IS THE MACROSCOPIC CLASSIFICATION OF FLINT USEFUL? A PETROARCHAEOLOGICAL ANALYSIS AND CHARACTERIZATION OF FLINT RAW MATERIALS FROM THE IBERIAN NEOLITHIC MINE OF CASA MONTERO

Archaeometry

51

, 2 (2009) 175–196 doi: 10.1111/j.1475-4754.2008.00403.x

*Received 30 November 2007; accepted 24 January 2008© University of Oxford, 2008

Blackwell Publishing LtdOxford, UKARCHArchaeometry0003-813X1475-4754© University of Oxford, 2008XXX

ORIGINAL ARTICLES

Flint raw materials from the Iberian Neolithic mine of Casa MonteroM. A. Bustillo

et al.

*Received 30 November 2007; accepted 24 January 2008

IS THE MACROSCOPIC CLASSIFICATION OF FLINT USEFUL? A PETROARCHAEOLOGICAL ANALYSIS AND

CHARACTERIZATION OF FLINT RAW MATERIALS FROM THE IBERIAN NEOLITHIC MINE OF CASA

MONTERO*

M. A. BUSTILLO,

1

N. CASTAÑEDA,

2

M. CAPOTE,

2

S. CONSUEGRA,

2

C. CRIADO,

2

P. DÍAZ-DEL-RÍO,

2

T. OROZCO,

3

J. L. PÉREZ-JIMÉNEZ

1

and X. TERRADAS

4

1

CSIC–MNCN (Dept. de Geología), C/ José Gutièrrez Abascal 2, 28006 Madrid, Spain

2

CSIC–IH (Dept. de Prehistoria), C/ Albasanz, 26-28, 28037 Madrid, Spain

3

Universitat de València (Dept. de Prehistòria), Avda. Blasco Ibáñez 13, 46010 Valencia, Spain

4

CSIC–IMF (Dept. de Arqueologia i Antropologia), C/ Egipcíaques 15, 08001 Barcelona, Spain

Casa Montero is a mining complex located outside Madrid (Spain), dated from the EarlyNeolithic (

c.

5400–5000 cal

BC

). An area of some 4 ha has been investigated and some 4000shafts recorded, of which 324 have been excavated. The characterization of its raw flintmaterials and the establishment of its diagnostic features are indispensable in thereconstruction of the distribution of the mine’s products beyond the immediate site. Thiswork reports the geological study of the mine’s Miocene flint layers and their petrologicalcharacterization. Archaeological samples from the mine’s shafts were classified accordingto macroscopic features and petrological characteristics.

KEYWORDS

: IBERIA, NEOLITHIC, PETROLOGY, MACROSCOPIC DESCRIPTION, CASA MONTERO, FLINT MINE, SILICEOUS RAW MATERIALS

© University of Oxford, 2008

INTRODUCTION

The Casa Montero site: a Neolithic flint mine

The Neolithic flint mine of Casa Montero was discovered in 2003 while performing theArchaeological Impact Assessment of Madrid’s M-50 highway belt. The site, located on ariver bluff south-east of Madrid, covers an area of at least 4 ha. The first three excavationcampaigns revealed four chronological phases, three prehistoric and one historic: MiddlePleistocene activity, Neolithic exploitation of the flint levels, a Middle Bronze Age settlement(

c

. 1700

bc

), and the exploitation of flint from the 19th century up to the present.The Neolithic flint workings may be the site’s most important feature; certainly they are

among the oldest in Europe and they are the most ancient in Iberia (Consuegra

et al.

2004,2005; Capote

et al.

2006; Díaz-del-Río

et al.

2006). These workings contain over 4000documented vertical shafts up to 9 m deep and with a mean width of 1 m (Fig. 1). A smallpercentage of these shafts contain chronologically diagnostic items, mainly impressed potteryand bone rings that suggest an Early Neolithic date (

c.

5400–5000 cal

bc

). This has beenconfirmed by two radiocarbon datings (Table 1). The mine seems to have developed as the

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result of reiterative, short-term, seasonal mining expeditions. The shafts rarely cut into anyprevious extraction pits, suggesting that this mining activity was probably only pursued over afew centuries.

The stone material recovered from inside the shafts represents the waste produced during theextraction and exploitation of the flint layers via striking and knapping. Both these processesleft behind abundant quantities of stone remains; only a small fraction of the mined material wasused, the rest was abandoned. Indeed, only a small percentage of the raw materials were trans-ported away from the mine (which are obviously not represented in the archaeological remains).

Figure 1 A plan of the shafts and location of Casa Montero flint mine.

Table 1 Datings for the Casa Montero mine shafts

Laboratory reference

Dating (bp)

Cal 2 sig. Material Origin Method

Beta 206512 6410 ± 40 5480–5320 bc Charcoal: Quercus ilex/Q. coccifera

Shafts: UE 2384 Stratum UE 2382

AMS

Beta 206513 6270 ± 40 5330–5070 bc Charcoal: Quercus ilex/Q. coccifera

Shafts: UE 2701 Stratum UE 2229

AMS

Flint raw materials from the Iberian Neolithic mine of Casa Montero

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Casa Montero flint has no evident aesthetic qualities. Its mineralogical composition andknapping qualities vary horizontally and vertically throughout the different types of flint layersand nodules at the site.

Aims of the present work

The Casa Montero site provides an excellent opportunity for studying in context the Neolithicprocurement of raw materials. The final aims of this study are to determine the reasons for themine’s exploitation, to reconstruct the means by which its contents were transformed and toestablish the distribution of its products. This information could help provide us with a pictureof potential circulation of mineral resources during the Neolithic of the Iberian Peninsula.However, these aims require that the geological features of the site be known (Bustillo andPérez-Jiménez 2005) and that the flint rocks cropped in prehistoric times be characterized.Clearly, this requires a series of discriminating criteria to be developed that allow the differenttypes to be identified in different Neolithic contexts—the aim of the present work. The flintraw materials at the Casa Montero site were therefore macroscopically characterized anddistinguished by examining the extraction remains. This allowed the different rocks to beclassified into types. The information was then used in the petrological characterization ofdifferent flint samples. Finally, the types of raw materials and their diagnostic attributes wereredefined on the basis of the comparison between the petrological and macroscopic data.

