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Preservation of anthropogenic materials under different geochemical processes:A mineralogical approach

Panagiotis KarkanasEphoreia of Palaeoanthropology-Speleology of Southern Greece, Ardittou 34b, 11636 Athens, Greece

a r t i c l e i n f o

Article history:Available online 4 November 2009

a b s t r a c t

Minerals that are preserved or formed inside the sediment or soil can be used to determine whetherprimary anthropogenic constituents (bones, phytoliths, organic matter, charcoal, etc.) would have beenaffected by diagenesis. Stability diagrams of the minerals that describe the chemical system underinvestigation can be constructed and then tested in the field. In this approach, it is assumed that thestability of the archaeological material of interest is known. Except for bone, there are not many studieson the stability of other archaeological materials. This approach does not provide information on theformer presence of an archaeological material in the sediment or soil but only whether these materialsare stable in such an environment. Some indirect indication may be provided by unique mineral asso-ciations, but much more has to be done in this direction.

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1. Introduction

Chemical reactions acting on soils or sediments before or afterburial are generally well known and studied extensively (Birkeland,1999; Bohn et al., 2001; Retallack, 2001, and references therein).This type of basic research is valuable for understanding soilforming processes (acidification, salinization, oxidation, hydration,etc.) that are related to environmental factors such as climate,activities of organisms and geomorphic processes (Retallack, 2001).As a consequence, they are also of major importance for archae-ology, because environmental changes are directly or indirectlyrelated to human biological and cultural evolution. Furthermore,chemical reactions in archaeological deposits produce alterationsthat may distort the original stratigraphic context. They producevolume changes, assimilation of layers or secondary layering,destroy interfaces and change the original locations of artifacts andthe relation between artifacts. Of equal importance is the preser-vation of the artifacts and generally the anthropogenic remains.Indeed, chemical reactions are by far the most destructive agent ofanthropogenic remains. Chemical reactions in soils are alsocontrolled by microorganisms which are the main destructiveagent of organic artifacts. Most of the materials of archaeologicalinterest are composed of minerals or organics whose generalstability in different chemical environments, is known. Forexample, it is well known that bones do not survive in acidicenvironments (Keely et al., 1977; Hedges and Millard, 1995).

However, except for this kind of general knowledge detailed studiesconcerning archaeological materials and their preservation underdifferent geochemical environments are limited, with the exceptionof bone (Weiner et al., 1993; Hedges and Millard, 1995; Karkanaset al., 2000; Berna et al., 2004). In most cases, there are no well-established indicators of whether a material of archaeologicalinterest originally was part of the archaeological deposit and hasbeen dissolved. Obviously this is fundamental for interpretingcorrectly the archaeological record.

There are two approaches for studying the preservation ofarchaeological materials:

a) To determine the past chemical conditions in order to see if thearchaeological material of interest was stable under theseconditions. In this step, it is supposed that the stability of theparticular archaeological material is known. Note that this willdetermine whether the primary anthropogenic constituents(such as bones, teeth, phytoliths, pollen, plant remains andcharcoal) would have been affected by diagenesis irrespectiveof whether or not they are or they were present in thesediments.

b) To find indications that the archaeological material was indeedpart of the archaeological deposit and has dissolved.

2. Theoretical foundation

It has long been known that stable minerals persist or tend toform secondarily in soils and sediments, while the unstable onesE-mail address: pkarkanas@hua.gr

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transform or dissolve away (for a review see Rai and Kittrick, 1989).From a set of minerals the most stable is the least soluble, thusstability sequences can be determined by mineral solubilities.Minerals that form within the sediments or minerals that arepreserved in the sediments can be used to determine key chemicalenvironmental parameters that prevailed in the sediments, such asacidity/alkalinity (pH), and oxidation conditions (Eh). In theory,stability fields of minerals provide an accurate way for determiningthe Eh and pH of soils and sediments. Each kind of archaeologicalmaterial can be considered to be chemically stable under certainchemical conditions. Thus, stability fields can in turn be used todetermine whether the primary anthropogenic constituents wouldhave been affected by diagenesis providing that their stabilityunder these conditions is known. For example, Retallack (1984) hasconstructed a diagram with the theoretical Eh–pH stability fields ofcommon kinds of terrestrial fossils preserved in paleosols.

