Mineral and textural transformations in aluminium-rich clays during ceramic firing

The aim of this study has been to analyse the mineralogical and textural transformations of a set of aluminium-rich shales of interest for refractory and ceramic uses, fired from 800 °C to 1300 °C. To that end, raw and fired samples were analysed by X-ray diffraction, transmitted light microscopy, field emission scanning electron microscopy, and transmission electron microscopy. Raw samples comprise variable proportions of illite, pyrophyllite, orthoclase, quartz, kaolinite, mixed-layer I-Sm, and organic matter. At temperatures below 800 °C, kaolinite, mixed-layer I-Sm, and organic matter are destabilized, indicating that they are the least stable phases in the firing process. Illite, pyrophyllite, and orthoclase remain until 1000 °C and show a broader stability field during firing than in natural environments. Quartz persists throughout the entire firing process, although it is partly replaced by vitreous phase. Hematite crystallizes at 900 °C. Vitrification begins at 1000 °C, marking the first significant textural change. From 1000 °C mullite starts to crystallize from the Si- and Al-rich vitreous phase. The mullite composition is not stoichiometric and probably as temperature increases Si is partially replaced by Al, Fe and Ti in the structure. Nevertheless, with the increase of the firing temperature, the mullite composition is closer to the theoretical composition and also to that of natural mullites. Furthermore its crystal thickness increases with temperature up to 70 nm

Transformaciones minerales durante los procesos de cocción de arcillas alumínicas y su relación con propiedades físicas de interés

Mineralogical and textural transformations and changes in physical properties of three shales fired from 800 to 1300ºC have been studied. Initially these samples were formed by variable proportions of illite, pyrophyllite, orthoclase, quartz, kaolinite, mixed layers I/S and organic matter. Raw and fired samples were analysed by X-ray diffraction, transmitted light microscopy, field emission scanning electron microscopy and transmission electron microscopy. Kaolinite, mixed layers I/S and organic matter were destabilized at temperatures below 800°C indicating that they are the least stable phases to the firing process. Illite, pyrophyllite and orthoclase remain until 1000ºC and they show a larger stability field during the ceramic process than in natural environments. Quartz remains during all the firing process, although it is partly replaced by vitreous material. When temperature increases, hematite and mullite crystallize and the vitrification starts from 1000ºC. This vitreous phase is the precursor of mullite. Changes in colour (grey-orange-brown) are due to the loss of organic matter, to the formation of hematite and its subsequent destabilization. Variations in linear contraction, density, total porosity and in average pore size are related to the destabilization of the different phases, the progress of vitrification and the crystallization of mullite

Mineralogical and Sedimentological Characterization of the Clay-Rich Sediments from Ases Cave (Cova Dets Ases, Mallorca, Spain): Origin and Classification

The Mallorca coastal caves present large amounts of speleothems that have been studied for decades. However, the sedimentary deposits also present in these cases have not been given the same attention. This work is the first study entirely focused on these deposits, specifically the ones found in the Ases cave. These deposits are formed by clay minerals (illitic phases, kaolinite, smectite, and chlorite), calcite and quartz, and minor proportions of dolomite, albite, orthoclase, hematite, and goethite. The grain size and the electron microscopy studies suggested the presence of different sedimentation processes (bedrock degradation, creep or saltation, and suspension) and different origins (authigenic and detrital origins) for the different sediments. Based on these differences, two types of deposits were characterized: autochthonous and allochthonous deposits. The first ones are located on the floor of chambers and corridors in subaqueous zones, indicating the stability of the mixing zone (and therefore the sea level) over time. The second ones appear filling voids on the walls and the ceiling in the terrestrial zone, evidencing the filling of the cavity in the presence of water (during a wet period). These results are very important to complete the understanding of the caves and their evolution and support the relevance of these materials in paleoenvironmental studies

Depósitos silexíticos en el terciario continental de la Cuenca de Calatayud: Caracterización y génesis

