm

defocus and 4.5-nm crystal thickness (Fig. 2a) demon-strate that in extremely thin crystals, the rows of dotscorresponding to alternating (100) layers ofA chains be-come unequal in intensity. Alternate layers of B chainslikewise become asymmetrical, though not so noticeably.However, the changes in spot size and intensity for thincrystals are minor compared to those for thicker crystals(Fig. 2b). This is because dynamical diffraction becomesmore important with increasing crystal thickness. Inspec-tion of the dynamical intensities indicates that the odd-order i00 intensities become appreciable for crystals 9-nmthick, and this is reflected in the pronounced asymmetryin spot intensities seen in some of the images of Figure2b. Yet, when the images calculated for the central beamtilted in the direction of the 004 and 004 beams (approx-imately 4 milliradians or 1.4' off-axis) are compared, thepositions of the large and small spots are reversed. Animage formed from a convergent beam (a cone of illu-mination containing a symmetrical range of angles of in-cident electrons) and a perfectly oriented crystal shouldthus be a summation of images of opposite intensity andtherefore should have spots ofeven sizes and intensitiesIying over all layers of A chains. We must conclude,therefore, that the asymmetry in many of our orthopy-roxene images is due to small crystal misorientations orslight misalignments in the illumination system. Such im-ages can still be interpreted with the aid of computer-generated image simulations.

Matches of simulated and experimental images

Calculated images of augite, pigeonite, and orthopy-roxene are compared with experimental images in Fig-ures 3a. 3b. and 3c. Since defocus values of -55 to -80

nm create images with high contrast and are near theoptimum (Scherzer) defocus value for the microscope,most high-resolution images were recorded at these con-ditions. The excellent matches for augite and pigeonitewere obtained at -63-nm defocus and 4.5-nm thickness.The experimental image of orthopyroxene in Figure 3ashows the asymmetrical spots mentioned above. An im-age calculated with the central electron beam parallel tothe orthopyroxene 802 beam direction, -65-nm defocus,and 9.0-nm thickness matches relatively well with theillustrated experi mental image.

The important features to note in images obtained un-der these conditions are as follows: (l) the white dots lieover the silicate chains; (2) the sizes of the dots can beused to distinguish between the A and B chains in pi-geonite and orthopyroxene; and (3) in slightly misorient-ed orthopyroxene, alternate A chains produce differentcontrast.

PvnoxnNp INTERFACES

As noted above, the Fe-rich augite LAC-6A containsabundant exsolution lamellae of pigeonite and relatively

t . 5 o

( 3 , 0 , 0 )


( 0 , 0 , 4 )

( 0 , 0 , 2 ) ( 2 , o , 2 \ (

( 0 , 0 , 0 )

b

ffi( 0 , 0 , 4 )

m-'{K( 8 , 0 , 4

( 0 , 0 , 0 ) ( 3 , 0 , 0 )

Fig.2. (a) Simulated tilt matrix showing the effects of slightcrystal misorientations on HRrEM images from a 4.5-nm-thickorthopyroxene crystal at a focus setting of -65 nm. Numbers inparentheses indicate the indices ofthe diffracted beam that wasparallel to the b axis for each simulation. (b) Simulated Hnrsr"itilt matrix as in part a, but for a 9-nm-thick crystal.

few orthopyroxene lamellae. At the level of resolution ofthe petrographic microscope, orthopyroxene lamellae arenot observed, but rElr observations indicate that narrowlamellae are present and are rigorously parallel to (100)of the host augite. With the petrographic microscope, itis observed that pigeonite lamellae lie in orientations thatdeviate slightly from (100) and (001), as described by Livi(1987); these are referred to as "l00" and "00l" lamellae.Such deviations of lamellar orientations from (100) and(001) are typical ofexsolved augites (Robinson, 1980).

