American Mineralogist, Volume 78, pages 775-781, 1993
Structure of magnesian calcite from sea urchins
Srunou J. TsrpunsrY, Pnlnn R. Busncr
Departmentsof Geologyand Chemistry,Arizona StateUniversity, Tempe, Arizona 85287-1404,U.S.A.
AssrRAcr
X-ray 0 scansand high-resolution transmission electron microscopy (HRTEM) imaging
indicate that the calcite of sea urchin spines and skeletal plates forms in mosaic crystals.
Both coherent and incoherent boundaries were observedbetweenslightly misaligned crystallites. Small domains show an ordered superstructure.Using HRTEM and both X-ray
and electron diffraction data, we found that the most appropriate model for this superstructurehas Ca:Mg: 3:l and alternatingCa layersand Ca and Mg layersalong c' We
did not find evidence for the presenceof protein moleculeswithin the calcite structure'
INrnooucrroN
Numerous organismsproduce calcium carbonate crystals in the oceanicenvironment, and the resulting biogenic minerals are important contributors to carbonate
sedimentson the oceanbottom (Lowenstamand Weiner,
1989).Beingabundantin sediments,thesecarbonatesare
involved in diagenetic processes,and their complete
characterization is important for understanding the
changesthat occur during diagenesis.Echinoderms,consistingof magnesiancalcite,are prominent sourcesof carbonatesand are the focus ofthis study.
In spite of extensiveinvestigationsof echinoid hard
parts, details of their structure are still not known with
certainty.Somescientistsconsiderthem to consistof single crystals(Raup, 1966;Donnay and Pawson,1969;Nissen, 1969),whereasothers believe they are polycrystals
(Pearseand Pearse,1975; O'Neil, l98l). Towe (1967)
proposedthat echinodermskeletonelementsare a combination of both single crystals and polycrystalline aggregates. Using high-resolution transmission electron microscopy(HRTEM), Blake et al. (198a) concludedthat
echinodermcalciteconsistsof a pervasivemosaic structure that crystallized "under strong organism control."
X-ray
Paquetteand Reeder(1990)obtainedsingle-crystal
structure refinements of echinoderm magnesian calcite,
and, in contrast to dolomite, they found no evidence for
Mg ordering.
An intriguing feature, and one that has given rise to
considerable speculation, is the unusual flexibility and
strength of echinoderm calcite. Its mechanical properties
have been explained by either the porous characteror the
hypothesized composite nature of the calcite. Currey
(1975), Burkhardt and Mdrker (1979),and Emlet (1982)
proposed that it is the porous structure that improves
properties such as flexural, torsional, buckling, and tensile strengths.On the other hand, Berman et al. (1990)
suggested,by analogywith composite materials, that protein molecules intergrown within the calcite are the critical factors,although Currey and Nichols (1967)and Kleyn
0003-004xi93i0708-077s$02.00
and Currey (1970) reported an absenceoforganic material in echinoderm calcite.
There are unresolved differencesof opinion regarding
biogeniccalcite.The purposeofthe presentinvestigation
is to examine echinoderm hard parts to obtain information about their structural states and to search for evidence of protein molecules within the biogenic crystals
that would indicate their composite character. We used
X-ray diffraction, electron microprobe analysis, and
transmission electron microscopy to study spines and
skeletonplatesof Slrongylocentrotus
franciscanus(Str.fr.),
a deep water species,and Strongylocentrotuspurpuratus
(Str.pr.), a shallowwaler species.
ExpnnllrnNr.LrPlatesand spinesof Str.fr. and Str.pr. were obtained
from fresh specimens.They were studiedusing a D/MAXIIB Rigaku X-ray diffractometer, operatedat 50 keV, 30
mA, and equipped with a CuKa source,curved graphite
monochromator,and a 0.15'receiving slit. Samplesfor
electron microscopy were oriented using X-ray diffraction measurements,and then thin sectionsparallel and
perpendicularto the [001] direction were cut, ground to
a 30-pm thickness,and ion milled. Microscopywas performed usinga Jeol JEM-4000EX electronmicroscopeat
a 400-keV acceleratingvoltage and with a point to point
resolution of 0. l7 nm. A 45-pm objective apertureand
150-pm condenseraperturewere used.
