Chemical weathering in a humid temperate environment

Chemical weathering in Maryland (USA) takes place in an open system flushed by through flowing meteoric waters. Weathering of a felsic schist results in thick saprolite composed of quartz and clay minerals ; where leaching is enhanced, gibbsite occurs. Weathering of serpentinite produces no saprolite due to lack of resistant minerals, and lack of alumina, but SiO is precipitated. Isovolumetric chemical weathering of mafic rocks results in loss of 30-60% of mass, bulk density decrease, and porosity increase. In well drained profiles plagioclase alters before hornblende, and large amounts of x-ray amorphous matter are produced. In poorly drained sites, hornblende and plagioclase alter simultaneously.Cleaves E.T. Chemical weathering in a humid temperate environment. In: Pétrologie des altérations et des sols. Vol. II : Pétrologie des séquences naturelles. Colloque international du CNRS, Paris 4-7 juillet 1983. Strasbourg : Institut de Géologie – Université Louis-Pasteur, 1983. pp. 47-55. (Sciences Géologiques. Mémoire, 72

Chemical weathering in a humid temperate environment

Chemical weathering in Maryland (USA) takes place in an open system flushed by through flowing meteoric waters. Weathering of a felsic schist results in thick saprolite composed of quartz and clay minerals ; where leaching is enhanced, gibbsite occurs. Weathering of serpentinite produces no saprolite due to lack of resistant minerals, and lack of alumina, but SiO is precipitated. Isovolumetric chemical weathering of mafic rocks results in loss of 30-60% of mass, bulk density decrease, and porosity increase. In well drained profiles plagioclase alters before hornblende, and large amounts of x-ray amorphous matter are produced. In poorly drained sites, hornblende and plagioclase alter simultaneously.Cleaves E.T. Chemical weathering in a humid temperate environment. In: Pétrologie des altérations et des sols. Vol. II : Pétrologie des séquences naturelles. Colloque international du CNRS, Paris 4-7 juillet 1983. Strasbourg : Institut de Géologie – Université Louis-Pasteur, 1983. pp. 47-55. (Sciences Géologiques. Mémoire, 72

Evolução geoquímica e mineralógica em perfis de alteração sobre rochas serpentinizadas no sudoeste de Minas Gerais Geochemical and mineralogical evolution in alteration profiles on serpentinized rocks in southwestern Minas Gerais, Brazil

Estudou-se a evolução geoquímica e mineralógica em três perfis distintos de alteração de rochas serpentinizadas que ocorrem nas imediações dos municípios de Alpinópolis e Fortaleza de Minas, no sudoeste do Estado de Minas Gerais, sob regimes de umidade e de temperatura údico e térmico, respectivamente. Nas condições atuais, o grau de evolução química e mineralógica é moderado em relação ao desenvolvido sobre outros tipos de rochas básicas e ultrabásicas da mesma área, caracterizando-se por uma importante perda de Na e Mg e, em menor proporção, de Ca e Si. O Al (localmente também o Fe) é o elemento menos móvel dos sistemas. O K é escasso no material de origem e nas zonas de alteração, e ocorre enriquecimento desse elemento nos horizontes superficiais por aporte externo. Os minerais primários mais facilmente intemperizáveis, como o talco, a tremolita e a clorita trioctaédrica, são abundantes ainda na fração argila desses solos tropicais com composição mineralógica pouco comum, mas são todos termodinamicamente instáveis. Do ponto de vista geoquímico, o processo de alteração atual pode ser definido como uma bissialitização, que pode coincidir com ferruginização, com formação de minerais trioctaédricos secundários por transformação direta de estrutura e também por neoformação, todos coexistindo com os minerais primários residuais. No entanto, as fases de maior evolução, em volumes com drenagem mais eficiente, tendem à monossialitização, com formação de caulinitas de diferentes graus de cristalinidade. A assembléia mineralógica existente evidencia a metaestabilidade e o caráter incipiente do sistema pedogenético.<br>The geochemical and mineralogical evolution was studied in three different alteration profiles of ultramafic (serpentine) rocks near Alpinópolis and Fortaleza de Minas, in southwestern Minas Gerais State (Brazil). Soil moisture and temperature regimes are udic and thermic, respectively. The current chemical and mineralogical evolution degree is moderate compared to other basic and ultramafic material of the same area and is characterized by significant losses of Na and Mg and, to a lesser extent, of Ca and Si. Very little K was found in the parent material and in the alteration zones, whereas the surface horizons are enriched by external addition. Aluminum (locally also Fe) are the least mobile elements of the system. The primary easily weatherable minerals, such as talc, tremolite and also trioctahedral chlorite, are abundant in the clay fraction and are all thermodynamically unstable in these tropical soils of rare mineralogical composition. The ongoing geochemical process can be defined as bisialitization that can coincide with ferruginization, with the formation of trioctahedric secondary minerals by direct transformation of the structure and also by neoformation, all coexisting with the residual primary minerals. However the most weathered phases observed in well-drained positions, tended to monosialitization with kaolinite formation of variable crystallinity degree. The mineralogical assembly evidences the metastability and incipient nature of the pedogenetic system

Astrobiology and the possibility of life on Earth and elsewhere…

Astrobiology is an interdisciplinary scientific field not only focused on the search of extraterrestrial life, but also on deciphering the key environmental parameters that have enabled the emergence of life on Earth. Understanding these physical and chemical parameters is fundamental knowledge necessary not only for discovering life or signs of life on other planets, but also for understanding our own terrestrial environment. Therefore, astrobiology pushes us to combine different perspectives such as the conditions on the primitive Earth, the physicochemical limits of life, exploration of habitable environments in the Solar System, and the search for signatures of life in exoplanets. Chemists, biologists, geologists, planetologists and astrophysicists are contributing extensively to this interdisciplinary research field. From 2011 to 2014, the European Space Agency (ESA) had the initiative to gather a Topical Team of interdisciplinary scientists focused on astrobiology to review the profound transformations in the field that have occurred since the beginning of the new century. The present paper is an interdisciplinary review of current research in astrobiology, covering the major advances and main outlooks in the field. The following subjects will be reviewed and most recent discoveries will be highlighted: the new understanding of planetary system formation including the specificity of the Earth among the diversity of planets, the origin of water on Earth and its unique combined properties among solvents for the emergence of life, the idea that the Earth could have been habitable during the Hadean Era, the inventory of endogenous and exogenous sources of organic matter and new concepts about how chemistry could evolve towards biological molecules and biological systems. In addition, many new findings show the remarkable potential life has for adaptation and survival in extreme environments. All those results from different fields of science are guiding our perspectives and strategies to look for life in other Solar System objects as well as beyond, in extrasolar worlds