1074

Augite-pigeonite interfaces

Pigeonite lamellae in LAC-6A augites are typical ofexsolved pigeonites found in other studies in that theyhave growth ledges, stacking faults, and dislocations attheir boundaries (Copley et al., 19741, Champness andCopley, 1976; Nord eI al., 1916:- Robinson et al., 1977.,Rietmeijer and Champness, 1982). These features will beaddressed in light of two-dimensional pyroxene latticeimaging. More unusual features such as bulging andhooking of lamellae will be discussed in a later section.

The pigeonite lamellae in LAC-6A augites exhibit arange of interface coherency with the host arrgite. A few


Fig. 3. Comparison of experimental nnrnu images with sim-ulated images (inset) of pyroxenes. (a) Orthopyroxene. Param-eters used in the simulation: crystal thickness: 9.0 nm, defocus: -65 nm, b axis oriented parallel to the 802 beam direction.(b) Pigeonite. (c) Augite. Parameters for the pigeonite and augitesimulations: thickness : 4.5 nm, defocus : -63 nm, D axisperfectly parallel to the microscope axis.

LAC-6A pigeonite lamellae termed "inverted" containwithin them lamellae of augite, orthopyroxene, and pi-geonite. These inverted pigeonites have incoherentboundaries. Large, "uninverted" pigeonite lamellae withhigh densities of (100) stacking faults are found to besemicoherent with a high density of dislocations, whereaspigeonites of intermediate size have semicoherent inter-faces with a lower density of dislocations, and thin la-mellae are coherent (with no interface dislocations). Fig-ure 4 shows the interface between a coherent "001"pigeonite lamella and the augite host. By following the(200) fringes across the boundary while sighting at a lowangle, it can be seen that the two (100) planes are at asmall angle to each other, while the (002) fringes in pi-geonite and augite are parallel. These relations show thatthin "001" pigeonites have their a axes parallel to thoseof the augite host, but the c axes are tilted. Similar imagesof thin "100" pigeonite lamellae show that the (200)fringes of host and lamellae are parallel, indicating thatthe c axes are parallel, but the 4 axes are not parallel.These orientation relationships between the two latticesare consistent with those observed previously with othermethods (Buseck et a1., 1980;Robinson, 1980).

Figure 4 also shows that the augite (200) fringes bendin a subtle "s" fashion over the space ofseveral unit cellsbefore reaching the augite/pigeonite boundary distin-guished by the meeting of the two pyroxenes' distinctive

Fig. 4. A D-axis unreu image showing relative axial orien-tation and bending offringes at the coherent interface ofa "001"pigeonite-augite exsolution lamella. The lower set of arrows in-dicates the trace of the boundary, and the upper set indicateswhere most of the bending of fringes takes place

image contrast. The major portion of the bending occursapproximately six augite unit cells from the boundary.This suggests that the strain of fitting the two structurestogether is distributed over a distance of a few nanome-ters from the actual boundary. This bending contrastswith the assumption of the exact-phase-boundary theoryfor pyroxenes, in which two rigid lattices are matched byvarying orientations of the interfaces and the lattices (e.g.,Robinson etaI., 1977). However, as shown by Livi (1987),the exact-phase-boundary hypothesis does explain thetrends among size, orientation, and history of pigeonitelamellae in LAC-6A augites. The exact-phase-boundarytheory therefore may not be seriously affected by latticesthat are not completely rigid and interface regions thatare several nanometers in width, rather than being a dis-crete interface plane. It obviously is not known whetheror not the boundary also involves a gradation or an abruptchange in chemistry.

Previous studies of augite-pigeonite grain boundarieshave demonstrated that "l00" pigeonite lamellae in au-gite hosts thicken by means of growth ledges (Champness

107 5

Fig. 5. A b-axis nnruu image ofdislocations with projectedBurgers vector l[001] (arrowed) on a very large growth ledge ofa " 100" pigeonite lamella. The " 100" lamella contains a pair of(100) stacking faults. The large defect in the center that is notarrowed is a combination of more than one dislocation and hasa resultant Burgers vector of [201].