Thermodynamic and solubility data can be used to predict theformation and weathering of minerals and the sequence ofmineral weathering, however the rate of formation cannot bepredicted. Kinetic factors (temperature, medium, inhibitors, cata-lysts etc.) influence the rate of reactions. For example, water andparticularly the rate of water flow through the soil or sedimentdetermines the rate of most reactions. Nevertheless, the thermo-dynamic approach has an advantage. Minerals in soils are rarely inequilibrium with one another. However, within the groupcompeting for common elements, individual minerals are usuallystable over a narrow range of solution compositions (Rai andKittrick, 1989). Thus it is possible to understand the soil system byconsidering the stability of only a few of its minerals. In addition,building a theoretical model based on thermodynamic data canproduce new insights and reveal what one must look for in thefield to validate them. This mutual interaction of theoretical designand field observations offers a good chance to advance knowledgeof preservation.

3. Methodological approach

It is imperative that a sedimentary contextual analysis is set upfirst. Micromorphological studies of undisturbed sediment and soilsamples will provide the textural relationships and degree ofalteration of the minerals (Courty et al., 1989; Delvigne, 1998). First,it should be noted that minerals in the process of alteration docu-ment the progress of chemical reactions, because chemical alter-ations are slow and seldom reach equilibrium. Cross-cuttingrelationships provide evidence for the sequence of mineral devel-opment in soils (Figs. 1–4). For example, direct indications ofchemical reactions are well formed crystals with sharp angularoutlines that may truncate pre-existing grain boundaries ofcorroded minerals or form on rounded and worn detrital grains(Figs. 1–4). For example, in Fig. 3, fresh sparry calcite crystals,pseudomorphic after diamond-shaped gypsum crystals, are grownon an isotropic mass of apatite. Most of the calcite crystals haveclear, sharp, angular outlines implying that they are not dissolvingby the formation of apatite. In contrast, they cross-cut the phos-phate mass. Apatite is usually forming together with gypsum(Shahack-Gross et al., 2004) but is not stable with calcite (seediscussion below). The fact that calcite has replaced gypsum furthersuggests that there is a reversal in the geochemical environment inwhich calcite is the stable mineral and not apatite.

Cross-cutting relationships are generally not difficult to identify,but what is usually passed over is spatial relationships and distri-bution. Since most minerals and archaeological constituentsdissolve completely, direct indications of chemical reactions arerarely found. However, the sequence of disappearance and/orappearance of certain minerals in space are indirect but secure

indications of chemical reactions that are responsible for thisspatial distribution. Nevertheless, minerals should have a chemicalrelationship, that is, in order to compensate for the appearance ofa mineral, another mineral has to dissolve, thus, providing the

Fig. 1. Photomicrograph in crossed-polarized light (XPL) of anhydrite crystals (A)growing on a diamond-shaped gypsum crystal that has been recrystallized toa gypsum aggregate (G). Note that the anhydrite crystals cross-cut the gypsum crystalsshowing also clear sharp angular outlines (see Karkanas and Goldberg, in press).

Fig. 2. Photomicrographs in plane-polarized light (PPL) (a) and in XPL (b) of bladedcalcite growing in the outer part of a dissolved diamond-shaped gypsum; only theouter rim of the latter is preserved delineated by phosphatized clay (confirmed also bybulk FTIR), better seen in PPL (see Karkanas and Goldberg, in press).

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necessary chemical components. Researchers must be aware of thisrelationship in order to recognize it.

Furthermore, instrumental mineralogical and microchemicaltechniques should be used in order to confidentially identify themineral phases. X-ray diffraction (XRD) analysis should be imple-mented for crystalline features and Fourier Transformed Infrared

