Micron and Microscopica Acta, Vol. 15, No. 2, pp. 85-90, 1984. 0739-6260/84 83.00+0.00 Printed in Great Britain. C) 1984 Pergamon Press Ltd.

ULTRASTRUCTURAL AND MICROANALYTICAL RESULTS FROM : IMPLICATIONS FOR AND DIAGENESIS OF SKELETAL MATERIAL

DAVID F. BLAKE, DONALD R. PEACOR

Department of Geological Sciences, University of Michigan, Ann Arbor, MI 48109, U.S.A. and

LAWRENCE F. ALLARD Department of Materials and Metallurgical Engineering, University of Michigan, Ann Arbor, MI 48109, U.S.A.

(Received 9 January 1984)

Abstract—Magnesian calcite skeletal elements ofthe modern crinoid echinoderm Neocrinus blakei were studied using high resolution TEM, high voltage TEM and STEM microanalysis. Unlike inorganic magnesian calcites which are compositionally heterogeneous, magnesium in these skeletal calcites is homogeneous to at least the 0.1 jim level. While a mosaic structure exists in echinoderm calcite, high voltage TEM reveals the absence of defects or dislocation features which should exist as a consequence ofthe structure. By comparison, inorganic magnesian calcites show a plethora of defects and dislocation features. High resolution lattice fringe images of the echinoderm calcite exhibit a kinking of fringes between mosaic domains, the boundaries ofwhich are largely coherent. Large scale dislocation structures are not observed. Such a ‘stressed’ lattice structure, if pervasive, explains conflicting observations concerning the ‘single crystal’ or ‘polycrystalline aggregate’ nature of echinoderm calcite. The microstructural and microchemical data demonstrate strong organismal control of skeletal deposition in Echinodermata. Both ultrastructural and compositional heterogeneity/homogeneity should be assessed when determining the susceptibility of skeletal material to diagenetic change. Index key words: Echinoderm calcite, biomineralization, analytical electron microscopy, skeletal diagenesis, magnesian calcite.

INTRODUCTION significant to an understanding of the general Echinodermata is a phylum of marine inverte- process of biomineralization. brates which includes such diverse organisms as While echinoderm hard parts appear to be sea stars (Asteroidea), sea urchins (Echinoidea) single crystals of calcite, fracture surfaces do not and sea lilies (Crinoidea). The hard parts of exhibit the { 1O.4} cleavage observed in inorganic have a porous microstructure, re- calcite (e.g. see Fig. 1(b)). Towe (1967) suggested ferred to as stereom (see Figs. 1(a—b)) and are that echinoderm calcite might possess a ‘mosaic’ composed of high-magnesium calcite (Weber, structure in which the skeletal calcite was made 1969). Each skeletal plate, with few exceptions. up of tiny crystallites, slightly misaligned with behaves optically as if it were a single crystal of respect to one another. Later fracture (Nissen, calcite (Raup 1966). It is this rather paradoxical 1969) and single crystal X-ray diffraction mix of organismal control of morphology and (Donnayand Pawson, 1969) studies appeared to ‘inorganic’ crystallographic features which has suggest that echinoderm hard parts were essen- prompted much recent work on echinoderm tially single crystals of calcite. Recent studies, hard parts, the growth processes of which are utilizing ‘stress-relaxed fracturing’ (O’Neill, 1981) and selected area electron diffraction (Blake and Peacor, 1981) suggest that a pervasive Address all correspondence to first author. mosaic structure is present. However, the actual 85 86 D. F. Blake, D. R. Peacor and L. F. Allard