and Lorimer , 197 3; Copley et aI., 197 4; Kitamura et a1.,198 1a). However, the dislocations found at the tips ofledges have not been fully characterized previously. Fig-ure 5 shows a two-dimensional lattice image of an un-usually wide growth ledge on a "100" pigeonite lamellain the LAC-6A augite. Enlargements of such images allowthe construction ofBurgers circuits around these defects.The projected Burgers vector thus determined to be as-sociated with " 100" growth ledges is l[001]. Nord et al./l97O have shown that unit dislocations with this Bur-gers vector provide good nucleation sites for "100" pi-geonite lamellae. Nucleation of secondary pigeonite la-mellae on these ledges at lower temperatures may thusaccount for the branching "100" lamellae observed inthis augite (see Fig. 4 of Livi, 1987).

Stacking faults and associated dislocations in"001'o pigeonite

Stacking faults rigorously parallel to (100) are foundcommonly in pigeonite lamellae. Many studies have in-vestigated the nature of similar stacking faults in a varietyof pyroxenes and have suggested that they can be de-scribed as unit-cell scale twins with displacement vectors'o/,2f001f (Champness and Copley, 1976; Coe and Kirby,1975; Iijima and Buseck, 1975; Mclaren and Etheridge,1976). Robinson et al. (1977) observed stacking faults in


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\

I

1076 LIVI AND VEBLEN: INTERFACES AND DEFECTS IN INTERGROWN PYROXENES

Fig. 6. (a) HnrErvr image of stacking faults in pigeonite terminating at a "001" pigeonite-augite boundary. The arrows point tothe dissociated partial dislocations at the tips of the stacking faults. Steps in the interface associated with the stacking faults areevident, and the rotation of c axes across the interface can be seen clearly by viewing at a low angle parallel to c.

"001" pigeonite lamellae that terminate at the interfacewith augite, similar to the faults observed in the presentstudy. They argued persuasively that the stacking faultsform during cooling, in order to minimize the strain en-ergy in response to the changing pigeonite a1d arrgite unit-cell parameters.

We have obtained HRrEM images from LAC-6A, as giv-en in Figure 6a, that show in detail the stacking faults,the dislocations at their terminations, the distribution ofaugite and pigeonite in the neighborhood of the termi-nations, as well as the lattice rotation previously dis-cussed. Burgers circuits constructed around stacking-faultterminations on the HRrEM images confirm a projectedBurgers vector of approximately '%r[001]. However, theimages also show that the partial dislocations pinned atthe boundaries of the lamellae are dissociated into two

equal partial dislocations with Burgers vectors of %r[00 I ].A schematic diagram illustrating such a Burgers circuitand the two partials is given in Figure 6b. The partialdislocations are generally separated by about 5 nm; thedissocation into partial dislocations probably has not beenobserved previously because the resolution of conven-tional bright- and dark-field rEM experiments is typicallyon the order of 5 nm. The dissociation of the interfacedislocations suggests that the formation of a stacking faultmay take place in two steps; the passage of each partialdislocation through the structure produces a relative shiftbetween two adjacent (100) sheets of oxygen atoms of%r[001]. The images also show that between the partialdislocations, the stacking faults separate augite on oneside from pigeonite on the other, producing steps in thepigeonite-augite interface.

LIVI AND VEBLEN: INTERFACES AND DEFECTS IN INTERGROWN PYROXENES 107'7

Fig. 6-Continued. (h) Schematic diagram showing Burgers circuits around the termination of a stacking fault in pigeonite,illustrating the splitting of the fault into two equal partial dislocations. The portion of the fault between the partials is a single-layered fault with structure + - +, whereas the stacking fault at the bottom ofthe figure has the more normal structure + - - +. (c)nnreu image of the termination of a stacking fault in which the region between the two dissociated partial dislocations involves anantiphase boundary.