The tertiary Calatayud Basin is a NW-SE elongated depression, extending from Calatayud (Zaragoza) to Montalbán (Teruel) and whose dimensions are approximately 130 km long and 20 km wide. This basin consists of two distinct sub-basins separated by the Daroca High (Calatayud Basin in the northern sector and Montalbán Basin in the sourthern sector). These Basins present a quite similar sedimentary evolution of the Neogene evaporitic and carbonatic lacustrine systems, that generally ocuppy central locations in both basins. Three main sedimentary units (Lower, Intermediate and Upper units) divided by two main sedimentary breaks are traditionally described in the Calatayud Basin. The lower and the Intermediate units have evaporitic sedimentation, whilst the Upper Unit is typically freshwater fluvio-lacustrine sedimentation. This work focuses on the study of carbonatic materials placed in the Cristo de Ribota hermitage area, which is localized in the Intermediate Unit of Calatayud Basin, to the Northern of Sierra de Armantes. The interest of this area lies in the presence of silexitic levels, whose thickness can be metric, exceptional in the geologic record and that it raises interesting troubles at the time of establishing a hypothesis on the possible origin of the silica that generates these sílex deposits, as well as in the establishment of a genetic model. The sílex levels in the Cristo de Ribota series appear mainly in carbonate rocks. Petrographic studies of the samples taken in this area show a replacement of silica by carbonate, as well as a silica cementation in karts cavities generated by epidiagenetic karts processes. These cavities acted as conduits for fluids saturated in silica which resulted in silex deposits. The fluids should necessarily have an episodic character and the fact that karstic cavities can be filled with silica or carbonate is an evidence of it. For the silica precipitation, not only the low temperature and a less than 9 pH are necessary, but also there must exist a lot of silica-supersatured water, as well as a high permeability. The source of silica deposits in this area is probably diagenetic, but also it would be necessary to consider a thermal origin corresponding to the contribution of silica by hydrothermal fluids

¿Cómo se abordan los minerales en la enseñanza obligatoria? Análisis del modelo de mineral implícito en el currículo y en los libros de texto en España

En la actualidad, los minerales forman parte de nuestro día a día. Sin embargo, a pesar de su relevancia, el interés que presentan los estudiantes sobre este tema suele ser reducido. Diversos autores señalan que puede deberse al tratamiento que reciben en el aula, bastante anquilosado y alejado de la realidad del alumnado. En este estudio se propone un modelo de mineral a partir del modelo de referencia y los modelos derivados de los currículos o ciales de Educación Primaria y Secundaria Obligatoria (modelo escolar) que incluye las relaciones que existen entre las propiedades, el ambiente de formación y los usos de los minerales. Una vez de nidos los modelos, analizamos a partir de éstos, cómo tratan el tema de los minerales los libros de texto que se utilizan en las aulas con la ley de educación vigente (LOMCE). En dicho análisis, se busca identi car qué tipo de relaciones intrínsecas al modelo de mineral promueven adquirir entre los estudiantes dichos manuales. Encontramos que existe una carencia de propuestas que permitan al alumnado establecer dichas relaciones lo que hace que, tras nalizar el sistema educativo, solo puedan adquirir un modelo incompleto del mismo

A time-space window between Eocene karst bauxite genesis and the first molasse deposition in the Dinaric Foreland Basin in the North Dalmatia, Croatia

Karst bauxite deposits in the North Dalmatian piggyback basin (NDPGB) are a part of the Mediterranean bauxite belt, which is the largest European bauxite deposit zone; however, there is a general lack of information regarding the genesis, age, and precursor of the bauxite deposits in this region. In this study, we combined detrital zircon U–Pb geochronology with compositional, mineralogical, and morphological data from four bauxite locations in the NDPGB to provide a new palaeogeographical and palaeoenvironmental evolution model for the Lutetian–Rupelian timeframe of the NDPGB. The Eocene climatic conditions began with the Palaeocene–Eocene Thermal Maximum event (∼56 Ma), followed by the Early Eocene Climatic Optimum (∼49 Ma) and Middle Eocene Climatic Optimum (∼40 Ma), and were completed as a cooling trend culminating around the Eocene/Oligocene boundary (∼34 Ma), with a shift towards an icehouse climate. These events were coeval with the continuous drift of the African continent towards Eurasia and the subsequent closure of the western part of the former Neo-Tethys Ocean associated with massive volcanic activity. Based on the bauxite deposits of the NDPGB, Early Eocene limestones formed in the last phase of the long-lasting Adriatic Carbonate Platform. The Middle Eocene orogenic activity resulted in an elevation in this area. High average temperatures, accelerated hydrological cycles and precipitation, and intensive continental weathering with increased volcanic carbon input resulted in favourable conditions for the development of karst bauxites at this time. Further Upper Eocene tectonic deformation of the NDPGB area resulted in the development of bauxite traps and enabled redeposition of the initial bauxite material. Subsequently, the bauxite deposits were covered with clastic carbonate molasse derived from the intensive erosion of the young Dinaric orogeny. The implications of this study are as follows. First, it provides new information on the timing of bauxitisation in the area by providing the first radiometric zircon geochronology, which refined and restricted the time window for bauxite formation in this region. Additionally, our results provide a new perspective on the possibility of aeolian precursors in karst bauxite formation and provide new constraints on the first tectonic marks of the initial Dinaric orogeny