The study of the mine’s geological context (Fig. 2) allowed the characterization of theoutcropping flint layers, and of their host rocks. This permitted the archaeological materialcollected to be correctly assigned to the different types.

Nowadays, research into the geographical distribution of the mine’s products is limited bytwo factors: first, the short time span of the prehistoric exploitation suggested by radiocarbondates, the fact that shafts do not cut into one another and the homogeneity of artefacts andtechnological traits; and, second, the lack of known contemporary occupations in the vicinityof the flint mine. This study provides the basis for future research aimed at establishing towhat point the mining strategy was determined by the differential exploitation of the flint layers,and the possible distribution of its flint products.

GEOLOGY AND PETROLOGY OF THE FLINT LEVELS IN THE CASA MONTERO AREA

Geological background

The Madrid Basin is known because of the abundance of flint sources of different properties.Although much of this flint is known to have been cropped during historical times, the specificityof the Casa Montero prehistoric mine rests in its location in an area of less than 5 km

2

, whereflint levels with remarkably specific properties outcrop. As a result of the limited extension ofthe outcrop, the presence of this flint in secondary alluvial deposits is scarce and, whenpresent, would not preserve its original knapping qualities.

The Casa Montero flint mine is home to several flint levels of the ‘green clays unit’ (Brell

et al.

1985). The latter forms the base of the Intermediate Unit of the Miocene (dated as Aragonian)in the Madrid Basin (Junco and Calvo 1983), and it also contains micaceous sand intercalationsand carbonates. In general, the clays are composed of illite (dioctahedral and trioctahedral)and trioctahedral smectites, with small amounts of dioctahedral smectites, kaolinite andsiliciclastic grains. The carbonates are in the form of white dolomitic marls and dolostones.

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The sedimentary rocks in the area of the archaeological site have been studied in somedetail (Bustillo and Pérez-Jiménez 2005). The sedimentary context in which these weredeposited was a shallow lacustrine system with precipitation or the neoformation of smectites.Shallowing-upward sequences finish with palustrine limestones that later became dolomitized(Pérez-Jiménez

et al.

2005). In some levels, the conditions were more evaporative (shown bythe presence of lenticular crystals of gypsum lenses) or more oxidizing (brown clays).Sometimes external inputs of detrital material was brought in by small streams. Subaerialexposure is made evident in different levels by the presence of pseudomicrokarst, bioturbationstructures or the formation of clay aggregates typical of vertisols.

Nomenclature and general concepts regarding the silica rocks and minerals

In the literature, the nomenclature of silica rocks is little standardized; names depend on thecountries in question and on whether they are used by geologists or archaeologists. Anotherproblem is that the term ‘opal’ is used in the literature to represent both a mineral and a rock.This paper uses the terms

flint

or

silica rocks

as general terms,

chert

is used to represent a

Figure 2 (a) The geological location of the Casa Montero archaeological site. (b) Logs of the studied sections, with descriptions of the lithologies and the main structures present.


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silica rock made mainly of quartz,

opal

to represent a silica rock made mainly of a number ofopaline minerals (opal-A, opal-CT etc.), and

opaline chert

to denote a silica rock in whichquartz and certain opaline minerals are present together.

Chert is mainly made of quartz, but it can show many textures under polarizing light (i.e.,in optical microscopy preparations). Common quartz can have different crystal sizes and canform crypto-, micro-, meso- or macrocrystalline mosaics. It is frequently associated with chalced-ony, a fibrous variety of quartz of different varieties: chalcedonite (length-fast chalcedony, inwhich the elongation of the fibres is perpendicular to the crystallographic

c

-axis), quartzine(length-slow chalcedony, in which the elongation is parallel), lutecite (another type of length-slow chalcedony, in which the fibre axis is inclined by approximately 30

°

), and helicoidalchalcedonite or zebraic chalcedony (which shows systematic helical twisting of the fibre axesaround the crystallographic

c

-axis). The varieties of chalcedony allow the definition of theformation environment as acid or non-sulphate (length-fast), or basic or sulphate/magnesium-richenvironments (length-slow) (see the review in Hesse 1990). Unfortunately, there are exceptionsto these rules and the strict application of these criteria can lead to errors of interpretation.

Moganite is a novel, monoclinic silica polymorph with a crystalline form structurallysimilar to that of quartz (Miehe and Graetsch 1992). The identification of moganite in thepresence of quartz is difficult. It can be detected, however, by detailed X-ray diffraction(XRD) experiments with Rietveld refinements, and other techniques such as Raman and NMRanalysis. The microscopic characteristics of moganite have not yet been studied in depth, butit is generally described as having uniformly microcrystalline mosaics and a flaky or fibrous(length-slow) texture. Its length-slow nature cannot be used to distinguish it from quartz(Pretola 2001), since quartzine is also length-slow. This mineral is found mixed with quartz inmany cherts, preferentially in those that developed in evaporitic environments (Heaney 1995).

Opaline rocks contain opaline minerals with different degrees of crystallinity, crystalstructure and proportions of water. Jones and Segnit (1971) classified opal minerals into threegroups according to their XRD patterns. Opals that produce an XRD with four moderatelybroad peaks that coincide closely with the position of the four most intense peaks of

α

-cristo-balite, plus minor evidence of

α

-tridymite, were termed opal-C; opals with patterns that show signsof both

α

-cristobalite and

α

-tridymite were designated opal-CT; while opals that yielded anXRD pattern that resembled that of amorphous silica were categorized as opal-A.

Opal-CT is the most common phase of all types of sedimentary, volcanic and hydrothermalrocks. Opal-CT can be considered a complex phase that contains stacked sequences of

α

-cristobalite/

α

-tridymite and a non-crystalline silica phase. The structure of this opal is ambiguousbecause of its variable water content, its small crystalline size, the differing amounts of cristo-balite/tridymite present and the stacking disorder within the silica framework (Guthrie

et al.