The stacking sequence of an unfaulted clinopyroxenecrystal can be represented as (+), indicating that all oc-tahedra have the same skew; the sequence of stackingvectors is placed in parentheses to indicate that it repeatsperiodically (see Thompson, 1981, p. 146, and Veblen,I 98 l, p. 194, for definitions and usage of + and - poly-type nomenclature). The normal stacking fault structurethat we observe in these pigeonites can be represented as* - - *, indicating that the fault involves reversal of theskew for two adjacent octahedral sheets (this sequence isnot placed in parentheses, indicating that it is a faultstructure and does not repeat periodically). We refer tothis type of stacking fault as a two-layer fault, or an even-layered fault, because it contains an even number of oc-tahedral layers. This type of stacking fault also could bedescribed as a single unit cell oforthopyroxene structure,since the normal orthopyroxene stacking can be repre-sented as a periodic repetition of the sequence (+ - - +).Between the two partial dislocations, however, the stack-ing fault has the structure *-+, which could be de-scribed as a single unit cell of protopyroxene structure,since the proto stacking sequence is (+-;. Because thislatter type of stacking fault involves skew reversal of only

a single octahedral layer, we refer to it as a oneJayer fault,which is one type of odd-layered fault. In a later section,we discuss the frequency ofoccurrence ofeven- and odd-layered faults in rock-forming pyroxenes.

Figure 6c shows an additional complication observedat one stacking fault termination at a "001" pigeonite-augite interface. Not only does the stacking fault disso-ciate into two partials, but the type of tetrahedral chain(A or B) appears to change between the two dislocations,producing a short antiphase boundary in the pigeonitealong the one-layer section of stacking fault. It is notknown how common this structure is, since the numberof terminating stacking faults found in areas thin enoughfor nnrnvr imaging was small.

Dissociated partial dislocations are common in metals,and their presence in these pyroxenes is not surprising,since the clinopyroxene structure permits splitting of thewhole dislocation. For a given Burgers vector direction,the energy of a dislocation is approximately proportionalto the square of the Burgers vector magnitude lbl'(e.9.,Hull, 1965, p. 78). Therefore, the total dislocation energymay be reduced by approximately one-half if the dislo-cation can be dissociated into two parts. Since both Bur-

r078

Fig.7. rEu image ofhooked "001" and bulging "100" pi-geonite lamellae in augite. By viewing at a low angle parallel tothe "100" lamella, it can be seen that bulging occurs on bothsides. Fine lines in this lamella are stacking faults.

gers vectors have the same sign, the partial dislocationsshould repel each other. The distance between the partialsis a balance between the repulsive forces that arise fromelastic interaction and the attractive force represented bythe energy of the + - * stacking fault between the par-tials. The distances observed between the paired partialswere relatively constant at l0 ro ll.5 x c, or 5.2 to 6.0nm, suggesting that the dissociated configuration adoptedby these interlace dislocations represents an energy min-imum. HRTEM or weak-beam rerra observations of dislo-cations at augite-pigeonite interfaces in other occur-rences, such as those shown in Figure 12 ofRobinson etal. (1977), would clarify whether or not such a dissociateddislocation structure is the usual one for "001" pigeonitelamellae with (100) stacking faults. Likewise, further workusing Hnrrr"r or weak-beam techniques will be necessaryto determine if the dislocations associated with stackingfaults involved in structural phase transformations in low-Ca clinopyroxenes and orthopyroxenes (e.9., Mtiller, 1974;Coe and Kirby, 1975; Buseck and Iijima, 1975; Mclarenand Etheridge, 197 6) are dissociated.

Morphology of pigeonite lamellae

Several studies have indicated that pigeonite exsolu-tion lamellae in Fe-rich augites can have complex mor-phologies not observed in more Mg-rich pyroxenes. Theseunusual lamellae either terminate with an obtuse-angled


hook or have peculiar bulges that are spatially related toneighboring lamellae (Kitamura et al., 198 lb; Rietmeijerand Champness, 1982; Livi and Veblen, 1984). Figure 7shows a typical pair of lamellae that exhibit these fea-tures. In LAC-6A augite, hooking occurs exclusively in"001" pigeonite lamellae that are near "100" pigeonitelamellae. Bulging is only present in "100" pigeonite la-mellae in the vicinity of hooking lamellae. This bulgingoccurs on both sides of the "l00" lamellae, even whenthere is a hooked "001" on only one side. It is observedthat "l00" lamellae impinging on "001" lamellae do notresult in bulging of the "00l" lamellae.