Chemical analyseswere obtained with an automated
electron microprobe atalyzer (Jeol-JXA 8600 Superprobe) using wavelength-dispersivespectroscopy.Samples were mounted on glassslides,polished,and coated
by vacuum evaporation with a thin conducting layer of
C. The microprobe was operatedat 15 kV and l0 nA,
with a l5-pm spot size. Count integration was for 30 s
for Mg and 5 s for Ca.
X-n-tv RESULTS
The positions and intensities of the reflections from
powder X-ray diffraction (XRD) patterns of all the mea-
7'16
TSIPURSKY AND BUSECK: MAGNESIAN CALCITE IN SEA URCHINS
use of XRD rocking-curve procedures (Az6roff et al.,
1974),which we will call d scansto distinguishthem from
standard 0/20 diffraction scans.During such a d scan, the
specimen is rotated slightly around the axis of an X-ray
powder diffractometer, with the X-ray source and detector retained at a fixed 20 value for a given Bragg peak.
This procedure is in contrast to a more conventional 0/20
scan, in which both crystal and detector are rotated synchronously. The resulting diffraction pattern contains information about both the sizes and orientations of the
crystalsin the specimen(Yukino and Uno, 1986).
Slabsof calcite crystals,roughly I x 5 x 5 mm in size,
were used as standards to test the d scan method. Three
types of samples were studied: (l) a transparent crystal
(Iceland spar) free of cracks visible with an optical microscope, (2) a transparent crystal with a single microscopic crack evident, and (3) a crystal containing multiple
microfractures. Slabs 2 and 3 were taken as models of
crystals having mosaic structures.The crystals were polished, mounted on glassslides, and then oriented on the
X-ray diffractometer.
We obtained both d and 0/20 scans in order to test
which is more sensitiveto crystal mosaicism. When using
a powder diffractometer to study a nominally single crystal, as we did for these measurements,it is necessaryto
determine the exact 0 orientation of the crystal before
obtaining a 0/20 scan. The d scan profiles of the 104 reflection, produced when a crystal fragment lies on its
cleavagesurface, were obtained for our three reference
crystals(Fig. lA). The differencesin width and peak complexity increase markedly with the extent of imperfections. Having determinedthe 0 position for the 104 peak
from the 0 scan, conventional 0/20 scanswere used to
obtain Braggdiffraction spectra(Fig. lB). In spite ofthe
striking diference among the d scan patterns (Fig. lA), a
comparison with the 0/20 patlerts (Fig. lB) shows no
marked broadening of reflections from a mosaic crystal
relative to those from a single crystal. It is evident that 0
scans are more sensitive to mosaic structures than are
0/20 scans.Thus, we used d scansfor studying the echinoderm samples.
Srnucrunp
AND
oF sEA uRCHTNSPTNES
SKELETAL
PARTS
Spines of Str. fr. and S/r. pr. werc sectioned so that
were both parallel and perpendicular to their lengths
they
29
30
20
for X-ray study. Optical microscopy in both reflectedand
Fig. l. (A) X-ray d scansof the 104reflectionfrom a: a nom- transmitted light showed that the sections were flat and
inally crack-freesingle crystal of calcite; b: a calcite crystal with
free of observablefractures.Sampleswere mounted onto
one crack visible with an optical microscope, and c: a mosarc
glass slides and oriented in the diffractometer. The 00/
calcite crystal. Note the changesin peak shapeswithin this series.
