Environ. Sci. Technol. 1983, 17, 518-522
(15) Prausnitz, J. “Molecular Thermodynamicsof Fluid Phase Equilibria”;Prentice-Hall: Englewood Cliffs, NJ, 1973;pp 340-344.
Received for review July 9,1982. Revised manuscript received
February 22, 1983. Accepted April 18, 1983. This work was supported in part by the Cooperative Research Program between the Great Lakes Environmental Research Laboratory (GLERL) of the National Oceanic and Atmospheric Administration, Department of Commerce, and The University of Michigan.
Weddellite on Limestone in the Venice Environment Marco Del Monte Istituto di Geologia, Universitl di Bologna, 40126 Bologna, Italy
Crlstlna Sabbloni”
.
Istituto Fisbat, CNR, 40126 Bologna, Italy
This is the first report of a large deposit of weddellite (CaC2O4-2H20), the formation of which is still in progress. Mineralogical and physical characteristics of this mineral were studied and the origins of its formation established. The presence of weddellite is rarely reported in natural outcrops, has controversial origins, and has always been found in very small quantities. Thus, in natural environments, the stable form appears to be calcium oxalate monohydrate (whewellite) of which there is evidence of wider diffusion. In our case, weddellite is the main component of the alteration affecting every marble surface on the Isle of Torcello in the Venetian Lagoon. Ita origin must be related to the presence of numerous Cyanophites,whose role is to provide the system with oxalic acid, which in the presence of a carbonatic substratum precipitates in the form of calcium oxalate dihydrate. The enormous proliferation of these perforating “algae”,as of other endemic species, is connected to characteristics that are most peculiar to the Venetian Lagoon.
of thick layers of weddellite (CaCzO4-2H20) has been found on the carbonatic surfaces. The presence of this oxalate is important for these reasons: (a) With the exclusion of the vegetable kingdom and human pathology, reports on the occurrence of this mineral are very rare and controversial. In fact, weddellite is a mineral commonly found in vegetable cells and in kidney stones, while its presence is rarely reported in natural environments, and many authors hypothesize that the formation of this mineral occurs after sampling, during storage. (b) This occurrence differs from the already existing, but scarce, information in literature for the percentage of weddellite found in the samples analyzed (80-90%), the large areas covered by this mineral, and the thickness of the layer observed (1-7 mm). ( c ) This type of alteration was never recorded before on marble monuments or on carbonatic surfaces in general. It is well-known that the most common mineral found in weathering processes is gypsum; dolomite is rare ( 4 ) and whewellite even rarer (5).
Introduction The interaction between atmosphere (hydrosphere and biosphere) and stone surfaces is a phenomenon known as weathering. An aspect both important and typical of weathering is the action of highly polluted urban atmospheres on stone used in the construction of buildings, monuments, and wall facings. One of the most investigated effects of weathering is the sulfation of calcium carbonates, noticeable on carbonatic surfaces such as limestone and marble in the form of gypsum crusts. This process of transformation depends on the chemical and physical characteristics of the stones, as well as on environmental parameters such as exposure, geometry of surfaces, and nature and composition of the local atmosphere (pollutant concentration, rain composition, etc.). The specific role played by airborne carbonaceous particles in sulfation processes taking place on carbonatic surfaces and the action of washout in relation to the different features of marble alteration have been investigated in previous papers (1-3). Observing different types of atmospheres (urban, suburban, rural, coastal, lagoon, and marine), we were able to find interesting analogies between the processes occurring in polluted and clean areas. In northern and central Italy, no solution of continuity seems to exist between the two. The object of this study is to discuss a particular kind of alteration observed in the Venetian Lagoon, an area of interesting transitional character, lying as it does between marsh and sea. From the study of facades of the wellknown Cathedral of Santa Maria della Assunta and the Church of Santa Fosca on the Isle of Torcello, the presence
Previous Works The two calcium oxalates, weddellite (CaC204-2H20) and whewellite (CaC204-HzO), have been mentioned in scientific literature since the 17th century. The Italians Malpighi (6) and Buonanni (7) studied them. Philipsborn (8) has extensively reviewed the works concerning this subject. Both oxalates have a wide diffusion in the vegetable world and in the cells of many higher and lower plants. Many findings are reported in the animal world, for instance, in the Malpighi vessels, in the moult of insects, in cocoons, inside bees’ intestines (€9,and in the gizzard plates of the deepwater gastropod (Scaphander cylindrellus) (9). Further examples are found in honey and in the urine of many mammals (in particular, herbivorous mammals). Furthermore, weddellite and whewellite are also the main components of kidney and bladder stones (10-13). In a natural nonbiological environment whewellite appears to be more stable than weddellite and, consequently, is more widespread. It has been found in coal mines, uranium mines (14), calcareous rocks, calcareous geodes, clay, septarian limestone (15,16), and weathering crusts of travertine (Coliseum) and marble monuments (Constantine’s Arch) in Rome (5). Reports concerning weddellite are controversial and scarce. The first concerns sediments dredged from the bottom of the Central Weddell Sea (Antarctica) at a depth of 4500-5000 m ( 1 7 ) . However, Arrhenius (18)points out how subsequent attempts to find weddellite in antarctic sediments have failed. He suggests the possibility that the weddellite reported by Bannister and Hey (17) might have been the result of alteration occurring during the storage of samples, due to the interaction of decomposing proteic
518
Envlron. Sci. Technol., Vol. 17, No. 9, 1983
0013-936X/83/0917-05 18$01.50/0
63 1983 American Chemical Society
I
1. (a)Detail 01 the oaapoMCshapcd Chuch 01 Santa Fosca (1011 A.D.). co*mns are pIswnt on We ot the aldes. These. Ike In the adbceni Chuch every o h part In marble or mestone. am wperRdally mated In w d d e h . T b M ~ O can be 01 Santa Marla. (b) CapHal ot a colurm heavily attacked by whke effbrescenag ot w&MHe unnamlv aponing the SMne. (c)Column almost uniformly mated wiiA w&kate. In me top lefthand mnsr,n can be seen how the t h l c k m of the weathered leyerreadraseveral mARneten. (d) Enlarged detail 01 area 01 weamalng of CaCO, In CaC,O,QH,O In ma 01 the columns. Weddellke presently covefs almwt 70-80% of the w!glnal calcareous area.
