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The Geology of Roofing Slate
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This paper reviews the peological factors linked to the quarrying and quality of roofing slate deposits, and gives recent research results on the loss of fissility by dewatering. and the oxidation of iron sulphides. Mineralogy, resultins from the ol-iginal protolith composition and the metamorphic grade, controls the colour, brightness and durability of slate tiles. The microtexture of the rock mainly controls the split fitness of slate (fissility). The structure (folding, slaty cleavage developmcnl, So-SI angle, lack of crenulation cleavage and kink-hands, joints and faults) controls the enploitability. Additional factors, such as bed thickness and changes in facies are also important. Both very cold environments, giving frost, and arid conditions. drying the rock enough to remove water, produce a loss of fissility in slate. Ostwald-ripening by dewatering and some collapse of the crystallograph~c structure of phyllosilicates cause the loss. A mineralogical study of the iron sulphides in some Spanish slate (pyrite, pyrrhotite. marcasire), encompnssing crystal habit and size, and their biochemical alterarion processes (stain spots), allow the oxidation behaviour of different ~ l ~ t e s la be predicted. The mining and processing of slate is an industry with a world production of about one million tonnes per year, roughly valued at €370 million (Euros). The EU is the largest producer , dominated by northwestern Spain. This paper reviews the roofing slate industry and the published technical literature , discusses the main geological features relevatlt to the extraction of roofing slates: andpives a detailed description of the major slate producing area in Spain, the TI-uchas Syncline. It also discusses sotne recent research on the dehydration of slate and the oxidation of lninor sulphides, sometimes found in slate products.
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... Slate has been used for centuries as a roofing material and the extraction of slate and other fissile rocks for roofing is probably as old as the art of building with stone [1]. There are some remains of slate roofs on Roman buildings in England [2] and many historical centers of European cities include Medieval and Renaissance buildings covered with slate, such as the monastery of El Escorial in Madrid (Spain). ...
... Spanish slate deposits did not begin to be exploited industrially until the 1970s [3] in response to the exhaustion of mining resources in European quarries and the modernization of the production chain in Spain [4]. Nowadays, Spain is the principal producer of roofing slate in the world, and the main areas of exploitation are in the Variscan belt in the northwest of the Iberian Peninsula [1,5]. ...
... Mining district S1/S0 Angle Lineation L0 GLU 2 0 No trace VLU 2 0 No trace VXE 2 0 No trace CAU 3 0 Slight LAS 3 0 Slight PEB 3 0 Slight QIA 3 0 Slight VIB 3 0 Slight 3C 5 0 Slight CAM 5 0 Slight CAP 5 0 Slight CST 5 It is important to note that the colorimetric study has been carried out on freshly exposed rocks and it is well known that colors of the roofing slates can change as a result of the weathering of minerals components such as carbonates and iron sulfides. Slates with high carbonate contents, as some Spanich slates or the Ligurian slate undergo gypsification processes, which whiten the slate surface [1,32], while high contents in iron sulfides lead to oxidation processes, which stain or redden the slate [1]. However, nowadays no studies concerning the objective characterization of color in roofing slates have yet been published and therefore there are no objective data on color variation, which would be helpful in discussing the limits of acceptability for the replacement of one type of slate by another. ...
a b s t r a c t Substitution of slate roofing tiles is a conventional operation during building restoration, since tiles are very difficult to restore or clean because of the high degree of alteration they suffer. Criteria for replace-ment of historical building stones must be based on geological, geotechnical and esthetic parameters, among which color is of great importance. In this sense, this paper constitutes a comprehensive and useful colorimetric study of roofing slates from the Iberian Peninsula, for the purposes of restoration. The color of 50 commercial varieties of roofing slate mined in quarries from the 12 mining districts in the Iberian Peninsula was analyzed with a spectrophotometer device, by considering the CIELAB color space. The results of the study were used to develop a protocol for characterizing the color of roofing slate and to define the color range of roofing slate from the Iberian Peninsula. In addition, the similarities and differences in the color and microstructure of the different commercial varieties of Iberian roofing slate were established and the limit of acceptability of replacement of one type of slate by another was determined. Parameter h ab was found to be the most important CIELAB color coordinate as regards the formation of homogeneous color groups, and the specular component excluded (SCE) mode was most sensitive as regards detecting color differences between two samples.