1995). Using transmitted light, the colour of opal ranges from light tan to various shades ofbrown, and presents high relief. Under polarizing light, all the opals are generally isotropic;the scanning electron microscope (SEM) shows them to have numerous microspheres(diameter < 600 nm). These microspheres are different to the lephispheres, which are bigger(diameter > 3

μ

m) and formed by platelets of opal-CT.

Petrology of the silica levels and their host rocks

The silica rocks of the Casa Montero mine form nodules and lenses arranged in discontinuousbeds (Fig. 3) and show great lateral variations in their arrangement (beds change to nodules),colour (brown, grey, beige and black), density and shape (nodular lobulate, the existence of

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rinds etc.). This variability is in part a direct consequence of the variability of the host rocks.Four episodes of silicification have been differentiated (Fig. 2). The first (episode I) is associ-ated with dolostones, while the other three are associated with clays.

Episode I is made up of two nodular beds of chert associated with hard dolostones withinbrown clays. The chert is grey, nodular and lobulated. The mineralogy is mainly quartz, with-out opaline minerals, and occasionally with minor amounts of moganite. Under the lightmicroscope, the chert is composed of mosaic quartz (of crypto- to macrocrystalline size),length-fast chalcedony (calcedonite), length-slow chalcedony (quartzine) and zebraic chalcedony.The texture of the chert is similar to that of the dolostones, and reflects intraclasts, bioturbation,desiccation cracks, gypsum lenticular crystal pseudomorphs and shell fragments (gastropods,ostracods, characea etc.).

Episodes II, III and IV comprise opals and opaline cherts (Bustillo and Pérez-Jiménez 2005)in argillaceous levels. These levels are mainly composed of magnesian smectites (trioctahedral).A very significant feature of the clays at the top of the studied column is the presence ofovoid pedogenic clay aggregates. These are common in vertisols, as described by Fitzpatrick(1983) and Rust and Nanson (1989), among others; after their formation they are transportedand incorporated into other clay sediments. The mineralogy of the opals is opal-CT withvariable amounts of magnesian smectites; the mineralogy of the opaline cherts is mainlyopal-CT and quartz in different quantities, with traces of Mg-smectites. The variability in theratio opal-CT/quartz of opaline cherts is a result of different degrees of ageing (Fig. 4). Ageingis a recrystallization process consisting of the dissolution of the opal and the rapid precipitationof quartz; this frequently begins in the inner part of the nodules and beds. When ageing iscomplete (i.e., all the opal has been recrystallized), the silica rock is formed mainly by agroundmass of micro-cryptocrystalline quartz.

The textures of the opals and opaline cherts resemble those of clays (Mg-smectites). Thelaminar structure of trioctahedral Mg-smectites can be observed in the opals, generatingstriated birefringence. Ovoid clay aggregates are also perceptible in the opals. Many cherts

Figure 3 An outcrop of silica episodes III and IV, crossed by an extraction shaft.


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and opaline cherts also show rounded shapes. This pattern may be a consequence of thesilicification of ovoid clay aggregates and of the subdivision into ovoids of the initially massiveopal during ageing.

Bioturbation structures, desiccation cracks and pores cemented by different types ofchalcedony (calcedonite, quartzine and lutecite) are common in the silica rocks of CasaMontero. It is important to note the presence of quartzine and lutecite (length-slow chalcedonies).These textures can be indicative of Mg-rich pore fluids (Folk and Pittman 1971); this is notsurprising, since these silica rocks developed over dolomites and magnesian clays.

Silicification has affected the carbonates (Fig. 2, episode I) and clays (Fig. 2, episodes II–IV) and reproduced the textural and structural characteristics of the host rocks, indicating themto be the result of a replacement process that took place under the level of the water table(Thiry 1997). Later transformations in a vadose environment (above the water table) explainthe generalized process of ageing seen, with transformation from opal to quartz.

METHODS, TECHNIQUES AND MATERIALS STUDIED

Archaeological materials studied and de visu classification

In a mining setting, in which large amounts of lithic remains are generated (approximately 60 Tnrecovered from 324 excavated shafts from Casa Montero), a petrological examination of all

Figure 4 (a) A silica nodule, showing the interior and exterior parts. (b) XRD patterns showing the internal quartz/opal-CT ratio to be higher. This is a consequence of opal to quartz ageing, which is more pronounced inside. The outer part still has relics of Mg-smectite (Sm), the main component of the host rock.

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those items collected is impossible. Such testing must therefore be selective, with the resultsof the mineralogical and petrological analyses linked to previously established types ofrock. Refitting analyses also allows the macroscopic variability of the raw flint material to beexamined.

A macroscopic description of the archaeological samples has two main advantages: itallows the fairly rapid analysis of large amounts of archaeological material and it is non-destructive. It is therefore ideal for characterizing the large quantities of lithic material foundin mining shafts. Macroscopic descriptions are somewhat subjective (Demars 1982; Terradas

et al.

1991), since they rely on qualitative criteria that, in turn, depend upon the person per-forming the study. However, this subjective component can be minimized by comparing theresults with those of a petrological study. The latter can qualify, correct and explain the priormacroscopic classifications made. Such a procedure was followed in the present work. Macro-scopic descriptions are commonly the first step in more-or-less intuitive

de visu

classifications(Barceló 1996), and allow the sampling method for later petrological analysis to be designed—theanalysis that eventually allows the restructuring and redefinition of the initial macroscopicclassification.

The criteria used in the macroscopic description of the different silica rocks from the CasaMontero mine were: type of material, colour, translucency, types of impurity, lustre, grain,cortex and knapping quality (Luedtke 1992). Using these criteria, seven types of siliceous rawmaterials were identified (see Table 2). Forty-three archaeological samples were randomlyselected for petrological analysis from the 5043 flint items collected during the first field sea-son. These archaeological samples were assigned to the seven types of silica material. Themore common types were therefore represented by more samples.

Refitting analysis allows the reconstruction of the reduction strategies followed. In thepresent work, refitting showed that some types of material always appeared mixed in the samenodule, whereas other types never appeared together.

