Pyrite

Summary

The mineral pyrite (ˈpaɪraɪt), or iron pyrite, also known as fool's gold, is an iron sulfide with the chemical formula FeS2 (iron (II) disulfide). Pyrite is the most abundant sulfide mineral. Pyrite's metallic luster and pale brass-yellow hue give it a superficial resemblance to gold, hence the well-known nickname of fool's gold. The color has also led to the nicknames brass, brazzle, and brazil, primarily used to refer to pyrite found in coal. The name pyrite is derived from the Greek πυρίτης λίθος (pyritēs lithos), 'stone or mineral which strikes fire', in turn from πῦρ (pyr), 'fire'. In ancient Roman times, this name was applied to several types of stone that would create sparks when struck against steel; Pliny the Elder described one of them as being brassy, almost certainly a reference to what is now called pyrite. By Georgius Agricola's time, 1550, the term had become a generic term for all of the sulfide minerals. Pyrite is usually found associated with other sulfides or

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Sulfidic mine tailings have to be classified as one of the major source of hazardous materials leading to water contamination. This study highlights the processes leading to sulfide oxidation and acid mine drainage (AMD) formation in the active stage of two tailings impoundments located in the southern part of the Apuseni Mountains, in Romania, a well-known region for its long-term gold-silver and metal mining activity. Sampling was undertaken when both impoundments were still in operation in order to assess their actual stage of oxidation and long-term behavior in terms of the potential for acid mine drainage generation. Both tailings have high potential for AMD formation (2.5 and 3.7 wt.% of pyrite equivalent, respectively) with lesser amount of carbonates (5.6 and 3.6 wt.% of calcite equivalent) as neutralization potential (ABA = -55.6 and -85.1 tCaCO(3)/1000 t) and showed clear signs of sulfide oxidation yet during operation. Sequential extraction results indicate a stronger enrichment and mobility of elements in the oxidized tailings: Fe as Fe(III)oxy-hydroxides and oxides (transformation from sulfide minerals, leaching in oxidation zone), Ca mainly in water soluble and exchangeable form where gypsum and calcite are dissolved and higher mobility of Cu for Ribita and Pb for Mialu. Two processes leading to the formation of mine drainage at this stage could be highlighted (1) a neutral Fe(II) plume forming in the impoundment with ferrihydrite precipitation at its outcrop and (2) acid mine drainage seeping in the unsaturated zone of the active dam, leading to the formation of schwertmannite at its outcrop. (C) 2011 Elsevier B.V. All rights reserved.

2011

The main objective of this thesis is to improve the understanding of the position and characteristics of karst conduits within a rock massif. Such a characterisation is an important issue in civil engineering and in hydrogeology. Today in practice dissolution voids are considered as random in most cases. However, it is obvious for karst researchers that dissolution void distribution is not random, but defined by parameters controlling the speleogenesis. We developed a method to analyse the 3D geometry of cave systems in order to demonstrate from a statistical point of view that karst conduits position is not random. The analysis of several among the largest cave systems in the World (more than 1500 km of analysed cave conduits) confirmed for the first time quantitatively that the development and position of karst conduits under phreatic conditions is strongly related to a restricted number of so called inception horizons. An inception horizon – a concept introduced by Lowe (1992) – is a part of a lithostratigraphic succession that is particularly susceptible to the effects of the earliest cave forming processes by virtue of physical, lithological or chemical deviation from the predominant carbonate facies within the formation. We demonstrate that probably less than 10 % of the existing bedding partings of a limestone sequence are inception horizons and guide more than 70 % of the phreatic conduits. Our analysis clearly confirms that the influence of these horizons onto the 3D geometry of cave systems is high. Based on the 3D analysis of cave systems as well as on field verifications, 18 inception horizons in six cave systems have been selected for field characterisation and sampling in order to identify the properties and processes that makes these particular lithostratigraphic horizons favourable to karstification. Around 200 rock samples from the horizons and the surrounding rock mass have been analysed. The results evidence that inception horizons have a thickness of some centimetres to decimetres and that it is possible to distinguish between 3 types of inception horizons: Inception horizons where the cave inception took place within the inception horizon (type 1); characterized by a slightly higher primary permeability, pyrite and quartz contents and lower matrix contents than the surrounding rock mass. Usually, fractures propagate through or occur within these horizons. Inception horizons where the cave inception took place at the contact with the inception horizon (type 2); characterized by a lower primary permeability and carbonate contents, but higher pyrite contents than the surrounding rock mass. Fractures usually are ending at these horizons. Inception horizons where the cave inception took place along bedding plane fractures (type 3); already a slippage of just a few millimetres, striation, brecciation and surface irregularities enhance openings along the sliding plane and cause a significant increase in permeability. Furthermore it can be assumed that for the inception of horizons of type 1 the primary permeability will be the relevant factor at beginning of karstification, whereas both the matrix and the pyrite contents are the key factors during the later phases of cave inception and gestation. Whereas for further cave development the total carbonate content will be crucial. For inception horizons of type 2, we can assume that the low primary permeability, the clogging of the pores by the clay minerals, the high content of pyrite (production of aggressive solutions within the horizon that concentrates the dissolution along the contact to the surrounding rock) and the ending of the fractures at the horizon are responsible for the enhanced karstification at the contact during cave inception and gestation phases. Using simple hydrogeological numerical modelling we show that an epigenic karstic rock massif can be subdivided into four speleogenetic zones: 1) vadose cave development zone above the water table, 2) the phreatic cave development and 3) gestation zone within the first tens of metres of the phreatic zone and 4) below them the inception zone. Each of these zones is characterized by typical speleogenetic processes as well as dissolution void distribution. Further, it was possible to explain and reproduce schematically the 3D pattern of different cave systems by using the position and orientation of the inception horizons and the history of the landscape evolution (i.e. the re- and discharge area). This forward analysis provides a first idea of the geometry of the conduits as well as a better understanding of the development of a karst system in time and space (vertical section). Finally, we evaluated the feasibility to combine the improved inception horizon hypothesis, to predict inception horizons, with other current applied methods to improve the prediction of dissolution voids. Furthermore, we proposed a scientific based risk assessment for underground engineering proposes. Essentially, it is now evidenced that it is possible to quantify the probability of karst occurrences inside a karst massif by reconstructing the hydrogeolgical history and identifying the few inception horizons that guide the karstification at a regional scale.

EPFL2009

Natural cement and monumental restoration

Christophe Gosselin

Natural cement, called "Roman" cement, was invented at the end of the 19th century and played an important role in the development of civil engineering works until the 1860s. More surprisingly, it was also used to restore historic buildings, such as gothic cathedrals. This paper deals with the mineralogy and the durability of natural cement in the particular case of the Bourges Cathedral in France. This study illustrates the interest of this material particularly adapted in stone repair or substitution. Contrary to traditional mortars, the present samples are made of neat cement paste, revealed by the absence of mineral additions as quartz or carbonate sand. Several combined techniques (SEM-EDS, TGA, XRD) were carried out to determine the composition of the hydraulic binder rich in calcium aluminate hydrates. The raw marl at the origin of the cement production contains oxidized pyrites which consist in a potential source of sulphate pollution of the surrounding limestone. The exposition of the cement in urban environment leads to some weathering features as atmospheric sulfation. Finally a petrophysical approach, based on water porosity, capillary sorption and compressive strength, has been performed to demonstrate the durability and the compatibility of natural cement applied as an historical building restoration mortar.

Springer Netherlands2009