matter and calcium aalta, thus excluding it is a primary mineral from the seabed. A further report of secondary weddellite eoncems the calcareous sediments in the area of Ten Thousand Island in Florida (19). In this instance alao, it is believed that the mineral was formed while the samples were being stored, over a period of 2 years. Lowenstan (9) reporta a second hding of weddellite in sediments from the Weddell Sea and c o n f m s the f i t hypothesis in which weddellite ia eonsidered an authigenic mineral in this area. However, the place of finding, type of sampling and storage have not been mentioned. Marlowe (20)reports the preaence of primary weddellite in sandy samples from the bed of the rivers Sanguenay and St. Lawrence in Canada In this case, the formation of the mineral is aasociated with the preaence of wood fibers rich in hydrated calcium oxalates, which might have favored the growth of weddellite crystals. Lastly, Tirelli (21) has found layers of weddellite and whewellite on ophiolitic rocks in the Italian Apennines. In this case, no mention of the presence of organic matter ia reported. It must be pointed out that in all the eases reported in literature both authigenic and secondary weddellite were present in amounts that never exceeded a small overall percentage of the total sample and that were always limited to a small area.
Experimental Data Torcello, a small bland in the Venetian Lagoon, lies approximately 9 km northwest of Venice. The Cathedral
of Santa Maria dell'Assunta and the Church of Santa Fosca testify to its ancient history as a flourishing trading center in the early Middle Ages. columns, marble facings, and decorations of both churches show, even on a superficial examination, their heterogeneous lithological nature and origin In fact, the material used for their construction dates back to Roman or earlier times and was salvaged from the ruins of Altino, a town destroyed by the Longbards at the beginning of the seventh century. Our observations dealt with all the external carbonatic surfaces of this complex and particularly with the marble columns, which are the main marmoreal architectural features of both churches. AU the carbonatic surfaces present visible alteration of varying intensities. The alteration of the columns is uniform from the base up to the capitals, weathering is between 1and 4-7 mm thick, white or yellow-white, and even at fmt glance, it seems to have grown a t the expense of the stone underneath, which is considerably eroded (Figure la,b). Approximately 100 samples were taken, some from the altered crust in order to study authigenic minerals, some from the weathered layer, plus a small portion of the unaltered underlying stone in order to study the relation between primary and secondary minerals. As previous reports (18.19) suggested the possibility that weddellite has occurred during storage, we analyzed the samples a few hours after sampling. Some were stored as they were found and the others were kept in a 70% alcohol solution. After reanalysis a year later, no differences in the three cases were found. The X-ray diffraction (XRD)results showed that the Emfon. Sd. Technal.. Vd. I?. No. 9. 1983 519
Table I. Measured d Spacings of X-ray Patterniof Weddellite fluvial ophiolitic marine sediments, rocb, sediments, urinary Canada FL(19) Italy ( 2 1 ) calculi ( 1 1 ) d. A
III.
6.23 4.45 3.93 3.68
90 70 40 60
3.36 3.09 2.98 2.78 2.50 2.41
10 50 20 100 10 60
2.35 2.24 2.20 2.12 2.07 1.96 1.83
80 20 50 10 60 60
d. A
Ill.
8.75 6.18 4.42 3.90 3.67 3.59 3.39 3.09 2.812 2.7765 2.6765 2.4245 2.400 2.353 2.281 2.237 2.208 2.119 2.022 1.958 1.9545 1.8955 1.835 1.7395 1.6915
20 90 60 30 25