spectrometry (FTIR) for amorphous and crystalline phases.However, in most cases these techniques cannot identify mineralphases that are less than a few percent in the sediment, particu-larly if many other minerals are present as well. Unfortunately thisis the case for several of the geochemical processes that are ofinterest in the archaeological record. There is the possibility of thetime-consuming physical separation of the different mineralogicalphases according to density and other properties or other moresophisticated techniques such as micro FTIR (Berna et al.,submitted for publication). There is also the possibility of usingScanning Electron Microscopy (SEM) and energy dispersion (EDSand EDAX systems) or the electron microprobe to identify theminerals based on their chemical composition. It is advisable tosupplement all these techniques with micromorphological analysisand if possible use the same thin section to do all these micro-chemical analyses. In the study of Grotte XVI (Karkanas et al.,2002) a transition alteration zone was found between an apatite-rich zone where bone is stable and a zone where Al-richphosphates occur and bone has been dissolved. The transitionalteration zone is dominated by non-stoichiometric apatite basedon extensive bulk FTIR analysis. However, through opticalmicroscopy it was found that there were traces of different phos-phate phases forming at the expense of the main one. These werelater identified as montgomeryite and leucophosphite throughSEM and EDS analysis (Fig. 4).

4. Constructing the stable mineral associations

In most of the cases the equilibrium chemistry and dissolutionrates of certain minerals that are of special interest to archaeologyare known. For example the equilibrium chemistry of mineralphases such as calcite, quartz, and gypsum are known. However, inother cases the equilibrium diagrams have to be constructed basedon meager thermodynamic data and many assumptions. Never-theless, this has an advantage of making a model of the expectedmineral associations and testing its validity in the field.

An example of stable mineral associations for the system Ca–P–Al–Kin caves was presented by Karkanas et al. (2000) (Fig. 5). This systemwas used, because the above constituents define the associatedminerals under investigation. That is hydroxyapatite [Ca10 (PO4)6(OH)2]containing Ca and P, crandallite [CaAl3(OH)6[PO3 O1/2 (OH)1/2]2], andmontgomeryite [Ca4Mg (H2O)12[Al4(OH)4(PO4)6]] containing Ca, Al andP and taranakite [H6K3Al5(PO4)8$18H2O] containing K, Al, and P.Montgomeryite also contains Mg, although it has also an end Al-richmember which it was used for simplicity.

Fig. 3. PPL (a) and XPL (b) photomicrographs of fresh sparry calcite crystals, some ofthe pseudomorphic after diamond-shaped gypsum crystals, replacing isotropic apatite(black in XPL) (see also text and Karkanas and Goldberg, in press). A void is labeledwith V.

Fig. 4. SEM image of white leucophosphite replacing a montgomeryite (grey) radialaggregate (see Karkanas et al., 2002).

Fig. 5. Stability relations for hydroxyapatite (A), montgomeryite (M), crandallite (C)and taranakite (T) after Karkanas et al. (2000). The activities of Al(OH)4

�, Ca2þ and Kþ

are fixed at 10�7, 10�4 and 10�4 respectively.

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From the produced mineralogical phase diagrams it wasdeduced:

1) Carbonated hydroxylapatite is the stable phosphate mineral inalkaline conditions and will react to form more complexaluminum-rich phosphate phases in more acidic environments(crandallite, montgomeryite and taranakite) (Fig. 5).

2) Fluctuating concentrations of the different cations has aneffect in expanding or decreasing the stability fields ofminerals and particularly montgomeryite. If, for example, thealuminum concentration is decreased, the stability field ofmontgomeryite increases. This is because crandallite, theother competitive Ca–Al phosphate mineral, is favored inhigher aluminum concentrations because it contains morealuminum than montgomeryite. In a same way an increase ofphosphorous will expand the field of montgomeryite.

3) Different transformation pathways can be followed dependingon the particular chemistry of the system.

None of the predicted reactions can be seen directly, as in mostcases there is complete dissolution of the precursor mineral beforethe formation of the new, stable mineral.

There is often a misconception reading these stability diagrams.Pathways and stability relations are not valid for every chemicalsystem but only for the particular system under consideration. Forexample, if the described system above does not contain Ca,implying that it is already decalcified before phosphate is intro-duced, then obviously phosphate minerals having Ca, like apatite,crandallite and montgomeryite are not stable at all and will not beformed. Therefore, for the same set of minerals only taranakite isstable under these chemical conditions providing of course thatthere is phosphate alteration. If iron is also part of the system,leucophosphite [K2[Fe3þ

4(OH)2(H2O)2(PO4)4]$2H2O], a Fe–K–Pmineral with or without Al, will be also stable together withtaranakite.