A time-space window between Eocene karst bauxite genesis and the first molasse deposition in the Dinaric Foreland Basin in the North Dalmatia, Croatia

Karst bauxite deposits in the North Dalmatian piggyback basin (NDPGB) are a part of the Mediterranean bauxite belt, which is the largest European bauxite deposit zone; however, there is a general lack of information regarding the genesis, age, and precursor of the bauxite deposits in this region. In this study, we combined detrital zircon U–Pb geochronology with compositional, mineralogical, and morphological data from four bauxite locations in the NDPGB to provide a new palaeogeographical and palaeoenvironmental evolution model for the Lutetian–Rupelian timeframe of the NDPGB. The Eocene climatic conditions began with the Palaeocene–Eocene Thermal Maximum event (∼56 Ma), followed by the Early Eocene Climatic Optimum (∼49 Ma) and Middle Eocene Climatic Optimum (∼40 Ma), and were completed as a cooling trend culminating around the Eocene/Oligocene boundary (∼34 Ma), with a shift towards an icehouse climate. These events were coeval with the continuous drift of the African continent towards Eurasia and the subsequent closure of the western part of the former Neo-Tethys Ocean associated with massive volcanic activity. Based on the bauxite deposits of the NDPGB, Early Eocene limestones formed in the last phase of the long-lasting Adriatic Carbonate Platform. The Middle Eocene orogenic activity resulted in an elevation in this area. High average temperatures, accelerated hydrological cycles and precipitation, and intensive continental weathering with increased volcanic carbon input resulted in favourable conditions for the development of karst bauxites at this time. Further Upper Eocene tectonic deformation of the NDPGB area resulted in the development of bauxite traps and enabled redeposition of the initial bauxite material. Subsequently, the bauxite deposits were covered with clastic carbonate molasse derived from the intensive erosion of the young Dinaric orogeny. The implications of this study are as follows. First, it provides new information on the timing of bauxitisation in the area by providing the first radiometric zircon geochronology, which refined and restricted the time window for bauxite formation in this region. Additionally, our results provide a new perspective on the possibility of aeolian precursors in karst bauxite formation and provide new constraints on the first tectonic marks of the initial Dinaric orogeny

High-Temperature Mineral Phases Generated in Natural Clinkers by Spontaneous Combustion of Coal

The aim of this study is to analyze natural clinkers (= calcined clays by coal combustion) from a lower Cretaceous coal outcrop in Ariño (Teruel, NE Spain) in order to describe mineral and textural transformations produced during the spontaneous combustion of coal. To achieve this aim, samples were analyzed using X-ray diffraction and optical and electron microscopy. Spontaneous combustion resulted in the melting of the surrounding clays, with the generation of an Al–Si-rich vitreous phase. Subsequently, high-temperature phases crystallized from this vitreous phase. These new minerals are interesting due to their similarity with those formed during ceramic processes, used in the manufacture of stoneware and ceramic tiles, as well as in refractory ceramics, and with natural events such as metamorphic and igneous processes. The studied natural clinkers are composed of vitreous phase mullite, hematite, hercynite, cristobalite, quartz, pyroxenes, cordierite, gypsum, pyrite, and calcium oxides. A trend from hematite to hercynite composition indicates compositional variations at sample scale, which evidence d-spacing differences in hercynite and may be related to the Al and Fe content in hercynite depending on its texture. The mullite shows higher Si/Al ratio (1.21) than the theoretical composition (0.35), indicating that this mullite is more Si-rich. Three pyroxene-type compositions (diopside-type, ferrosilite-type, and a Ca–Al-rich pyroxene) were found. Both the mullite and the pyroxenes are nonstoichiometric

