Equatorial rain forest lateritic mantles : a geomembrane filter

ABSTRACT The superimposed weathering layers in equatorial rain forest lateritic mantles from Gabon, AtXca, function as interactive compartments forming a dynamic semipermeable geomembrane filter. Selectivity of the filter is controlled by a progressive downward disappearance Of Connected macropore pathways created by bioturbation and dissolution. The natural balance of root activity, translocation, dissolution, deformation, and pore evolution leads to the development of porous and permeable, mature, open geochemical weathering systems at the expense of the lithosphere. These conclusions can be useful in modeling the fate of lateritic soils, which cover one-third of the emerged area of the world and which are economically important both as metal deposits and agricultural soils

Onion morphology and microstructure of polyhedral serpentine

We describe the shape and internal structure of polyhedral spheroids found in serpentinized peridotites which correspond to a new serpentine microstructure. Serpentine spheroids resemble geodesic domes made of c. 160 to 180 triangular facets. At facet edges, the nested layers bend by c. 14° along their three crystallographic directions, resulting in an onion-like structure with lateral continuity of the layers. The stacking of the serpentine layers within sectors is controlled by interlayer bonding. These polyhedral onions are made of a novel type of spherical nanostructure for layered materials

Electronic, vibrational, and thermodynamic properties of ZnS (zincblende and rocksalt structure)

We have measured the specific heat of zincblende ZnS for several isotopic compositions and over a broad temperature range (3 to 1100 K). We have compared these results with calculations based on ab initio electronic band structures, performed using both LDA and GGA exchange- correlation functionals. We have compared the lattice dynamics obtained in this manner with experimental data and have calculated the one-phonon and two-phonon densities of states. We have also calculated mode Grueneisen parameters at a number of high symmetry points of the Brillouin zone. The electronic part of our calculations has been used to investigate the effect of the 3d core electrons of zinc on the spin-orbit splitting of the top valence bands. The effect of these core electrons on the band structure of the rock salt modification of ZnS is also discussed.Comment: 33pages, 16 Figures, submitted to Phys. Rev.

Afro Re-Existence in the School Of Arts

This article adopts an ethnographic approach to describe a learning experience aimed at making visible the works of African and Afro-descendant artists among undergraduate Visual Arts students at Universidad Veracruzana (in Xalapa, Veracruz, Mexico). From a decolonial perspective, an analysis is conducted of how their knowledge has increased in terms of Africa and its diaspora, the contemporary art of African artists, the work of Afro-Mexican artists and the gaps in their education regarding knowledge of the African continent and the history of Afro-descendance in Mexico. Furthermore, an exploration is made of the terms in which they have reflected about how blackness is represented in art, the place occupied by Africa and its diaspora in their education and how they relate to the ethnic category “Afro-descendant” with which, in some cases, singular processes of Afro-Mexican ethnogenesis were triggered

Combinatorial molecular optimization of cement hydrates

Despite its ubiquitous presence in the built environment, concrete’s molecular-level properties are only recently being explored using experimental and simulation studies. Increasing societal concerns about concrete’s environmental footprint have provided strong motivation to develop new concrete with greater specific stiffness or strength (for structures with less material). Herein, a combinatorial approach is described to optimize properties of cement hydrates. The method entails screening a computationally generated database of atomic structures of calcium-silicate-hydrate, the binding phase of concrete, against a set of three defect attributes: calcium-to-silicon ratio as compositional index and two correlation distances describing medium-range silicon-oxygen and calcium-oxygen environments. Although structural and mechanical properties correlate well with calcium-to-silicon ratio, the cross-correlation between all three defect attributes reveals an indentation modulus-to-hardness ratio extremum, analogous to identifying optimum network connectivity in glass rheology. We also comment on implications of the present findings for a novel route to optimize the nanoscale mechanical properties of cement hydrate.National Ready Mixed Concrete Association (Research sponsorship)Education Foundation (N.J.) (Research sponsorship)Portland Cement Association (Research sponsorship

Combinatorial molecular optimization of cement hydrates

Despite its ubiquitous presence in the built environment, concrete’s molecular-level properties are only recently being explored using experimental and simulation studies. Increasing societal concerns about concrete’s environmental footprint have provided strong motivation to develop new concrete with greater specific stiffness or strength (for structures with less material). Herein, a combinatorial approach is described to optimize properties of cement hydrates. The method entails screening a computationally generated database of atomic structures of calcium-silicate-hydrate, the binding phase of concrete, against a set of three defect attributes: calcium-to-silicon ratio as compositional index and two correlation distances describing medium-range silicon-oxygen and calcium-oxygen environments. Although structural and mechanical properties correlate well with calcium-to-silicon ratio, the cross-correlation between all three defect attributes reveals an indentation modulus-to-hardness ratio extremum, analogous to identifying optimum network connectivity in glass rheology. We also comment on implications of the present findings for a novel route to optimize the nanoscale mechanical properties of cement hydrate.National Ready Mixed Concrete Association (Research sponsorship)Education Foundation (N.J.) (Research sponsorship)Portland Cement Association (Research sponsorship