(B) The 0/20 scansof the 104 reflection from crystals a and c, reflections were obtained for the transversesections.The
above. The increasedsensitivity of the d scansrelative to 0/20 d scansof the 0,0,12reflectionfrom many samplesof Slr.
y'. spines all produced broad, complex, and asymmetric
scansis evident.
peaks having full widths at half maximum (FWHM) of
-0.9" (Fig. 2A). Having determined the proper d position
sured samples are typical of calcite. We were interested
in assessing the crystal perfection of this calcite. Yukino
for the 0,0,12 reflection, we obtained d/2d scans.Figure
and Uno (1986) showed that such measurements could
28 shows the 0/20 scanof the 00/ reflectionsof the crystal
be made of either powders or larger crystals through the
used for Figure 24. Similar 0/20 profrles were obtained
TSIPURSKY AND BUSECK: MAGNESIAN CALCITE IN SEA URCHINS
Fig.2. (A) A d scanofthe 0,0,12reflection
of a Slr.y' spine.
(B) A 0/20scanof the 00/ reflectionsof a Str.fr. spine,with the
peaks(006and0,0,I 2) shownwith intensities
two strongest
scaled
so that the peakpositionsareevident.The intensityofthe 003
peakis 300counts(curvea), whereas
that ofthe strong0,0,12
peak(b) is 200000counts.
when the crystal was oriented for any position within the
FWHM range shown in Figure 24.
Typically, calcite 00/ and h00 reflections are weak on
powder XRD patterns becauseof the preferred orientation along {104} cleavageplanesand the correspondingly
strong 104 reflections. Our patterns, which are not of
powderedsamples,show strong,narrow 006 and 0,0,12
peaks,togetherwith weaker003 and 009 peaks(Fig. 2B).
The strong peaks have d values of 0.2827(2) and
0.14136(4) nm, and the weak ones have values of
0.5543(6)and 0.18467(7)nm. Thesepeaksarenot simple
submultiples of a single c parameter, presentingan interestingproblem. In addition, 003 and 009 reflectionsare
forbidden by the R3c spacegroup of calcite. We interpret
this situation as one resulting from the superpositioning
of reflections that arise from two structures having different symmetries-R3c and R3 @r C2/m), with the latter being a minor component. The patterns arise from
calcite and dolomite-like structures,with the strong calcite peaksnormally obscuringthe weak 006 and 0,0,12
-l
777
01a0
Fig. 3. X-ray scansof 300-typereflectionsfrom a Str.fr.
(b)0 scanofthe 300,
spine.(a)A0/20scanofthe 300reflection,
030. and 330 reflections.and (c) d scanof the 300 reflection.
The peakwidth in c hasbeenexpandedrelativeto b in orderto
showdetailswithin the peak.
dolomite-like peaks. Calculated c dimensions corresponding to the two sets of peaks are 1.696 and 1.662
nm, respectively.
In order to examine the effects of mosaicism in other
directions, we reoriented the crystals to investigate a set
of ft00 reflections.An entire Str.fr. spine was mounted
on the XRD goniometer, with the long spine axis coincident with the axis of rotation of the goniometer. By
using conventional 0/20 scansaround c, we found sharp
300 reflections that are well resolved into Ka, and Ka,
peaks (Fig. 3a). The d scansaround c show three welldefined peaksthat are separatedby 60'and that result
from the {300} planes (Fie. 3b), as is typical of welldeveloped single crystals. However, there is considerable
dispersion(-0.6" FWHM) evident in the 0 scans(Fig.
3c), showing that this is, in fact, a mosaic crystal.
TSIPURSKY AND BUSECK: MAGNESIAN CALCITE IN SEA URCHINS
778
-6
-5
-4
-3
-2
-l
2Lg
from Slr. pr.
Fig. 4. Two 0 scansof the 0,0,12reflections
Curvea showstwo wideskeletonplatesfrom a singlespecimen.
peaks,and curveb showstwo overlappingpeaks.
ly separated
Otherplatesshowdifferentpeakseparations.
