Devonshire marbles—the greatest British decorative stones?

‘Devonshire marbles’ were once the pre‐eminent British decorative stones, installed in dozens of Victorian buildings in Britain and elsewhere. There are numerous varieties, none of which are true marbles for they retain the fossils and sedimentary textures of the original Devonian limestones. Quarried and manufactured across south Devon and used both internally and externally, the stones were part of a passing nineteenth century architectural fashion. There are 20 or so main varieties, mostly now unobtainable, but excellent examples are found in the panels, shafts and floors of some exceptionally fine buildings. These reveal much about the fossils, their depositional environments and the changes that affected them during the Variscan orogeny. Now rediscovered and distinguished for the first time using geological criteria, Devonshire marbles represent a valuable resource for geological, conservation, heritage and architectural research.     

British decorative stones: finding the UK top ten

British geology reveals many good looking rocks, both ‘soft’ and ‘hard’, some of which were once highly prized for their polished decorative uses. They were quarried, cut, shaped and finished in a locally‐based British decorative stone industry that flourished twice, first in the Middle Ages, based largely on Purbeck marble from Dorset and also alabaster from Derbyshire, and then again in the nineteenth century when diverse sources of coloured and textured stone were pursued to the far corners of the British Isles. Today only the finished products survive; the pillars, panels and pavement adorning some fine but dusty architecture, but the stones commonly languish unrecognized and unappreciated. This anonymity is quite out of line with the heritage status of their settings and it will take a bit more geological awareness to put that right. A recent project aimed at rediscovering just one regional category of British decorative stones, the Devonshire marbles, has revealed how diverse and extensively‐used the fuller range of British decorative stones actually is. Not only do they embellish buildings from the merely modest to some of our finest, they were latterly chosen, specified, designed and coordinated by some of our greatest architects. Much work needs to be done to recognize British decorative stones in architectural settings and to restore them to their proper place in our national heritage. It is hoped that this article will throw light on the task ahead.     

Preliminary results of a first record of gold and uranium in marble from Central Eastern Desert, Egypt: a witness for (syn- and post-?) metamorphic mineralization in metasediments

This paper records, for the first time, the mineralization of gold (0.98–2.76 ppm) and uranium (133–640 ppm) in marbles from the Arabian-Nubian Shield of the Eastern Desert of Egypt. These auriferous and uraniferous marbles are hosted by sheared and altered ophiolitic serpentinized ultramafic rocks of Gebel El-Rukham (ER), Wadi Daghbag (DG), and Wadi Al Barramiyah (BM). They occur as massive or banded in pod-like or bedded shapes. The ER and BM-mineralized marbles are impure calcitic, whereas the DG marble is impure calcitic to impure dolomitic. Their protolith are pure limestones and dolomitic limestones with probable argillaceous components (BM marble), and their metamorphism (Pan-African) was retrograde. Peaks of metamorphism were at granulite-amphibolite facies for the ER and BM marbles, forming diopside (Al₂O₃?=?0.17–1.07 wt.%) at 600–900°C and augite (Al₂O₃?=?2.45–9.40 wt.%) at 825–975°C, and at the amphibolite facies for DG marble, recrystallising the carbonate minerals and forming tremolite. The lowest temperatures of metamorphism were at the upper subgreenschist facies as chlorite (ER and BM marbles) and kaolinite (DG marble) were formed. Metamorphic fluids were, most probably, essentially binary H₂O–CO₂ mixtures with low NaCl and HF concentrations. Gold in the studied mineralized marbles occurs as native nuggets (10–35 μm) having globule, rod, crescent, and streak shapes, in pores, vugs, and fissures. The source of gold in all marbles is mostly the country ultramafic rocks. Timing of gold mineralization relative to the marblization and metamorphism of the country source ultramafic rocks was both syn- and post-metamorphic. Concerning the ER and DG marbles, it was syn-metamorphic, where Au liberation and transportation were mostly by the metamorphic fluids. The composition and temperature of these fluids were most probably inappropriate for formation of the sulfide complexes of gold. The gold mineralization of BM marble, on the other hand, was mostly post-metamorphic. The mineralising fluid was of surficial origin under oxidizing conditions. The encountered uranium minerals are of secondary origin such as autunite, uranophane, and carnotite. These minerals occur as fine oval aggregates and irregular grains (10–50 μm) usually filling fissures and vugs. The uranium mineralization can be classified as surficial of ages <1.5 Ma. It is proposed that the U was transported from its source (might be flesite and trachyte dikes for the ER and DG marbles and granite rocks for BM marble) to the marble rocks by surface and/or underground water related to the pluvial periods in Egypt. In BM marble, U and Au have mutual mineralizing fluid but different paragenesis.     

