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DOLOSTONES USED IN MIDDLE AGE IN FRIULI (NE ITALY)

SPADEA, P. Dept. GEOTER, University of Udine, Via Cotonificio, 114, 33100 Udine Italy. PERUSINI, T. ' EUCORE, 33051 Pavia di Udine, Italy;

FRANGIPANE, A. Dept. GEOTER, University of Udine, Via Cotonificio, 114, 33100 Udine, Italy.

SUMMARY

Rare exan:iples of the use of dolostones in historical buildings have been recognized in Friuli among the materials used for ?ecoration of religious buildings from the 14th century. They include the Major Portal a~d the Coro~at1~n ~orta,I of the Duomo. of Udine, ~culptured elements from an archeological excavation at S. Mana d1 Rive d Arcano, and minor decorations from S. Daniele del Friuli near Udine. The stones are mostly fine-grained arenites composed of dolomite, calcite and quartz, less frequently conglomerates and ~iltstones similar in mineral assemblage. Because no document on the provenace of the dolostones exists, the possible stratigraphical relations and geographical provenance of these rocks wer~ investigat~d by comparison with dolomite-bearing rocks from the nearby area. Petrographical comparison has shown that the rocks are dissimilar from dolostones or dolomite­bearing stones from known quarries in the Friuli Prealps and the Karst. A possible provenance from some unknown quarry(ies) once existing in the hills from the piedmont area of Friuli to the NW of Udine is suggested.

INTRODUCTION

Dolomitic rocks are relatively rare stones used for sculptures and decorations worldwide in architectural works. Reasons for the paucity of works in dolomitic rocks are probably related with the properties of the rocks themselves, that are hardness of dolomite, which is much higher than that of calcite, and relatively high fragility. Moreover, among the carbonate rocks, dolostones are less abundant than limestones and consequently Jess frequently exploited. In Italy, the use of dolostones in historic architectural works is mostly restricted to the northern area with important examples in Lombardia where the use of pure dolomitic rocks (Alessandrini et al., 1995) is recorded. In the Friuli region (NE Italy), limestones are the most used carbonate stones for building and decoration since the Roman age. Their provenance is mostly well known, from local (eastern hills) or farther (Karst and lstria) quarries. Rare dolostones have been recognized recently for the first time in Friuli in monuments and artworks from the 14th centrury. The most important is the Major Portal of the Duomo of Udine which is in a dolomitic-calcareous arenite (Berlasso et al., 1991; Spadea, 1992; Spadea, 1995). Also a coheval minor portal and a corbel from the Duomo are in a similar stone. Other occurrences of dolostones have been recognized in decorations from an archeological excavation at the church of S. Maria di Rive d'Arcano near Udine (Spadea, in press), where remnants of a Middle Age building were discovered. Preliminary investigations in Udine and the surrounding area suggest that the use of dolostones in construction and decoration is restricted mostly to the Middle Age. No document on the provenance of these stones has been found, even for the Duomo of Udine Major Portal (Perusini, 1995). This study addresses firstly the problem of a single or multiple provenance of the dolostones by means of petrographical analysis. Secondly the possible local source of the stones is considered by petrographical comparison with dolomitic rocks from the geological units which appear to be realable candidates for the quarried materials.

MATERIALS AND METHODS

About 40 samples of dolostones were selected for analysis under the petrographical microscope w!th the aim of mineralogical and textural characterization. The identification of the ca.rbonat~s (d.olom~te and calcite) was confirmed by X-ray diffraction analysis (in the Dept. Geater of ~dine Urnvers1tx, with a Philips PW1390 diffractometer mounting a Cu tube). X-ray analysis also provided an evaluation of

the calcite/dolomite ratio. The selected samples include some previously described from the Major Portal of the Duomo of Udine (Spadea, 1995) and from the Rive D'Arcano archaelogical excavation (Spadea in press). Newly

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studied samples were collected again from the Major Portal of the Duomo of Udine, during the restoration works commenced in June 1996, from the Coronation Portal of the Duomo and from a corbel holding the statue of Madonna which is placed on the eastern side of the bell tower of the Duomo. In S. Daniele to the NE of Udine samples were collected from a window frame of the Parsonage and in Flagogna, a little village in the piedmont area N to S. Daniele, from window frames of two houses. The selected reference stones include the Aviano stone from the Carnian Prealps and the Rupingrande stone from the Karst of Trieste.