The results from skeletal plates differed slightly from
those of spines.The d scansof 0,0,12 reflectionsshow
both dispersion (Fig. 4a) and apparent angular separation
of up to 5" into distinct crystals, each of which shows
distinct mosaicism(Fig. ab). Closeexaminationof Figure
4 shows additional minor structural complexity in the
weak peripheral peaks.
ANnr-vrrclr-
AND TEM oBSERvATIoNS
All of the calcite that we analyzedis magnesian.Based
we eson a seriesof electronmicroprobe measurements,
timate the MgCO3 content as ranging from 2.4 to 4.5
molo/ofor spines of Str. pr. and Slr. fr. and for skeletal
plates of Str. fr. and MgCO. content from 4.2 to 7.8
molo/ofor skeletalplates of Slr. pr. The estimatederror for
any given analysisis about 3olo(relative) for Mg and about
lolofor Ca. Neither systematic variations in Mg content
nor appreciableamounts of other elementswere observed.
Calcite is sensitiveto radiation damageduring HRTEM
investigations,and so rapid operation was required. To
minimize the damage, it was useful to prepare calcite
sectionshaving the desired[00] and [001] orientations
for imaging. We used0 scansto find theseorientations.
SAED patterns of certain regions reveal weak c-type
extra reflections(Reederand Wenk, 1979) halfway between the fundamental calcite reflections (Fig. 5a). We
found that the intensitiesofthe c reflectionsvary considerably from place to place. Images obtained from areas
in which the c reflections are most pronounced contain
round domains exhibiting a set of parallel, widely spaced
fringes that suggesta superstructure.These domains are
<10 nm in diameter (Fig. 6). The orientations of the
superstructurefringes in different domains appear to be
rotated by 120'to one another,as would be expectedif
their orientations were controlled by the host carbonate.
An SAED pattern and image of diageneticmagnesiancal-
Fig. 5. SAEDpatternsof the sameStr.y'. spineas is illusare along(a) [001],(b) tl00l,
tratedin Fig. 3. The projections
and (c) [20]. The arrowsshowthe positionsof superstructure
spots.
cite from a Jurassicammonite, similar to Figures5a and
6, are shownby Wenk et al. (1991).
In contrast to SAED patterns of magnesiancalcite from
sediments(Wenk et al., l99l), some of our [00] and
[ 20] SAED patterns contain dolomitelike 00/ reflections, suchas 003,009, and 0,0,15(Fig. 5b, 5c).Tilting
experiments were performed to determine whether they
might arise from dynamical effects. Although they became weaker and so may have some dynamical contribution, these reflectionswere not eliminated by tilting,
and therefore they are at least permissive of R3 or lower
symmetry. Moreover, since these same reflections were
TSIPURSKY AND BUSECK: MAGNESIAN CALCITE IN SEA URCHINS
779
Fig. 7. A U00l imageof a slightlybentcrystalof the ,S/r.y'.
to the whiteline (arrowed)showsthe misalignspine.Reference
ment.A regionof strain(S)and a dislocation(D) areindicated.
dimensions.Figure 9a and 9b provide perspectiveviews
and [001] projectionsof the z and 7 models that are of
specialinterestfor our purposes.It is convenientto discuss them with referenceto calcite (a structure) and dolomite (0 structure).In dolomite the Ca and Mg cations
lie in layers that altemate along c. Each cation layer in
Fig.6. A [00I]imageof the,S/ry'. spine.The outlinedregion the z structurehas a compositionwith the dolomite ratio
showsoneof thedomainsthardisplaysanorderedsuperstructure. of Ca:Mg : l: I and containsrows of Ca and rows of Mg
alternatingalong one of the hexagonala axes.Thus, all
layers in the v structure are compositionally identical,
observed in the X-ray patterns, we believe that they are whereasthose of dolomite are compositionallydistinct.
real and thereby indicate the presenceof dolomite-like The 7 structure can then be described as a composite of
structure, rather than simply being the result of multiple the calcite and / structures, with a regular alternation of
diffraction.
