Search

Glauberite (Na2Ca[SO4]2) is an evaporitic mineral which is used in the industries of detergents, paper, glass, pharmacy, etc. Glauberite rocks are seldom found cropping out because they are very sensitive to weathering processes; for this reason their prospection is conducted by means of boreholes. Nowadays, geophysical techniques are not used to support the characterization of glauberite deposits due to the lack of knowledge of their physical properties. In this study geoelectrical methods are proposed as alternative techniques in the early stages of glauberite prospecting. Several glauberite units have been studied in different parts of the Ebro basin (Spain) by means of electrical resistivity tomography sections. The electrical resistivity range showed by glauberite deposits has been found to be low (10–100Ω·m) when the matrix component (clay and microcrystalline carbonates) is above 45% of the bulk composition of the rock. This type of rocks has been studied in Montes de Torrero (Zaragoza) and is the most common glauberite deposit case. Besides matrix-rich glauberite rocks, an exceptional case of a pure glauberite layer has been studied in Alcanadre (La Rioja). From this site, it has been estimated that deposits with glauberite crystal fraction close to 100% show a resistivity range of at least 3×103 Ω·m. Using this extreme value as reference, the Hashin–Shtrikman bounds have been calculated for glauberite rocks considering that they are constituted of four phases (glauberite, gypsum, anhydrite and matrix). When the matrix fraction represents 45% or more of the bulk rock, the resistivity range will be that of the lower Hashin–Shtrikman bound, which is similar for any combination of sulfate (glauberite, gypsum and/or anhydrite) composition; hence, it can be considered as a two-phase system (matrix and sulfate). For rocks with less than 30% of matrix fraction, the upper Hashin–Shtrikman bound trend must be considered; however, the resistivity values overlap, making it impossible to establish a classification. Between 30 and 45% of matrix fraction, there is a transitional domain. Additionally, some theoretical models representing the most common structures in sulfate rocks have been elaborated in order to help in the interpretation of the inverted resistivity images obtained from the field data. Some artifacts generated by the complexity of the resistivity distribution of the terrain have been identified in both data sets. [ABSTRACT FROM AUTHOR]

Alabaster is a white and pure gypsum rock classically used in sculptures or for ornamental purposes, but its characteristic white colour is sometimes masked by the development of noticeable reddish stains over the surface of the rock. The main altarpiece of the Santa Maria de Poblet monastery (Tarragona, NE Spain; 16th century, Damià Forment) is the focus of this study. Red-to-ochre patinas with diameters of 2–20 cm are found on the alabaster surface of this altarpiece. The origin of such patinas are discussed, as they are the result of degradation processes of minerals present in alabaster rocks, contrary to what happens in the formation of most patinas. The patinas consist of two differentiated layers that may occur occasionally mixed. The Lower Layer contains iron compounds, which have precipitated around the gypsum crystals of the rocky support and have provided the characteristic red-to-ochre colour to the surface of the altarpiece. The formation of this layer was driven by the (bio)oxidation of the pyrite, which is disseminated over the alabaster surface. The formation of this film of iron-rich particles was conducted by a series of destructive and penetrative processes, promoting disaggregation and crystal reduction (mechanical and/or by dissolution) of the matrix minerals (gypsum, celestite, barite, calcite-dolomite…). The Upper Layer was grown by means of constructive (agglutination of particles by accretion) and destructive (destruction of the alabaster matrix and incorporation into the Lower Layer) mixed processes. Thus, the presence of small crystals (1–10 μm) of gypsum, quartz, calcite, celestite, barite, clay minerals and pyrite in this upper patina are mainly related to the residual products of the alabaster rocks. Moreover, some of the calcite, quartz and clay particles could also be considered atmospheric dust. Oxalates (weddellite and whewellite), portlandite and coal particles are not related to the formation of iron-rich patinas. Such studied patinas might date from the 19th century due to the partial destruction of the monastery after the approval of the Law of Confiscation of Religious Properties in Spain (1835) and no later than the end of the ninetieth century, when the monastery was abandoned. The entry of rainwater and presence of humidity inside the monastery would promote the pyrite oxidation processes. Currently the patinas seem to be stabilized, the pyrites are no longer in contact with water, which is needed for oxidation. [ABSTRACT FROM AUTHOR]