... All images are taken in Ourense, Spain. Exploration, extraction and production processes have been greatly improved since the beginning of industrial slate mining (Lombardero et al., 2002). Mathematical models used in other fields of mining have been applied specifically to slate Taboada et al., 2005). ...
... Treatment and use of waste is another field of research which has been the object of research in recent years. Due to the characteristics of the production process, huge amounts of materials are dumped, between 85 and 90% of the slate volume extracted at the quarry (Lombardero et al., 2002). Whilst this waste is not chemically active, and does not represent an environmental risk, the dust emissions (6,7,8,9) Dimensional control Standardized dimensions of the slate tile Customer requirements EN 12326 (10) Bending strength Mechanical resistance of the tiles None EN 12326 (11) Water absorption Susceptibility to weathering by freeze-thaw Less than 0.6%, if higher must agree with freeze-thaw test EN 12326 (12) Freeze-thaw Susceptibility to weathering by freeze-thaw No significant reduction of the bending strength EN 12326 (13) Determination of calcium carbonate Occurrence of weatherable components None EN 12326 (13) Determination of elemental carbon Occurrence of weatherable components Less than 2% EN 12326 (14) Exposure to SO2 Weathering of harmful minerals No splitting or cracking of the edges and swelling, softening or surface flaking. ...
Roofing slate is one of the world’s most popular dimension rock products. This special type of slate can be split into regular and thin tiles forming an exceptional covering material. Many historical heritage buildings along Europe use slate covers. Although slate has been quarried worldwide for centuries, in the second half of the past century the production of roofing slate increased hugely, especially in Europe, where the largest outcrops known to date are located.
Despite its importance as a construction material, roofing slate has not been the target of a proportional number of scientific publications, as compared to other materials such as granite, sandstone, marble or limestone. This could be due to the general perception that roofing slate is a rather simple rock with a relatively unvarying mineralogy consisting of quartz, chlorites and mica, and having a monotonous structure dominated by the slaty cleavage and various fracture planes.
This paper deals with the main features of the roofing slate industry, i.e. petrological and mineralogical characterization, and their application on slate production, as well as additional phenomena and significant factors affecting quality rarely considered, such as weathering and durability.
... La pizarra es una roca común, producto de metamorfismo de bajo grado, distinguida por la presencia de una esquistosidad penetrativa, denominada slaty cleavage. Las pizarras de techar son rocas de grano muy fino, caracterizadas por una alta fisibilidad debida a la orientación preferencial de los filosilicatos, la cual está condicionada por el tamaño de los elementos no planares y por la homogeneidad textural de la roca (1)(2)(3)(4). Las mejores pizarras de techar tienen índices de cristalinidad de la illita (5, 6) entre 0,15 y 0,25º2θ, lo que indica condiciones metamórficas de facies de esquistos verdes (7,8). ...
... Slate is a common, widespread product of low-grade regional metamorphism, distinguished by a slaty cleavage. Roofing slates are very fine grained rocks, characterised by a high fissility due to a preferred orientation of the phyllosilicates, and conditioned by the size of the non-planar elements and by their textural homogeneity (1)(2)(3)(4). The best roofing slates have illite crystallinity values (5,6) of between 0.15 and 0.25º2θ, which indicate a geological greenschist metamorphic facies (7,8). ...
The fissility and durability of representative samples of commercial roofing slates from ten deposits in the NW of Spain have been evaluated using transmitted and reflectedlight optical microscopy, scanning electron microscopy, chemical testing and geomechanical procedures.