Investigación descolonizada y formación crítica en ciencias sociales

This paper addresses the significance and the challenges of various decolonized practices in research and in social science training methods in Mexico and, particularly, in Chiapas, based on a Latin American approach to the field of studies conducted by intellectuals and activists involved in social movement struggles. We analyze different forms of intercultural dialogues used in decolonizing research in terms of social, ethnic and gender relationships, according to the contexts and the knowledge production of popular and indigenous movements facing politics, education and epistemic racism.Este artículo aborda los alcances y los retos de diferentes prácticas descolonizadas en la investigación y la formación en ciencias sociales en México y, en particular, en Chiapas, a partir de un acercamiento latinoamericano al campo de los estudios realizados por intelectuales y activistas involucrados en las luchas de los movimientos sociales. Se analizan las formas comprometidas de diálogo intercultural en trabajos de investigación descolonizada en función de relaciones sociales, étnicas y de género, propias de los contextos y los conocimientos que se producen en movimientos populares e indígenas ante la política, la educación y el racismo epistémico

Zapatismo y educación autónoma: de la rebelión a la dignidad indígena

A partir de finales de los años 90, las comunidades mayas que pertenecen a los municipios autónomos zapatistas de Chiapas están poniendo en práctica el derecho de los pueblos indígenas a construir y administrar sus escuelas según sus modos propios de organización interna. Esto implica que las familias militantes tengan la capacidad de seleccionar, formar y evaluar al educador bilingüe. En la cotidianidad de su rebelión educativa, las bases de apoyo del movimiento zapatista en la región rural de las Cañadas de la Selva Lacandona se comprometen internamente mediante prácticas de asamblea y delegaciones de cargos comunitarios. En oposición al centralismo de la política indigenista nacional, la organización comunitaria de la educación autónoma contribuye a fortalecer la dignidad colectiva de sus actores directos, los cuales se enorgullecen de poder construir y tener en sus manos el control local de la educación formal en los territorios rebeldes del sureste mexicano

La construcción de la demanda educativa en los pueblos originarios del sureste mexicano

This article analyses processes of social construction that shape the demands for education in the Cañadas (canyons) of Ocosingo, in the Lacandon Jungle, south-eastern Chiapas in Mexico. The relations among educational actors and peasant communities, mainly Tseltales, are marked since the emergence of schools in the twentieth century by an organizational context of negotiation and confrontation with the State, involving processes of politicization of ethnic identities and social appropriation of school education. Under the conditions created by political conflict, the Maya peoples have generated and extended new experiences of community schools since the end of the 80s and over the last two decades, in accordance with renewed practices of self-government that give meaning to an alternative project that they have assumed as their own autonomous undertaking. Based on socio-anthropological research methods, ethnographies and interviews, the article studies the generation of autonomous educational practices in a multicultural region in conflict, whose territories are reorganized twenty years after the creation of new municipalities by the Zapatistas. Moreover, in the two decades preceding the emergence of the Zapatista movement’s schools, the demand for education is also constructed in relation to self-organizing peasants struggling for the dignity of the Mayan cultures. The current alternative practices are presented as a product of self-managed educational policies by indigenous actors, embeded in structures of communal and regional power that are unrecognized and excluded within the framework of national education policies.Este artículo analiza procesos de construcción social que estructuran las demandas de educación en las Cañadas de Ocosingo de la Selva Lacandona, al sureste de Chiapas en México. Las relaciones entre actores educativos y comunidades de campesinos, principalmente tseltales, están marcadas desde la aparición de la escuela en el siglo xx por un contexto organizativo de negociación y confrontación con el Estado, en el cual ocurren procesos de politización de las identidades étnicas y la apropiación social de la educación escolar. En las condiciones dadas por el conflicto político, los pueblos mayas generan y extienden experiencias novedosas de escuelas comunitarias, desde el final de los años ochenta y en las dos últimas décadas, de acuerdo con prácticas renovadas de autogobierno que dan sentido a un proyecto alternativo asumido como propio y autónomo. A partir de métodos de investigación socio-antropológica, etnografías y entrevistas, el artículo estudia la generación de prácticas educativas autonómicas en una región multicultural en conflicto, cuyos territorios están reorganizados a veinte años de la creación de nuevos municipios por el zapatismo. Además, en las dos décadas que preceden el surgimiento de las escuelas del movimiento zapatista, la demanda de educación se construye asimismo en relación con la autoorganización campesina en lucha por la dignidad de las culturas mayas. Las actuales prácticas alternativas se presentan como producto de políticas educativas autoadministradas por actores indígenas incrustados en estructuras de poder comunal y regional que son desconocidas y excluidas en el marco de las políticas nacionales de educación