Petrological methods and techniques

Samples from the beds of the outcrop and of the 43 archaeological samples were studied usingdifferent petrological and mineralogical methods. XRD analysis was performed using aPhilips X-ray diffraction system, operating at 40 kV and 30 mA with monochromatic Cu K

α

radiation. All samples were powdered and mounted in a conventional aluminium holder. Asemi-quantitative analysis of the mineral composition of the samples was thus obtained. Theinside, opaline exterior and patinas of the archaeological samples were analysed.

XRD diagrams were also used to calculate the index of crystallinity of the quartz present(Murata and Norman 1976). This index is based on the degree of resolution of the 68

°

2

θ

zone(Fig. 5). To obtain the correction factor

F

required in order to express the index of crystallinityon a standard scale of < 1 to 10, a euhedral quartz crystal was analysed and a value of 6.3obtained. The ratio 6.3/10—or 1.58—was then used as the correction factor applied to the data.

Thin sections of the samples were studied by optical microscopy (OM) under plane andcross-polarized light to observe the mineralogical composition and define the petrologicalgroups shown in Table 3 and Figure 6. A gypsum plate was used to obtain information regardingthe elongation of the chalcedony observed (positive or negative).

Scanning electron microscopy (SEM) was performed with a FEI QUANTA 200 system,working at 30 kV and a employing a working distance of 10 mm. The microscope was operatedin high- and low-vacuum modes, using secondary and backscattered detectors (SSD and BSE,

Flint raw

materials from

the Iberian Neolithic m

ine of Casa M

ontero

183

© U

niversity of Oxford, 2008,

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Table 2

Types of raw materials and their main features as established by macroscopic observation of the archaeological lithic record of Casa Montero

Type Raw material Colour Translucency Grain size Lustre Veins Other observations %*

1 Chert Brown Translucent Dense Matt Spots, geodes Variable quantity of opal in the outer part 70.312 Chert Grey Opaque Dense to fine Matt Several impurities A little opal in the outer part 14.543 Opaline Chert White/beige Opaque Fine to medium Matt Altered 1.474 Opaline Chert Several Opaque Fine Intense waxy 0.875 Opaline Chert Several Opaque Dense Light waxy 3.896 Opaline Chert Dark grey Opaque Dense Light waxy Many impurities 0.157 Opaline Chert Beige/ light grey Opaque Dense Light waxy Ferruginous impurities Pleistocene material 0.32

* The relative frequency in a population of 5043 objects collected during the first field season.

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respectively) in both. The microscope used incorporated an energy-dispersive X-ray spectroscopy(EDS)-type Analytical-INCA (Oxford Instruments) device for semi-quantitative compositionalanalysis. Analytical determinations were standardized to an area of 125 μm × 125 μm.

Several archaeological samples were fractured and observed in high-vacuum mode (thesamples having previously been covered with gold); this provides the most detailed view. Inaddition, non-destructive observations in low-vacuum mode were made to determine thecharacteristics of the samples’ unaltered insides, patinas and opaline exteriors. The featuresobserved in both modes were compared to determine whether low-vacuum observations wereof any use in characterizing the archaeological samples and patinas (Figs 7 and 8).

The EDS analysis was focused on Mg. This element was thought to be of great importancefor characterizing the supply sources at Casa Montero, since the silicified rocks had smallamounts of Mg and can also contain Mg-clays. Metal-coated (high-vacuum) and non-metal-coated (low-vacuum) samples were analysed by EDS, and the results compared.

RESULTS: CHARACTERISTICS OF THE ARCHAEOLOGICAL SAMPLES

Mineralogical composition and crystallinity index of quartz

Table 3 shows the XRD results. Type 1 artefacts were made of quartz (> 95%), traces ofphyllosilicates (illites and smectites), and occasional gypsum (Table 3). The only exceptionwas sample CM-4, which was composed of 15% opal-CT. Curiously, this was the only Type 1sample with a white patina. The XRD study of the opaline exteriors of samples CM-8 andCM-17 showed that their opal content was also of the CT type, accompanied by smectitesand relic quartz (Table 3). The amount of opal in the exterior part might be underestimated,since it is difficult not to mix the central and outside parts during powdering if they are thin.

The Type 2 samples were similar to those of Type 1; that is, they were formed largely ofquartz, sometimes with opaline exteriors (although these were very thin). The Type 3 samples,defined de visu as opals, corresponded to very altered rocks (as shown by their cross-section)with an unaltered nucleus of the same shape as the outside (Fig. 8 (a)). XRD analysis mainlydetected quartz plus a maximum opal-CT content of 15%, contradicting that suggested by de visu

Figure 5 X-ray diffraction patterns of cherts with different quartz crystallinity indices (I.C.): (a) CM-29, I.C. = 3.74; (b) CM-23, I.C. < 1.

Flint raw materials from the Iberian Neolithic mine of Casa Montero 185

© University of Oxford, 2008, Archaeometry 51, 2 (2009) 175–196

Table 3 Mineralogical composition and index of crystallinity (I.C.) as determined by X-ray diffraction analyses, andcorrelation between macroscopic types (Types 1–7) and petrological groups (Groups A, B, C or D) of archaeological samples

Sample Mineralogical composition I.C. Group defined under optical microscopy

Type 1CM-1 BCM-2 A with opaline exteriorCM-3 A with opaline exteriorCM-4 pat. Q = 80, Op = 20, Ph = 5 2.2 A + CCM-5 B with opaline exteriorCM-6 Nd, with opaline exteriorCM-7 Q = 95, Ph = In, Fd = In < 1 ACM-8 inside Q = 95, Op = 5, Ph = In 1.70 C with opaline exterior (D)CM-8 exterior Q = 5, Op = 90, Ph = 5CM-9 A + B with opaline exteriorCM-10 B with opaline exteriorCM-11 Q = 100, Ph = In < 1 B with relics of opaline exteriorCM-12 Q = 100, Ph = In 2.45 ACM-13 A + BCM-14 B with opaline exteriorCM-15 Q = 95, Ph = In, Fd = In 1.30 A + B with opaline exteriorCM-16 Q = 95, Ph = In, Fd = In < 1 A with opaline exteriorCM-17 inside Q = 95, Ph = In, Gy = In 2.72 B with opaline exteriorCM-17 exterior Q = 90, Op = 10, Ph = In < 1CM-18 A with opaline exteriorCM-19 A with opaline exteriorCM-20 BCM-26 A + BCM-28 A with opaline exteriorCM-29 Q = 100, Ph = In 3.74 B