Another misconception has to do with the idea of lack of equilib-rium between mineral phases. However, transition zones whereminerals are in an arrested form are actually more informative on theundergoing chemical reactions and they do not violate the predictedpathways and preservation of the materials of interest. Most of thetime, transition zones exhibit an internal zoning that shows clearlythe reaction pathway but it is hard to be seen in the scale of a thinsection, for example. They need a special approach in their study; oneis based on the sequence of disappearance and/or appearance ofcertain minerals in space as crossing the alteration zone.

5. Stability of archaeological materials

Based on the mineral stabilities above described, it is possible topredict when bone will be preserved in the archaeological record.Bone, composed of carbonated hydroxylapatite, will be not stable ina chemical environment in which the mineral carbonated apatite isnot stable. An indication of the instability of apatite according to theconstructed phase diagrams of Karkanas et al. (2000) will bea phosphate mineral that contains Al (e.g. taranakite, crandallite,montgomeryite). This has been seen in all studied archaeologicalcave sites (Weiner et al., 1993, 1995, 2002, 2007; Schiegl et al., 1996;Karkanas et al., 1999, 2000, 2002; Quattropani et al., 1999; Shahack-Gross et al., 2004). Open-air sites are not well studied with respectto their mineralogy and bone preservation. Such sites do not nor-mally contain large amounts of phosphate-bearing guanocompared to caves (cf. Shahack-Gross et al., 2004) and, therefore,detectable phosphate minerals are not easily formed. However,since phosphate is a weakly soluble anion (Bohn et al., 2001, p. 44),phosphate minerals should be formed in the soil environment after

dissolution of bone (see for example Simpson et al., 2000).Furthermore, in anthropic sediments associated with animal dungand human waste phosphate minerals and features are frequent(Macphail, 2000; Shahack-Gross et al., 2003; Macphail et al., 2004).In such environments Al phosphates have been recorded and it issuggested that they are related to bone alteration (Schaefer et al.,2004).

At this point, it has to be stated that the mineral stabilitydiagrams are not constructed to demonstrate that apatite or boneare not stable in an acidic environment; something that is wellknown (Gordon and Buikstra, 1981; Berna et al., 2004; Nielsen-Marsh et al., 2007). It is for finding indicators in the sediment (i.e.an Al-phosphate mineral) demonstrating clearly that if bone waspresent it has been dissolved. An alternative method would be tomeasure the pH of the sediment or soil. However, in addition tosome difficulties and ambiguities in measuring soil pH (Bohn et al.,2001, p. 268; Matthiesen, 2004), the present pH does not neces-sarily record the former pH of the sediments because pH tends tochange after burial (Retallack, 2001).

A more detailed study on the stability of the bone itself pre-sented insights into the details of the process. Berna et al. (2004)were able to show experimentally that bone is stable in sedimentswith pH above 8.1, and that between pH 8.1 and 7.4 it will undergore-crystallization and be replaced by more stable forms of apatite.Below pH 7 fresh or recrystallized bone will rapidly dissolvedepending on the water flow (see also Hedges and Millard, 1995).Berna et al. (2004) also confirmed that in the presence of calcitebone is stable, something that can be also predicted by the stabilitydiagrams and already was shown in case studies (Weiner et al.,1993).

Note that bones exposed on the surface will interact with soilpore water (Trueman et al., 2004), and also directly with rain water(or dripping water in caves). In the latter case the mineralogy of thesoil or sediment might not always be decisive and dissolutionmight happen irrespective of the minerals present. Dissolution ratewill depend on the pH, amount, and rate of the rain water and thehydrological parameters of bones (Hedges and Millard, 1995). Butnevertheless, the only available study (Trueman et al., 2004) hasshown that soil chemistry very quickly influences the mineralogy ofthe exposed bone and thus determines its dissolution.