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Fig. 1. (a) Low magnification SEM micrograph ofN. hlakr’i columnal. ~ie~ed looking down Oie morphological axis ofthe columnal. The five-fold pseudosymmetry, rounded stereom and fenestrate microstructure are typical ofskeletal elements ofEchinodermata ( x 17). (b) SEM micrograph ofcrinoid stereom showing rounded stereom structure and conchoidal (c) and irregular (I) fracture surfaces typical of echinoderm calcite (x 3800). imperfections which relate to the mosaic struc- magnesian calcite of sponge spicules is homo- ture have never been directly observed. geneous at the level of resolution of EMPA, The microstructure of the skeletal calcite must despite the curved nature of the spicules. result from original skeletal growth processes. In addition, the susceptibility of skeletal materialto diagenetic processes may be influenced as much MATERIALS AND METHODS by skeletal ultrastructure as by composition. Columnals of N. blakei were prepared in the Insight into both the biomineralization process following manner: ‘crushed grain’ mounts were and the diagenetic potential of such metastable prepared by grinding whole columnals in a phases may be gained by studying these ultra- mortar and pestle under liquid nitrogen. Crushed structural features in detail, grain material was then distributed onto holey O’Neill (1981) suggested that variations in Mg carbon support films on beryllium electron content could explain the morphologically microscope grids. A thin film of carbon was curved surfaces present in the echinoderm evaporated onto the grids to eliminate charging stereom, since magnesium incorporation in the by the electron beam. These crushed grain crystal structure would serve to reducethe calcite mounts were used for microanalysis and lattice lattice parameters. Magnesium heterogeneity has fringe imaging. ‘Ion-thinned’ specimens for been reported in some echinoderm skeletal HVEM imaging were prepared from petrologic elements (Weber, 1969; Davies et a!., 1972), but thin sections of skeletal calcite. A 3 mm support this heterogeneity appears to be the result of washer was attached to the material, and the temperature induced changes in growth rate, surrounding thin section ground away. The These large scale variations do not explain the washer with its attached disc of skeletal material morphologically curved surfaces present in the was removed from the slide and further thinned stereom of echinoderm skeletal elements, in an argon ion mill. A thin film of carbon was Furthermore, the data of Jones (1969, 1972) then evaporated onto the specimen to prevent indicate that the distribution of Mg in the charging. TEM/STEM Microanalysis of Echinoderm Calcite 87

Scanning Transmission Electron Microscopy the sub-i.im level. That is, the magnesium is not (STEM) microanalytical studies were performed preferentially distributed with respect to the in order to determine the distribution of mag- imperfections in structure known to exist from nesium in the skeletal calcite. Crushed grain indirect measurements. Furthermore, these data mounts were observed in a JEOL JEM-100CX demonstrate that organismal control of Mg AEM. Analyses for Mg and Ca were recorded in incorporation cannot be responsible for the STEM mode and data were quantified using the complex stereom microstructure ubiquitous in Cliff—Lorimer (ratio) technique (Cliff and Echinodermata as has been previously hypo- Lorimer, 1972). Quantitative analyses were per- thesized (O’Neill, 1981). While the echinoderm formed at 100 kV accelerating voltage and spec- calcite is extremely homogeneous with respect to tra were collected over200 s counting timeswith Mg incorporation, inorganic magnesian calcite a beam current of 0.5 nA. Areas chosen for cements analysed in analogous fashion are not. analyses met the ‘thin film criterion’ (Goldstein, Table 1 shows statistical summaries of the 1979) which holdsfor Mg/Cadata in materials of echinoderm skeletal data and two examples of this type which are of 300 nM thickness or less. inorganic magnesian calcite data. (Data are from Results were analysedstatistically by a technique Blake et a!., 1982a and Blake and Peacor, 1983.) developed for use with data of this type (Blake et As seen in Table 1, the inorganic magnesian a!., 1983). Utilizing the STEM technique it is calcites are highly heterogeneous in Mg incor- possible to achieve a lateral spatial resolution poration when compared to the biogenic high approaching 30 nM in thin samples. However, magnesian calcite. carbonates are very susceptible to mass loss The mosaic structure of echinoderm calcite has (electron beam damage) under these conditions been seen on natural surfaces (Towe, 1967) and and analyses reported here were recorded by demonstrated by rapid (Towe, 1967; Nissen, scanning the beam over 100 nM square areas of 1969) and stress-relaxed (O’Neill, 1981) fractur- material. ing, as well as by selected area electron diffraction (Blake and Peacor, 1981). However, the specific imperfections giving rise to the mosaic structure RESULTS have never been observed in TEM micrographs Microareas of N. b!akei skeletal calcite ana- of the skeletal material. This contrasts markedly lysed using the STEM technique are remarkably with modern low-temperature inorganic mag- uniform in composition. For example, for a nesian calcites we have examined. The inorganic sample size of N = 25, the mean composition is cements, which are mosaic composites of sub-j.tm Ca0 894Mg0 106CO3, with a standard deviation sized crystallites, show a plethora of imperfection of 0.011. This value agrees well with microprobe features not observed in the echinoderm skeletal results for which an average composition of material. Ca0 88Mg0 12CO3 was determined. An F- of It is possible, however, that defects present in the STEM data yields a value of F =0.1232 the echinoderm calcite are annealed out of the (degrees of freedom (24, 72)), which supports the thin regions (those areas less than about 500 nM conclusion that the magnesium is uniformly thick) observable at 100 keY accelerating distributed within individual skeletal elements at voltage. We therefore utilized high voltage elec-