Suitable explanations for this texture should satisfac-torily take into account all ofthese features. The investi-gators mentioned above all suggest that changes in unit-cell parameters contribute to changes in preferred growthorientations (either "100" or "001"). Rietmeijer andChampness (1982) also speculated that solute-depletedzones at the borders of "100" lamellae interfere with thegrowth of approaching "001" lamellae. The hooks arethen added at a lower temperature when the growth di-rection switches from "001" to "100." This hypothesisdoes not explain why hooking is always accompanied bybulging, which might not be expected to form in a solute-depleted zone.

Kitamura et al. (1981b) attributed lamellar hooking toa thermal interval (called the morphological change tem-perature, MCT) in which the direction of favored growthswitches from "00l" to "100." I-amellae growing in thistemperature range will exhibit both "00 I " and " 100" ori-entations. However, the MCT hypothesis does not ac-count for the spatial relationship between hooked lamel-lae and " 100" lamellae, nor does it explain why the " I 00"lamellae bulge in the presence of impinging "001" la-mellae.

Although the models of Rietmeijer and Champness(1982) and Kitamuraetal. (l98lb) may explain hookingin some occurences, the spatial relationship between"00 I " hooking and " l 00" bulging suggests that these fea-tures result at least partially from the interacting strainfields of the two lamellae and from the resulting changesin energetically favored interface orientation. Similarstrain field interactions arising between different twin ori-entations have been invoked to explain unusual, non-planar lamellae in alkali feldspars (Salje et al., 1985). Inthe present case, a change in interface morphology couldbe due to local, strain-induced changes in the unit-cellparameters of the augite host that lies between the " 100"and impinging "001" lamellae. If the strain field from a"00 I " lamella extends through the " 100," then this couldexplain why bulging occurs on both sides of the lamellae.It is also possible that solute depletion contributes tohooking of "001" lamellae, as suggested by Rietmeijerand Champness (1982), whereas " 100" bulging is the re-sult of the strain field from the impinging "00l."

As new growth proceeds, the hooked and bulged la-mellae can join to form a hinge structure, as shown inFigure 8. All such hinge structures that we have observed


Fig. 8. (a) rer'r image of intersecting " l 00" and "00 1 " pigeonite lamellae. Boxed region is enlarged in part b. (b) Enlargementof the hinge region of one of the lamellar intersections shown in part a. The large arrows point to notches of augite included in thepigeonite hinge. The small arrows indicate a low-angle boundary between the "l00" and "00l" pigeonite lamellae; the boundaryforms when the lamellae intersect, because "l00" and "001" lamellae are oriented differently in the augite host.

r079

in LAC-6A include a small sliver of host augite in theacute side of the hinge. The dislocations associated withthese "notches" have Burgers vectors of l[001], identicalto those associated with growth ledges on " 100" lamellae.These notches may represent Ca-rich pyroxene caughtbetween impinging pigeonite lamellae.

Augite-orthopyroxene interfaces

Little orthopyroxene is present in LAC-6A, with thelargest lamellae being only a few tens ofnanometers wide.Figure 9a shows one lamella terminating in the augitehost. A circuit constructed around the tip (shown sche-matically in Fig. 9b) indicates that the augite lattice isundisturbed, and there is no evidence of strain contrastin the augite. The growth of orthopyroxene from a mono-clinic pyroxene host is thought to involve the propagationof '0lr[001] partial dislocations (e.g., a Burgers vector of%[001] as suggested by Champness and Copley, 1976).Unfortunately, our HRTEM images do not shed any furtherlight on the exact dislocation structure involved in thistransformation. However, we can state that the interfaceat the lamellar tip is very tight, and the apparent absenceofstrain suggests that the strain fields ofany dislocationsinvolved in the growth of the orthopyroxene in LAC-6Amay cancel each other, at least partially.