Wood ash, which consists mainly of calcite (Wattez and Courty1987; Canti, 2003), is also an archaeological material that can easilydissolve or alter under acidic conditions and high water flow. It isobvious that a decalcified soil would not preserve ash or any othercalcareous material of archaeological interest, like shell forexample. Since calcite is expected to be found almost everywhereeven as dust component (Birkeland, 1999, p. 131) its absence is anindication for the dissolution of ash or other calcareous materials ofinterest. Fresh ash depending on the temperature of burning hasa pH of 9–13.5 (Etiegni and Campbell, 1991). The high pH values aremainly due to hydroxides of calcium and probably soluble potas-sium and sodium compounds that decrease with increasingtemperature of burning. Nonetheless, calcium hydroxide readilyabsorbs atmospheric carbon to become calcite, whereas dissolutionof most of potassium and sodium in ash is rapid in the presence ofwater (Etiegni and Campbell, 1991; Canti, 2003). Therefore, verysoon after burning (on the time scale of weeks in temperateclimates: Canti, 2003) the pH of fresh wood ash is regulated bycalcite and this is the reason why all mineralogical analyses ofarchaeological pristine ash do not show any other minerals exceptcalcite, biogenic silica and siliceous aggregates (Weiner et al., 1995;Schiegl et al., 1996; Canti, 2003). Re-crystallization of ash hasserious implications in the interpretation of the archaeologicalrecord. Re-crystallization initially produces a continuous cementedmicritic mass rich in black microcontrasting particles (charred

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material) that gradually transforms to well-crystallized sparrycalcite. However, careful microscopic examination might reveal thepresence of calcitic-ash crystals and wood-ash cellular pseudo-morphs (Karkanas et al., 2007). Furthermore, charcoal, oxidized(burnt) soil aggregates as well as a close association with burntbone and flint are further indications. In the presence of phosphateswood ash will be altered to carbonated hydroxylapatite and withincreasing diagenesis to the same mineralogical phases alreadydiscussed above for bone (see also Schiegl et al., 1996; Karkanaset al., 1999, 2000, 2002; Weiner et al., 2002, 2007).

Unfortunately this type of detailed information is not availablefor other archaeological remains of interest, for example dungspherulites, phytoliths, chert, charcoal and organic matter.

Dung spherulites are another form of calcium carbonate foundin large quantities in several archaeological sites (Canti, 1997, 1999).However, their mineralogy is not yet fully deciphered; it can beeither monohydrocalcite (Shahack-Gross et al., 2003) or calcite(Canti, 1997). It has been shown experimentally that spherulites donot survive at pH lower than approximately 7.7 and that they aremore soluble than calcite (Canti, 1999), something that has beenalso observed in archaeological sediments (e.g. Albert et al., 2008).The reason might be that spherulites are composed of mono-hydrocalcite and not calcite, the former being more soluble thancalcite (Shahack-Gross et al., 2003; Albert et al., 2008).

Opal which is a hydrous, amorphous silica polymorph (bettercalled opal-A) has a constant solubility up to 8.5 pH and increasesrapidly above pH 9 (Krauskopf, 1979). Phytoliths, which area biogenic form of opal, are a primary material of archaeologicalinterest. Dissolution of phytoliths in fluctuating degrees is assumedif calcite is present, an indicator of a generally alkaline environment(Piperno, 1988; Albert et al., 2006). However, calcite itself is notalways an indicator of such a high pH. Halite, gypsum, and clayminerals like palygorskite or sepiolite and certain zeolites arecharacteristic of saline soils and highly alkaline environments.Some of these minerals are indicative of an arid environment,where normally there is not much water for the dissolution ofphytoliths. However, frequent alternations of wet and dry periodswill result in pH fluctuations, dissolution of phytoliths andprecipitation of amorphous silica, palygorskite and sepiolite (c.f.Ringrose et al., 2008). A drop in pH will result in precipitation of thedissolved silica in the form of opal and Si-rich clays, such askaolinite (c.f. Blanco et al., 2008). Nonetheless, the solubility ofamorphous silica and phytoliths is almost an order of magnitudehigher than that of quartz (Fraysse et al., 2009). Therefore, thesolubility of phytoliths appears to be high enough that they candissolve in the normal pH range (4–8) of the soil environment,assuming a high rate of water flow through the sediment (Fraysseet al., 2009). The problem becomes more complicated becausesolubility studies of biogenic opals and phytoliths with differentphysical and chemical properties show differences, with somerelated to pH (Wilding et al., 1979; Bartoli and Wilding, 1980; Dixitand Van Cappellen, 2002; Fraysse et al., 2006; Loucaides et al.,2008). However, in a more recent experimental study (Fraysse et al.,2009) all analyzed phytoliths show the same dissolution rates. Onthe other hand, this is in contrast with was it is observed in naturalsoil systems (Albert et al., 2006). Fraysse et al. (2009) have alsoshown that dissolution rates of phytoliths are an order of magni-tude higher than all typical soil and rock minerals. This mightindicate that the silica originated from the dissolution of phytolithsplays a significant role in the formation of authigenic silica-richminerals, such as the Si-rich clay kaolinite (cf. Lucas et al., 1993).However, more studies are needed to relate dissolution of phyto-liths with the formation of certain minerals in the soil environment.