Prediction of weathering by thermal degradation of a coarse-grained marble using ultrasonic pulse velocity

Engineering properties of building stones can vary because of degradation by weathering agents. Thermal fluctuation is one of the most important agents on deteriorations such as sugaring, bowing, cracking and spalling of marble. As a result, physical and mechanical properties of marble used in the construction of old and/or modern structures are adversely affected by time. On the other hand, some properties of building stones are always required for decision of rehabilitation works. Several kinds of conventional tests have been suggested for characterization of stones, to measure their properties or to evaluate conservation or repair works. However, in most cases, sampling from historical buildings is not possible. Therefore, nondestructive testing methods are often suggested for the prediction of weathering grade of building stones. One of the most practical methods for similar purposes is ultrasonic pulse velocity measurement, which can be performed easily. The main goal of this study is attainment of sound empirical correlations between the ultrasonic pulse velocity and engineering properties of previously deteriorated marble. Experimental works were conducted on seven different specimen categories of a coarse-grained marble having different micro-crack frequencies induced by both cyclical heating–cooling and freeze–thaw actions. The experimental results indicated that physical and mechanical properties of Mu?la marble can be reliably estimated for different environmental cases by ultrasonic pulse velocities. P-wave velocities in dry and saturated cases are two sound indicators of both the apparent porosity and the coefficient of capillary absorption, and whereby the sugaring type of deterioration for coarse-grained marbles.     

Geology and production of construction material resources of Lebanon: a preliminary study

Construction material resources of Lebanon are discussed in terms of geology, distribution and production. This is a first attempt with a systematic approach. Crushed rocks form the bulk of total production, and are used for cement, concrete and pavement. They are all produced from limestones ranging in age from Jurassic to Tertiary. Sands, other than those coming from crushed rocks, are produced from Quaternary deposits, or from the basal Cretaceous sandstone formation (Gres de Base). They are used for cement, concrete, pavement and glass. Dimension and face stones are also produced from the limestones. The mosaic and pottery industry uses clays which are mostly in seams or pockets within the basal Cretaceous unit. This study points out to the need for a systematic compilation of pertinent data to facilitate evaluation of supplies for future development.     

Analysis of the surface of different marbles by X-ray photoelectron spectroscopy (XPS) to evaluate decay by SO2 attack

Atmospheric pollution is one of the main agents of decay in monuments and other works of art located in industrialised urban centres. SO₂ is a permanent and abundant component of air pollution and, although it does not have an immediate visual effect, after continuous exposure, it can cause irreversible damage to building materials. Marble is one of the most commonly used ornamental stones in historical monuments and its mineralogical composition makes it very susceptible to damage caused by exposure to SO₂. To measure the chemical reactions caused on marble by the action of atmosphere rich in SO₂, selected calcitic and dolomitic samples were altered by weathering accelerated test. For this, seven marble types (four calcitic and three dolomitic) were exposed to high concentration of sulphur dioxide for 24 h in a climate chamber under controlled temperature and humidity conditions (20 °C and > 90 % HR). Changes on marble surfaces caused by reactions of SO₂ with calcite and dolomite were studied using two non-destructive techniques: chromatic change by means of colorimetry and chemical analysis using X-ray photoelectron spectroscopy (XPS). The development of new mineral phases was also observed by scanning electron microscopy. Colorimetric analysis revealed a decrease in lightness and chromatic parameters suggesting that these changes were due to the development of new mineral phases in all marbles. The XPS technique, which is generally used in the analysis of metals, is relatively new in the field of stone deterioration. It enabled us to recognise the development of sulphites and sulphates on marble surfaces with high precision, after just 24 h of exposure to high SO₂ concentrations and to distinguish different decay paths for calcitic and dolomitic marbles.     