Previous studies of augite exsolution lamellae in ortho-pyroxene have suggested that the lamellae grow by amechanism in which narrow ledges that are only one tofour unit cells wide propagate along the lamellar interface(Champness and Lorimer, 1973; Vander Sande andKohlstedt. 1974: Kohlstedt and Vander Sande. 1976).

However, the termination of the entire orthopyroxene la-mella in Figure 9a suggests that in this case of orthopy-roxene exsolving from augite, the lamellae formed by col-lective growth of a larger number of orthopyroxene unitcells. Failure to observe ledges far from the tips of la-mellae suggests that they did not coarsen following initialgrowth at the lamellar tip. However, such ledges couldhave been missed, given the low density of orthopyrox-ene lamellae in LAC-6A.

Srnucrunn AND spAcING oF sTACKING FAULTS rNROCK-FORMING PYROXENES

There have been numerous observations of (100)stacking faults in low-Ca clinopyroxenes and ofthopyrox-enes (for review, see Buseck et al., 1980). Such stackingfaults commonly have been discussed in terms of thestacking of 0.9-nm structural slabs parallel to (100), forexample, by Buseck and Iijima (1975). Such slabs, whichare most conveniently defined as being bounded by planeswithin the layers of silicate chains of the pyribole struc-tures, contain two layers of octahedral strips. As a result,we refer to stacking faults that can be fully described withsuch 0.9-nm slabs as evenJayered faults. Two commonexamples of even-layered stacking faults in pyroxenes in-clude (l) pairs ofoctahedral layers having reversed skewin clinopyroxenes, resulting in isolated blocks with+ - - + structure, and (2) extra pairs of octahedral layerswithout skew reversal between the A chains oforthopy-roxenes, resulting in blocks with *----+ structure.In addition, interpretation of the structure of pyroxenes

1080

with highly disordered stacking typically has been basedon the assumption that they consist of 0.9-nm slabs.

While most stacking faults and stacking disorder thathave been observed in pyroxenes (and other pyriboles)can be described in terms ofeven-layered fault structures,there is no topological reason why one-layer or other odd-layered faults cannot occur in pyriboles. Odd-layered faults


Fig. 9. (a) A D-axis nnrnn image ofthe tip ofan orthopy-roxene lamella in augite. (b) Schematic diagram of orthopyrox-ene growth in augite. The dark line traces a Burgers circuit thatshows there is no observable disruption ofthe augite host.

cannot be described in terms of 0.9-nm slabs, but insteadrequire 0.45-nm (100) slabs. Such faults include, for ex-ample, (l) faults with +-+ structure in clinopyroxeneshaving the ideal stacking sequence (+) and (2) faults with+ - + or + - - - + structure embedded in otherwise nor-mal orthopyroxene with ideal stacking (+ - - +).

Although odd-layer faults are structurally permissible(i.e., one can build structure models having such faultswith no difficulty), they have not been reported previ-ously. Indeed, in the present study of LAC-6A, the mostcommon stacking faults by far are of the evenlayeredtype, such as the + - - + faults in pigeonite shown inFigure 6a. However, as noted above, the short bands ofstacking fault between the dissociated partial dislocationsin this figure are odd-layered faults having the structure+-+. These faults also are peculiar because they areassociated with steps in the "001" augite-pigeonite inter-face and because they separate augite on one side ofthefault from pigeonite on the other.

Stacking faults also occur in the orthopyroxene lamel-lae of LAC-6A. Again, most of these faults are of theeven-layered type, but oddJayered faults also occur. Thelattice image shown in Figure l0a, obtained under im-aging conditions in which dark bands parallel to (100)emphasize the normal L8-nm orthopyroxene periodicity,shows both types. The 2.7-nm block of structure (six 0.45-

Oop, = 2 ?",,s Sin R

LIVI AND VEBLEN: INTERFACES AND DEFECTS IN INTERGROWN PYROXENES 108 |

ls

Fig. I 0. (a) Odd- and even-layered stacking faults in orthopyroxene. Numbers refer to the structural width between dark verticalfringes in units of0.45-nm (100) layers. All such units in unfaulted orthopyroxene have widths of4. (b) EvenJayered faults inpigeonite with odd and even numbers of chains between them.

