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Materials Science and Engineering C 23 (2003) 593–595

A qualitative description of preferred orientation in porous carbonate

matrices of marine origin

Yuval Golana,*, David Dahana, Razi Vagob

aDepartment of Materials Engineering, Ben-Gurion University of the Negev, Beersheba 84105, IsraelbThe Institute for Applied Biosciences and the Department of Biotechnology Engineering, Ben-Gurion University of the Negev, Beersheba 84105, Israel

Accepted 26 June 2003

Abstract

Porous aragonite matrices of marine origin exhibit a prominent preferred orientation in which the (221) crystal planes are aligned

perpendicular to the principal growth direction of the organism. Since the aragonite crystallites that compose the matrix appear to be

organized perpendicular to the spherical pore surfaces, these apparently conflicting findings can be explained by a bimodal distribution of the

crystallites into pore and bulk crystallite populations. Analysis of X-ray diffraction data obtained from matrices taken from eight different

organisms was carried out. The validity of the bimodal distribution model was confirmed by correlation with porosity data.

D 2003 Elsevier B.V. All rights reserved.

Keywords: Microstructure; Porosity; X-ray diffraction; Aragonite

Recently, there has been considerable interest in porous

carbonate matrices of marine origin for biomedical applica-

tions such as bone replacement [1]. These materials, which

are biofabricated by marine organisms such as corals, were

reported to be highly biocompatible under in-vitro and in-

vivo conditions and are easily recognized and colonized by

various cells and tissues [2,3]. One of the main reasons for

the biocompatibility is the morphological similarity of the

porous structure of the biomatrices to the porous structure of

bone [4]. We have conducted comprehensive microstruc-

tural studies of a variety of coral skeletons and found similar

microstructural guidelines: (i) All matrices were crystalline

according to X-ray diffraction measurements (XRD) and

showed the aragonite structure. (ii) The crystals were found

to be organized perpendicular to the matrix surface, and

were thus viewed edge-on when imaged by scanning

electron microscopy (SEM) as seen in Fig. 1. While the

full details of these studies will be published elsewhere, it is

important to portray another common feature in these

matrices, which is the prominent preferred crystallographic

orientation in which the (221) planes are preferentially

aligned perpendicular to the principal growth axis of the

0928-4931/$ - see front matter D 2003 Elsevier B.V. All rights reserved.

doi:10.1016/S0928-4931(03)00054-7

* Corresponding author. Tel.: +972-7-6461474; fax: +972-7-6472944.

E-mail address: ygolan@bgumail.bgu.ac.il (Y. Golan).

organism. Thus, while the (111) reflection is the strongest

reflection in the aragonite powder diffraction file [5], X-ray

diffraction spectra taken from cross-sections (samples cut

perpendicular to the principal growth axis) of matrices

obtained from eight different organisms showed that the

(221) reflections were clearly stronger than the (111) reflec-

tions. This preferred orientation was initially surprising

since if all crystallites are organized perpendicular to the

spherical pore surfaces (as seen in Fig. 1), this should result

in the averaging of all crystal orientations and subsequent

loss of preferred orientation in the X-ray diffractograms. We

hypothesized that this discrepancy can be explained by a

bimodal distribution of the crystallites into bulk and surface

crystallites. The oriented bulk crystallites facilitate the

growth of the organism along the principal growth axis,

while at the same time, exposure of well-defined aragonite

c-planes at the pore surface can be advantageous for surface

recognition and adhesion of cells [6]. If proven true, this can

be another manifestation of nature’s remarkable ability to

construct ‘‘smart’’ complex materials.

Therefore, in this work, we have tested the assumption

that the crystallites in these matrices are composed of two

major populations. A population fraction of crystallites that

are organized normal to the pore surface, Nav, and hence

with averaged orientations, and a second population fraction

of crystallites, Nb, which are in the bulk of the matrix and

Fig. 1. Scanning electron micrographs of a porous carbonate matrix

obtained from the organism P. lutea. (a) Low magnification, showing the

porous structure of the matrix. (b) Higher magnification, showing the

organization of the aragonite crystallites normal to the pore surface.

Table 1

True porosity and anisotropy parameter (a) values calculated from XRD for

cross-sections obtained from the eight marine organisms studied in this

work

Species Direction a True porosity (%)

P. lutea cross-section 0.57 60.7

P. digitata cross-section 0.47 57.4

M. dichotoma cross-section 0.54 54.3

Acropora sp. cross-section 0.58 51.5

S. pistillata cross-section 0.59 47.2

A. palifera cross-section 0.58 44.7

T. reniformis cross-section 0.64 41.2

F. simplex septal 0.75 27.5

Fig. 2. (a) Photograph showing the multiseptal structure of F. simplex. (b)

Scanning electron micrograph showing the organization of the crystallites

parallel to the septal surface.