Petrographical and geochemical investigation of the Triassic marbles associated with Menderes massif metamorphics,Kavaklıdere,Muğla,SW Turkey

Vast marble deposits occur in a cover sequence of the Menderes Massif, SW Turkey. Four major marble deposits are recognized in Mu?la province based on the stratigraphic levels. These are Permo-Carboniferous aged black marbles (1), Triassic aged marbles (2), Upper Cretaceous aged marbles (3), and Paleocene aged pelagic marbles (4). This study deals with Triassic aged marbles of the southern part of the Menderes Massif. The Triassic marbles from SW Turkey consist of two big marble horizons in the Çayboyu (ÇM) and Kestanecik (KM) regions. The characteristic samples are collected from different stratigraphic levels in marble deposits in the ÇM and KM horizons. Mineralogical and major, trace, and rare earth element (REE) analyses of marble, limestone, and schist were conducted on these samples to reveal their petrographical and geochemical characteristics. The ÇM horizon is represented by calcitic marble layers. Nickel, cobalt, manganese, and iron elements filled in fractures, fissures, and intergranular spaces of calcite crystals and these elements give the pinky colour to the marble from the ÇM horizon. KM marbles were deformed, metamorphosed, and recrystallized under greenschist facies P–T conditions. As a result of the metasomatic reaction of magnesium and manganese rich fluids with marbles, dolomite, and manganese, minerals such as rhodochrosite and pyrolusite have crystallized along vein walls and layers in the KM horizon. Dolomitization was determined in KM marbles, whereas ÇM marbles show the character of limestone. MgO, MnO, Fe₂O₃, Ni, and Zn contents of marbles from the KM horizon are higher than those of ÇM marbles due to metasomatic reactions. The Sr content in white coloured marbles ranges between 11.20 ppm and 112.20 ppm and this concentration reaches up to 272.70 ppm due to metasomatic reactions and fluid intake. The REE content of Triassic marbles is independent of the abundance of carbonate and the REE enrichment observed due to syn-metamorphic fluid flow. The significant negative Eu anomaly in REE patterns indicates that the protoliths of Triassic marbles are carbonate rocks of sedimentary origin.     

Marbles from Roman Hispania: stable isotope and cathodoluminescence characterization

Pure white marble has been considered a valuable ornamental and architectural material since ancient times. Many scientific techniques have been used to create an extensive data base of “finger-prints” characterizing white marbles from the major classical quarries. However, determining the provenance of white marbles is a difficult task due to their similarity in physical and chemical parameters. Three techniques (petrography, cathodoluminescence and stable C and O isotopes) have been used to characterize white marbles from the ancient quarries of the Iberian Peninsula. Maximum grain size, texture and isotopic composition can be used to identify the different quarries. Each area is generally represented by several cathodomicrofacies, but quantitative CL analysis is also helpful in distinguishing those quarries for which the data provided by other techniques are not sufficiently diagnostic. The database and the discriminating criteria presented in this study have been tentatively applied to some ancient sculptures from the National Museum of Roman Art in Merida (Spain).     

Marbles and carbonate rocks from central Morocco: a petrographic,mineralogical and geochemical study

Petrographic, mineralogical, and stable isotopes (δ¹³C, δ¹⁸O values) compositions were used to characterise marbles and sedimentary carbonate rocks from central Morocco, which are considered to be a likely source of ornamental and building material from Roman time to the present day. This new data set was used in the frame of an archaeometric provenance study on Roman artefacts from the town of Thamusida (Kenitra, north Morocco), to assess the potential employment of these rocks for the manufacture of the archaeological materials. A representative set of samples from marbles and other carbonate rocks (limestone, dolostone) were collected in several quarries and outcrops in the Moroccan Meseta, in a region extending from the Meknes–Khenifra alignment to the Atlantic Ocean. All the samples were studied using a petrographic, mineralogical and geochemical methods. The petrographic and minerological investigations (optical microscopy, electron microscopy, X-ray diffraction) allowed to group the carbonate rocks in limestones, foliated limestone, diagenetic breccias and dolostone. The limestones could be further grouped as mudstones, wackestones–packstones, crinoid grainstones, oolitic grainstone and floatstones. Textural differences allowed to define marbles varieties. The stable carbon and oxygen isotope composition proved to be quite useful in the discrimination of marble sources, with apparently less discriminatory potential for carbonate rocks.     