Type 2CM-21 Q = 95, Ph = In, Fd = In < 1 ACM-22 Q = 90, Ph = In, Gy = In, Fd = In < 1 BCM-23 Q = 100, Fd = In < 1 A + B plus impuritiesCM-24 B with opaline exteriorCM-25 Q = 100, Fd = In 2.81 A + B plus impuritiesCM-27 A + BCM-30 A + B plus impurities with opaline exterior

Type 3CM-31 pat. Q = 95, Ph = In , Fd = In < 1 Nd CM-32 pat. Q = 100, Ph = In < 1 C CM-33 pat. Q = 95, Op = 5 < 1 C CM-34 Q = 80, Op = 20 < 1 C

Type 4CM-35 pat. Q = 20, Op = 80 C with opaline exterior (D)CM-36 Q = 65, Op = 35 < 1 C with Nd exterior CM-37 C with Nd exterior

Type 5CM-38 Q = 90, Op = 10 3.47 C + Nd exterior CM-39 pat. Q = 85, Op = 15, Ph = In < 1 CCM-40 Q = 85, Op = 15, Ph = In < 1 C with opaline exterior (D)CM-41 pat. C

Type 6CM-42 Q = 90, Op = 5, Ph = In, Fd = In < 1 C

Type 7CM-43 Q = 90, Ph = In, Fd = In, Do = In 2.2 C

Q, quartz; Op, opal-CT; Ph, phyllosilicates; Gy, gypsum; Do, dolomite; Fd, feldspars; In, traces; Nd, does not correspond to any

of the defined groups; pat., with patination.

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Figure 6 Thin-section photomicrographs of different types of cherts and opals found in the archaeological samples. (a) Group A chert. The groundmass is homogeneous microcrystalline quartz, showing gridwork extinction. Fibrous crystals of quartzine cement cross the image. The image was digitally modified to enhance the NE–SW and NW–SE grid pattern, originally marked by blue and red colours. Sample CM-21; cross-polarized light (CPL) and gypsum plate. (b) Group B chert. The groundmass shows a granular texture. Sample CM-22; plane-polarized light (PPL). (c) Group C opal. The groundmass is composed of opaline ovoids (globular opal, Op). An old pore cemented with an opaline rim (R) aged to quartz, and later with helicoidal chalcedony (Ch). Sample CM-36; PPL. (d) The same view as in (c): isotropism of opal generates the black colour. (e) The ageing of Group C globular opal produces microquartz granules. Isotropic opal (Op) remains between granules. Sample CM-37; CPL. (f) An undefined chert type with a mosaic quartz groundmass (Qtz), an opaline border (Op) and cements of macrocrystalline quartz mosaic (M), fibrous crystals of quartzine (Qn) and chalcedonite (Ch). Sample CM-6; CPL.



Figure 7 SEM views of the different petrological groups defined by the archaeological samples. (a) Group A chert. A fresh fracture surface under high-vacuum conditions and at high magnification. This group shows a flaky microstructure with a groundmass of microspheres. Sample CM-4. (b) Group A chert. Sample surface under high-vacuum conditions and at high magnification. The groundmass pattern of microspheres is still evident despite the smooth surface. Sample CM-7. (c) Group B chert. A sample surface under low-vacuum conditions, showing the granular general pattern that characterizes this group. Sample CM-1. (d) Group C opaline chert. A sample surface under low-vacuum conditions, clearly showing the typical opaline globular pattern. The image includes an EDS analysis showing small amounts of Mg. Sample CM-34. (e, f) The surface of a Group C with opaline border observed under (e) low- and (f) high-vacuum conditions. The edges of the globules are sharper in the former conditions. Sample CM-14.



Figure 8 Petrological features of the patinas seen in the archaeological samples. (a) A section of a hand specimen of a highly altered sample with a darker non-altered core. Sample CM-33. (b) A thin-section of an altered zone with widespread opaque impurities. Sample CM-31; PPL. (c–f) SEM views of different features found in patinas and non-altered zones: (c) non-altered zone and patina under high-vacuum conditions and after fresh fracture—the patina is very porous; (d) a close-up view of the patina in the previous image, showing the microspheres and a high degree of porosity (sample CM-41); (e) the surface of the sample (top) and fresh fracture (bottom) of a very altered Group C sample under low-vacuum conditions—the globular pattern is clearly visible on the surface of the patina; (f) a close-up view of the surface of the sample in the previous image—dissolution has marked out the globular pattern of the sample, since this process preferentially affects the contacts between globules or between globules and cements (sample CM-33).



analysis, which suggested these samples were of opal. The Type 4 samples contained muchmore opal-CT, which was detected in both XRD and thin-section analysis. In Types 5 and 7,quartz was again dominant; the proportion of opal-CT was small since it recrystallized toquartz during ageing.

The index of crystallinity of all the archaeological samples was generally < 1 (Table 3 andFig. 5 (b))—a very low figure according to the scale of Murata and Norman (1976). In thosesamples in which quartz was in a proportion of < 50%, the index could not be calculated.Differences were seen in the crystallinity indices of Types 1, 2 and 3, the first accumulatingthe greatest number of samples with an index of over 1 (Table 3 and Fig. 5 (a)). Generally, thelarger the size of the crystals (in small cementations), the greater was the index of crystallinity.Comparisons were difficult for the remaining types, since the quantity of data available wassmall and the values recorded very discrepant.

