Journal of
Archaeological
SCIENCE
Journal of Archaeological Science 30 (2003) 1211–1214
http://www.elsevier.com/locate/jas
Discussion
Calcite crystals, starch grains aggregates or.POCC? Comment on
‘calcite crystals inside archaeologial plant tissues’
Jacques E´. Brochier a,*, Michel Thinon b
a
UMR 6569 du CNRS, Laboratoire de Pale´ontologie Humaine et de Pre´histoire, Faculte´ des Sciences, Centre St Charles, Baˆt. 10,
13331 Marseille Cedex 3, France
b
UMR 6116 du CNRS, Faculte´ de Sciences de Saint Je´roˆme, Institut Me´diterrane´en d’ E´cologie et de Pale´oe´cologie, case 461,
avenue Escadrille Normandie-Niemen, 13390 Marseille Cedex 20, France
Abstract
The microscopic objects presented by F.O. Freitas and P.S. Martins as ‘recrystallized calcite crystals’ and ‘starch grain
aggregates’ are both compound crystals, end products of the transformation of plant calcium oxalate crystals into calcite in wood
fires. These calcareous silty particles are the major component of the wood ashes familiar to archaeologists. Petrographic microscope
remains the more eﬃcient tool for this kind of research.
2003 Elsevier Ltd. All rights reserved.
Keywords: Calcium oxalate crystal; Oxalic phytolith; POCC; Wood ashes
In a recent paper F.O.Freitas and P.S. Martins describe
calcite crystals and starch grain aggregates ‘in’ the
reserve tissues of samples of Zea mays mays and
Manihot esculenta coming from Brazilians calcareous
rock-shelters. Macro-botanical remains were packed in
braided palm-leaf baskets themselves covered by the
same braided palm leaves and buried in wood-ashes and
soils in silo-like structures.
The good preservation of the remains allows them
to recognize these two cultivated plants. The specific
morphology of the starch grains, observed by SEM,
allows them to confirm the identification of the cassava
sample. The 14C estimated age of the samples shows
that these crop samples are among the oldest found in
tropical South America. As geoarchaeologist and
pedoanthracologist, we would like to make some comments on the two types of objects they described i.e.
the so-called ‘calcite crystals’ and ‘aggregated starch
grains’.
* Corresponding author.
´ . Brochier).
E-mail address: [email protected] (J.E
E-mail address: [email protected] (M. Thinon).
0305-4403/03/$ - see front matter 2003 Elsevier Ltd. All rights reserved.
doi:10.1016/S0305-4403(02)00031-6
1. Calcite crystals
During the SEM search of starch grains, calcite
crystals were consistently found. They are interpreted as
calcite recrystallization within seed and tuber tissues.
The mineralogy of these crystals is based on X-ray
microanalysis (large amount of calcium), X-ray powder
diﬀractometry and HCl eﬀervescence testing. Unlike
what is said in the text and in caption of Fig. 6, it is not
the X-ray microanalysis spectra which is shown but an
X-ray powder diﬀractometry diagram of the material
extracted from inside the seed or tuber. The spectra is
compared with reference synthetic calcite (card number
5-0586) shown as horizontal lines against the 2 axis.
There is no doubt that calcite is present in the sample.
Equally, there is no doubt that other minerals, including
quartz dust, are present. How to explain the presence of
a detrital mineral, quartz, inside the seed? The best
explanation is to consider that the sample analysed is
a mixture, in an unknown proportion, of seed-tuber
material and surrounding sediment.
There is no argument which clearly demonstrate that
the ‘calcite crystals’ shown in the figures are those which
gave positive HCl eﬀervescence test, those which gave
the major calcite diﬀraction peak of Fig. 6. The only
strong argument is the large amount of calcium revealed
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J.E´. Brochier, M. Thinon / Journal of Archaeological Science 30 (2003) 1211–1214
Fig. 1. Experimental micritic calcite pseudomorphs after calcium oxalate (POCC) obtained by burning Quercus coccifera wood and bark in a furnace
at 500 (C. Note the granular texture, the dark colour (brown viewed in polarized light), the habitus and the size similar to those of Freitas and
Martins’s ‘calcite crystals’.
by X-ray microanalysis, but a lot of other minerals can
yield the same results.
In spite of the lack of evidence, all the ‘crystals’
shown, well known to geoarchaeologists, are made of
calcite. Their observation with a petrographic microscope, the best suited tool for mineralogical study, would
have pointed out that they are not calcite mono-crystals
but a dense packing within a crystal-shaped outline of
tiny micritic calcite crystals at the origin of the granular
texture observed. The optical axes of this multitude of
tiny calcitic crystals, generally randomly oriented, lead
to their permanent illumination when the stage of the
microscope is rotated between crossed nicols.