The dominant sulphides in the different slate samples are pyrite and pyrrhotite, the weathering potential of which can be highly variable. The dominant carbonate mineral is ankerite, which explains the low reactivity of these slates in acidic media. The minimum commercial plate thickness varies between 3.5 and 5 mm, depending on the microtexture of the rock.
The methodology used in this paper is proposed in order to eliminate the subjectivity of input data that are used in current methods of evaluation and modelling of slate deposits, thus producing an improvement in the profitability of mining operations and a reduction in waste materials.
... Spain is the main producer of roofing slate in the world, and the principal areas of exploitation are in the Hercynian areas in the northwest of the peninsula[1,2]. The presence of iron sulphides in different types of slate is currently one of the main problems facing the sector as oxidation of these compounds produces colour changes and rust marks on the slate. This defect is purely aesthetic and the slate rarely becomes deteriorated as a result of oxidation. ...
... The area affected by pyrite oxidation after the SO 2 test was 5 times greater than before the test, and was between 1.5 and 2 times larger than the area affected by the thermal cycle test (significant difference, p < 0.001), which indicates that the conditions generated in this test are more favourable to oxidation (Fig. 5). During this test an acidic environment is created that may be partially neutralized by mineral weathering, mainly of carbonates[2], however extremely acidic conditions may be produced on the surface of the slate, thus favouring the presence of Fe 3+ and an increase in the velocity of oxidation. Despite this, the results of this test also demonstrated the protective effect induced by the treatments with sodium silicate and potassium phosphate. ...
The aim of the present study was to evaluate the efficacy of different methods of sulphide microencapsulation in terms of inhibiting sulphide oxidation in roofing slate. For this, a preliminary test was carried out with ground pyrite and the methods that provided the best results were applied to samples of roofing slate. Protection against oxidation was measured by the amount of iron released after enforced oxidation with H2O2 in the tests with pyrite, and by thermal cycle alteration, SO2 and saline spray tests (EN 12326-2:2000 and EN 14147:2004) in the tests with the slate samples. The results indicate that treatments with potassium phosphate and potassium silicate, proposed for controlling acid mine drainage, were the most effective at protecting pyrite against oxidation, and that these methods are also effective when applied to slate.
... The 432 existence of water in its interlayered space is not generally 433 accepted, but some authors (Brown and Norrish 1952;Gaudette 434 et al. 1966;Gualtieri et al. 2008;Nieto et al. 2010) in the interlayered spaces of illite, according to Gaudette et al. 439 (1966), would be much higher (between 100 and 200°C) than 440 those reached by slate exposed to air at the quarry. 441 2. The XRD data presented here did not reproduce the large 442 deformations of the 10-Å peak of illite-chlorite as reported 443 by García-Guinea et al. (2000) and Lombardero et al. (2002). 444 Moreover, as the values of these displacements were limited 445 to thousandths of Å, it can be concluded that there were no sig-446 nificant changes in this peak. ...
... This foliation is microscopically defined by stacking of mica layers. Slate is formed under low or very low metamorphic grade conditions (Kubler Index 0,25), with pressures of 2-3 kbar and temperatures of 300-400°C (Lombardero et al., 2002). These values are included in the range of the metamorphic facies of the greenschists. ...
The European norm UNE 12326 for roofing slate defines the mechanical properties of a slate tile through the Bending Strength test, which defines the modulus of rupture (MoR) of the tile. This MoR is related to the durability of the slate, and is one of its main commercial quality properties. During the prospection task previous to open a new slate quarry, it is crucial to define the MoR. The Bending Strength test needs proper slate tiles and a flexion press. To supply these slate tiles it is frequently necessary to open experimental quarry fronts, which are very expensive, and do not guarantee the viability of the quarry. Therefore, a method which could reduce the costs and time for the mechanical characterization would be very useful. In this work we have tried to relate the MoR to the mineralogical and petrographic characteristics of slate, using the Petrographic Examination test, also part of UNE 12326. This test contemplates the finding of a numerical index called Mica Stacking Index (MSI) which quantifies the mica layers fabric. We collected fifteen samples of roofing slates from the Northwest of the Iberian Peninsula and we studied them by using the Bending Strength and Petrographic Examination tests, plus mineralogical quantification using X-ray diffraction and measure of the illite crystallinity (Kubler Index) to determine the metamorphic grade. We have found a strong positive correlation between MoR values and MSI, and a negative correlation between MoR and Mica percentages. These correlations may be used to deduce the MoR, reducing costs during the exploitation labours in roofing slate mining, and also can be used in the quality controls on a working slate quarry.