Table 1. Summary of STEM microanalytical data for high magnesian calcite (HMC) cements from inorganic and biogenic sources

Mean value S.D. F-test Type ofcalcite No. of Obs. MgCO3 MgCO3 results*

Fresh water HMCt 202 2l.79~ 6.75~ 12.32 (201, 48) Marine HMC~ 97 14.1 3.41 1.29 (96, 96) Echinoderm HMC 25 10.6 1.1 0.12 (24, 72)

* Numbers in brackets refer to degrees of freedom for F-test. ~ Data are from Bear Creek No. 1 cement, Blake and Peacor (1983). ~ Data are from ‘rip-up clast’ cement, Blake et a!. (l982a). 88 D. F. Blake, D. R. Peacor and L. F. Allard tron microscopy (HVEM) to image the ion- oriented so that three different sets of planes thinned columnals, in order to determine the (01.2, 11.2, and 10.4) are in contrast, while on the presence or absence of dislocations in thicker right side only one set of planes (10.4) is in regions of the calcite. Figure 2 is a representative contrast. By viewing the figurewith the pageheld micrograph of the material, recorded at 1.0 MeV parallel to the direction of view, and along the accelerating voltage. Dislocations and crystalline direction of the 10.4 planes, a slight but sharp defects are conspicuously absent, even in bulk change in orientation of the 10.4 planes is clearly regions of the material. The ‘mottling’ of contrast visible along the boundary where the other two seen in Fig. 2 (particularly along bend extinction planes go out of contrast. We interpret this contours) is also observed in micrographs taken structure as two ‘mosaic blocks’ or crystallites of at 100 keY. This heterogeneity of contrast may the mosaic structure which are slightly mis- be due to unrequited stress in the crystalline oriented one tothe other. The boundary between material, the two blocks is largely coherent and resembles High resolution lattice fringe images of the a low angle grain boundary. Arrows point to crinoid calcite were taken to determine the dislocations which accommodate the slight misfit nature of the mosaic boundaries which should be between the planes. visible in lattice fringe images of the material. It is unlikely that a structure of this sort is an Figure 3 shows a lattice fringe image taken with artifact of sample preparation or sample treat- axial illumination, with the crystal oriented so ment and we interpret it to be a consequence of that several crystallographic planes are in con- the mechanism of growth of the skeletal material. trast. The inset diffraction pattern shows the Such a structure, ifpervasive, could give rise toall diffracting condition used to construct the image. of the observations noted in previous reports of On the left side of the micrograph, the crystal is echinoderm fracture surfaces. The low angle

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Fig. “. High voltage (1.0 MeV) bright field electron micrograph of N. h!ukei columnal. While indirect methods have shown that a pervasive mosaic structure exists, HVEM micrographs show a notable absence ofdislocation networks which should exist as a consequence of the structure. The fine‘punctate’ structure in the micrograph is due to electron beam damage and is not an original crystallographic feature of the calcite ( x 38.000). TEM/STEM Microanalysis of Echinoderm Calcite 89