Petrological groups under optical microscopy

A number of petrological groups were defined, taking into account the mineralogical, texturaland structural characteristics of the samples. The XRD and thin-section analyses recordedlarge differences in the opal contents, with XRD generally returning smaller values. Thisoccurred because: (1) in thin-section analysis, the quartz areas due to ageing were cryptocrystallineand still conserved some of their colour-optical properties, their relief and the isotropismtypical of opal—and were thus recorded as such; and (2) in XRD analysis, the opal canenclose a proportion of amorphous silica that is difficult to evaluate.

The types defined by macroscopic analysis were redefined as the petrological groups below(see Table 3).

Group A: homogeneous micro-cryptocrystalline chert The groundmass is formed by a mosaicof crystals of micro-cryptocrystalline quartz. These samples are usually colourless in transmittedlight. With crossed nicols, very small crystals (< 25 μm) can be seen. Even the largest crystalslack well-defined boundaries; sometimes their shape tends to be elongated as though they werelaminas or fibres.

Gridwork extinction is usually seen with crossed nicols—a phenomenon even more evidentwhen the gypsum plate is inserted. This is due to the difference in the elongation of theperpendicular microcrystals (Fig. 6 (a)). A compact set of small chalcedonies generally < 25 μmcaused the gridwork extinction; this was less apparent when these were larger.

Small areas of striated birefringence are sometimes seen with crossed nicols, showing thepresence of silicified clays.

The general fabric is micro-cryptocrystalline. However, some areas (no more than 30% of thetotal) are speckled with pores cemented by quartz, silicified bioturbations and lenticular shapesreminiscent of gypsum pseudomorphs. In these areas the crystal mosaics are larger, and fibroustextures with negative (calcedonite) and positive (quartzine and lutecite) elongation can be seen.

As well as having an opaline exterior, this chert can show brown opal relicts either disseminatedor located in certain bioturbations. In transmitted light these do not generally show the highrelief corresponding to opal, due to their almost complete transformation into quartz.

Group B: granular chert With crossed nicols, spherical or ovoid components with diametersof 60–600 μm (termed granules) were seen (Fig. 6 (b)). The interior texture of the granules iscommonly represented by mosaics of micro-cryptocrystalline quartz (from crystals that are



practically indistinguishable up to 25 μm), although sometimes these can show quartz crystalsof larger size. The extinction of the granules as a whole is variable. Sometimes the extinctionmodel shows two preferred directions in a gridwork extinction pattern. The granules can bein contact, slightly separated or dispersed in a groundmass, which itself shows different structuresand textures. The crystals making up the groundmass were generally mosaic quartz, usually< 40 μm. The groundmass pores were cemented by calcedonite, quartzine and lutecite, andmosaic quartz of larger size. The pores are largely the product of bioturbation and desiccation,and therefore show very different morphologies.

Both Groups A and B can have an opaline exterior. Some samples are ‘mixed’; that is, partsof them are micro-cryptocrystalline in nature (Group A) and parts are granular (Group B). Insome cases, the large amount of cement hinders classification (Fig. 6 (f )).

Group C: opaline cherts These are formed from mixtures of opal-CT and quartz in veryvariable proportions. The fabric of the opal can be homogeneous, although it normally showsa globular structure (Figs 6 (c) and 6 (d)). Thin-section observations revealed the presence ofsignificant quantities of opal ghosts, although XRD analysis indicated only small amounts tobe present. The quartz formed by ageing conserves the optical properties of the opal, as highrelief, a brown colour and a globular structure. Only small amounts of quartz were formeddirectly by cementation of the pores (Figs 6 (c) and 6 (d)). The difference between Group Cand Group A or B samples with opaline exteriors is that the opal in Group C is homogeneouslydistributed throughout the sample—not in the outer area alone.

Group D: argillaceous opal This type of opal is somewhat atypical, since it shows almost norelief; it also accumulates many opaque impurities. It is homogeneous (there are no globules),and with crossed nicols it shows a certain amount of striated birefringence corresponding to thepresence of silicified clays. Type D opal was only seen in sample CM-31, a completely patinatedartefact, and in part of samples CM-8 and CM-35, in which it appears to form an opaline exterior.

Characterization of the petrological groups under SEM with EDS

Group A: homogeneous micro-cryptocrystalline chert This group, essentially represented bythe micro-cryptocrystalline quartz samples, is difficult to study by SEM due to its homogeneouspetrological characteristics; large magnifications—around 4000×—are needed. The resultsacquired under high-vacuum conditions and by examining a fresh fracture reveal a flakystructure and a network of microspheres of < 0.5 μm diameter (Fig. 7 (a)). These features,however, were not clearly visible under low-vacuum conditions at the same magnification. Thechemical composition revealed by EDS included no Mg, regardless of whether or not thesamples had been metal-coated.

The surface of the archaeological samples also showed a flaky structure and network ofmicrospheres under high-vacuum conditions. However, both features were less marked than inthe fresh fracture, because of the smoothness of the surface (Fig. 7 (b)). Under low-vacuumconditions, the microspheres became less clear and the flaky structure invisible. Analysesperformed with and without metal-coating returned evidence of Mg (Table 4).

Group B: granular chert Petrographically, the recognition of Group B chert is based on itsgranular pattern; high magnification is not necessary for this to be apparent. Low-vacuumobservations at low magnification allowed the granular structure to be observed (Fig. 7 (c)).



High-vacuum conditions did not improve the results; indeed, the granular structure was often moredifficult to see due to small surface details becoming more apparent. EDS under low-vacuumconditions and without metal-coating of the sample revealed the presence of Mg (Table 4).

When the samples showed an opaline exterior, the globular structure was better seen inbackscattering mode; increasing the vacuum did not improve the results. Magnesium wasdetected in all EDS analyses of the opaline exterior, while only half of these revealed itspresence in the interior (Table 4).

Group C: opaline cherts Group C was analysed by studying samples that showed a largewhite patina or an opaline exterior with characteristics of Group D. Observations under low-vacuum conditions and with backscattered electrons clearly showed the globular structure ofthe opal (Fig. 7 (d)), even though the samples had to be viewed through their patinas.