ROLE OF METASOMATIC PROCESSES IN RUBY-BEARING MARBLES FORMATION OF THE SOUTHERN URAL MOUNTAINS

Ruby-bearing marbles of the Southern Ural Mountains are developed in the metamorphic perimeter of granites-gneisses domes where high grade metamorphic granitization and diaphthoresis have occurred.Geological research into the development and occurrence of ruby-bearing marbles indicate that they formed as a result of repeated transformation.Their substrate consisted of an organogenous marine limestone containing Visean faunal remains.Intensive Mg metasomatism of limestone during early progressive stages of metamorphism resulted in a substrate of dolomite composition containing faunal remains with a calcite composition.Increased temperature and pressure resulted in metamorphism of early Mg metasomatites,turning them into fine-grained marble containing Visean faunal remains.Tensional stresses near the intrusive domes resulted in dedolomitization of early Mg metasomatites,giving rise to light,coarse-grained calcite marbles having polygonal-grained structure.Such carbonaceous marbles became metamorphosed around the perimeter of granite-gneiss massifs.Their rheological properties allowed for plastic flow in these marbles into areas of tectonic neutrality,forming bodies of rheomorphic marbles,sometimes even among marbled limestone.Relict bodies of Mg metasomatites underwent boudinage and rotation.Flow cleavage occurred in axial blocks of inter-dome structures and in their steep walls.Therefore platy jointing(banding,pseudo-lamination)formed in the marbles.Thickness of the plates is from several millimeters to 2-3m.Calcite underwent recrystallization with extension of grain size in the central parts of plates,sometimes amounting to 15-25cm in cross-section.Ruby-bearing marbles consist of Mg-calcite.The rock is coarse-grained,with a panidiomorphic texture.Schistosity is often observed in the plane of cleavage.Mg-calcite marble occurs among and grades into background calcite marbles,which are controlled by cleavage of flowing.It is supposed that the ruby-bearing Mg-calcite marbles bodies are elongated in the direction of dip.Their formation is caused by recrystallization under the action of rising metamorphogenic fluids at high temperatures and higher pressures(amphibolitic and epidote-amphibolitic facies).Ruby-bearing marbles formed at the end of the prograde stage of metamorphism.The early retrograde stage of metamorphism is defined by a new wave of Mg metasomatism and formation of calcite-dolomitic marbles with a poikiloblastic texture from calcite or Mg-calcite marbles.Usually the boundaries of the bodies are clear,planar,and controlled by cleavage.Studies of small bodies have shown that they are elongated in the direction of dip.Usually they contain pink corundum and/or pink spinel;red corundum is replaced by red spinel.Thus the initial marine limestones were transformed into various marbles and completely lost their primary composition and bedding as a result of metamorphism,deformation,and metasomatic transformation.Ruby-spinel mineralization in marbles is controlled by cleavage.     

Persistence of pre-metamorphic C and O isotopic signatures in marbles subject to Pan-African granulite-facies metamorphism and U–Th mineralization (Tranomaro, Southeast Madagascar)