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Fig. 3. High resolution TEM lattice fringe image of N. hlakei columnal. Inset diffraction pattern shows the diffracting condition used to construct the image. The image is interpreted as two ‘mosaic blocks’ ofthe calcite, which are slightly misaligned one to the other. The boundary iscoherent, with the exception ofthe areas marked by arrows. Arrows mark dislocations which act to reduce the stress caused by the abruptchange in orientation of the 10.4 planes as they cross the boundary between the two mosaic domains (x 4,180,000). boundaries, constituting zones of weakness, typical of all echnoderm skeletal elements, is could be the areas of crack nucleation and therefore not the result of organismal control of growth during stress relaxed fracturing. Under variations in Mg incorporation. other conditions, fracture surfaces could exhibit We speculate that the pervasive mosaic struc- conchoidal or ‘mixed’ fracturing depending on ture is the result of small changes in the the magnitude and direction of the force applied, orientation of sub-i.tm-sized crystallites, which The slight misorientation of the crystallites despite their differencesin orientation, are largely would give rise to the mosaic spread of diffrac- coherent. The presence of semi-coherent boun- tions reported previously. Lastly, such a semi- daries between mosaic domains, and the absence coherent structure, if regularly repeated over of major dislocation networks, indicate that the large volumes, could easily give rise to the structure is in a state of stress. From our continuous change in orientation of crystal- observations of the heterogeneous and imperfect lographic axes, as noted previously in the am- ultrastructure and microchemistry of inorganic bulacral and interambulacral skeletal plates of magnesian calcites, it is clear that bio- some echinoids (Raup, 1966). mineralization as it occurs in Echinodermata is not strictly aprocess ofphysico-chemical precipi- tation. There must be strong organismal control DISCUSSION of bothcomposition and structureat the sub-l.lm We conclude from the STEM microanalytical level during the biomineralization process. data that Mg distribution in columnals of N. The presence of such a ‘stressed’ lattice struc- blakei is strikingly homogeneous. The rounded ture in echinoderm calcite implies that the morphology present in N. b!akei stereom, and skeletal material is in a higher energy state than 90 D. F. Blake, D. R. Peacor and L. F. Allard would be a perfect crystal of the same composi- REFERENCES tion. The mechanisms involved in the stabiliz- Blake, D. F. and Peacor, D. R., 1981. Biomineralization in ation of biogenic and abiogenic magnesian crinoid echinoderms: Characterization of crinoid skeletal calcite under diagenetic conditions are not well elements using TEM and STEM microanalysis. SEM/1981/III, 321 -328. understood by carbonate petrologists. It is well Blake, D. F., Kocurko, M. J. and Peacor, D. R., 1982a known, however, that solution and chemical Crystallography of Holocene magnesian calcite cements alteration of minerals and other inorganic com- from the gulfcoast of Louisiana. Geo!. Soc. Am. Abstr. W. pounds is initiated at sites which are either Prog., 14 (7): 445. chemically or structurally inhomogeneous. ~f Blake,sequenceD. F.,andPeacor,mechanismD. R. andofWilkinson,low-temperatureB. H., 1982b.dolomiteThe diagenesis occurs in chemically homogeneous formation: Calcian dolomites from a Pennsylvanian echi- materials, then the sites of crystal imperfection noderm. J. Sedim. Petrol., 52: 59-70. are attacked first. Blake et a!. (1982b) present Blake, D. F., Isaacs, A. I. and Kushler, R. H., 1983. A evidence from echinoderm fragments sug- statistical method for the analysis ofquantitative thin-film microanalytical data. J. Microsc., 131 (2): 249—256. gesting that diagenesis of primary high mag- Blake, D. F. and Peacor, D. R., 1983. Microstructure and nesian calcite takes place via micro- microchemistry of Holocene freshwater magnesian calcite dissolution.-reprecipitation events, commonly and dolomite cementsfrom the Coast Range of California. with retention of Mg from the original skeletal Geol. Soc. Am. Abstr. W. Prog., 16 (6): 527. material in microdolomite inclusions. It is highly Cliff, G. and Lorimer, G. W., 1972. The quantitative analysis of thin metal foils using EMMA-4--—The ratio technique. unlikely that such microdolomites could result Proc. F~flhEuropean Congres.s of EM, Inst. 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An investigation of some calcareous environment, an ultrastructural as well as micro- sponge spicules by means of electron probe micro-analysis. chemical characterization of the initial mineral Micron, 1: 34-39. phase is required. Jones, W. C., 1972. Examination of the large triacts of the calcareous sponge Leuconia nitea Grant by scanning electron microscopy. Micron, 3: 196-210. Acknowledgements—We thank Dr. Kenneth Towe of the Nissen, H., 1969. Crystal orientation and plate structure in Smithsonian Institution, Dr. Richard Reeder of SUNY. echinoid skeletal units. Science, 166: 1150—1152. Stony Brook. Dr. Daniel Fisher of the Museum of O’Neill, P. L., 1981. Echinoderm calcite and its fracture Paleontology, University of Michigan and Dr. B. H. mechanics. Science, 213, 646-648. Wilkinson of the Department of Geological Sciences, Raup, D. L., 1966. The . In: Physiology of University of Michigan for reviewing the manuscript. We Echinodermata, Boolootian, R. A. (ed.), Interscience, New thank Dr. Anthony Taylor and the staff of the DOE HVEM- York, 379-395. Tandem National User Facility, Argonne National Towe, K. M., 1967. Echinoderm calcite: Single crystal or Laboratory, for assistance and machine time on the HVEM. polycrystalline aggregate. Science, 157: 1048—1050. Support under NSF EAR81-07529 (D.R.P.) and a Rackham Weber, J. N., 1969. The incorporation of magnesium into the predoctoral fellowship (D.F.B.) is gratefully acknowledged. skeletal calcite of echinoderms. Am. J. Sci. 267: 537-566.