However, the opaline exterior of Group C hinders visibility. The globular structure was onlyvisible in certain areas; no good images were obtained.

Group D: argillaceous opal Argillaceous opal is difficult to define using SEM, since itshows no clear or typical characteristics.

White patinas

The characteristics of the patina observed by the naked eye (Fig. 8 (a)) and under opticalmicroscopy (Fig. 8 (b)) were studied using SEM. Generally, the best results were achievedunder low-vacuum conditions and with backscattered electrons, because the shape of thecrystals was smoothed out while other structures and features became clearer. The patinasappeared as rinds covering the unaltered parts of the samples, and were either rough or smooth

Table 4 The number of EDS analyses returning significant amounts of Mg in archaeological samples

Sample

Presence of Mg (dispersive energy)

Petrological group

Low vacuum High vacuum

Number of analyses

With Mg Number of analyses

With Mg

CM-1 (sample surface) 2 1 1 1 B

CM-4 (patina) 2 2 2 1 –CM-4 (interior) 2 0 2 0 A + C

CM-7 (sample surface) 1 1 1 1 A

CM-14 (opaline exterior) 2 2 CCM-14 (interior) 2 1 B

CM-33 (patina) 4 4 2 2 –CM-33 (interior) 1 0 C

CM-34 (opaline exterior) 4 4 C

CM-41 (patina) 4 2* 4 4 –CM-41 (interior) 2 0 CTotal number of analyses 23 17 15 9



and showed darker, more porous areas alternating with lighter, more compact zones. The mostporous parts showed a flaky structure and masses of microspheres (< 500 nm in diameter) thatare general features of the unaltered parts of the archaeological samples. The globularstructure of Group C samples was also visible in the patinas—in fact, even more clearly thanin the unaltered parts, since the contact zones between the globules, or between the globulesand the cements, was accentuated there due to dissolution (Figs 8 (c) and 8 (d)).

To compare the patina and unaltered areas at greater magnification, the archaeologicalsamples were fractured, coated in gold and observed under high-vacuum conditions. Thepatina was the most porous, appearing dissolved and corroded with respect to the unalteredarea (Fig. 8 (e)). It sometimes showed a flaky structure, and it was sometimes accompanied bymicrospheres of 100–200 nm in diameter (Fig. 8 (f )).

The EDS analyses performed under low-vacuum conditions showed the chemical composi-tion of the patina to be variable. The smoothest and most compact areas were made of Sialone, while the porous areas always contained Mg, and on occasion Ca, Al and Fe. Underhigh-vacuum conditions, comparisons of the unaltered area with the patina showed the formerto be composed of Si alone, while the patina also contained Mg.

DISCUSSION

SEM conditions

The SEM–EDS study of the artefact surfaces was performed under both high- and low-vacuum conditions. While the former conditions usually provided the best results for silicarocks, this was not always the case for the archaeological samples. For example, low-vacuumconditions with backscattering of electrons provided better results (i.e., more distinguishingcharacteristics were seen) when examining the patinas. In general, if the characteristics to bestudied can be seen without high magnification, low-vacuum conditions appear to be the mostappropriate, since the small details tend to become lost to the benefit of larger-scale features.In addition, low-vacuum conditions are appropriate for examining archaeological samplessince no metal-coating is required—they are therefore less destructive. Further, minorityelements are better detected, since their signals are not diminished by gold-coating.

Sources of the archaeological samples

The most outstanding outcome of the comparison between the artefacts and the silica rocks samplesfrom the different flint layers of the mine was that the majority of the former were mainlycomposed of quartz. In contrast, the flint layers of the outcrops, with the exception of episodeI, were composed of opal and quartz. Therefore, it would appear that prehistoric populations selectedthe parts of the layers that had recrystallized to quartz (aged). This is confirmed by the refittingresults (Castañeda et al. in press), which show that quartz was preferentially worked compared toopaline chert. The textures and structures found in the archaeological samples indicate that theartefacts were cut from the rocks belonging to episodes II, III and IV, and especially II and IV.

Typical differential features of the Casa Montero raw materials

Shackley (1998) defined five steps for determining probable sources of flint: (1) define theproblem and problem area, (2) know the geological region, (3) identify the regional chert



sources, (4) determine which techniques or procedures are valid for their characterization and(5) match artefacts to sources. In Casa Montero, these steps can be easily performed.

Several common distinguishing characteristics were recorded for the raw materials. Although each ofthe following might be seen in rocks of different areas, their combinations provide a diagnostic feature:(1) The granular pattern of the cherts (under OM).(2) The gridwork extinction of the micro-cryptocrystalline cherts (under OM).(3) The mixtures of opal and quartz caused by ageing (under OM and XRD).(4) Length-slow chalcedony in cements of opals or cherts (under OM).(5) The opaline exterior (under OM).(6) Low or very low crystallinity indices (XRD).(7) A granular pattern (under SEM) and the presence of Mg (EDS). The determination of Mgcan also be undertaken by non-destructive techniques; for example, mass spectrometry withlaser sampling (Moroni and Petrelli 2005).

These characteristics are the result of the fact that, geologically, the type of chert and opallayers that crop out at Casa Montero are not very common in the Madrid Basin, where opal ismore commonly found with sepiolite and is usually little aged.

Correspondence between macroscopic types and petrological groups of flint: is macroscopic classification useful?

The types established macroscopically can be explained by the results of the XRD andthin-section analyses.

Type 1 flints have a > 95% quartz composition (not taking into account the patina or the opalineexterior) and an index of crystallinity generally greater than 1. Type 2 flints have a less developedopaline exterior and generally show an index of crystallinity of less than 1. Type 3 flints areall composed of intensely patinated opaline cherts, favouring their confusion with opal in thede visu classification. Type 4 flints include significant amounts of opal-CT, which correspondswith the poor knapping quality noted in macroscopic assessment. It may also be responsiblefor the intense waxy lustre that was noted. Types 5, 6 and 7 flints are all composed of quartz thatshows optical properties typical of opal. This corresponds with the good knapping quality observedin the macroscopic assessment, and with the waxy lustre not seen in other types of opal (Table 2).