... In its product specification section, the Spanish UNE-EN12326-1 standard [1] defines the defects that characterize roofing slate. Roofing slate quality, furthermore, has been the subject of a number of studies [2,3]. The defects analysed in this study are described as follows: ...
Dimensional control based on 3D laser scanning techniques is widely used in practice. We describe the application of a hybrid 3D-2D laser scanning system to the characterization of slate slabs with structural defects that are difficult for the human eye to characterize objectively. Our study is based on automating the process using a 3D laser scanner and a 2D camera. Our results demonstrate that the application of this hybrid system optimally characterizes slate slabs in terms of the defects described by the Spanish UNE-EN 12326-1 standard.
Slate and zinc have traditionally been employed on the roofs of historical heritage as roofing materials. This is the case of the roof of the Church tower of Saint Mary of Aranda de Duero (Burgos), upon which in undated repair work, the original slate roof was replaced by zinc. The roof material was highly deteriorated due to seasonal weathering and the proximity of the chimney used for the evacuation of combustion gases from the boiler of the heating system of the temple. As part of the church roof restoration process, a comparative study of the durability of zinc and slate roof tiles was completed, by exposing them to laboratory tests in a Kesternich chamber with SO2 gas, with the objective of determining which material best withstands these aggressive pollutant agents and meteorological weathering. The results of the tests showed that the slate had a better behavior than the zinc roof tile. Moreover, the most faithful image of the original monument is restored with the slate roof, at the same time as using a material with very favorable characteristics for improving the durability and the conservation of the built heritage. So, this study sets out the modus operandi used for the analysis and the diagnosis of the deterioration process of an historical monument, through objective study procedures that followed a multidisciplinary approach, in such a way that the technical experts in charge could take the most suitable decisions for the conservation of the architectural heritage.
We present the results of an application of Bayesian networks to the evaluation of the quality of roofing slate. Using data from borehole samples of a slate deposit, two networks constructed with different levels of expert knowledge input were evaluated for their capacities for inference and prediction of the quality of slate for roofing.It can be concluded from the results that Bayesian networks are an extremely useful automated tool for evaluating the quality of a resource such as slate for the following reasons: they allow final quality to be assessed immediately and in probabilistic terms with a tolerable degree of uncertainty; they enable the probability of obtaining different final qualities to be estimated when new data is introduced into the network; they speed up the evaluation process by simulating and optimising the work of the expert (during field data collection and borehole description) in identifying the parameters with the greatest influence on final quality; and finally, they have a satisfactory capacity for prediction.
Raman spectroscopy was applied to the microbially mediated dissolution of pyrite by high density (more than 109 cells) of Thiobacillus ferrooxidans. In the range 600-200 cm-1, peaks corresponding to secondary minerals such as elemental sulfur and jarosite were observed in addition to those of pyrite. Small amounts of elemental sulfur and poorly crystalline jarosite were detected by Raman spectroscopy, but are undetectable by X-ray diffraction and Fourier transform infrared spectroscopy. Ammoniojarosite was predominantly formed in microbially mediated dissolution of pyrite with a high density of cells, whereas jarosite was formed in the sterilized control. This finding is probably ultimately due to the much greater amount of jarosite precursors (Fe3+ sulfate complexes) formed by the bacterial oxidation of iron. Initially, and in both cases, the kinetically favored jarosite appeared, but in the presence of bacteria, there was so much precursor present that the available potassium was exhausted. Ammoniojarosite subsequently was formed. In the sterilized control, precursors were less readily formed, and jarosite formed. There was no evidence for direct involvement of bacteria in the formation of jarosite. The changes in mineral compositions are correlated with those in the solutions. Raman spectroscopy was used to follow compositional changes in minerals during the microbially mediated dissolution of pyrite.