One of us has described since 1983 [1,2,6] the genesis
of these very common compound crystals in archaeological contexts. Gymnosperms and dicot Angiosperms
(some other plants like lichens too) contain in their
tissues (principally in bark and leaves but also in wood)
calcium oxalate phytoliths [7–10,12,13]. When the wood,
or the leaves, are burnt, the calcium oxalate crystals
(monoclinic whewellite, CaC2O4 . 2H2O) or quadratic
weddellite, CaC2O4 . 2H2O) are converted into calcium
carbonate when the temperature reach 430–510 (C [12].
This chemical transformation upon heat does not aﬀect
the habitus of the former oxalic crystal. The microcrystalline aggregates can thus be named calcitic pseudomorphs after calcium oxalate or more simply POCC,
an acronym for their French name: pseudomorphose
d’oxalate de calcium en calcite (Fig. 1).
If the temperature exceeds 600 (C, a common temperature in a domestic hearth, the POCC are converted
into lime which after hydration and carbonation gives
rise to very fine calcite crystals.
The POCC are rarely characteristic of a peculiar
vegetal specie. Nevertheless, there are some exceptions:
for instance, the very elongated prisms present in Pinus,
the specific prisms of the two species Tilia cordata and
Tilia platyphyllos, the raphides of the genus Vitis [3–5].
Produced in great quantity in wood fire, widely
dispersed on the living floor, the POCC, calcareous dust
15 µm wide on average, give their ashy appearance and
their high fine carbonate content to holocene calcareous
rock-shelter anthropogenic deposits. In old open-air
sites, on acidic soils, these very fine calcitic crystals no
longer survive. Only macro and microscopic charcoal
are thus observed.
The ‘crystals’ presented by F.O. Freitas and P.S.
Martins are all very common shapes of POCC present
J.E´. Brochier, M. Thinon / Journal of Archaeological Science 30 (2003) 1211–1214
1213
Fig. 2. (a,b) Two kinds of POCC from archaeological samples (Belesta cave, France; (a) Iron Age, (b) Middle Neolithic). Dark and micritic calcitic
POCC are the usual forms. Hyaline microspar calcitic POCC have all the external morphologic characters of Freitas and Martins’s ‘starch
aggregates’.
in a great number of vegetal species. Their shape
and texture are typical of heated vegetal calcium
oxalate crystals converted into calcite. Bearing in mind
that the samples were buried in ashes of fire wood,
and that the analysed seed-tuber samples are polluted
by external dust and therefore by wood ashes, the
simplest interpretation is to consider that the observed
‘crystals’ are POCC adhering on the seed-tuber surface.
They are not recrystallized calcite crystals in vegetal
tissues.
2. Aggregated starch grains
F.O. Freitas and P.S. Martin found in their maize
and cassava archaeological samples aggregates of starch
grains. They emphasize that these aggregates do not
occur in fresh material, that the starch aggregates are of
similar size and shape to the so-called calcite crystals and
that calcite acts as a cementing agent.
All these features are compatible with those
observed on a common archaeological variety of POCC
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J.E´. Brochier, M. Thinon / Journal of Archaeological Science 30 (2003) 1211–1214
(Fig. 2a, b). The use of a petrographic microscope would
have shown that the so-called starch aggregates have
calcite high interference colours and not the starch low
interference colours. It would also have shown the lack
of the starch characteristic extinction cross observed
between crossed nicols.
This variety of POCC is common in holocene
rock-shelter archaeological deposits. It has never been
obtained through experimental heating of vegetal
calcium oxalate crystals. Their shape and size, as noted
by the authors, are similar to those of classical POCC.
Nevertheless, their crystalline outline, always recognizable (Figs. 7 and 8 of the authors), is less sharp. The
starch-like surface structure can be observed on any
shape (there are as many shapes of POCC as of habitus
of plant calcium oxalate crystals) of archaeological
POCC.
We do not know exactly the genesis of this variety of
POCC. A recrystallization phenomenon of the packed
micritic crystals forming the usual POCC is the most
likely hypothesis.
Although SEM technics are irreplaceable in modern
research, optical microscopy is a necessary first step that
should not be omitted. The major advances of the last
decade in geoarchaeology, phytolith studies or starch
analysis [11,14] rely upon this old tool.
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