Ca layersand Ca and Mg layersalong [001], resultingin
We found numerous instancesof crystal misorienta- Ca:Mg: 3:1. The z structurecan also be consideredas
tions within the urchin calcite similar to those observed a compositeof the dolomite and / structures,with all Mg
by Blake et al. (1984),who describeda coherentbound- layersof dolomite replacedby z-type layers.As in doloary betweentwo slightly misoriented blocks in magnesian mite, adjacentlayershave differentcompositions.
calcite.Local areasof strain and dislocationsare associFigure 9c showssimulated [001] images.For this oriated with suchmisorientation,as seenin [00] projection entation, the severalmodels yield distinct ordering pat(Fig. 7).
terns and diffraction patterns. In distinction to the idenOur samples also contain crystallographicdefects that tical cation site occupationsand hexagonalorganization
produce incoherent domains. These defects include of the cationsin calciteand dolomite, the z and y models
stacking faults (Fig. 8a), dislocations, and complicated show doubled spacingsalong one ofthe a axesand nonplanar defects of composite character that seem to be hexagonalordering. Such doubling and ordering are comtypical of the urchin calcite. Figure 8b shows examples patible with the domains in Figure 6 and presumablygive
ofelongate, slightly winding defectsthat cut acrossregions rise to the c reflections in Figure 5a. They occur in three
of well-formed calcite from Slr. fr. Calcite from Str. pr. directions as a result of the several orientations of the
also displays such long (>130 nm) subgrainboundaries domains within the host calcite(Reeder,1992).
(Fig. 8c).
The simulated [001] imagesof the,y and z models appear similar (Fig. 9c), as do their [001] diffraction patDrscussroN
terns. However, for orthogonal orientations, their ditrracBoth the diffraction patterns and HRTEM images of tion featuresdiffer. Specifically,the 7 structure yields odd
our samplesindicate the presenceof superstructuraldo- 00/ (003, 009, . . .) reflections(asdoesdolomite),whereas
mains in regions -10 nm in diameter. Superstructures the v structure does not. The X-ray data suggesttwo dishave been reported from many diageneticrhombohedral tinct c parameters,1.696 and 1.662 nm, corresponding
carbonates(Reeder and Wenk, 1979; Gunderson and to calcitecontainingminor Mg and to carbonatecontainWenk, l98l;Van Tendelooet al., 1985;Wenk andZhang, ing an increasedamount of Mg, respectively.The odd 00/
1985;Frisia Bruni and Wenk, 1985;Miser et al., 1987; reflections seen in X-ray and SAED patterns, taken together with the experimental and simulated HRTEM imBarberandKhan, 1987;Reksten,1990a,1990b).
Wenk et al. (1991) proposedseveralordering models ages,suggestthat the domains in our material have the 7
to explain the superstructuresin magnesiancalcite. Four structure.However, we cannot excludethe possibility that
of their models result in a doubling of the 4r and a, cell regions having the z structure could also exist, given that
TSIPURSKY AND BUSECK: MAGNESIAN CALCITE IN SEA URCHINS
780
Fig. 8. Three [00] images of subgrain boundaries. (a) A stacking fault (SF) and (b) misaligned blocks characterizedby offset
fringeson opposite sidesof the subgrainboundary of a Str.fr. spine, (c) a long (> I 30 nm) defect zone separatingtwo mosaic blocks
in a Slr. pr. spine. The arrows show shifts in the fringe contrast. A dislocation (D) is indicated.
^l
3:1
1:1
Ca:Mg
B
@
ffi
ffi
ffi/vv
zw9F
@
*o1
@,44-
ffi
ffi
/J
ffi
@
I
.
O
l€
of"iffi
o=---H
t----H
otsNr7o
o ./^.