Chert, flint and other silica artifacts are usually composed ofmicro-quartz with variable amounts of chalcedony and opal

(Luedtke, 1992). Quartz is practically non-soluble below pH 9, butopal and the structurally disordered chalkedony are much moresoluble than quartz (Krauskopf, 1979; Sheppard and Pavlish, 1992;Burroni et al., 2002). Therefore, the silica mineral composition ofartifacts will determine the dissolution rate which also depends onthe amount of water flow through the sediment and generally thehydrological regime (see also discussion on phytoliths above).Nevertheless, silica artifacts have usually impurities that showdifferent solubilities than silica minerals. For example, calcite isa frequent minor constituent of chert artifacts. Therefore, localdissolution of artifacts is expected under different pH regimes (cf.Sheppard and Pavlish, 1992; Burroni et al., 2002). If chert showssings of local chemical alteration a petrographic analysis willindicate the nature of the inclusions and their alteration products.For example, calcite inclusions may have dissolved or transformedto phosphate minerals (see above).

Charcoal until recently was assumed to be stable and not alteredby chemical processes (Nichols et al., 2000). It was just recentlyfound that charcoal can undergo diagenetic changes and this mighthave serious implications for C-14 dating (Cohen-Ofri et al., 2006).Charcoal is a difficult material to characterize because it does nothave a fixed mineralogy and composition. Recently, with the adventof the ABOX charcoal pretreatment method (acid-base-wet oxida-tion) for C-14 dating, it appears that the problem of chemicalalteration of the charcoal is taken under consideration (Bird et al.,2002). However, based on the study of the diagenetic changes offossil charcoal by Cohen-Ofri et al. (2006) but also on publishedC-14 data obtained with the ABOX method (Kuhn et al., 2009) it canbe suggested that the problem of charcoal alteration is not at allsolved with ABOX or probably any kind of known method ofcharcoal pretreatment (see also Rebollo et al., 2008). It should bementioned that in some archaeological sites, like Theopetra cave(Karkanas et al., 1999) and Grottte XVI (Karkanas et al., 2002),where phosphate alteration is prominent, severe oxidation of burntlayers has occurred to the stage that thick hematite-rich layers havebeen secondarily formed. The oxidation of the organic matterproduces reducing conditions which result in an increase in thesolubility of iron. The latter will then re-oxidize and precipitate inthe form of hematite as diffuses away from the organic-rich layerand encounters oxygen (Karkanas et al., 2002). Therefore, it is veryprobable that oxidation of charcoal has happened in these burntlayers. Indeed, there are indirect indications that the most stablephase of charcoal, the graphite phase, can be oxidized and be ‘‘self-humified’’ and not just contaminated by humics from externalsources. Cohen-Ofri et al. (2006) suggested that a basic environ-ment might accelerate the chemical alteration of charcoal but thisremains to be proven.

Microorganisms decompose and oxidize organic matter miner-alizing its compounds. Therefore, preservation of organic matter isrelated to the oxidation state of the soil or sediment. The oxidationstate can be estimated by the iron and manganese minerals (Collinsand Buol, 1970). Defining the iron mineralogy of sediments andsoils is not part of a routine procedure because often the content islower than the sensitivity of the usual methods of mineral identi-fication and iron minerals are not well-crystallized. However,understanding the distribution and preservation of organic matteror probably charcoal in a site requires identification of the ironmineralogy. Mossbauer analysis in combination with magneticstudies can provide information on the iron mineralogy of a site(Longworth et al., 1979).