4 4 46 444

2 4 2 6 2 7 2 2 2

nm layers) in this figure corresponds to an even-layeredfault of the type + +. (All even-layered faults willproduce blocks of structure having widths of 2n x 0.45nm.) Also shown. however. is a 1.35-nm block. which isconsistent with an odd-layered fault unit of + - + struc-ture inserted into the orthopyroxene. (All odd-layeredfaults will produce blocks of structure having widths of[2n + ll x 0.45 nm.) The low abundance of odd-layeredstacking faults, compared to the even-layered types, un-doubtedly indicates that they have higher energies underthe relatively low temperatures at which most pyroxenemicrostructures form. At high temperatures in the pureMgSiO, composition, however, it might be expected thatodd-layered faults would occur more commonly, sinceprotoenstatite possesses * - * units as part of its basic(+ -) stacking sequence.

It is also observed that the stacking faults in LAC-6Apigeonite lamellae are nearly always separated by regionsof pigeonite that are 2n x 0.45 nm wide. Out of dozensof observed separations, only two cases were found inwhich stacking faults were separated by slabs of pigeonite(2n + l) x 0.45 nm thick; one ofthese cases is presentedin Figure 10b, in which two of the faults are separated byseven 0.45-nm slabs of structure. This observation im-plies either that (l) the two octahedral sheets of anoma-lous skew forming the fault always nucleate in the un-faulted pigeonite structure (arrangement a, Table l), sothat they are centered on planes of pigeonite A chains(arrangement b, Table l), or that (2) they are centered on

planes of B chains (arrangement c, Table l). unrru im-ages, such as that in Figure 6a, show that the latter pos-sibility is the one that actually occurs (image interpreta-tion is based on computer-simulated images showing thatthe more prominent rows of white spots in the imagecorrespond to the layers ofA chains, as noted above).

If one considers this type of + - - + stacking fault tobe a single unit cell oforthopyroxene structure, with theorthopyroxene B chains between the octahedral layershaving negative skew, then these observations further im-ply that the orthopyroxene A chains replace the pigeoniteA chains and that the orthopyroxene B chains replace thepigeonite B chains. This structural control on the positionof formation of stacking faults in pigeonite may be relatedto the structural similarities between the pigeonite andorthopyroxene chains: the A chains in both structures arerelatively straight, whereas the B chains ofboth pigeoniteand orthopyroxene are more rotated (Cameron and Pa-pike, 1980). These similarities appear to control thestacking-fault positions, in spite of the fact that the pi-geonite A chains lie between octahedral layers of like skew,whereas those oforthopyroxene are between layers withopposite skew.

If these + - - + stacking faults had formed in a cli-nopyroxene in which all of the silicate chains were sym-metrically identical (e.e., C2/c pigeonite or augite), thisstructural control would be absent, and the stacking faults(both odd- and even-layered) would be separated by botheven and odd numbers of 0.45-nm (100) slabs. This in-

1082 LIVI AND VEBLEN: INTERFACES AND DEFECTS IN INTERGROWN PYROXENES

TABLE 1. Consequences of various fault arrangements

a. Unfaulted pigeonite

b, Evenlayered fault centered on A chains

c. EvenJayered fault centered on B chains

d. OddJayered fault without APB

e. OddJavered fault with APB

A + B + A + B + A + B + A + B + A + B

A + B + A + B _ A _ B + A + B + A + Bl o p x l

A + B + A + B + A - B _ A + B + A + Bl o p x l

A + B + A + B + A - B + A + B + A + B

A + B + A + B + A _ A + B + A + B + AI

APB

A/ote: See text for further discussion. A and B indicate the oositions of A and B tetrahedral chains, and + and - indicate the skew (stacking vectordirections) of the octahedral layers between the silicate chains.