The status of fluid regimes during the Pan-African granulite-facies metamorphism in Southern Madagascar was examined by means of a mineralogical and stable isotope study of marble lithologies. In the granulitic Tranomaro area, which is made up of metasedimentary rocks (metapelitic, leptynitic (felsic) and calc–silicate metasediments) and intruded by syn-granulitic granitic bodies, syn-granulitic mineralization (Th–U) and metasomatic transformations (marbles→pyroxenites) demonstrate that fluid circulation has occurred. The Tranomaro marbles can be subdivided into two compositional groups, either Mg-poor or Mg-rich. This division reflects pre-metamorphic compositional differences rather than different histories. On a regional scale, the marbles display a large variation in (from +19 to +6.5‰ relative to SMOW) but have a more restricted range (from +1.4 to −2.5‰ relative to PDB). The lowest values are observed in magnesian marbles, whereas calcic ones do not have values lower than +13.5‰. Both initially low and high marbles may have locally been infiltrated by the syn-metamorphic fluid flow which gave rise to pyroxenites and Th–U ores (thorianite). values of carbonate in Mg-rich marbles evolve from +7 up to 8.5‰ towards the contact with a metasomatic pyroxenite in a meter-scale profile across an infiltration zone. The mineralogy, as well as the large and small-scale isotopic results argue for: (i) a pre-metamorphic origin for the isotopic variations, probably related to pre-granulitic dolomitization, (ii) a lack of pervasive fluid infiltration capable of erasing pre-metamorphic isotopic compositional differences, (iii) an absence of mantle-derived C-bearing fluids in the infiltrated zones. For the area as a whole, the present data are consistent with the decarbonation of impure siliceous limestones and dolostones coupled with fluid release from syn-metamorphic granitic intrusions.     

Geological parameters affecting the marble production in the quarries along the southern flank of the Menderes Massif, in SW Turkey

The Mugla province is one of the major marble producing regions located in the southern flank of the Menderes Massif in SW Turkey. The Menderes Massif is a regionally metamorphosed massif with an old Pan-African core and cover successions from the Permo–Carboniferous to Paleocene. There are four major metamorphic carbonate horizons in the cover successions exploited for the marble production. These horizons are located within the Permo–Carboniferous, Triassic, Upper Cretaceous and Paleocene successions along the southern flank of the Menderes Massif. Here the world wide known marbles with names such as the Mugla Black, the Milas White, Veined, Pearl, Aubergine, Lilac and Lemony, the Mugla White and the Aegean Bordeaux are found.

Detailed geological studies were carried out in selected marble quarries representing the different stratigraphic levels to determine the geological parameters affecting the marble production in the southern flank of the Menderes Massif in SW Turkey. The geological parameters such as bedding, joints, schist interlayers and mica filled joints affecting the block production from the marble beds are considered to be primary features. The presence of dolomite bands and lenses, abnormal sized calcite crystals and emery minerals which affect the slab and the production qualities and appearances are considered to be secondary geological parameters. The primary and secondary geological parameters affecting the marble productions at different stratigraphical levels in SW Turkey, are determined and the practical aspects of these findings are discussed.     

On the mechanical and physical properties of ten hellenic marbles

In the present work the most characteristic mechanical and physical properties of ten Hellenic marbles were investigated. The marbles, five of which were white and the other five coloured, are the most representative ones found in Greece. Among those studied are the famous marbles of Paros and Penteli, the latter being the basic material of the Parthenon. After a rough classification and the determination of their composition, the following properties of each marble were determined: specific gravity; apparent density; behaviour in water; coefficient of thermal conductivity; coefficient of linear thermal expansion and its variation with temperature; strength in various loading forms; and dynamic modulus of elasticity as well as the resistance to frost and corrosion. All the results obtained are given in diagrammatic and tabular forms. On the basis of these results, useful conclusions regarding the utility of each marble for various applications are derived.     

Direct observation of microcrack development in marble caused by thermal weathering

One of the properties that makes marble such an excellent construction and ornamental material is its low porosity. It is very difficult for water or decay agents to penetrate the internal structure of materials with no or few pores, so enhancing the durability of these materials. However, environmental temperature fluctuations bring about significant physical changes in marbles that result in an increase in porosity, due to the appearance of new microcracks and the expansion of existing ones. These cracks offer new paths into the marble which make it easier for solutions containing pollutants to penetrate the material. Thermal expansion tests were performed on three different types of marble known as White, Tranco, and Yellow Macael (Almeria, Spain), after which an increase in porosity (from 17 to 73% depending on marble type) was observed, mainly due to crack formation. The structural changes occurring during thermal expansion tests were more significant in the case of White Macael samples, a fact that is not only related to its mineralogical composition but also to the morphology of the grains, grain boundaries and crystal size. Our research suggests that thermally weathered White Macael marble could be more susceptible to decay by other contaminant agents than Tranco or Yellow Macael. The use of hot-stage environmental scanning electron microscopy is proposed as a valid tool for observing, both in situ and at high magnification, changes in the fracture system of building stones induced by thermal stress.     