Finally, the issue of determining if the archaeological materialwas indeed part of the archaeological deposit and has beendestroyed must be addressed. This is hardly considered. Forexample it is important to be able to determine, with high accuracy,areas where bones have been dissolved. In order to answer this

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question, indications for the presence of bones must be found,rather than merely determining the past geochemical conditions. Insome cases, there are some indirect indications. For example ifa layer contains bones and they abruptly disappear along a disso-lution front as determined by mineralogical changes, then it isreasonable to assume that in the direct vicinity of this dissolutionfront bones were present and have been dissolved. However, itcannot be determined if several meters away in the same layer andwith the same mineralogical assemblages that bones were alsopresent. Karkanas et al. (2002) suggested that a certain phosphatemineral, montgomeryite, might be an indicator of the presence ofbones in phosphatized cave sediments. Montgomeryite is a Ca–Al–phosphate mineral. This mineral is reported only in caves witharchaeological remains that have undergone chemical alterationand never in natural cave sediments. Crandallite, which is also a Ca–Al phosphate mineral, is known to be the mineral that is found innatural cave and open-air environments rather than mont-gomeryite. Furthermore, according to the stability diagrams ofKarkanas et al. (2000), it appears that montgomeryite formsthrough a very specific alteration pathway. This might be explainedby a later additional input of phosphorous in the system, not relatedto the initial or continuous input of phosphorous due to the decayof organic materials such as guano. This source could be thephosphate of the dissolving bone. In order to identify if mont-gomeryite is really an indication of the past presence of bones incaves, the spatial distribution of crandallite and montgomeryite inrelation to anthropogenic and non-anthropogenic layers that haveundergone chemical alteration must be determined. Anothersimilar indirect indication is probably leucophosphite and Fe-richtinsleyite for the presence of phosphatized wood ash. Schiegl et al.(1996) pointed out the relation of leucophosphite and altered ash.Leucophosphite is a ferric potassium phosphate mineral and itspresence might have to do with the oxidation of carbon inside theburnt layers. A Fe-rich tinsleyite (the Al end member of the leu-cophosphite group) was also found to be related with altered ash(Karkanas et al., 1999). Taranakite, the other commonly foundpotassium aluminum phosphate mineral in caves, is found in thesurrounding clastic, usually clay-rich layers that normally alsocontain potassium and aluminum but not much carbon.

An indication that might be related to other kinds of environ-ments and not only to caves is the identification of certain chemicalchanges in the sediments or soils that can be explained by the pastpresence of an anthropogenic remain. For example, it is known thatbones act as uranium sink and are preferably enriched in uraniumand rare earth elements from the soil environment (Millard andHedges, 1995; Trueman et al., 2004). Depending on the water flowor the type of chemical alteration it is possible that this enrichmentmight remain in the surrounding soil after the dissolution of boneas insoluble uranium salts (cf. Krestou et al., 2004), thus offering anindirect indication of its past presence. For example, it is nowknown, based on isotopic analyses, when iron minerals are formedby the oxidation of organic matter (Brantley et al., 2001; Wieder-hold et al., 2007). It is thus conceivable that detailed chemical andisotopic analysis of the sediments and soils that have undergonechemical alteration might reveal in future studies which materialsof archaeological interest have been dissolved.

6. Conclusions

Understanding the past geochemical conditions is a prerequisitefor the study of the preservation of archaeological materials.Stability fields of minerals thought to have been original or laterauthigenically formed provide an accurate way for determining thepalaeochemistry of the sediments. However, stability diagramsmust be carefully constructed bearing in mind that each chemical

system defines the relationship of the minerals that are describedonly by this system. In studying the geochemical processes thespatial relationship of the different mineral phases has to bedetermined through petrographic/micromorphological studies andinstrumental mineralogical analysis.

With the exception of bone, there are not many studies on thestability of archaeological materials under different geochemicalenvironments. Nevertheless, the above approach does not provideevidence for the former existence of an archaeological material inthe sediments. It only determines whether the primary anthropo-genic constituents would have been affected by diagenesis irre-spective of whether or not they are or were present in thesediments. Although, it is suggested that there may be some indi-rect mineralogical indications for the former presence of certainarchaeological materials, there are not secure indications for this.Trace element and isotopic analysis of the sediment might bea future solution to this problem.

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