Y. Golan et al. / Materials Science and Engineering C 23 (2003) 593–595594

oriented with respect to the principal growth axis of the

organism. We assume that the bimodal distribution accounts

for the entire population of the samples, so that Nav +Nb = 1.

Since the preferred orientation of the crystallites is man-

ifested with respect to the (111) reflection that is the

strongest reflection in the aragonite crystal structure [5], it

is thus reasonable to assume that the (111) reflections are

effected only by the pore population, Nav, and therefore the

contribution of Nb to the measured X-ray diffraction inten-

sity of the (111) reflections can be neglected. Hence, we can

assume that the intensity of the (111) reflections, Imeas(111),

corresponds solely to Nav. On the other hand, the (221)

reflection is effected by crystallites from both the Nav and

the Nb populations. This can be used in order to separate

the diffraction intensities for each of the two populations in

the total intensity measured for the (221) reflection,

Imeas(221). The contribution of the intensity corresponding

to the pore crystallites in the (221) reflection, Iav(221), can

be given by normalization of the measured (111) intensity,

Imeas(111), according to the theoretical ratio expected for

these two reflections in the corresponding powder diffrac-

tion file (pdf) [5]:

Iavð221Þ ¼ Imeasð111ÞIpdf ð221ÞIpdf ð111Þ

ð1Þ

We can now subtract the contribution of the intensity

obtained from the pore crystallites from the total measured

Fig. 3. Anisotropy parameter values calculated from XRD data of cross-

sections (Table 1) plotted vs. the true porosity of the matrices.

Y. Golan et al. / Materials Science and Engineering C 23 (2003) 593–595 595

intensity of the (221) reflection to obtain the intensity

contribution of the bulk crystallites:

Ibð221Þ ¼ Imeasð221Þ � Iavð221Þ ð2Þ

Now we define the anisotropy parameter, a, which

represents the fraction of the ordered crystallites out of the

total population:

a ¼ Nb ¼Ibð221Þ

Imeasð221Þð3Þ

The analysis outlined above was carried out for cross-

sections of carbonate matrix samples obtained from eight

different organisms, and the results are presented in Table 1.

All spectra were obtained in the h/2h geometry using CuKa

radiation. The APD X-ray data analysis package was used

for all analyses. Prior to peak integration, the background

was subtracted for each spectrum. Note that cross-sections

were measured for all samples except for Fungia simplex, a

solitary coral with a multi-septal structure (see photograph

in Fig. 2a), since in this particular matrix, the crystallites are

organized parallel to the septal surface as shown in the

SEM image in Fig. 2b [7]. As expected, the (221) preferred

orientation in this particular matrix is observed in the septal

section rather than the cross-section, and therefore the

septal section was measured and analyzed. Note that from

Fig. 2b it is strongly suggested that the porosity of the F.

simplex matrix is expected to be relatively small (compare

with Fig. 1a).

The results of this simple model were tested by correlat-

ing the X-ray data with true porosity measurements carried

out using the Archimedes method.1 Table 1 shows the

anisotropy parameter (a) values that were calculated accord-

ing to the analysis detailed above, and the results are plotted

vs. the porosity data and showed in Fig. 3. A linear

correlation (R2) of 90% was obtained, which validated the

bimodal distribution of the crystallites in the eight matrices

studied in this work. This degree of linear correlation is

clearly significant keeping in mind the variability in bio-

logical systems, and that a sharp interface between the two

populations is rather unlikely. Moreover, note that signifi-

cant deviation from linearity is obtained only for the two

most porous matrices, Porites lutea and Psammocora

digitata. Due to the spherical geometry of these two coral

colonies, the preparation of well-aligned cross-sections

becomes much harder and the error in the alignment of

the specimen with respect to the principal growth axis

becomes consequently larger. Thus, as the matrix becomes

more porous, the anisotropy parameter decreases due to an

increased contribution of the averaged pore crystals to the

diffracted intensity of the (221) reflection. It is important to

1 For details on true porosity measurement using the Archimedes

method see, e.g., Ref. [8].

note that no simple quantitative relationship exists between

X-ray profile intensity ratios and the population fractions of

oriented crystals. For a fully quantitative analysis, a more

rigorous treatment must be carried out by using, e.g., the

March analysis [9,10].

In summary, we have investigated the prominent pre-

ferred orientation in which the (221) planes are aligned

perpendicular to the principal growth direction of the organ-

ism. This finding was initially surprising since the orienta-

tion of the crystallites is expected to be averaged as they were

seen to be aligned normal to the spherical pore surface. A

bimodal distribution of the crystallites into pore (isotropic)

and bulk (aligned) populations was confirmed by correlating

the results of the XRD analysis with porosity data. We note

that this approach is not limited to carbonate matrix bio-

materials, and can be generally applied for the study of other

cases of preferred orientation in complex microstructures.

Acknowledgements

We are grateful to Youli Li from the Materials Research

Laboratory, UC Santa Barbara for inspiring discussions.

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