The mineralogy and microstructure, which affect the slaty cleavage, are linked with the strong preferred orientation of phyllosilicates and this enables the rock to be split into large, thin, flat sheets. Roofing slate samples with different commercial fissilities have been analyzed by radioluminescence (RL), thermoluminescence (3DTL), by X-ray diffraction (XRD), by scanning electron microscopy (SEM) using the back-scattered mode (BSEI) and by electron microprobe (EMP). They are made up of white micas, chlorite, quartz, detrital feldspars, ilmenite, pyrite, rutile apatite and tourmaline. Texturally, all consist of silt-sized clasts of detrital quartz, feldspars, chlorite-mica stacks, muscovite and ilmenite in a recrystalline, lepidoblastic matrix of white micas and chlorite with quartz lenses, all showing a very strong preferred orientation. The luminescence emission centers are a low broad blue band around the 400 nm spectra positions linked with alkali losses and formation of [AlO<sub>4</sub>]º defects; a peak at 473 nm interpreted as a the first thermal step (150-300ºC) of a non-isothermal dehydroxylation of the slate phyllosilicates; and a 568 nm peak which agrees with Mn<sup>2+</sup> point defects in aluminosilicate lattices. The studies on the slaty cleavage could be significant because Spain is the largest producer of roofing slate tiles in the world (87% of world production).
La exfoliación de las pizarras depende fundamentalmente de su mineralogía y microestructura, especialmente de la fuerte orientación de los filosilicatos. Esta propiedad permite hendir o abrir las pizarras de techar en láminas muy grandes, delgadas y planas. Se han analizado varias pizarras de techar con diferentes calidades comerciales, correspondientes a diferentes grados de físibilidad, por radioluminiscencia (RL), termoluminiscencia espectral (TL3D), difracción de rayos X (DRX), microscopía electrónica de barrido (MEB) utilizando el modo backscattered (BSEI) y microsonda electrónica (ME). Las muestras están compuestas por micas blancas, clorita, cuarzo, feldespatos detríticos, ilmenita, pirita, rutilo, apatito y turmalina. Texturalmente, se trata de fenocristales de cuarzo, feldespatos, interestraficados clorita-mica, moscovita e ilmenita dentro de una matriz microcristalina de micas blancas y clorita con lentejones de cuarzo, todo ello mostrando una fuerte orientación preferencial. Los centros luminiscentes corresponden a una pequeña banda de emisión espectral alrededor de 400 nm asociada a pérdidas alcalinas y formación de centros [AlO<sub>4</sub>]º; un pico a 473 nm interpretado como un primer escalón termal (150-300°C) en un proceso de deshidroxilación no-isotermal característicos de los filosilicatos y un pico a 568 nm que corresponde con defectos puntuales de Mn<sup>2+</sup> en redes de aluminosilicatos. Este tipo de estudios sobre fisibilidad pueden ser interesantes porque España es el mayor productor de pizarras de techar (un 87% de la producción mundial).