The textural and structural observations made also have a certain correspondence with theinitial macroscopic classification (Table 2). The samples of Types 1 and 2 correspond topetrological Groups A (homogeneous micro-cryptocrystalline chert) and B (granular chert).This was noticed when they were recognized as chert in the initial de visu classification. Thesamples of Types 4, 5, 6 and 7 correspond to petrological Group C.

Although the petrological results tend to group the macroscopically characterized types,macroscopic examination does reveal evidence that is very useful when trying to distinguishbetween different nodules. In addition, this type of assessment provides a certain degree oftechnological information. Thus, the types identified by macroscopic classification can be usedin the analysis of the site’s lithic material, while the types identified by the petrological classificationcan be used to identify materials from the Casa Montero mine found at other archaeological sites.

Patination

White patinas, or complete alteration of the sample, was observed in some archaeologicalsamples belonging to Group C. The patinated samples have opal or can be shown to have contained



it previously, although they are now completely made of quartz (Type 3 flints). Since opal iseasier to dissolve than quartz (Williams et al. 1985), it would be the first mineral to be modified.Thus opals and opaline cherts ought to be more likely to suffer modification than cherts. Opticalmicroscopy revealed no distribution of impurities of the sort described by Hurst and Kelly (1961).The opaque materials in the altered samples might be interpreted as oxides of iron, but iron wasnot generally detected by EDS. The SEM analysis made it quite clear that the white patinasmainly form due to dissolution processes, as suggested by Burroni et al. (2002). In the opalinechert samples, the opal preferentially dissolved, leaving quartz crystals with intercrystal pores,and therefore it is more easily degraded to a powder. In turn, the different discontinuitiesmarking the borders of the ovoids in the globular structure of the opals favoured modification byoffering dissolution routes. Opals with such a structure are therefore more likely to show patinas.

The SEM study showed the presence of microspheres in the patinas. These may be theconsequence of the local reprecipitation of silica after its dissolution (Burroni et al. 2002).This process would tend to render the patinas more heterogeneous in composition. The Mgcontent of the patinas was clearly greater than that of the unaltered samples, perhaps indicatingthat it accumulates during dissolution–reprecipitation.

CONCLUDING REMARKS

The comparison of the macroscopic and petrological studies shows that:• The greater number of flint types established by the macroscopic analysis reflects the greatermacroscopic than microscopic variability of the studied material. The results of the refittingwork showed that Types 1 and 2, even though they are structurally and mineralogically similar,are usually not found in the same nodule (Castañeda et al. in press); these types correspond tothe replacement of host rocks of different flint levels. • Petrological classification reduces the seven macroscopic types into four groups. This ispossible because of the great similarities between the genetic process that produced the differentsilica episodes. Similar process produced homogeneous petrological characteristics, althoughthe colour, translucency and so on were different.

Thus, neither refitting nor petrological/mineralogical analysis rules out the validity of macroscopiccharacterization and classification. Clearly, petrological and mineralogical analyses appear toprovide a solid base upon which to compare hypotheses regarding the exploitation of the flintraw material of Casa Montero, but it should be noted that macroscopic analysis offers a goodmeans of making a preliminary classification of these siliceous rocks. In addition, the latteris easier when dealing with large numbers of archaeological remains. Refittings can providea lithological understanding of the variability of these raw materials. An exhaustive com-parison of the two classification methods can explain some of the macroscopic criteria used,such as greater or lesser opacity, the type of lustre or the different response to intentionalfracturing.

The geological study shows that the uniqueness of the flint levels of Casa Montero is theconsequence of two processes: silicification and ageing. The silicification or replacement ofMg-smectites produced opaline minerals that were later recrystallized to micro-cryptocrystallinequartz. Both processes produced compact flints of petrological types and compositionsuncommon in the Madrid Basin—but which are very easy to knap. The petrological charac-teristics and the features of the outcrops, formed by interlayered claystones and flints, may helpus understand why intensive Neolithic mining was practised at Casa Montero: the mine isunique in terms of its compact flint layers and their accessibility.



The abundance of silica rocks within this regional framework is well known (Bustillo1976). It would therefore be logical to think that they could have been obtained relatively easilyduring prehistoric times without the need for excavation. The nature and availability of the rawmaterial at Casa Montero may partly explain the intensity of Neolithic mining activity andthe profitability of digging shafts. However, it cannot entirely explain the reason why suchactivity was undertaken; the clue may lie in discovering the technological, subsistence andeconomic reasons that led early Neolithic societies to produce such a mine.

In conclusion, the results of this work provide the basis for future research at Casa Montero.Diagnostic attributes for recognizing the raw materials in the geological context of this site,and their assignment to the different siliceous episodes, are established here. This should allowthe recognition of this mine’s products at other archaeological sites, which in turn will allow usto trace their distribution, determine how far they travelled, and estimate their economicimportance and social cost.

ACKNOWLEDGEMENTS

This research was undertaken as part of the agreement between the Consejería de Cultura yDeportes de la Comunidad de Madrid, the CSIC, and Autopistas Madrid Sur ConcesionariaEspañola, S.A. for the research, preservation and diffusion of the Casa Montero archaeologicalsite, and the projects ‘Formación de sílex, encostramientos y otras transformaciones en ambi-ente vadoso/freático e hidrotermal’, ‘Minería de sílex y poblamiento neolítico en la Mesetapeninsular: dinámicas de explotación y asentamiento’ and ‘Determinación de los objetivos dela explotación minera prehistórica a partir del estudio de los desechos de producción lítica’,under the auspices of the Dirección General de Investigación of the Ministerio de Educacióny Ciencia (references CGL 2005-05953-CO2-01, HUM2005-05732-C02-01 and HUM2005-05732-C02-02, respectively).

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IS THE MACROSCOPIC CLASSIFICATION OF FLINT USEFUL? A PETROARCHAEOLOGICAL ANALYSIS AND CHARACTERIZATION OF FLINT RAW MATERIALS FROM THE IBERIAN NEOLITHIC MINE OF CASA MONTERO