PETROGRAPHICAL CHARACTERISTICS OF THE DOLOSTONES

All dolostone samples are carbonate rocks consisting of both calcite and dolomite in rather variable proportions, with significant or moderate contents of quartz. Texturally and mineralogically different types are distinguished. 1. Dolomite-calcite arenite, contaning up to about 15% quartz. In hand specimen the rock is whitish, pale yellow, or pale pink yellow and fine grained. Occasionally rounded granules of carbonate rocks (micrite and siltite) are visible. Under the microscope the arenite clasts are in the 0.5-0.01 mm range, with average size of 0.15 mm in most samples, and consist of: a) dolomite in single rhombohedral crystals or aggregates of few crystals, mostly clear; b) micritic limestone, sometimes partially recrystallized (dismicrite); c) quartz in single crystals and polycrystalline aggregates; d) chert, pale brown or turbid due to opaque inclusions; e) rare accessory minerals including albite, muscovite, magnetite, epidote and zircon. The matrix is dominantly micrite with scarce clay minerals. 2. Dolomite-calcite siltite, quartz-bearing. Pale yellow of whitish, with finely laminated or gradate structures in hand specimen. Texture is siltose, with grain size in the range 0.03-0.01 mm. Clasts are similar mineralogically to those of type 1 stone. 3. Dolomitic-calcareous micro-coglomerate. This type rock type occurs associated and grading to the arenite at the Major Portal of the Duomo of Udine. It consists of small pebbles of dolomite-calcite arenite and siltite included in an arenite matrix. 4. Dolomite-calcite-quartz arenite. The rock is light gray and fine-grained in hand specimen. Under the microscope the clasts are 0.15 mm as average size and consist mostly of dolomite as single crystals, recrystallized limestone, partly dolomitized limestone, mono- and poly-crystalline quartz, and chert. Minor components are albite, chlorite, muscovite and iron oxides. Very scarce calcite occurs as cement. With the exception of a dolomite-calcite arenite from Rive D'Arcano, the samples resulted to be not fossiliferous; also in the micrite clasts no microfossil occurs. The only fossil found is a Globotruncana sp. (only one specimen found in a thin section) , rather badly preserved. It is doubtful wether this fossil has the same age of the sediment or is reworked. The second possibility seems to be more probable.

REFERENCE STONES

Different types of Aviano stone analyzed by X-ray and under the microscope (on a total of 13 samples) are dolomitic limestone and dolomite composed of micrite, often fossiliferous, and secondary dolomite. Dolomite is shown in fact replacing the micrite at variable stages as rhombohedral crystals often showing growth structures. The Rupingrande stone, studied similarly to the Aviano stone on 8 samples, consists also of dolomitic limestone and dolomite and shows evidence of secondary dolomite like the Aviano stone.

PROVENANCE

Petrographic analysis has clearly shown that the dolomitic rocks from this study derive from elastic sediments composed mostly of sand and silt in which dolomite occurs as detrital component. Petrographic comparison with stones of known provenance has also clearly shown that these rocks are dissimilar from the Aviano and Rupingrande stones, both consisting of micritic limestones at various stages of dolomitization. The provenance of the . dolomitic rock.s from this study represents therefore an open problem. Howe~er, the petrograph1? study has evidenced close similarities of the analyzed stones, suggesting an unique provenance (with some doubts for the quartz-rich stone from the bell tower of the Duomo of Udine). Excluding a provenance from known quarries in the Friuli Prealps and the Karst, which are located rather far away from the places of use, also an origin from more distant localities outside the Friuli region seems unlikely. In fact stones of similar quality are common in Friuli .