Seven strains of heterotrophic iron-oxidizing acidophilic bacteria were examined to determine their abilities to promote oxidative dissolution of pyrite (FeS2) when they were grown in pure cultures and in mixed cultures with sulfur-oxidizing Thiobacillus spp. Only one of the isolates (strain T-24) oxidized pyrite when it was grown in pyrite-basal salts medium. However, when pyrite-containing cultures were supplemented with 0.02% (wt/vol) yeast extract, most of the isolates oxidized pyrite, and one (strain T-24) promoted rates of mineral dissolution similar to the rates observed with the iron-oxidizing autotroph Thiobacillus ferrooxidans. Pyrite oxidation by another isolate (strain T-21) occurred in cultures containing between 0.005 and 0.05% (wt/vol) yeast extract but was completely inhibited in cultures containing 0.5% yeast extract. Ferrous iron was also needed for mineral dissolution by the iron-oxidizing heterotrophs, indicating that these organisms oxidize pyrite via the "indirect" mechanism. Mixed cultures of three isolates (strains T-21, T-23, and T-24) and the sulfur-oxidizing autotroph Thiobacillus thiooxidans promoted pyrite dissolution; since neither strains T-21 and T-23 nor T. thiooxidans could oxidize this mineral in yeast extract-free media, this was a novel example of bacterial synergism. Mixed cultures of strains T-21 and T-23 and the sulfur-oxidizing mixotroph Thiobacillus acidophilus also oxidized pyrite but to a lesser extent than did mixed cultures containing T. thiooxidans. Pyrite leaching by strain T-23 grown in an organic compound-rich medium and incubated either shaken or unshaken was also assessed. The potential environmental significance of iron-oxidizing heterotrophs in accelerating pyrite oxidation is discussed.
In the case of pyrite bioleaching by Leptospirillum ferrooxidans, an organism without sulfur-oxidizing capacity, besides the production of tetra- and pentathionate, a considerable accumulation of elemental sulfur occurred. A similar result was obtained for chemical oxidation assays with acidic, sterile iron(III) ion-containing solutions. In the case of Thiobacillus ferrooxidans, only slight amounts of elemental sulfur were detectable because of the organism's capacity to oxidize sulfur compounds. In the course of oxidative, chemical pyrite degradation under alkaline conditions, the accumulation of tetrathionate, trithionate, and thiosulfate occurred. The data indicate that thiosulfate, trithionate, tetrathionate, and disulfane-monosulfonic acid are key intermediate sulfur compounds in oxidative pyrite degradation. A novel (cyclic) leaching mechanism is proposed which basically is indirect.
Sommaire
Dans certains schistes ardoisiers les minéraux possèdent, dans le plan de schistosité, une direction préférentielle d'allongement déterminant une microlinéation visible seulement au microscope. La méthode de mesure proposée consiste à définir, sur des lames minces orientées, la direction statistique d'extinction des minéraux grâce à un dispositif photométrique. Sur un exemple pris dans Massif armoricain, il est montré que la microlinéation ainsi déterminée est parallèle aux axes B des mégastructures dans la région considérée.
Studies have been made in two laboratories on metapelites from the Manx Group, Isle of Man, showing the distribution of white mica "crystallinity', using the Kubler index (Δ°2θ) and the Weber index (Hb rel). Two isocryst maps, which show broadly similar distributions of metamorphic grade, are illustrated and a correlation between the laboratories is presented. The mineralogy of the < 2 μm fraction in relation to "crystallinity' has been determined. The distribution of metamorphic grade over central and eastern parts of the Isle of Man, as depicted by the Kubler index isocryst map, is related to subjacent granite plutons, but these contact metamorphic effects overprint an earlier, largely anchizonal, regional metamorphism. Localized, late, retrograde effects are related to low temperature hydrothermal mineralization and to cold re-working in a large fault zone. -from Authors
The approach used most commonly to prevent pyrite oxidation in the field is the use of limestone. This approach, however, has a short span of effectiveness because the pyrite particle surfaces remain exposed to the atmospheric O2 after treatment. The objective of this study was to examine the feasibility of controlling pyrite oxidation in bench-scale studies by creating a ferric hydroxide-silica coating that would prevent either O2 or Fe3+ from oxidizing pyrite further. Ferric hydroxide-silica coating formation involved leaching pyrite at room temperature using a 10-mm-i.d. chromatographic column with a solution containing H2O2, Na-acetate (NaOAC), and soluble silicate at a flow rate of 0.43 mL min-1. The results of this bench-scale study show that formation of a ferric hydroxide-silica coating was induced on the pyrite surface and that it inhibited pyrite oxidation under acid conditions.