Metamorphism of sedimentary rocks recognised by 19th century French naturalists: A case study from the Chavanon sequence marbles,Massif Central,France

The Chavanon metamorphic sequence in the Variscan French Massif Central contains marble lenses that have been exploited since at least the 18th century to produce lime. They provided an opportunity to some major pioneers among nineteenth century French geologists to understand how metamorphic rocks are formed. The intimate association between marbles and gneisses led them to propose that the initial pile of rocks was deposited as a single unit under water. They thoroughly described the attitude, folding and mineralogy of the marbles, making their works of great historical value. Originally written in French, their translation into English and the reproduction of original figures from the works they published will be very useful to the international community of geologists interested in the history of their science.     

Zircon SHRIMP U–Pb dating,C and O isotopes for impure marbles from the Jiaobei terrane in the Sulu orogen: Implication for tectonic affinity

Middle Neoproterozoic carbonates are found in the western part of Shandong Pennisula (i.e., the Jiaobei terrane) that is located in the northwestern part of the Sulu orogen in east-central China. For the first time, a successful SHRIMP U–Pb dating, coupled with CL imaging, was conducted on two samples of impure marble from the Fenzishan Group in this tectonic unit. The results yield consistent ages of 786 ± 67 and 240 ± 44 Ma for igneous and metamorphic zircons, respectively. Positive δ¹³C values as high as +5.6‰ are measured for both pure and impure marbles, consistent not only with the worldwide Neoproterozoic limestones in connection with the Sturtian ice-age, but also with the marbles associated with UHP metamorphic eclogites in the Dabie orogen. O isotope fractionation between calcite and garnet from one sample gave a temperature of 680 °C, pointing to upper amphibolite-facies metamorphic conditions. These results indicate that protolith of the marbles is a kind of limestone that was synchronously deposited with volcaniclastic rocks in the mid-Neoproterozoic rift basin of continental margin. Like the UHP metamorphic rocks in the Dabie-Sulu orogenic belt, both mid-Neoproterozoic magmatism and Triassic metamorphism are recorded in the impure marbles. Therefore, protolith of the impure marbles corresponds to the sedimentary limestone of rift basin developed during the mid-Neoproterozoic breakup of supercontinent Rodinia, but it was the sedimentary cover along the northern margin of the South China Block prior to its Triassic subduction. The occurrence of the mid-Neoproterozoic limestone with the Triassic metamorphism in the southern margin of the North China Block thus indicates tectonic overthrust by a crustal detachment between the sedimentary cover and the Precambrian basement during the continent subduction. As a result, the marbles in affinity to the South China Block were northward thrusted over the basement of the North China Block.     

两种晶粒大理岩的力学性质研究

矿物结构是影响岩石力学特性的根本因素。对晶粒0.5～1 mm细晶微风化大理岩和晶粒0.2～0.5 mm粉晶风化大理岩,进行了超声波测试和单轴、常规三轴压缩试验。粉晶大理岩晶粒尺度较小,滑移界面较多,粘接强度稍有变化就显著影响变形,即单轴压缩强度离散性较小而杨氏模量离散性较大,杨氏模量是细晶大理岩的1/3 左右,而单轴压缩强度仅低20 %。细晶大理岩的杨氏模量与围压无关,粉晶大理岩的杨氏模量随围压而增大,但进入延性变形阶段后两种大理岩的三轴强度完全相同。岩样的剪切破坏角都是随围压增大而减小,最后达到Coulomb准则确定的数值。在较高围压下,试样的承载极限与应力路径、材料强度以及内部的缺陷关系不大。晶粒的摩擦决定了大理岩的强度、变形特性。     

Metacarbonatite or marble? — the case of the carbonate,pyroxenite, calcite–apatite rock complex at Borra,Eastern Ghats,India