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It can be therefore supposed that the dolostones have a local provenance from old unknown quarry(ies) not distant from Udine and nearby localities. The piedmont area to the NW of Udine seems to be most probable for the location site of the quarried material in view of the findings of these stones. A geological approach to the problem of provenance should be based mostly on the lithological and sedimentological characteristics of the stones. The unique fossil found could indicate a Late Cretaceous age, but, if reworked, should provide only a maximum age. It is therefore possible that the stones belong to a stratigraphic unit of Late Cretaceous age or younger. In the Friuli piedmont area, units composed of detrital sedimentary rocks are known from the Late Cretaceous to the Miocene. Preliminary investigations on the strata of probable Miocene age outcropping in the lower Arzino valley near Flagogna allowed to identify an outcropping sequence of mostly conglomerates and interlayered arenites. The latter consist of dolomite, calcite and minor quartz and are similar to the studied stones. These indications could suggest a local provenance from Arzino Valley. This inference needs however to be confirmed with further data on distribution of the dolostones, history of the constructions and location of historic quarries from the region.

CONCLUSIONS

This study summarizes the occurrences of dolostones used for architectural decoration in Friuli during the Middle Age resulting from previous investigations and new identifications. The collected data indicate an unique, local provenance from the piedmont area of the Friuli region to the NW of Udine. This inference should be confirmed with further data on both the distribution of these stones in artworks and a detailed comparison with geological units from the supposed provenance area. A significant contribution to the knowledge of the use and trade of materials for architecture during the Middle Age is expected.

REFERENCES

Alesandrini , G., Toniolo, L., Sansonetti, A., Terrani, M.and Cesana, A., 1995. Geochemical characterization of an Italian dolostone: the "pietra d'Angera". Sc. Techn. for Cultural Heritage, 4: 51-57.

Berlasso, M. T. , Bruschi, A., Longega, G., Pagani, T., Perusini, T., Proietti , A., Biscontin, G., Repaci, G., Maravelaki, P., Zendri, E., Galeazzo, G., and Spadea, P., 1991. Studi preliminari per un intervento conservativo del Duomo di Udine. Atti Conv . Le pietre nell'Architettura struttura e superfici, Bressanone 1991, Libreria Progetto, Padova, 923-942.

Perusini, T ., 1995. Analisi storico-artistica e ricerca documentaria. In: Duomo di Udine. Ricerca per ii restauro del Portale della Redenzione., Restauro del Friuli-Venezia Giulia, 3: 13-54.

Spadea, P., 1992. Mineralogical and petrographical factors of the ~lter~tion of the carbon~te stones from the Duomo of Udine Portal. Proc. 7th Intern. Congr. on Detenorat1on and Conservation of Stone,

Lisboa, 2: 158-164, Lisboa.

Spadea, P., 1995. Studio petrografico dei materiali lapidei e del.le malt~ . In:. D.uomo di Udine. Ricerca per ii restauro del Portale della Redenzione., Restauro del Friuh-Venez1a G1uha, 3: 95-116.

Spadea, P., 1996. Studio petrografico d.ei camp!oni.di roc~ia deg.Ii eleme.nti scultorei alto-medievali. In: S. Maria a Rive d'Arcano: archeolog1a e stona d1 una p1eve fnulana (in press) .

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Figure 2. Dolomite-calcite arenite · '. ~IDU!P

(sample US324 406349: S. Maria di · : .. ·.---~ _ ~~ .. -. ~ • · ... ~ ""~ t , ,

Rive D'Arcano) . Clasts of dolomite, , .. : ..... : ."'" nucnte and chert (large white fragment) and a Globotruncana sp. shell (center of the figure) . Micro­photo with plane polarized light; length of field 1 mm.

Figure 1. Dolomite-calcite arenite (sample PD 19: Major Portal of the

r1 Duomo of Udine). Arenitic texture, with clasts of dolomite (white crystal at the center of the figure), mi critic limestone (turbid) and quartz. Microphoto with plane polarized

· light; length of field 2 mm.

3. Dolomite-calcite quartz arenite (sample PC2/1 : Coronation Portal of the Duomo of Udine). Arenitic texture, with clasts of dolomite as rhombohedric crystals (one shown at the center of the

·,. figure), mi critic limestone, quartz · - and altered opaques. Microphoto

with plane polarized light; length of field 3 mm.