a. Normal pigeonite structure.b. An even-layered fault consisting of two octahedral layers with anomalous skew, centered on the A chains.c. An evenlayered fault consisting of two octahedral layers with anomalous skew, centered on the B chains. This is the common stacking fault

structure in the pigeonite of LAC-6A.d. An oddJayered fault consisting of a single octahedral layer with anomalous skew.e. An oddJayered fault consisting of a single octahedral layer with anomalous skew, accompanied by an antiphase boundary with respect to the

arrangement of silicate chains. The antiphase boundary is indicated by the juxtaposition of two layers of A chains.

dicates that the stacking faults formed below the pigeon-ite C2/c - P2,/c phase-transition temperature, which isestimated to be 730 "C for the LAC-6A pigeonite (Livi,I 983). This is consistent with the suggestion of Robinsonet al. (1977) that stacking faults in "001" pigeonite la-mellae of augites form below the C - P transition, as aresult ofstrain induced by changes in unit-cell parametersduring cooling.

Odd-layered faults may be less stable than even-lay-ered faults in pigeonite because they place a layer of Bchains between octahedral layers of opposite skew (ar-rangement d, Table 1), although topological constraintsforce chains in such a position to be relatively straight(Thompson, 1970; Papike etal., 19731, Veblen and Burn-ham, 1978). It would thus be topologically impossible forthis layer of B chains to assume its favored, relativelyO-rotated conformation. However, an antiphase bound-ary (APB) that juxtaposes two A chains might stabilizean odd-layered fault, since it would place the relativelystraight chains in both positions where octahedral skewis reversed (arrangement e, Table l). The rarity of odd-layered faults is consistent with the absence of APBs inthe well-annealed LAC-6A pigeonite lamellae. In pigeon-ites that contain both APBs and (100) stacking faults, wewould predict that odd-layered stacking faults would bestabilized by and coincide with APBs parallel to (100).

CoNcr,usroNs

We have used conventional rnu observations and nn-rnu images interpreted with the aid of computer simu-lations to elucidate a number of new details about pyrox-ene interfaces and defect structures. These microstructuresformed by exsolution processes during subsolidus equil-ibration of an augite (LAC-6A) from the Laramie Anor-thosite Complex.

The observations confirm that defects such as dislo-cations and stacking faults can play a critical role in exso-lution piocesses, and they show that there can be greatcomplexity in the ways in which interface morphologies

interact with these defects. In addition, interactions be-tween lamellae, probably through their strain fields, cansubstantially alter the morphology of the lamellae.

HRTEM observations show that stacking faults in pi-geonite and orthopyroxene typically involve reversal ofthe skew of even numbers of octahedral layers. However,oddJayered faults also occur but in lesser numbers, dem-onstrating that a full description ofpyroxene stacking re-quires the use of (100) slabs that are 0.45-nm thick, asopposed to the 0.9-nm slabs that have been used by someauthors. HRrEM images indicating the detailed structuresand spacings of stacking faults in the pigeonite lamellaeof LAC-6A show that the faults nucleate symmetricallyabout the B chains ofthe pigeonite structure. This showsthat the detailed conformation of the silicate chains cancontrol the resulting defect structure and that the stackingfaults form below the C2/c - P2,/c phase transition assuggested previously.

AcrNowr,rocMENTS

We thank Gordon Nord and Masao Kitamura for helpful reviews ofthis paper. We also thank Richard J Reeder for access to the Stony BrookrEM that was used for the early part of this study, Michael A. O'Keefe for

supplying the sHI-r software, Judith Konnert for supplying a modified

output routine for these programs, and George Guthrie for his wizardry

on the Mac and insightful discussions. This research was supported byNSF g,rant EAR86-09277. High-resolution microscopy was performed atThe Johns Hopkins University using instrumentation obtained with par-

tial support from NSF instrumentation grant EAR83-00365.

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MeNuscnrsr REcETvED Decrrrasen 12, 1988M.lxuscnrpr AccEPTED JuNe 2, 1989


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