Carbonatites are often of economic importance, which raises the problem of distinguishing carbonatites from limestones when either are metamorphosed to high-grade marbles. They can be of similar appearance, particularly those from the Proterozoic and Archaean of the Indian Subcontinent. This study also contributes to solving the problem of determining the frequency of alkaline and carbonatitic magmatism during the early history of the Earth.The mineral assemblage of apatite–magnetite–phlogopite–calcite is common to marbles of both carbonatite and limestone origin. If pyrochlore is present that identifies the rock as carbonatite; if anorthite, fassaite, scapolite or spinel then it was formerly a limestone. If these minerals are absent, then trace element analysis can supply the critical Sr and REE data, which are both normally high in carbonatitic rocks and low in former limestones. These distinguishing factors are applied to the metamorphic carbonate, pyroxenite, calcite–apatite rock complex at Borra, Eastern Ghats, India, which has been variously interpreted as formerly a carbonatite and as a limestone. The evidence shows that the Borra rocks are meta-sedimentary.     

Fluid flow and mass transport at the Valentine wollastonite deposit, Adirondack Mountains, New York State

The Valentine wollastonite skarn in the north-west Adirondack Mountains, New York, is a seven million ton deposit which resulted from channellized infiltration of H₂O-rich, silica-bearing fluids. The wollastonite formed by reaction of these fluids with non-siliceous calcite marble. The skarn formed at the contact of the syenitic Diana Complex and was subsequently overprinted by Grenville-age granulite facies metamorphism and retrograde hydrothermal alteration during uplift. Calcite marbles adjacent to the deposit have generally high δ¹⁸O values (c. 21‰), typical of Grenville marbles which have not exchanged extensively with externally derived fluids. Carbon isotopic fractiona-tions between coexisting calcite and graphite in the marbles indicate equilibration at 675d? C, consistent with the conditions of regional metamorphism. Oxygen isotopic ratios from wollastonite skarn are lower than in the marbles and show a 14‰ variation (-1‰ to 13‰). Some isotopic heterogeneity is preserved from skarn formation, and some represents localized exchange with low-δ¹⁸O retrograde fluids. Detailed millimetre- to centimetre-scale isotopic profiles taken across skarn/marble contacts reveal steep δ¹⁸O gradients in the skarn, with values increasing towards the marble. The gradients reflect isotopic evolution of the fluid as it reacted with high δ¹⁸O calcite to form wollastonite. Calcite in the marble preserves high δ¹⁸O values to within <5 mm of the skarn contact. The preservation of high δ¹⁸O values in marbles at skarn contacts and the disequilibrium fractionation between wollastonite skarn and calcite marble across these contacts indicate that the marbles were not infiltrated with significant quantities of the fluid. Thus, the marbles were relatively impermeable during both the skarn formation and retrograde alteration. Skarn formation may have been episodic and fluid flow was either chaotic or dominantly parallel to lithological contacts. Although these steep isotope gradients resemble fluid infiltration fronts, they actually represent the sides of the major flow system. Because chromatographic infiltration models of mass transport require the assumption of pervasive fluid flow through a permeable rock, such models are not applicable to this hydrothermal system and, by extension, to many other metamorphic systems where low-permeability rocks restrict fluid migration pathways. Minimum time-integrated fluid fluxes have been calculated at the Valentine deposit using oxygen isotopic mass balance, reaction progress of fluid buffering reactions, and silica mass balance. All three approaches show that large volumes of fluid were necessary to produce the skarn, but silica mass balance calculations yield the largest minimum flux and are hence the most realistic.     

Exotic marbles on Cheshire cheese: High-street geology in Bath

Most shoppers in Bath ignore the cladding on the front of the'Co-op'. Those pausing to look probably consider it marble. In fact, it is Larvikite, a type of syenite commonly used to clad high-street stores throughout Britain. A survey of shop fronts, banks and other services in this city reveals a wide variety of exotic imported marbles and granites attached to local Bath Stone at street level. Modern stone cladding is also found to extend beyond the post-war shopping area. Stromatolitic limestones and porphyritic granites peep out beneath Victorian shop windows in Milsom Street. Moreover, not all the cladding in this street is found to be modern. Scottish and Shap Granites, introduced into Bath in the nineteenth century, clad former banks and a Victorian department store.