¿Cómo tratamos a los minerales? Análisis del modelo de mineral en la Educación Secundaria Obligatoria

Los minerales son los materiales que forman las rocas de la corteza y del interior de la Tierra. Gracias a ellos se ha obtenido una gran cantidad de información acerca de muchos de los procesos geológicos que actúan en nuestro plantea. Además, se utilizan en la fabricación de muchos de los productos que consumimos en nuestro día a día, desde alimentos hasta aparatos electrónicos. Dada la importancia que tienen los minerales, constituyen un tema interesante para tratar durante toda la escolarización. Sin embargo, diversos estudios muestran que ya desde Educación Primaria, los alumnos presentan dificultades a la hora de entender el modelo de mineral en su totalidad (Gallegos, 1998; Monteiro, Nóbrega, Abrantes y Gomes, 2012; Regueiro, 2008). Estas dificultades pueden deberse, entre otros factores, a la variedad de definiciones que existen sobre el concepto de mineral y a la transposición didáctica de este término a los libros de texto (Mateo et al., 2017). Además, los minerales constituyen un tema que se suele tratar de manera teórica en las aulas y, en muchas ocasiones, no se tienen en cuenta las ideas alternativas de los alumnos, algo que resulta fundamental, especialmente en didáctica de las ciencias. Para llevar a cabo este trabajo se ha analizado el currículo oficial de enseñanza de Educación Secundaria y una serie de libros de texto de Educación Secundaria Obligatoria de la Ley Orgánica 8/2013, de 9 de diciembre, para la mejora de la calidad educativa (LOMCE). El objetivo que se persigue es analizar el grado de conocimiento que actualmente pueden llegar a adquirir los alumnos sobre el modelo de mineral según como se establecen los contenidos en el currículo oficial de enseñanza y cómo se abordan en los libros de texto que utilizan los profesores en el aula. Además de este análisis, se proponen posibles mejoras a la hora de tratar los diferentes contenidos que engloba el modelo de mineral. Para realizar este análisis se ha utilizado como referencia un modelo científico de mineral basado en las cuatro dimensiones que proponen Cañal, García-Carmona y Cruz- Guzmán (2016) para lograr una alfabetización científica. Las dimensiones propuestas por estos autores han servido de base para establecer los elementos competenciales que se consideran necesarios para comprender el modelo de mineral en su totalidad. Todo ello será comparado con lo expuesto en el currículo oficial de enseñanza y en los libros de texto analizados

Micro- and Nanotexture and Genesis of Ball Clays in the Lower Cretaceous (SE Iberian Range, NE Spain)

Ball clay deposits in the SE of the Iberian Range (NE Iberian Peninsula) consist of Albian clays and siltstones with greyish and blackish colors, interbedded with subbituminous coals. The ball clays are nowadays mined for the manufacture of white color ceramics. The mineralogy of these deposits consists mainly of kaolinite, illitic phases, and quartz. The euhedral to sub-euhedral morphology of the kaolinites suggests their in-situ origin. The anhedral morphology of the illites and the presence of frayed illites suggest a detrital origin. At the micro-scale, authigenic kaolinite booklets are observed filling pores and forming mica/kaolinite intergrowths, in which the kaolinite grows between the cleavage sheets of pre-existing detrital mica. At nanometer scale, illite/smectite (IS) phases are detected forming interlayers with mica and kaolinite, and evidence of the replacement of mica by kaolinite is observed. The matrix consists of defective illite and kaolinite, and random mixed layers of kaolinite-I/S (Kln-IS), illite-I/S (Ilt-IS), and I/S-smectite (IS-S). The textures of illite and the presence of different types of mixed layers suggest that the expandable phases and kaolinite are products of mica alteration. The effectivity of the alteration was probably a consequence of the low pH that occurred in the environment due to the presence of abundant organic- and acidic- rich fluids