Mercedes Doval

Occurrence of silica polymorphs nanocrystals in tuffaceous rocks, Province of the Mesa Central, Mexico, and their formation from subcritical Si-rich fluids

Abstract Cristobalite-tridymite blade nanocrystals cemented by SiO2-glass and tridymite nanocrystals fill separately SiO2-glass spherules released by explosive volcanism, in rhyolitic tuffs from the Province of the Mesa Central, Mexico. This paper presents the mineralogy of silica polymorphs, occurrence, and process of formation in the Province of the Mesa Central. The understanding of the origin of these pure SiO2-polymorphs, their association with high-temperature minerals, fractionation of magmas, and role of volatiles contribute to our knowledge on silica minerals, their technological implications and damaging health effects. Results indicate that a precursor magma, from which kyanite crystallized, partitioned into an immiscible Fe-rich magmatic liquid that crystallized Fe-cordierite, Fe-amphiboles, and fayalite and into a siliceous melt that led to low-temperature glasses of 78.22-80.01 wt% SiO2 and Si/Al ratio 4.07-5.65. Presence of amphiboles, sulfur in cristobalite-tridymite crystals and alunite suggest association of volatiles. The crystallization of silica polymorphs is associated with the dissolution of water vapor and volatiles in the precursor magma, establishing a silicate melt-water system of two critical points, one of them at pressure and temperature near the critical point of water and another close to the critical point of SiO2, and defining a critical curve between them and supercriticality at pressures and temperatures lower than the critical point of SiO2. Decreasing the ambient conditions from supercritical to subcritical would have allowed the separation of liquid and gases and the crystallization of cristobalite-tridymite and tridymite nanocrystals from the vapor phase. Cristobalite single crystals were not formed and transformation of cristobalite and trydimite to quartz did not occur. We conclude that glasses did not crystallize cristobalite or tridymite. Components in excess of the pure phases precipitated as nanoparticles of siliceous glass forming agglomerates, some containing iron hydroxides and alunogen.

Clay minerals associations in palaeoweathering profiles from Central Spain: genesis and implications

Abstract This study examines part of the thick palaeoweathering mantle that formed on the northern area of the Spanish Central System. The study of a compound profile indicates that despite weathering processes, the primary structure of the metamorphic rocks is preserved, and is only partially lost in some intervals of the upper part of the compound profile. Macro/micromorphology, mineralogy and geochemical changes within the profiles revealed two weathering paths. In the first path, Fe-chlorite weathered to chlorite-smectite mixed-layer/smectite/kaolinite + iron oxides. In the second path, biotite and/or muscovite weathered to kaolinite + iron oxides. The profiles show a progressive decrease, from base to top, in mica and mixed-layers and an increase in smectite and kaolinite. Thus, the profiles only comprise the lower or intermediate zones of the weathering mantle. The weathering occurred under humid climates; the lower zones of the profiles were poorly drained, whereas the topmost zones were better drained and more oxidizing. The results obtained indicate that detailed mineralogical studies are very useful to reconstruct the characteristics of the weathering mantles, and as palaeogeographic and palaeoclimatic indicators.

Geochemistry of reversible hydratable tephra from the Trans Mexican Volcanic Belt

Abstract Rhyolitic glass of high, reversible adsorption water (to 12.63 wt%) occurs in pyroclastic rocks from the La Malinche stratovolcano in the Mexican Volcanic Belt. The glass constitutes 98 vol% of the pyroclastics. It is a heterogeneous glass that dehydrates reversibly at 72°C, composed of sodic and non-sodic glasses of surface activity caused by IVAl substituting in Q4(1Al) and Q4(2Al) positions, minor VAl, tetrahedra terminating in NBOs, and insufficient Na and Ca to charge balance Al in the glass network. Adsorption is of molecular water H2Om in interstitial sites, H-bonded to silanol groups, to the silica network, and to other H2Om molecules. Sodic glasses contain 71.80-77.77 wt% SiO2, are partially devitrified to crystallites (~5 nm size) of Na-plagioclase and clinopyroxene, and exhibit minor low-grade metamorphism to <1 vol% crystals of mazzite (~10 μm size). Sodium-free glasses are more siliceous, with 74.84-83.88 wt% SiO2, show partial devitrification to crystallites (~5 nm size) of Ca-plagioclase and clinopyroxene, with minor low-grade metamorphism of glass and plagioclase to <1 vol% crystals of laumontite (~10 μm size)

Estudio de las ofitas del Triásico y de su influencia en la Mineralogía de los sedimentos arcillosos del Keuper.

In this paper the influence of igneous basic rocks on the mineralogical composition of Keuper materials, has been studied in the North border of Iberian Range. Te mineralogy of igneous and sedimentaryrocks, has beendeterminated by X-ray diffraction and optical microscopyand also her chemical composítion. From the experimental data, it has been establishedan agradationprocess leading to the formation of chlorites and micas of a good cristalinity, in the outcrops nearof basic igneousrocks. This agradationprocess,can be related to the local rises of temperature and also to a probable issue of alkalis and magnesiunfrom the igneous materials. CARAcTERÍSTICAS GEOLÓGICASGENERALES La zona estudiadaestá situada en la provincia de Soria, dentro de los términos de Toranzo y Cuevas de Agreda. Desdeel punto de vista geológico correspondeal borde septentrionalde la rama aragonesade la Cordillera Ibérica, en las estribacionesoccidentalesde la Sierra del Moncayo. Como en el resto de la Cordillera Ibérica el Triásico de esta zona se presentaen facies germánica.El Buntsandsteinestá constituidoen su basepor un conglomeradocuarcítico de 3-5 metros de potencia,sobre el que se encuentran areniscas,arcillas y limos rojos, constituyendouna compleja sucesión de algo más de 300 metros. Los tramos detríticos finos están compuestos por cuarzo y filosilicatos en proporciones muy variables, junto con plagioclasasy dolomíaen pequeñacantidad.Los filosilicatos estánconstituidos ca si exclusivamentepor ifltn acompañadapor inter..fl~A-~ (lOt El Muschelkalk está representadopor calizas dolomíticas grises, tableadas y, frecuentementerecristalizadas.Su potencia es bastantereducidaen esta zona, no rebasandoen ningún punto los 20 metros. Por último, el Kcuper se encuentraconstituido por arcillas rojas y margasyesíferas,entre las que se encuentranfinas intercalacionesde do]omias cavernosasy calizas dolomíticas con anhidrita, cuya frecuenciaaumentahacia su techo, pasándoseinsensiblementea tramos dolomíticos y brechoidesde color beige claro, que muy probablementerepresenteya la base del Jurásico.Litológicamenteel límite Keuper-Liásico es imposible de precisar.La potenciade los tramos asignadosal Keuper de forma convencionalse puede cifrar en unos 50 metros. No obstante,debido a la erosiónpor una parte y a su carácter plástico por otra, la potencia original fue probablementebastante superior. Un poco más al Este de esta zona, en la región de TrasobaresTierga, RIBA et al (1971) le asignanuna potenciade 100-225 metros. Ruiz CRUZ (1974) en su estudio sobre la mineralogíadel Triásico de la Cordillera Ibérica, describe detalladamenteel Keuper de esta zona, que denominacomo serie de Noviercas-Borobia.Consideraque los tramos detríticos arcillosos del Keuper estánconstituidos fundamentalmentepor cuarzo (10-40%) y filosilicatos (50-70 %), junto con plagioclasas(0-25 %), calcita (0-5 %), dolomita (0-5 3⁄4)y yeso en porcentajesmuy variables. Los filosili-