^\./^.
r "/rY o "/o
a
alYa-va
ffi ffiffiffi
Fig. 9. The effectsof ditrerent Ca and Mg ratios on the carbonate structure. (a) Models showing different ordering schemes
and the corresponding (b) t00ll projections, and (c) computer
simulatedimages(Scherzerfocus).The scaleof c is 1.33 times
that of b. The large solid circles represent Ca, the small open
circles indicate Mg, and the large open circles indicate columns
containing both Ca and Mg; CO. is not shown.
their diagnosticfeaturesare not unique where / occurs
togetherwith 7.
Our conclusioncontrastswith the analysisof Wenk et
al. (1991),who indicated the z structureis plausibleand
the .y structure is prohibited as domains within calcite.
They point out that ^ycan occur within dolomite and can
also occur as a secondaryorderingproduct ofz, although
they indicate that origin is unlikely. Wenk et al. (1991)
studied material that had been diagenetically processed
and found the 7 structure as domains only within dolomite and the z structure within magnesiancalcite. On the
other hand, we investigatedbiogeniccalcitethat had not
experiencedany geological processingand observed the
? structure as domains within calcite. Our results, although complementaryto theirs, differ and are incompatible with their theoreticalanalysisof possibletransformations of the structure models. The difference in
origin of the two sets of samplesmay contain the answer
to this apparentdiscrepancy,but a resolutionremainsin
the future.
Our X-ray and HRTEM studies show that a pronounced mosaicism is characteristicof urchin carbonates.Blakeet al. ( 1984) speculatedthat the "mosaic structure [of biogenic calcite] is a result of small changesin
the orientation of submicrometer-sizedcrystallites,which
despite their differencesin orientation, are largely coher-
TSIPURSKY AND BUSECK: MAGNESIAN CALCITE IN SEA URCHINS
ent." Finding an absenceof dislocations, these authors
concluded that "the structure is in a state of stress." Our
investigations of the imperfections in biogenic calcites
show numerous coherentand complex incoherent boundaries between slightly misoriented mosaic blocks (Figs. 7,
8. and 9). Most of thesedefectzonescontain dislocations
that relieve stressesin the structure.
There are several possible explanations for the development of mosaicism during crystal growth. Davies et al.
(1972) showedthat multiple long, thin spinesdevelopin
parallel packets and are subsequentlyconnectedby narrow bridges of calcite. He concluded that growth is not
synchronous acrossthe crystals. Moreover, localized environmental conditions have a profound influence on the
perfectionofthe skeletalhard parts ofsea urchins.Their
growth rate and Mg content depend on the temperature
of the sea water and the food supply (Weber, 19691,Davies et al. 1972;Pearseand Pearse,1975).Deviations in
these conditions are reflected in the internal structure of
the hard parts and may result in uneven crystal growth
and mosaic structureswith intermediate defective zones,
in spite of strong organismal control. Contrary to the results of Berman et al. (1990), we found no evidenceof
organic matrix either along the subgrain boundaries or
within the calcite structure.
It is apparent that our biogenic calcite, which has not
been processedgeologically,and material that has undergone diagenesis(Wenk et al., l99l) show diferent features. It remains for a future study to determine how
generalthese differencesare and whether they arise from
aging or from other processes.
AcxNowr-nncMENTS
Samplesof Str. fr. were kindly provided by D. Chandler of Arizona
State University (ASU) and those of Str pr from Marinus, Inc. (l,ong
Beach,Califomia). We thank A. Berman and K. Towe for reprints as well
as samplesand R. Reeder, K Towe, and R. Angel for helpful reviews.
This study was supported by NSF grant EAR-87-0529. The X-ray and
electron microprobe investigation was performed in the Department of
Chemistry, and electron microscopy was done at the ASU Facility for
HREM, which is supported by NSF and ASU. The electron microprobe
was purchasedwith the aid of NSF grant EAR-84-08163 We are especially grateful to those families who provided private donations to help
supportthe researchofS.J T at ASU.
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APnIr 4, 1993
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